Showing papers in "Economic Geology in 2010"

Porphyry Copper Systems

Abstract: Porphyry Cu systems host some of the most widely distributed mineralization types at convergent plate boundaries, including porphyry deposits centered on intrusions; skarn, carbonate-replacement, and sediment-hosted Au deposits in increasingly peripheral locations; and superjacent high- and intermediate-sulfidation epithermal deposits. The systems commonly define linear belts, some many hundreds of kilometers long, as well as occurring less commonly in apparent isolation. The systems are closely related to underlying composite plutons, at paleodepths of 5 to 15 km, which represent the supply chambers for the magmas and fluids that formed the vertically elongate (>3 km) stocks or dike swarms and associated mineralization. The plutons may erupt volcanic rocks, but generally prior to initiation of the systems. Commonly, several discrete stocks are emplaced in and above the pluton roof zones, resulting in either clusters or structurally controlled alignments of porphyry Cu systems. The rheology and composition of the host rocks may strongly influence the size, grade, and type of mineralization generated in porphyry Cu systems. Individual systems have life spans of ~100,000 to several million years, whereas deposit clusters or alignments as well as entire belts may remain active for 10 m.y. or longer. The alteration and mineralization in porphyry Cu systems, occupying many cubic kilometers of rock, are zoned outward from the stocks or dike swarms, which typically comprise several generations of intermediate to felsic porphyry intrusions. Porphyry Cu ± Au ± Mo deposits are centered on the intrusions, whereas carbonate wall rocks commonly host proximal Cu-Au skarns, less common distal Zn-Pb and/or Au skarns, and, beyond the skarn front, carbonate-replacement Cu and/or Zn-Pb-Ag ± Au deposits, and/or sediment-hosted (distal-disseminated) Au deposits. Peripheral mineralization is less conspicuous in noncarbonate wall rocks but may include base metal- or Au-bearing veins and mantos. High-sulfidation epithermal deposits may occur in lithocaps above porphyry Cu deposits, where massive sulfide lodes tend to develop in deeper feeder structures and Au ± Ag-rich, disseminated deposits within the uppermost 500 m or so. Less commonly, intermediate-sulfidation epithermal mineralization, chiefly veins, may develop on the peripheries of the lithocaps. The alteration-mineralization in the porphyry Cu deposits is zoned upward from barren, early sodic-calcic through potentially ore-grade potassic, chlorite-sericite, and sericitic, to advanced argillic, the last of these constituting the lithocaps, which may attain >1 km in thickness if unaffected by significant erosion. Low sulfidation-state chalcopyrite ± bornite assemblages are characteristic of potassic zones, whereas higher sulfidation-state sulfides are generated progressively upward in concert with temperature decline and the concomitant greater degrees of hydrolytic alteration, culminating in pyrite ± enargite ± covellite in the shallow parts of the litho-caps. The porphyry Cu mineralization occurs in a distinctive sequence of quartz-bearing veinlets as well as in disseminated form in the altered rock between them. Magmatic-hydrothermal breccias may form during porphyry intrusion, with some of them containing high-grade mineralization because of their intrinsic permeability. In contrast, most phreatomagmatic breccias, constituting maar-diatreme systems, are poorly mineralized at both the porphyry Cu and lithocap levels, mainly because many of them formed late in the evolution of systems. Porphyry Cu systems are initiated by injection of oxidized magma saturated with S- and metal-rich, aqueous fluids from cupolas on the tops of the subjacent parental plutons. The sequence of alteration-mineralization events charted above is principally a consequence of progressive rock and fluid cooling, from >700° to <250°C, caused by solidification of the underlying parental plutons and downward propagation of the lithostatic-hydrostatic transition. Once the plutonic magmas stagnate, the high-temperature, generally two-phase hyper-saline liquid and vapor responsible for the potassic alteration and contained mineralization at depth and early overlying advanced argillic alteration, respectively, gives way, at <350°C, to a single-phase, low- to moderate-salinity liquid that causes the sericite-chlorite and sericitic alteration and associated mineralization. This same liquid also causes mineralization of the peripheral parts of systems, including the overlying lithocaps. The progressive thermal decline of the systems combined with synmineral paleosurface degradation results in the characteristic overprinting (telescoping) and partial to total reconstitution of older by younger alteration-mineralization types. Meteoric water is not required for formation of this alteration-mineralization sequence although its late ingress is commonplace. Many features of porphyry Cu systems at all scales need to be taken into account during planning and execution of base and precious metal exploration programs in magmatic arc settings. At the regional and district scales, the occurrence of many deposits in belts, within which clusters and alignments are prominent, is a powerful exploration concept once one or more systems are known. At the deposit scale, particularly in the porphyry Cu environment, early-formed features commonly, but by no means always, give rise to the best ore-bodies. Late-stage alteration overprints may cause partial depletion or complete removal of Cu and Au, but metal concentration may also result. Recognition of single ore deposit types, whether economic or not, in porphyry Cu systems may be directly employed in combination with alteration and metal zoning concepts to search for other related deposit types, although not all those permitted by the model are likely to be present in most systems. Erosion level is a cogent control on the deposit types that may be preserved and, by the same token, on those that may be anticipated at depth. The most distal deposit types at all levels of the systems tend to be visually the most subtle, which may result in their being missed due to overshadowing by more prominent alteration-mineralization.

Iron Formation: The Sedimentary Product of a Complex Interplay among Mantle, Tectonic, Oceanic, and Biospheric Processes

Abstract: Iron formations are economically important sedimentary rocks that are most common in Precambrian sedimentary successions. Although many aspects of their origin remain unresolved, it is widely accepted that secular changes in the style of their deposition are linked to environmental and geochemical evolution of Earth. Two types of Precambrian iron formations have been recognized with respect to their depositional setting. Algoma-type iron formations are interlayered with or stratigraphically linked to submarine-emplaced volcanic rocks in greenstone belts and, in some cases, with volcanogenic massive sulfide (VMS) deposits. In contrast, larger Superior-type iron formations are developed in passive-margin sedimentary rock successions and generally lack direct relationships with volcanic rocks. The early distinction made between these two iron-formation types, although mimimized by later studies, remains a valid first approximation. Texturally, iron formations were also divided into two groups. Banded iron formation (BIF) is dominant in Archean to earliest Paleoproterozoic successions, whereas granular iron formation (GIF) is much more common in Paleoproterozoic successions. Secular changes in the style of iron-formation deposition, identified more than 20 years ago, have been linked to diverse environmental changes. Geochronologic studies emphasize the episodic nature of the deposition of giant iron formations, as they are coeval with, and genetically linked to, time periods when large igneous provinces (LIPs) were emplaced. Superior-type iron formation first appeared at ca. 2.6 Ga, when construction of large continents changed the heat flux at the core-mantle boundary. From ca. 2.6 to ca. 2.4 Ga, global mafic magmatism culminated in the deposition of giant Superior-type BIF in South Africa, Australia, Brazil, Russia, and Ukraine. The younger BIFs in this age range were deposited during the early stage of a shift from reducing to oxidizing conditions in the ocean-atmosphere system. Counterintuitively, enhanced magmatism at 2.50 to 2.45 Ga may have triggered atmospheric oxidation. After the rise of atmospheric oxygen during the GOE at ca. 2.4 Ga, GIF became abundant in the rock record, compared to the predominance of BIF prior to the Great Oxidation Event (GOE). Iron formations generally disappeared at ca. 1.85 Ga, reappearing at the end of the Neoproterozoic, again tied to periods of intense magmatic activity and also, in this case, to global glaciations, the so-called Snowball Earth events. By the Phanerozoic, marine iron deposition was restricted to local areas of closed to semiclosed basins, where volcanic and hydrothermal activity was extensive (e.g., back-arc basins), with ironstones additionally being linked to periods of intense magmatic activity and ocean anoxia. Late Paleoproterozoic iron formations and Paleozoic ironstones were deposited at the redoxcline where biological and nonbiological oxidation occurred. In contrast, older iron formations were deposited in anoxic oceans, where ferrous iron oxidation by anoxygenic photosynthetic bacteria was likely an important process. Endogenic and exogenic factors contributed to produce the conditions necessary for deposition of iron formation. Mantle plume events that led to the formation of LIPs also enhanced spreading rates of midocean ridges and produced higher growth rates of oceanic plateaus, both processes thus having contributed to a higher hydrothermal flux to the ocean. Oceanic and atmospheric redox states determined the fate of this flux. When the hydrothermal flux overwhelmed the oceanic oxidation state, iron was transported and deposited distally from hydrothermal vents. Where the hydrothermal flux was insufficient to overwhelm the oceanic redox state, iron was deposited only proximally, generally as oxides or sulfides. Manganese, in contrast, was more mobile. We conclude that occurrences of BIF, GIF, Phanerozoic ironstones, and exhalites surrounding VMS systems record a complex interplay involving mantle heat, tectonics, and surface redox conditions throughout Earth history, in which mantle heat unidirectionally declined and the surface oxidation state mainly unidirectionally increased, accompanied by superimposed shorter term fluctuations.

Iron Oxide Copper-Gold (IOCG) Deposits through Earth History: Implications for Origin, Lithospheric Setting, and Distinction from Other Epigenetic Iron Oxide Deposits

Abstract: The iron oxide copper-gold (IOCG) group of deposits, initially defined following discovery of the giant Olympic Dam Cu-U-Au deposit, has progressively become too-embracing when associated deposits and potential end members or analogs are included. The broader group includes several low Ti iron oxide-associated deposits that include iron oxide (P-rich), iron oxide (F- and REE-rich), Fe or Cu-Au skarn, high-grade iron oxide-hosted Au ± Cu, carbonatite-hosted (Cu-, REE-, and F-rich), and IOCG sensu stricto deposits. Consideration of this broad group as a whole obscures the critical features of the IOCG sensu stricto deposits, such as their temporal distribution and tectonic environment, thus leading to difficulties in developing a robust exploration model. The IOCG sensu stricto deposits are magmatic-hydrothermal deposits that contain economic Cu and Au grades, are structurally controlled, commonly contain significant volumes of breccia, are commonly associated with presulfide sodic or sodic-calcic alteration, have alteration and/or brecciation zones on a large, commonly regional, scale relative to economic mineralization, have abundant low Ti iron oxides and/or iron silicates intimately associated with, but generally paragenetically older than, Fe-Cu sulfides, have LREE enrichment and low S sulfides (lack of abundant pyrite), lack widespread quartz veins or silicification, and show a clear temporal, but not close spatial, relationship to major magmatic intrusions. These intrusions, where identified, are commonly alkaline to subalkaline, mixed mafic (even ultramafic) to felsic in composition, with evidence for mantle derivation of at least the mafic end members of the suite. The giant size of many of the deposits and surrounding alteration zones, the highly saline ore fluids, and the available stable and radiogenic isotope data indicate release of deep, volatile-rich magmatic fluids through devolatization of causative, mantle-derived magmas and variable degrees of mixing of these magmatic fluids with other crustal fluids along regional-scale fluid flow paths. Precambrian deposits are the dominant members of the IOCG group in terms of both copper and gold resources. The 12 IOCG deposits with >100 tonnes (t) resources are located in intracratonic settings within about 100 km of the margins of Archean or Paleoproterozoic cratons or other lithospheric boundaries, and formed 100 to 200 m.y. after supercontinent assembly. Their tectonic setting at formation was most likely anorogenic, with magmatism and associated hydrothermal activity driven by mantle underplating and/or plumes. Limited amounts of partial melting of volatile-rich and possibly metal-enriched metasomatized early Precambrian subcontinental lithospheric mantle (SCLM), fertilized during earlier subduction, probably produced basic to ultrabasic magmas that melted overlying continental crust and mixed with resultant felsic melts, with devolatilization and some penecontemporaneous incorporation of other lower to middle crustal fluids to produce the IOCG deposits. Preservation of near-surface deposits, such as Olympic Dam, is probably due to their formation above buoyant and refractory SCLM, which resisted delamination and associated uplift. Most Precambrian iron oxide (P-rich) or magnetite-apatite (Kiruna-type) deposits have a different temporal distribution, apparently forming in convergent margin settings prior to or following supercontinent assembly. It is only in the Phanerozoic that IOCG and magnetite-apatite deposits are roughly penecontemporaneous in convergent margin settings. The Phanerozoic IOCG deposits, such as Candelaria, Chile, occur in anomalous extensional to transtensional zones in the Coastal Cordillera, which are also the sites of mantle-derived mafic to felsic intrusions that are anomalous in an Andean context. This implies that special conditions, possibly detached slabs of metasomatized SCLM, may be required in convergent margin settings to generate world-class IOCG deposits. It is likely that formation of giant IOCG deposits was mainly a Precambrian phenomenon related to the extensive mantle underplating that impacted on buoyant metasomatized SCLM. Generally smaller and rarer Phanerozoic IOCG deposits formed in tectonic settings where conditions similar to those in the Precambrian were replicated.

Sediment-Hosted Lead-Zinc Deposits in Earth History

Abstract: Sediment-hosted Pb-Zn deposits can be divided into two major subtypes. The first subtype is clastic-dominated lead-zinc (CD Pb-Zn) ores, which are hosted in shale, sandstone, siltstone, or mixed clastic rocks, or occur as carbonate replacement, within a CD sedimentary rock sequence. This subtype includes deposits that have been traditionally referred to as sedimentary exhalative (SEDEX) deposits. The CD Pb-Zn deposits occur in passive margins, back-arcs and continental rifts, and sag basins, which are tectonic settings that, in some cases, are transitional into one another. The second subtype of sediment-hosted Pb-Zn deposits is the Mississippi Valley-type (MVT Pb-Zn) that occurs in platform carbonate sequences, typically in passive-margin tectonic settings. Considering that the redox state of sulfur is one of the major controls on the extraction, transport, and deposition of Pb and Zn at shallow crustal sites, sediment-hosted Pb-Zn ores can be considered a special rock type that recorded the oxygenation of Earth’s hydrosphere. The emergence of CD and MVT deposits in the rock record between 2.02 Ga, the age of the earliest known deposit of these ores, and 1.85 to 1.58 Ga, a major period of CD Pb-Zn mineralization in Australia and India, corresponds to a time after the Great Oxygenation Event that occurred at ca 2.4 to 1.8 Ga. Contributing to the abundance of CD deposits at ca 1.85 to 1.58 Ga was the following: (1) enhanced oxidation of sulfides in the crust that provided sulfate to the hydrosphere and Pb and Zn to sediments; (2) development of major redox and compositional gradients in the oceans; (3) first formation of significant sulfate-bearing evaporites; (4) formation of red beds and oxidized aquifers, possibly containing easily extractable Pb and Zn; (5) evolution of sulfate-reducing bacteria; and (6) formation of large and long-lived basins on stable cratons. Although MVT and CD deposits appeared for the first time in Earth history at 2.02 Ga, only CD deposits were important repositories for Pb and Zn in sediments between the Great Oxygenation Event, until after the second oxidation of the atmosphere in the late Neoproterozic. Increased oxygenation of the oceans following the second oxidation event led to an abundance of evaporites, resulting oxidized brines, and a dramatic increase in the volume of coarse-grained and permeable carbonates of the Paleozoic carbonate platforms, which host many of the great MVT deposits. The MVT deposits reached their maximum abundance during the final assembly of Pangea from Devonian into the Carboniferous. This was also a time for important CD mineral deposit formation along passive margins in evaporative belts of Pangea. Following the breakup of Pangea, a new era of MVT ores began with the onset of the assembly of the Neosupercontinent. A significant limitation on interpreting the secular distribution of the deposits is that there is no way to quantitatively evaluate the removal of deposits from the rock record through tectonic recycling. Considering that most of the sedimentary rock record has been recycled, most sediment-hosted Pb-Zn deposits probably have also been destroyed by subduction and erosion, or modified by metamorphism and tectonism, so that they are no longer recognizable. Thus, the uneven secular distribution of sediment-hosted Pb-Zn deposits reflects the genesis of these deposits, linked to Earth’s evolving tectonic and geochemical systems, as well as an unknown amount of recycling of the sedimentary rock record.

Lithospheric, Cratonic, and Geodynamic Setting of Ni-Cu-PGE Sulfide Deposits

Abstract: The location of magmatic Ni-Cu-PGE sulfide deposits is related to lithospheric architecture, particularly that of the subcontinental lithospheric mantle (SCLM). At crustal levels, this relationship is manifest by a close proximity to craton and paleocraton margins. Deposits are associated with mafic-ultramafic rocks and many show a close spatial relationship with a coeval large igneous province (LIP). Metal quantities and tenors observed in deposits require segregation of a magmatic sulfide melt from a large volume of parental ultramafic melt. Generation of these parental melts requires melting of upwelling mantle rising to depths of 100 km or less. The timing and tectonic setting of deposits indicates that this most likely occurs when mantle plumes impact on the base of the SCLM and are channeled laterally to areas of thinnest SCLM, where they undergo decompression melting. Alternatively, the setting of some smaller deposits suggests that upwelling may be induced by syn-to post-collisional lithospheric delamination. Craton margins are generally zones of relatively thin lithosphere and are the focus of strain during regional tectonism, providing points of dilation along active translithospheric faults. These faults facilitate melt introduction into the crust. The craton margin zone of thin lithosphere and active faulting may be adjacent to a neighboring block of continental lithosphere (paleocraton margin) or adjacent to a flanking narrow marginal basin (underlain by asthenosphere). All deposits form during periods of active regional tectonism, most commonly under mildly compressional to transpressional conditions. These different settings and conditions may result in differing depths of melting and differing depths and degrees of crustal interaction. The latter is believed to be a key factor for the development of a metal-rich sulfide melt, which is ultimately emplaced in the deposit environment. These variations can account for the observed range of ore and host rock types. Most large deposits are associated with intracontinental settings or with (former) passive margins at the edge of small marginal basins. No significant deposits are associated with the margins of large oceans or with supra-subduction zone environments, possibly reflecting poor preservational potential or (with the latter) limitations to plume interaction with continental lithosphere. A craton-margin model for the genesis of magmatic Ni-Cu-PGE deposits is proposed. This model provides a framework for further examination of deposit-forming processes and also provides a coherent predictive framework for mineral exploration.

The Bingham Canyon Porphyry Cu-Mo-Au Deposit. III. Zoned Copper-Gold Ore Deposition by Magmatic Vapor Expansion

Abstract: Fluid inclusion microthermometry and laser-ablation ICPMS microanalysis are combined with geological and textural observations to reconstruct the spatial and temporal evolution of magmatic fluids that formed the subvolcanic porphyry Cu-Au(-Mo) ore deposit at Bingham Canyon, Utah. The Bingham Canyon orebody is exposed over ~1.6 km vertically and has the shape of an inverted cup with distinct metal zoning. Fluid inclusions in the barren but highly veined and potassically altered deep center of the system have intermediate density (~0.6 g cm−3) and a salinity of ~7 wt percent NaCl equiv. They have subequal concentrations of Na, K, Fe, and Cu and contain minor CO2. The intermediate-density fluids were trapped as a single phase, mostly at >500°C and >800 bars. The Au-Cu-rich center near the top of the orebody contains low-density vapor inclusions (~0.2 g cm−3) coexisting with brine inclusions containing ~45 wt percent NaCl equiv. The vertical transition of different inclusion types indicates phase separation of the single-phase input fluid upon volume expansion associated with a pressure drop to 200 ± 100 bars. Mass-balance calculation based on all analyzed inclusion components indicates that the mass of the vapor phase exceeded that of the brine by ~9/1. The vapor contained Cu as its dominant cation (~1.5 wt %) and contributed about 95 percent of the total amount of copper transported to the base of the orebody. Bornite, chalcopyrite, and native gold were precipitated in a narrow temperature interval from 430° to 350°C, into secondary pore space created by local redissolution of vein quartz as a result of retrograde quartz solubility in the vapor-dominated fluid system. Intermediate-density fluid inclusions in the deepest parts of the peripheral copper ore zone have identical density and composition, including similar gold contents, as those in the deep center. Microthermometry and statistical estimation of phase proportions in the inclusions show that the vapor in the peripheral Cu-rich but Au-poor ore zone remained denser, and the separating brine was less saline (~36 wt % NaCl equiv), compared to vapor and brine in the central Au-Cu ore zone. This indicates that the peripheral fluids experienced a lower degree of phase separation, due to slightly higher fluid pressure at equivalent temperature, compared to more strongly expanding fluids in the center of the system. The systematic zoning of Au/Cu within the ore shell, despite compositionally similar input fluids, is interpreted to have resulted from slightly different pressure-temperature-density evolution paths of magmatic fluids. Copper was selectively precipitated in the peripheral ore zone, in contrast to complete coprecipitation of Au and Cu in the central upflow zone of the vapor plume. The formation of particularly rich Cu-Au ore in the center of the upward-expanding fluid plume is consistent with published experimental data, showing that the solubility of metals in hydrous vapor decreases sharply with falling pressure, due to destabilization of the hydration shell around metal complexes in expanding vapor. This interpretation supports the classic vapor plume model for porphyry copper ore formation but additionally emphasizes the role of sulfur-bearing complexes as a key chemical control on magmatic-hydrothermal metal transport and the deposition of Cu and Au in porphyry ores. Our interpretation of selective Cu ± Au precipitation as a function of vapor density can explain the more general observation that most gold-rich porphyry copper deposits are formed in shallow sub-volcanic environments, whereas deeper seated porphyry Cu-(Mo) deposits are generally gold poor.

Connections between Sulfur Cycle Evolution, Sulfur Isotopes, Sediments, and Base Metal Sulfide Deposits

Abstract: Significant links exist between the sulfur cycle, sulfur geochemistry of sedimentary systems, and ore deposits over the course of Earth history. A picture emerges of an Archean and Paleoproterozoic stage of the sulfur cycle that has much lower levels of sulfate (<200 μM ), carries a signal of mass-independent sulfur, and preserves evidence for temporal and spatial heterogeneity that reflects lower amounts of sulfur cycling than today. A second stage of ocean chemistry in the Paleoproterozoic, with higher atmospheric oxygen and oceanic sulfate at low millimolar levels, follows this stage. The isotopic record in sedimentary rocks and in sulfide-bearing ore deposits suggests abundant pyrite burial and implies a missing 34S-depleted pool that may have been lost via deep ocean deposition and possibly subduction. Proterozoic ocean chemistry appears to be quite complex. The surface waters of the Proterozoic oceans are believed to have been oxygenated, but geologic evidence from ore deposits and sedimentary rocks supports coexistence of significant sulfidic and nonsulfidic, anoxic, intermediate water and deep-water pools in the Mesoproterozoic. This stage in ocean chemistry ends with the second major global oxidation event in the latest Neoproterozoic (~600 Ma). This event started the transition to more oxygenated intermediate and deep waters, and higher but variable oceanic sulfate concentrations. The event set the scene for the formation in the Phanerozoic of the first significant MVT deposits and possibly is reflected in changes in other sedimentary rock-hosted base metal sulfide deposits.

Composition of the marginal rocks and sills of the Rustenburg Layered Suite, Bushveld Complex, South Africa: Implications for the formation of the Platinum-group element deposits

Abstract: The Bushveld Complex contains large ore deposits of platinum-group elements (PGE), V, and Cr. Understanding how these deposits formed is in part dependent on estimates of the compositions of the magmas that filled the Bushveld chamber. Over the past 20 years, estimates for the major oxides and some trace elements in the magmas have been made using the marginal rocks of the intrusion. However, data for most of the trace elements have not been available. This paper presents the results for a full range of trace elements, including the platinum-group elements. The marginal rocks of the Lower and lower Critical zones (B-1 magmas) are tholeiitic Mg-rich basaltic andesites with Mg# 71. It had been suggested that they are boninites but their mantle-normalized incompatible lithophile trace element patterns (spidergrams) resemble those of the upper continental crust and the concentrations of the elements are much higher than those of boninites. The patterns resemble siliceous high magnesium basalts. An unusual feature is that the Pt/Pd ratios are >1.5. The Pt contents of the B-1 rocks (15–25 ppb) are slightly higher than those observed in most primary mantle melts, suggesting that the high Pt/Pd ratio is due to Pt enrichment rather than Pd depletion. The crystallization order and composition of the minerals formed in equilibrium with the B-1 magma matches that of the Lower and lower Critical zones and thus this magma appears to be representative of the parental magma of these zones. The marginal rocks to the upper Critical zone (B-2) are tholeiitic basalts in terms of major element composition, with Mg# 55. The spidergrams show some similarities with E-MORB; however, the B-2 rocks have strong positive Ba and Pb anomalies and negative P, Ti, Hf, and Zr anomalies, and thus they more closely resemble lower continental crust. The B-2 rocks have lower and more variable Pt + Pd contents than the B-1 magma, suggesting that some of the samples have experienced sulfide saturation, but in common with the B-1 magmas, the Pt/Pd ratios are high, in excess of 1.5. The crystallization order of the Upper Critical zone cannot be modelled by the B-2 magma alone. However, mixtures of B-2 magma and B-1 magma satisfy the crystallization order and mineral composition of the upper Critical zone. The marginal rocks of the Main zone (B-3) are also tholeiitic basalts in terms of major element composition but have a higher Mg# (62) than the B-2 rocks. Trace element patterns in part resemble those of B-2 magmas but are depleted in most incompatible elements with large positive Ba, Pb, and Eu anomalies and negative Nb, Ta, Hf, and Zr anomalies, suggesting the rocks contain a plagioclase component. The PGE contents of the B-3 rocks are lower than those of the B-1 magma and less variable than those of the B-2 magma, but in common with both the other magmas, they have high Pt/Pd. The crystallization order and composition of the minerals in equilibrium with the B-3 magma matches that of the Main zone. Two processes have been suggested to explain the compositions of the Bushveld magmas: mixing of primitive mantle melts with partial melts of continental crust and mixing of primitive mantle melts with melts derived from the subcontinental lithospheric mantle (SCLM). The trace element concentrations of the magmas can be modelled by crustal contamination. This interpretation is supported by oxygen isotopes, initial 87Sr/86Sr ratios and ɛNd of the cumulate rocks. However, the high Pt/Pd ratios of all of the magmas and the overall higher than normal Pt concentrations of the B-1 magma are difficult to explain by mixing of primary mantle melt with crustal components. The SCLM has high Pt/Pd ratios and mixing of primary mantle magma with SCLM-derived magma could account for the high Pt concentrations and high Pt/Pd ratios. This interpretation is supported by recent work on Os isotopes of the Kaapvaal SCLM. It should be kept in mind that the two processes are not necessarily exclusive. A magma with a SCLM component could have been emplaced into the crust and subsequently have been contaminated by partial melts of the crust.

The Chemistry of Manganese Ores through Time: A Signal of Increasing Diversity of Earth-Surface Environments

Abstract: Almost all economic manganese ores are ultimately or directly derived from hydrothermal vents associated with intermediate volcanic rocks. This source is in contrast to deep-sea nodules, which likely have a larger component of sediment-derived manganese and whose volcanic sources are more mafic. Manganese deposits can be divided into sedimentary rock-hosted, volcanic rock-hosted and karst-hosted, in order of predominance. Two genetic types of sedimentary rock-hosted deposits can also be identified: those with Mn derived via upwelling from oxygen-minimum zones and those formed on the margins of euxinic basins. Most of the large tonnage deposits appear to form by the euxinic mechanism. Manganese ores, like those of Fe, show a strong concentration of deposits in the Paleoproterozoic and a lesser occurrence in the Neoproterozoic, but, unlike Fe, there is an additional strong peak in the Oligocene. Therefore, Mn is not controlled entirely by the level of oxygen in the Earth’s atmosphere. At each peak of Mn deposition, the associated ore deposits are concentrated in a few districts, suggesting a more local than global control on manganese metallogenesis. Age trends can, however, be discerned in some chemical properties of manganese deposits. Overall, there is a trend to progressive increases in chemical diversity from the Archean to the Recent, with a particularly steep increase in the Neoproterozoic-Early Cambrian, corresponding in time to the radiation of metazoans. Also beginning in the Cambrian is the development of upwelling-linked deposits. There is another sharp increase in chemical diversity at the Jurassic-Cretaceous boundary, which includes increased SiO 2 /Al 2 O 3 ratios and corresponds to the radiation of diatoms. There is a conspicuous gap in sedimentary rock-hosted Mn deposits between 1800 and 800 Ma that may correspond to a monotonous, low-oxygen ocean, but one without sulfidic deep water. Alternatively, Mn may have been precipitated entirely in the deep ocean, beneath a sulfidic oxygen minimum layer. The positive Eu anomalies, which in iron formations are equated to vent-sourced metals, are not seen in most Mn deposits, although they are found in Mn-rich iron formations. By contrast, Fe deposits interbedded with major Mn ores lack the usual Eu signal. Therefore, mechanisms of transport between hydrothermal vents and the sites of deposition differed for Fe and for Mn deposits in the Archean-Paleoproterozoic. The dominant pattern in the time trend of Mn deposition is increasing chemical diversity, which reflects an increasing compartmentalization of the Earth’s depositional environments. This compartmentalization was a response to, but also provided a spur to, the diversification of life forms.

Formation of Sedimentary Rock-Hosted Stratiform Copper Deposits through Earth History

Abstract: Sedimentary rock-hosted stratiform copper deposits form by movement of oxidized, copper-bearing fluids across a reduction front that results in the precipitation of copper sulfides. Large-scale production of such oxidized fluids, as well as the formation of mobile hydrocarbons (oil) has probably been common since the formation of the first red beds in the Paleoproterozoic, and deposits of this type occur in rocks from the Paleoproterozoic to the Tertiary. However, supergiant deposits are currently recognized in only three basins: the Paleoproterozoic Kodaro-Udokan basin of Siberia, the Neoproterozoic Katangan basin of south-central Africa, and the Permian Zechstein basin of northern Europe. The paucity of data regarding the Udokan deposit makes understanding this system difficult in terms of Earth history events. Both the Neoproterozoic and the Permian were times of supercontinent breakup with major landmasses at low latitudes. This global tectonic framework favored the formation of failed rifts that subsequently became significant intracratonic basins with basal, synrift red-bed sequences overlain by marine and/or lacustrine sediments and, in some basins located at low latitudes, by thick evaporitic strata. The intracratonic setting of these basins allowed the development of a hydrologically closed basinal architecture in which highly oxidized and saline, moderate-temperature basinal brines were produced that were capable of supplying reduction-controlled sulfide precipitation over very long time periods (tens to hundreds of millions of years). The length of time available for the mineralizing process may be the key factor necessary to form supergiant deposits. However, examination of the absolute ages for the Kupferschiefer (Zechstein basin) and Katangan deposits allows speculation that other factors may also have been important. Both the Neoproterozoic and Permian were times of major glacial events. Glaciation may also be conducive for the formation of supergiant sediment-hosted stratiform copper deposits. Glacial periods correspond to magnesium- and sulfate-rich oceans that could have been responsible for additional sulfur in basinal brines developed during evaporite formation and would then be available during the long mineralization process.

Secular Variation of Magmatic Sulfide Deposits and Their Source Magmas

Abstract: Magmatic sulfide deposits are divisible into two major groups, those that are valued primarily for their Ni and Cu and that are mostly sulfide rich (>10% sulfide), and those that are valued primarily for their PGE and tend to be sulfide poor (<5% sulfide). Most members of the Ni-Cu group formed as a result of an interaction of mantle-derived magma with the crust that gave rise to the early onset of sulfide immiscibility. Of the different classes of deposit in this group, the komatiite-related class ranges from 2.7 to 1.9 Ga in age, the Flood basalt-related class from 1.1 to 0.25 Ga, and the Mg basalt- and basalt-related group from the Archean to the present. There is only one example each of anorthosite complex- and impact-related deposits, so that one cannot generalize about their secular distribution, except to say that anorthosite complexes are Proterozoic. Ural-Alaskan intrusions are dominantly Phanerozoic (some Archean deposits have been included with this group), but as yet no examples have been found with economic sulfide bodies. Seventy-five percent of known PGE resources occur in three intrusions—the Bushveld, Great Dyke, and Stillwater, the rocks all of which have crystallized from two magma types, an unusual, high SiO2, MgO, and Cr and low Al2O3 type (U-type) that was emplaced at an early stage and a later, normal tholeiitic-type magma (T-type); the PGE are concentrated in layers close to the level at which the predominant crystallization switches from one magma type to the other. The U-type magma is interpreted as a PGE-rich, komatiitic magma (possibly the product of two-stage mantle melting) that has interacted to varying degrees with the crust, becoming SiO2 enriched in this way. These three intrusions are Neoarchean to Paleoproterozoic in age. All known examples of komatiites, with one exception, are Paleoproterozoic or older and their secular distribution is thought to be due to cooling of the Earth. Known deposits do not occur in the oldest (>3.0 Ga) komatiites but appear at around 2.7Ga in continental (Kambalda, Western Australia) or island-arc (Perseverance-Mount Keith, Western Australia) environments, possibly because it was these environments that offered the opportunity for interaction with felsic rocks. It is suggested that the development of these environments in the Archean was an additional control on the age distribution of these deposits. It is postulated that the restricted secular distribution of PGE-enhanced intrusions is also due to the need for a hot mantle to give rise to U-type magmas.

Evolution of Uranium Fractionation Processes through Time: Driving the Secular Variation of Uranium Deposit Types

Abstract: Uranium deposit types have evolved considerably from the Archean to the Present. The major global drivers were (1) change of geotectonic conditions during the Late Archean, (2) strong increase of atmospheric oxygen from 2.4 to 2.2 Ga, and (3) development of land plants during the Silurian. Other significant variations of uranium deposit types are related to unique conjunctions of conditions such as those during phosphate sedimentation in the Cretaceous. Earth’s uranium fractionation mechanisms evolved through four major periods. The first, from 4.55 and 3.2 Ga, corresponds to formation of a thin essentially mafic crust in which the most fractionated trondheimite-tonalite-granodiorite (TTG) rocks attained uranium concentrations of at most a few parts per million. Moreover, the uranium being essentially hosted in refractory accessory minerals and free oxygen being absent, no uranium deposit could be expected to have formed during this period. The second period, from about 3.1 to 2.2 Ga, is characterized by several widespread pulses of highly fractionated potassic granite strongly enriched in U, Th, and K. Late in this period peraluminous granite was selectively enriched in U and to a lesser extent K. These were the first granite and pegmatite magmas able to crystallize high-temperature uraninite. The erosion of these granitic suites liberated thorium-rich uraninite which would then be concentrated in placer deposits along with pyrite and other heavy minerals (e.g., zircon, monazite, Fe-Ti oxides) within huge continental basins (e.g., Witwatersrand, South Africa, and Bind River, Canada). The lack of free oxygen at that time prevented oxidation of the uraninite which formed the oldest economic uranium deposit types on Earth, but only during this period. The third period, from 2.2 to 0.45 Ga, records increased oxygen to nearly the present atmospheric level. Tetravalent uranium from uraninite was oxidized to hexavalent uranium, forming highly soluble uranyl ions in water. Uranium was extensively trapped in reduced epicontinental sedimentary successions along with huge quantities of organic matter and phosphates accumulated as a consequence of biological proliferation, especially during the Late Paleoproterozoic. A series of uranium deposits formed through redox processes; the first of these developed at a formational redox boundary at about 2.0 Ga in the Oklo area of Gabon. All known economically significant uranium deposits related to Na metasomatism are about 1.8 Ga in age. The high-grade, large tonnage unconformity-related deposits also formed essentially during the Late Paleoproterozoic to early Mesoproterozoic. The last period (0.45 Ga-Present) coincided with the colonization of continents by plants. The detrital accumulation of plants within continental siliciclastic strata represented intraformational reduced traps for another family of uranium deposits that developed essentially only during this period: basal, roll front, tabular, and tectonolithologic types. However, the increased recognition of hydrocarbon and hydrogen sulfide migration from oil or gas reservoirs during diagenesis suggests potential for sandstone-hosted uranium deposits to be found within permeable sandstone older than the Silurian. Large uranium deposits related to high-level hydrothermal fluid circulation and those related to evapotranspiration (calcretes) are only known during this last period of time, probably because of their formation in near-surface environments with low preservation potential.

Diamonds through Time

Abstract: Diamonds form in the upper mantle during episodic events and have been transported to the Earth’s surface from at least the Archean to the Phanerozoic. Small diamonds occur as inclusions in robust minerals in tectonically activated, ultrahigh-pressure metamorphosed crustal rock, establishing an association with subduction processes and recycled carbon, but providing no economic deposits. Diamonds in economic deposits are estimated to be mainly (99%) derived from subcontinental lithospheric mantle and rarely (approx. 1%) from the asthenosphere. Harzburgite and eclogite are of roughly equal importance as source rocks, followed by lherzolite and websterite. Diamonds which provide evidence of extensive residence time in the mantle are, with minimal exceptions, smooth-surfaced crystalline diamonds (SCD) with potential commercial value. The oldest prolific SCD formation event documented on the world’s major diamond producing cratons occurs in Archean lithospheric mantle harzburgite, metasomatized by likely subduction-related potassic carbonatitic fluids. Disaggregation of the diamondiferous carbonated peridotite on decompression during volcanic transit gives rise to the association between diamonds, G10 garnets, and diamond inclusion-type chromites, well used in diamond exploration. Within the mantle domains of diamond stability, there have been repeated episodes of further diamond crystallization and/or growth. These are associated with old, often Proterozoic, subduction-related melt generation, metasomatic fluid migration, and reaction with preexisting mantle eclogite, websterite, and peridotite. Using improved methods of isotope analysis, diamond formation ages can be correlated with specific major processes such as craton accretion, craton edge subduction, and magmatic mantle refertilization. Fibrous cuboid diamond and fibrous coats on SCD are rough-surfaced diamonds with abundant fluid inclusions. They have low mantle residence time, forming rapidly from late stage metasomatic fluids in diamond stable domains that may already contain SCD. The symbiotic relationship between formation of fibrous diamond and magmatic sampling and transport of diamonds into the crust suggest that the associated fluids contribute diamond-friendly volatile loading of the deep lithospheric mantle shortly before the triggering of a volcanic eruption, continuing a process of volatile enrichment in the lithospheric mantle already identified in the Archean harzburgite diamond event. Mantle-derived SCD commonly shows evidence of resorption, illustrating that diamond-unfriendly processes, including lamproite and kimberlite generation, are also active and may have a substantial negative effect in extreme cases on SCD crystal size. Exposure of SCD to a long period of changing conditions during mantle residence contributes to the difficulty of assigning specific parageneses and ages to individual inclusion-free diamonds with our current state of knowledge.

The Geology and Metallogeny of Volcanic-Hosted Massive Sulfide Deposits: Variations through Geologic Time and with Tectonic Setting

Abstract: Analysis of metallogenic data, including grade and tonnage, host-rock succession, ore and alteration mineralogy, and lead and sulfur isotope data, indicates significant secular changes in the character of volcanic-hosted massive sulfide (VHMS) deposits, which appear to be related to changes in tectonic processes, tectonic cycles, and changes in the composition of the hydrosphere and atmosphere. The distribution of these deposits, whether measured in number of deposits, tons of ore, or tons of metal, is episodic, with major peaks at 2740 to 2690, 1910 to 1840, 510 to 460, and 390 to 355 Ma. These peaks correspond to the assembly of major continental land masses, including Kenorland, Nuna, Gondwana, and Pangea, respectively. Periods when fewer VHMS deposits formed correspond to periods of supercontinent and/or supercraton stability. The VHMS deposits do not form during supercontinent and/or supercraton breakup; rather, these intervals are associated with deposition of clastic-dominated sediment-hosted zinc-lead deposits. The main exception to these generalizations is the amalgamation of Rodinia, which was not accompanied by significant VHMS formation. Rodinian amalgamation may have been dominated by advancing accretionary orogenesis, whereby the overriding plate did not go into extension. In this case, slab rollback and the associated extension to form back-arc basins would not have been common, a setting typically conducive to the formation and preservation of VHMS deposits. Large ranges in source 238U/204Pb (μ) that characterized VHMS deposits in the Archean and Proterozoic indicate early (Hadean to Paleoarchean) differentiation of the Earth. A progressive decrease in μ variability may indicate homogenization with time of these differentiated sources. Secular increases in the amount of lead and decreases in 100Zn/(Zn+Pb) relate to an increase in felsic rock-dominated successions as hosts to deposits and to an apparent absolute increase in the abundance of lead in the crust with time. The increase in the abundance of barite and other sulfate minerals in VHMS deposits, from virtually absent in the Mesoarchean and Neoarchean to relatively common in the Phanerozoic, relates to the progressive oxidation of the atmosphere and hydrosphere. The total sulfur content of the oceans also increased, resulting in the enhanced importance of seawater sulfur in VHMS ore fluids with time. In Archean to Paleoproterozoic deposits, the bulk of the sulfur was derived by leaching rocks underlying the deposits, with little contribution from seawater, resulting in uniform, near-zero per mil values of δ34Ssulfide. In contrast, the more variable δ34Ssulfide values of younger deposits reflect the increasing importance of seawater sulfur in the hydrothermal systems. Unlike Mesoarchean and Neoarchean deposits, Paleoarchean deposits contain abundant barite. This sulfate is inferred to have been derived from photolytic decomposition of atmospheric SO2 and does not reflect overall oxidized oceans. Archean and Proterozoic seawater was significantly more saline than that in the Phanerozoic, particularly upper Phanerozoic seawater. The VHMS ore fluids reflect this, being on average more saline in Archean and Proterozoic deposits. This variability introduces uncertainty into genetic models advocating brine pools or magmatic- hydrothermal contributions based on high-salinity ore fluids.

Lateritization and Bauxitization Events

Abstract: Laterites and bauxites are produced in tropical soils by weathering, which enriches iron (of laterites) and alu- mina (of bauxites)—as well as trace elements such as nickel, gold, phosphorus, and niobium—to ore grade. La- terites and bauxites can be redeposited into sedimentary sequences, and remain as ores if not transported far and diluted with other materials. The age of redeposited laterites and bauxites, and of bauxitic and lateritic pa- leosols, can be established from the geologic age of overlying rocks, an approach especially effective in pale- osols within sequences of isotopically datable volcanic rocks. Lateritic profiles can also be dated by paleomag- netic inclination in special cases in which land masses such as in Australia and India drifted long distances northward during Cenozoic time. In addition, cryptomelane and other K-Mn oxides can be dated by K-Ar and 40Ar-39Ar techniques to obtain multiple ages from different crystals in a single relict paleosol. Compilation of new and more accurate laterite and bauxite ages reveals unusually widespread and intense laterite and bauxite formation during events of less than 100 k.y. duration at 2, 12, 16, 35, 48, 55, 65 and 100 Ma. Such events can also be inferred at times older than 100 Ma from paleolatitudinal distribution of laterites and bauxites, but these are poorly sampled. Laterite and bauxite peaks were coeval with times of global high warmth and pre- cipitation, elevated atmospheric carbon dioxide, oceanic anoxia, exceptional fossil preservation, and mass ex- tinction. These CO2 greenhouse events and attendant titration of carbonic acid with soils are interpreted as transient fluctuations in the atmosphere produced by meteorite impact, flood basalt volcanism, and methane outbursts. Concentration of bauxite and laterite resources, in particular stratigraphic horizons formed during greenhouse crises, suggests the usefulness of an event stratigraphic approach to exploration and exploitation of these and related ores.

Copper mineralization prevented by arc-root delamination during alpine-himalayan collision in central iran

Abstract: Contrasting geochemical signatures of copper ore hosting Eocene and some barren undeformed Miocene diorites to granites in central Iran temporally overlap with the Alpine-Himalayan collision and provide key implications on the existence and lack of Cu mineralization during collisional magmatism. High Sr and low Y (and Yb) contents of Eocene arc rocks in the Natanz arc segment reflect thickened, Andean-type orogenic arc crust (~45 km), whereas barren Miocene Natanz arc rocks (21–19 Ma) indicate thin arc crust similar to collisional volcanism in Anatolia. Geochemical modeling indicates a change in the mineralogy of the melt residual, from precollisional Eocene basaltic garnet-bearing (5–30%) amphibolite to syn- or postcollisional Miocene metasomatized mantle peridotite, which can be explained by collision-induced delamination of the arc lithospheric root. Subsequent recharge of hot asthenosphere and melting of metasomatized mantle peridotite and lack of interaction with a garnet-bearing arc crustal keel explain the low Sr and high Y (and Yb) contents, the relatively enriched initial Sr isotope ratios of postcollisional Miocene Natanz rocks, and the lack of copper mineralization in postcollisional Miocene Natanz arc rocks. Arc-root delamination removes the copper- and sulfur-enriched metasomatized lithospheric arc root and hydrous cumulate reservoir required to form copper ore deposits. Lack of the dense melt residues also provides an alternative explanation for the elevated, thin crustal Iranian back-arc plateau (38 km) as a result of uplift by isostatic rebound rather than uplift by anomalous shortening. Miocene arc-root delamination implies a minimum age of >21 Ma for the Alpine-Himalayan collision in central Iran.

Secular Variation in Economic Geology

Abstract: The temporal pattern of ore deposits on a constantly evolving Earth reflects the complex interplay between the evolving global tectonic regime, episodic mantle plume events, overall changes in global heat flow, atmospheric and oceanic redox states, and even singular impact and glaciation events. Within this framework, a particular ore deposit type will tend to have a time-bound nature. In other words, there are times in Earth history when particular deposit types are absent, times when these deposits are present but scarce, times when they are abundant, and still other times for which we lack sufficient data. Understanding of such secular variation provides a critical first-order tool for exploration targeting, because rocks that have formed or were deformed during a certain time slice may be very permissive for a given deposit type, whereas identification of rocks of less favorable ages would help eliminate large areas during exploration programs. Secular analysis, therefore, is potentially a powerful tool for mineral resource assessment in poorly known terranes, providing a quick filter for favorability of a given deposit type using age of host rocks. Factors bearing on the known age distribution of a particular type of deposit include the following: (1) uneven preservation, (2) data gaps, (3) contingencies of plate motions, and (4) long-term secular changes in the Earth System. The present special issue of Economic Geology is focused on the latter factor, although all of these are interrelated. The selective preservation of certain mineral deposit types and the greater susceptibility for shallowly formed ores in tectonically active environments to be lost to erosion define a pattern that is superimposed on the secular formational trends (e.g., Groves et al., 2005a, b; Kerrich et al., 2005). With improved geochronological methods and the availability of information on important mineral deposits from most parts of the …

The Genesis of Distal Zinc Skarns: Evidence from the Mochito Deposit, Honduras

Abstract: The Mochito deposit, located in Honduras, is a distal Zn(-Pb-Ag) skarn in which economic mineralization (sphalerite and subordinate, argentiferous galena) is found in mantos and chimneys dominated by garnet and pyroxene that replaced limestone and a mixed limestone-siliciclastic unit. Except for a few variably altered, unmineralized diabase dikes, evidence of igneous activity is conspicuously absent. The nearest felsic igneous rocks, volcanic rock units, crop out ~13 km from the deposit. Early skarn and skarn proximal to faults consist dominantly of garnet that evolved from grandite, with a composition of ~Ad55 to andradite (≥Ad90), whereas later skarn, or skarn distal to faults, is mainly made up of pyroxene (Hd70). Magnetite and pyrrhotite locally form between the garnet and pyroxene skarns. Analyses of the whole-rock chemistry show that formation of grandite skarn involved large additions of all major elements (except Ca) and most trace elements, whereas formation of andradite skarn (from grandite skarn) involved losses of most of these elements; the notable exceptions are Ca and Fe, which were added. Mass changes in pyroxene skarn were not evaluated because of the heterogeneous nature of the precursor. Primary fluid inclusions in grandite and associated low-iron sphalerite are interpreted to have been trapped at ~370°C and a pressure of 500 bar. These inclusions have a mean salinity of 14 wt percent NaCl equiv. By contrast, primary inclusions in pyroxene and associated high-iron sphalerite were trapped at ~400°C and have a mean salinity of 5 wt percent NaCl equiv. Fluid inclusions could not be observed in andradite. Small proportions of CO2, CH4, and N2 were detected by gas chromatographic analyses of the fluids released by crushing small samples of the host mineral; CO2 was the most abundant of these gases. Based on LA-ICPMS analyses of individual fluid inclusions, Na and Ca were the principal metals in the fluids (median concentrations of 3.2 wt %), followed by K (0.9 wt %) and Mn (0.3 wt %). Median concentrations of ore metals Zn, Pb, and Ag were 6,000, 900, and 50 ppm, respectively. Analyses of phase equilibria and related thermodynamic calculations indicate that the log f O2 during the grandite and pyroxene skarn stages was >–30.3 and <–30.2, respectively. The pH during the grandite stage was ~5.0. In the absence of reliable data on bulk fluid chemistry, a pH for the pyroxene stage could not be estimated. Based on the calculated Si content of the fluid and the mass addition of Si during grandite skarn formation, the fluid/rock ratio was between 500:1 and 1,000:1. Evaluation of the solubility of sphalerite, galena, and argentite based on the physicochemical characteristics of the putative ore fluid indicate that the ore metals were transported dominantly as chloride complexes and deposited in response to an increase in pH. We propose a model in which relatively oxidizing hydrothermal fluids, exsolving from magma at a depth of >4 km, interacted with graphitic limestones in the Mochito graben during an episode of mid-Tertiary intraplate extension. These fluids rose through faults created by the extension and were cooled by the overlying sedimentary succession. Early skarn formed in an environment of high-fluid flux proximal to the faults and was dominated by grandite because of the oxidizing nature of the fluids. With continued heating of the rocks by subsequent pulses of fluid, temperature increased and the locus of interaction expanded into unaltered limestone distal to the faults, where lower fluid/rock ratios and the presence of graphite promoted buffering of the fluid to lower f O2 and formation of pyroxene skarn. Ore mineral deposition (dominantly sphalerite), which began during or after the formation of grandite skarn, reached its maximum after hydrothermal activity was focused in the lower fluid/rock ratio regime of pyroxene skarn formation, occurring in response to the sharp drop in pH that accompanied neutralization of the fluid by limestone.

A Review of Fluid Inclusion Constraints on Mineralization in the Irish Ore Field and Implications for the Genesis of Sediment-Hosted Zn-Pb Deposits

Abstract: Many fluid inclusion studies have been carried out in the Irish Midlands basin ore field (Lower Carboniferous) since the earliest work by Ed Roedder in the late 1960s. Results show that, in the ore deposits, the total range in fluid salinity is 4 to 28 wt percent NaCl equiv but with the majority billing in the moderate-salinity range between 8 and 19 wt percent. This variability is interpreted in terms of mixing between moderate-salinity ore fluids and low-temperature brines during ore formation. The most northerly ore deposits of Navan and Abbey-town are, distinct in containing fluids of both lower and higher salinity than is typical of the Waulsortian-hosted deposits farther south (Tynagh, Silvermines, Lisheen, and Galmoy). Subeconomic prospects tend to display a narrower range, in salinity mostly at the lower end of the range observed in the ore deposits. In some prospects, and cm the margins of some ore deposits, evidence for dilution is observed, interpreted to reflect mixing between hydrothermal fluids and unmodified seawater. This process is inferred to be unfavorable for mineralization.Homogenization temperatures, a reasonable proxy for true trapping temperatures in the ore field, range from 70 degrees to 280 degrees C but with the majority falling between 130 degrees and 240 degrees C. There is no evidence for systematic stretching or leakage of inclusions related to the postentrapment heating implied by elevated thermal maturity indicators. The highest temperatures are observed in the Waulsortian-hosted systems, with peak temperatures of -280 degrees C supported by local, high-grade Cu and Ni mineralization. in the Navan and Abbeytown deposits, lower temperature fluids appear to have been more prevalent. The subeconomic prospects formed over essentially the same temperature range as the ore deposits (90 degrees-270 degrees C), with the exception of the morphologically and texturally distinct Mississippi Valley-type (MVT) systems in the region (e.g. Kinnitty, Harberton Bridge) that formed at lower temperatures (50 degrees-100 degrees C).Similar hydrothermal fluids to those recorded in both deposits and prospects are widely observed in dolomite (and sometimes calcite) cements within Courceyan-Arundian-age rocks, indicating that hydrothermal fluid activity occurred over an extremely large area (>30,000 km(2)) and probably over an extended time period. There is a broad regional division in fluid properties, suggesting that the northwestern and southeastern provinces, separated by the trace of the Iapetus suture zone, may represent partly decoupled, large-scale flow regimes. Up to three, low-temperature brine types are also recorded by cements in the host-rock sequence, indicating that a complex range of evaporation and fluid-rock interaction processes were ongoing in the shallow basin succession during the period of hydrothermal activity.Halogen data show that fluids involved in mineralization were originally seawater-derived brines, produced by evaporation to varying degrees. Relatively high temperature, basement-interacted hydrothermal fluids were derived from partially evaporated seawater (molar Cl/Br = 559-825). Their compositions can be explained by dolomitization in the Carboniferous succession prior to circulation to depth; alkali exchange, reduction, and metal-leaching from the lower Paleozoic basement; and mixing with low-temperature brines that locally penetrated the upper parts of the basement rock package. Fertile ore fluids appear to be characterized by higher delta O-18 (+7 to +9 parts per thousand), lower delta D (-25 to -45 parts per thousand) and much higher metal contents than otherwise similar fluids sampled in basement-hosted feeder veins distal to deposits. This may reflect highly efficient metal scavenging in deeper and/or higher temperature reaction zones that underlie the principal deposits. In the ore deposits, these fluids mixed with Br-enriched bittern brines (Cl/Br similar to 290) produced by evaporation of Carboniferous seawater past halite saturation. It is inferred that bittern brine generation occurred in the shallow marine shelf regions in the footwalls to the synsedimentary fault systems that controlled the localization of mineralization. These brines then migrated into hanging-wall depressions where they ponded within permeable sediments and became enriched in H2S via bacteriogenic sulfate reduction. The coincidence of structurally controlled, high-temperature reaction zones, brine-producing footwalls, and hanging-wall traps, with bacterial blooms above upwelling plumes of hydrothermal fluids, can be interpreted as a self-organizing system that locally converged on ore-forming conditions. Understanding the first-order structural control of the ore systems will therefore be critical for predicting new deposits.The Irish ore field presents arguably the best database available on the thermal and chemical characteristics of hydrothermal fluids involved in sediment-hosted ore genesis. The system shares much of the variety and complexity observed in other intracratonic basin-hosted Zn-Pb(Ba) ore districts. This includes the coexistence of contrasting styles of mineralization that are typically observed in the more distal and platform-marginal parts of the basinal environment. The thermal and chemical fluid heterogeneity observed is typical of modern intracratonic basin systems and should be expected in large paleohydrothermal systems where recharge of surface-derived fluids is involved.

The Bingham Canyon Porphyry Cu-Mo-Au Deposit. I. Sequence of Intrusions, Vein Formation, and Sulfide Deposition

Abstract: The Bingham Canyon porphyry copper-gold-molybdenum deposit is one of the largest and highest-grade porphyry orebodies in the world. This study focused on the northwest side of the deposit where quartz mon-zonite porphyry (QMP), the first and largest porphyry intrusion, hosts the bulk of the high-grade copper-gold ore (>1.0% Cu, >1.0 ppm Au). The north-northeast–trending, high-grade zone had pre-mining dimensions of 1,500 m strike, >300 m vertical, and 500 m width and contained more than 500 million tonnes (Mt) of ore associated with potassic alteration and abundant quartz veins. The lack of superimposed sericitic alteration yielded ideal exposures in which to study the early, high-temperature stages of ore formation, a style of mineralization that in many porphyry deposits represents the major period of copper introduction. We mapped multiple porphyry dikes in the sequence: (1) QMP, (2) latite porphyry (LP), (3) biotite porphyry (BP), (4) quartz latite porphyry breccia (QLPbx), and (5) quartz latite porphyry (QLP). Porphyry dikes, faults, and quartz veins are steeply dipping and have two dominant orientations; north-northeast– and northwest-striking. Dikes have a north-northeast strike but they thicken and develop northwest-trending apophyses and host high-grade copper-gold zones at intersections with northwest-faults, indicating that magmatic-hydrothermal fluids were focused by these structural intersections. Each porphyry intrusion was accompanied by a similar sequence of veins, potassic alteration, and sulfides. Biotite veinlets were followed by fractures with early dark micaceous (EDM) halos of sericite, K-feldspar, biotite, andalusite, and local corundum containing disseminated bornite-chalcopyrite-gold. EDM halos are cut by multiple generations of A-quartz veins representing the main Cu-Au ore-forming event. Postdating all intrusions are quartz-molybdenite veins followed by quartz-sericite-pyrite veins. Cathodoluminescence (CL) petrography identified distinct A-quartz veinlets consisting of dark-luminescing quartz filling fractures and dissolution vugs in earlier A-quartz veins and adjacent porphyry wall rock. These veinlets contain abundant bornite and chalcopyrite and minor K-feldspar and are closely linked in time to the introduction of the bulk of the copper and gold. Although a similar sequence of veins was repeated on emplacement of all porphyry intrusions, the vein density and intensity of potassic alteration declined with time. The youngest porphyry, QLP, is mostly weakly mineralized and locally unaltered. These observations indicate that magmatic-hydrothermal fluids underwent a similar physiochemical evolution during and immediately following emplacement of each of several porphyry dikes. The relationship between EDM veins and A-quartz veins requires that the flux of magmatic fluid from the magma chamber occurred in an episodic manner as opposed to a continuous discharge. Vein truncation relationships coupled with abrupt changes in copper-gold grades, sulfide ratios, and potassic alteration intensity at porphyry intrusive contacts indicate that the mass of introduced copper and gold decreased significantly during successive porphyry intrusive-hydrothermal cycles, presumably due to depletion of metals and volatiles in the underlying magma chamber.

Baogutu Porphyry Cu-Mo-Au Deposit, West Junggar, Northwest China: Petrology, Alteration, and Mineralization

Abstract: Baogutu is the first porphyry Cu-Mo-Au deposit discovered in West Junggar, Xinjiang, China. It is part of the Central Asian orogenic belt. Baogutu is associated with a Carboniferous intrusive complex that was emplaced into lower Carboniferous volcano-sedimentary sequences of the Baogutu and Xibeikulasi Groups. The intrusive complex is made up of main stage equigranular to porphyritic diorites and quartz diorites, and minor late stage diorite porphyries. Intrusive activity occurred at about 325 Ma based on U-Pb (SHRIMP) analyses of zircons. The main stage diorites host the bulk of the Cu-Mo-Au mineralization at Baogutu. They have been overprinted by three alteration assemblages, including an early potassic (biotite) assemblage that occurs in the center of the deposit. A propylitic assemblage surrounds the potassic zone concentrically. Both of these alteration assemblages have been overprinted locally by phyllic alteration (quartz-sericite-pyrite), which is associated with the highest Cu-Mo grades. Mineralized and hydrothermally cemented breccias have disrupted the main stage diorites. The late stage diorites have undergone moderate potassic alteration and contain weak Cu mineralization. Matrix-rich breccias with very weak mineralization have cut the intrusive complex and disrupted the ore-body. Re-Os dating of molybdenite indicates that mineralization occurred at about 310 Ma.

Direct Analysis of Ore-Precipitating Fluids: Combined IR Microscopy and LA-ICP-MS Study of Fluid Inclusions in Opaque Ore Minerals

Abstract: Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in combination with near-infrared microscopy of fluid inclusions hosted by ore minerals that are opaque to visible light can provide the composition of ore-precipitating fluids. We applied the two techniques to well-constrained fluid inclusion assemblages hosted by pyrite, enargite, and quartz to trace the source and evolution of the fluids in high-sulfidation epithermal veins overprinting a porphyry copper deposit at Rosia Poieni, Romania. Despite some analytical limitations caused by the sulfide host minerals, the data demonstrate that fluids trapped in apparently cogenetic quartz and ore minerals are chemically different. Systematic changes in major and trace element ratios between liquid-vapor, vapor-rich, and brine fluid inclusion assemblages in the three minerals record an evolving fluid source at the porphyry to epithermal transition. Regarding their Cs/(Na + K) ratios, most of epithermal quartz-hosted fluid inclusion assemblages form a well-defined cluster, which coincides with the narrow range of the porphyry-stage fluids trapped in early quartz of the porphyry stockwork veins. Their Cu/(Na + K) ratios are 10 to 100 times lower compared to the pyrite-hosted inclusions and correspond to the lowest Cu/(Na + K) ratios recorded for the porphyry-stage fluids. By contrast, pyrite-hosted, vapor-rich fluid inclusions have the highest Cu/(Na + K) similar to the highest Cu/(Na + K) ratios measured in the porphyry-stage fluid inclusions. The results led to the conclusion that the gangue and ore minerals in the high-sulfidation epithermal veins at Rosia Poieni formed by successive pulses of chemically distinct hydrothermal fluids that were successively exsolved from residual melt batches of a progressively crystallizing magma at greater depth. These results are consistent with detailed textural observations, but petrography alone could not have led to this unambiguous conclusion.

Platinum Group Element Geochemistry of Mineralized and Nonmineralized Komatiites and Basalts

Abstract: Platinum-group elements (PGE) are strongly chalcophile and are therefore potentially sensitive indicators of processes involving segregation and accumulation of sulfide melts from silicate magmas. Over 500 new high-precision PGE data for komatiites and komatiitic basalts, spanning a wide range of emplacement and crystallization histories, have been combined with literature data on PGE in magmatic systems from other barren and variably mineralized environments, to test the effectiveness of PGE geochemistry as an indicator of processes forming magmatic sulfide ores. Results show that PGE depletion in S-poor komatiites and komatiite basalts spatially and genetically associated with Fe-Ni-Cu sulfide mineralization is not as common or as strong as expected: samples displaying orders of magnitude depletion in PGE represent less than 10 percent of any given data set from any location. The data confirm that most, if not all, komatiites were sulfide undersaturated when they separated from their sources and remained undersaturated on eruption. Some ore-bearing komatiite sequences display no detectable depletion, and the degree of PGE depletion is commonly less than expected based on modeling using experimentally determined partition coefficients. PGE enrichment is more common and spatially widespread than PGE depletion, commonly representing a better approach to lithogeochemical exploration, even where samples containing anomalous Ni or S contents are absent. PGE enrichment and/or depletion associated with sulfide enrichment and/or segregation can be discriminated from secondary hydrothermal and/or metamorphic processes by covariance of all PGE, with the exceptions in some cases of Ir, Ru, and Os whose abundances may be complicated by the presence of saturation in and accumulation of Ir-Os-rich liquidus phases. Variations attributable to other magmatic processes, such as olivine accumulation and fractionation, can be distinguished by variations in PGE/Ti ratios and strong correlations between Pt/Ti, Pd/Ti, and Rh/Ti ratios in mineralized systems. The degree of PGE depletion is consistent with the relatively low R factor estimated for many komatiite-hosted deposits, which fall in the range of 20 to 200 for Thompson, 100 to 500 for Kambalda, and 300 to 1,100 for Raglan, implying that the volume of silicate magma that interacted with sulfide liquid was relatively small. This is also consistent with the relatively small proportion of komatiites displaying PGE depletion within ore-bearing flow sequences, as only magmas in ore-forming channels or conduits will interact with sulfides. False negatives, i.e., mineralized komatiite sequences with no detectable PGE depletion, are associated with systems characterized by high R factors. Basalts and komatiitic basalts show more complex patterns of variation, which can broadly be divided into three categories: (1) systematic PGE depletion over a range of Mg numbers, as in MORB suites, consistent with retention of sulfide in the mantle during partial melting; (2) increasing PGE depletion with decreasing Mg numbers in large igneous province (LIP)-associated basalts, interpreted to reflect attainment of sulfide saturation during fractionation with subsequent cotectic olivine-sulfide segregation; and (3) variable PGE depletion over a range of Mg numbers in komatiitic basalts (e.g., Raglan) interpreted to reflect ore-forming sulfide incorporation and segregation processes. The results of this study confirm that the PGE geochemistry of komatiites and basalts is a powerful indicator of sulfide saturation and ore-forming processes, but that it must be interpreted with the context of physical volcanologic and fluid dynamic processes.

The Timing and Formation of Platinum-Group Minerals from the Creighton Ni-Cu-Platinum-Group Element Sulfide Deposit, Sudbury, Canada: Early Crystallization of PGE-Rich Sulfarsenides

Abstract: Platinum-group elements (PGE) are typically hosted in base metal sulfides and by platinum-group minerals (PGM) in Ni-Cu-PGE sulfide deposits. At Sudbury, it appears that the majority of PGE are hosted in PGM. In order to understand why this is the case we have investigated the origin of PGM from the 402 trough ore-bodies of the Creighton deposit located on the South Range of Sudbury. These predominantly pyrrhotite-rich sulfides, with low (Pt + Pd)/(Os + Ir + Ru + Rh) whole-rock ratios, represent cumulates of monosulfide solid solution (MSS) that crystallized early from the sulfide melt, collected in troughs and embayments at the base of the Sudbury Igneous Complex, and formed small pendants of ore in the footwall country rock. The majority of PGE (Ir, Rh, Pt ± Os, Ru) show a stronger affinity for the sulfarsenide phases than the cocrystallizing sulfide phases which are strongly depleted in these PGE. The precious metal mineralogy is dominated by PGE sulfarsenides (86%) with subordinate sperrylite (PtAs2: 9%), michenerite (PdBiTe: 5%), and electrum (AgAu2: 0.1%). These discrete minerals are predominantly hosted within pyrrhotite and pentlandite except, however, a large proportion of michenerite is hosted either entirely by silicates and/or juxtaposed against silicates. The PGE sulfarsenides are euhedrally zoned with an irarsite (IrAsS) core, an outer layer of hollingworthite (RhAsS), and a PGE-rich Ni cobaltite rim (CoAsS). Rhenium sulfides, some of which are Os bearing, are documented for the first time at Sudbury. Platinum-group minerals may crystallize directly from sulfide melt, form by exsolution during cooling of the base metal sulfides or recrystallize from them during metamorphism. We propose that zoned PGE sulfarsenides and sperrylite crystallized from a sulfide melt at high temperatures (1,200°–900°C) and were subsequently surrounded by MSS cumulates, even by disseminated sulfides, that crystallized from the now Ir, Rh, Pt ± Os, Ru-depleted immiscible sulfide liquid. The base metal sulfides recrystallized with secondary hydrosilicates at a late magmatic and/or hydrothermal stage (<540°C) at which time michenerite formed. The magmatic zoning of the PGE sulfarsenides was preserved during later deformation in shear zones but these PGM were corroded, fractured, and juxtaposed against silicates.

Age and Origin of Orogenic Gold Mineralization in the Otago Schist Belt, South Island, New Zealand: Constraints from Lead Isotope and 40Ar/39Ar Dating Studies

Abstract: The Otago Schist belt of southern New Zealand hosts numerous orogenic gold deposits that formed in a range of structural and lithological settings during and after Mesozoic metamorphism. Previous 40Ar/39Ar dating studies in the Otago Schist belt indicate formation ages of ~106 to 101 Ma for late, postmetamorphic, gold-bearing quartz veins and shear zones. Samples of hydrothermal muscovite from several mineralized quartz vein systems were dated using the 40Ar/39Ar method. Muscovite from a gold-bearing quartz stockwork vein at the Macraes mine, which is a relatively old mineralized system that formed during the latter stages of metamorphism, yields a well-defined plateau age of 135.1 ± 0.7 Ma. Fine-grained muscovite from a quartz shear vein at Macraes gives a rising age spectrum with a maximum age of 135.7 Ma. Five 40Ar/39Ar dates from the scheelite-bearing vein swarm in the Glenorchy district are interpreted to indicate that mineralization occurred between 142 and 134 Ma, and that relatively slow cooling and/or minor thermal overprints have disturbed the age spectra of some of the samples. Lead isotope compositions for a total of 42 sulfide samples from 17 separate mineralized zones in the Otago Schist belt are relatively radiogenic, consistent with an entirely upper crustal source for the Pb (and presumably Au). The Pb data define a less radiogenic and a more radiogenic cluster, with no overlap. Lead isotope compositions from the sulfides from the Macraes group of deposits form part of a less radiogenic cluster, together with Pb isotope compositions for sulfides from scheelite-rich veins in the Glenorchy camp. Lead isotope compositions for sulfides from younger, postmetamorphic quartz veins and shear zones fall exclusively in the more radiogenic cluster. The new 40Ar/39Ar dating results and Pb isotope studies, in conjunction with previously published work, demonstrate that most orogenic gold mineralization within the Otago Schist belt formed during two discrete mineralizing events—one at 142 to 135 Ma and a later one at approximately 106 to 101 Ma. We interpret the two pulses of gold mineralization to reflect sequential extraction of ore-forming components from the deeper part of the Otago Schist belt during short-lived thermal events. The younger event was probably associated with subduction of an actively spreading ridge, and similar ridge subduction may have been involved in the older event.

Longevity of porphyry copper formation at quellaveco, peru

Abstract: Metal introduction at the late Paleocene to early Eocene Quellaveco porphyry copper-molybdenum deposit in southern Peru spans several phases of quartz monzonite porphyry emplacement and is bracketed by a precursor granodiorite pluton and a late-mineral porphyry body that postdates essentially all copper introduction. Together, the U-Pb ages of zircons from these intrusive rocks show that 1.08 ± 0.58 m.y. elapsed between the precursor pluton and initiation of stock emplacement; the porphyry system was active intermittently for at least 3.25 m.y. (4.07 ± 0.82 m.y.); and at least three-quarters of the copper inventory was deposited in a maximum of 3.12 m.y. (2.51 ± 0.61 m.y.). Recent U-Pb zircon dating of several other major central Andean porphyry copper deposits, in combination with other isotopic techniques, suggests that 2.5- to 4-m.y. life spans are commonplace. The longevity of porphyry copper systems implied by these studies appears to reflect the protracted time gaps between the multiple intrusions that intermittently replenished porphyry stocks. Other precise isotopic methods (Re-Os, 40 Ar/ 39 Ar) typically document shorter life spans because it is more difficult, if not impossible, to date the full sequence of events involved in porphyry copper formation.

Crustal-Scale Fluid Pathways and Source Rocks in the Victorian Gold Province, Australia: Insights from Deep Seismic Reflection Profiles

Abstract: Deep seismic reflection data collected across the western Lachlan orogen of southeast Australia have provided important insights into crustal-scale fluid pathways and possible source rocks in one of the richest orogenic gold provinces in the world. The profiles span three of the most productive structural zones in Victoria: the Stawell, Bendigo, and Melbourne zones. zone-scale variations in the age and style of gold deposits correspond with differences in crustal structure and composition. The bilateral distribution of gold production in the Stawell and Bendigo zones is related to the V-shaped crustal-scale geometry of the two zones in cross section. Major first-order faults, like the east-dipping Moyston fault and a set of west-dipping listric faults within the Bendigo zone, were probably major fluid conduits in the lower to middle crust during gold mineralization. First-order faults in the Stawell and Bendigo structural zones appear to have accommodated large-scale thickening down to the lower crust. The faults converge in a region beneath the western Bendigo zone where a thick section of mafic volcanic rocks and lesser sedimentary rocks are identified as a likely common source of gold-bearing fluid that was largely generated by Late Ordovician to Early Silurian metamorphism. The areas with the greatest gold endowment lie above lower crustal regions that have preserved the thickest succession of “fertile” mafic igneous rocks up to about 25 km thick. They also correspond to a region of thin or absent Precambrian lithosphere. Mafic rocks in the Stawell zone and far western Bendigo zone were probably partly consumed by Cambrian west-dipping subduction. Similar rocks in the rest of the Bendigo zone lay outside the influence of Cambrian subduction-accretion and were deformed later, probably beginning in the Late Ordovician-Early Silurian Benambran orogeny during crustal-scale imbrication. This imbrication preserved much of the mafic rocks to form the lower to middle crust. First-order listric faults in the Bendigo zone are interpreted as major controls on the locations of goldfields, even though they are largely unmineralized near the present surface. The shallow-dipping segments of first-order listric faults were favorably oriented for reactivation at the time of gold mineralization and acted as major fluid conduits in the lower to middle crust. In contrast, the upper steeply dipping segments of first-order listric faults were unfavorably oriented for reactivation and were poor fluid conduits. The seismic data show that the transition from predominantly shallow- to steeply dipping fault segments occurs in the middle to upper crust near the boundary between thick imbricated metavolcanic rocks that lie immediately below 6 to 15 km of folded metasedimentary rocks. This transition may have coincided with fluid escape zones that aided the transfer of permeability away from first-order faults and into the overlying fold-dominated turbidites. This transfer of permeability was enhanced by the growth of subvertical, fold-related fault and fracture meshes in the upper-crustal turbidites. The fault and fracture meshes consisted of bedding-parallel faults, limb thrusts, and tension vein arrays that developed along fold hinges. In the Bendigo zone, individual fold hinges and regional fold culminations were important controls on the distribution of fluid flow. Fluid flow was partly syndeformational but overlapped into the immediate postdeformational period of the Benambran orogeny. Later reactivation of first-order faults in the Late Silurian to Early Devonian and again in the Late Devonian led to further, although less important, mineralizing events as fluids exploited the preexisting fault architecture. Deeply penetrating, north-dipping listric faults in the less gold rich Melbourne zone cut into inferred Proterozoic basement and may have been fluid conduits during a Late Devonian mineralizing event.

Multistage Intrusion, Brecciation, and Veining at El Teniente, Chile: Evolution of a Nested Porphyry System

Abstract: The El Teniente copper-molybdenum deposit is hosted by the late Miocene Teniente Mafic Complex, a largely subvolcanic package of primarily basaltic-andesite porphyry sills and stocks that were emplaced within the mid-late Miocene Farellones Formation. In the late Miocene-Pliocene, a series of intermediate felsic plutons were intruded into the Teniente Mafic Complex. These are spatially associated with magmatic-hydrothermal breccias and multiple vein types that form individual mineralized complexes.Here we present detailed observations on mineralogy, textures, and intrusion, breccias. and vein crosscutting relationships to constrain the nature and relative timing of magmatic and hydrothermal events. Our revised classification defines 13 vein types, divided lilt three main stages: (1) premineralization biotite and/or K-feldspar +/- quartz-anhydrite-albite-magnetite-actinolite-epidote veins that formed prior to emplacement of mineralized intrusions and breccias; (2) main mineralization stage veins that grade from gangue-dominated quartz-anhydrite veins +/- potassic alteration halos into sulfide-dominated veins with phyllic alteration halos; and (3) late mineralization veins containing sulfosalts. Five breccia types have been observed in the deposit, typically forming individual, vertically zoned complexes, spatially associated with individual intrusions and overlapping in time: (I) igneous-cemented breccias, (2) K-feldspar-cemented breccias, (3) biotite-cemented breccias, (4) anhydrite-cemented breccias, and (5) tourmaline-cemented breccias. Breccia cements display 1 similar paragenetic evolution to main mineralization stage veins, indicating a close genetic link between them. A distinction can be made between early premineralization vein types (types 1-2) that represent veins formed, possibly deposit-wide, prior to emplacement of intrusion-breccia complexes, late pre- and main mineralization vent types (types 3-8), which are interpreted to reflect the repeated cycle of fluid release associated with each mineralized intrusive complex, and late mineralization vein types (types 9-10) that represent it single event linked to the emplacement of the Braden Breccia Pipe.The geologic evidence indicates a close spatial and temporal relationship between emplacement of shallow felsic-intermediate pipelike intrusions and the development of igneous mid mineralized magmatic-hydrothermal breccias and vein halos. The magmatic-hydrothermal transitions observed in these complexes indicate that the deposit formed from a series of localized pulses of magmatic-hydrothermal activity which followed rather similar evolution paths. Single, deposit-wide models of Fluid evolution and mineralization are therefore inappropriate. We conclude that El Teniente represents a nested but otherwise rather typical porphyry Cu-Mo system, unusual only in thia the Teniente Mafic Complex pi is it particularly efficient physical trap in terms of pervasive fracturing during intrusion of magmatic-hydrothermal breccia complexes and its an effective chemical trap for deposition of sulfides. The overlapping of mineralized envelopes from successive fertile intrusions and the absence of barren, intermineral porphyries resulted in its unusual size and high hypogene grades.

Evolution of Calc-Alkaline Volcanism and Associated Hydrothermal Gold Deposits at Yanacocha, Peru**

Abstract: Clusters of high-sulfidation epithermal deposits containing more than 50 Moz of gold are hosted by advanced argillic-altered Miocene volcanic rocks in the Yanacocha district, northern Peru (lat. 6°59'30" S, long. 78°30'45" W). We describe the nature of the volcanism and its relation to the gold ores on the basis of new district-scale geologic mapping, 69 40Ar/39Ar ages on igneous rocks and hydrothermal alunite, and petrologic and geochemical investigations. Volcanic rocks of the Calipuy Group are the oldest Cenozoic rocks at Yanacocha, and include the Huambo Cancha andesite, andesitic lahars of Tual and Chaupiloma (19.5–15.9 Ma), and the dacitic Cerro Fraile pyroclastics (15.5–15.1 Ma). The younger Yanacocha Volcanics (14.5–8.4 Ma) form a cogenetic series of lavas and pyroclastic rocks with a cumulative volume of ~88 km3 that represents eruption from a moderate-size volcanic center. Early pyroxene>hornblende-bearing lavas of the Atazaico Andesite (14.5–13.3 Ma) erupted from small stratovolcanoes progressively younger from southwest to northeast. Dacitic dikes followed that are spatially associated with gold deposits at Quilish and Cerro Negro (~7 Moz Au) and stage 1 alunite (13.6–12.6 Ma). The Colorado Pyroclastics erupted in the center of the district and include the hornblende- and biotite-bearing andesitic to dacitic Cori Coshpa (12.6 Ma) and Maqui Maqui (12.5–12.4 Ma) ignimbrites. The Colorado Pyroclastics are overlain by hornblende>pyroxene-bearing andesitic to dacitic lavas, flow-domes, and minor pyroclastic rocks of the Azufre Andesite (12.1–11.6 Ma). Widespread stage 2 alunite (11.5 Ma) and minor gold deposition (~0.5 Moz) closely follow. The San Jose Ignimbrite (11.5–11.2 Ma) overlies the Azufre Andesite and stage 2 alunite and includes three members of hornblende-pyroxene (biotite) dacite and andesite that erupted in the center of the district and flowed southward. Mineralogically similar domes were emplaced into the inferred vents. The Coriwachay Dacite (10.8–8.4 Ma) forms the youngest and most silica rich igneous rocks in the district, and includes intrusions and flow domes of dacite to rhyolite at Corimayo (10.8 Ma), Cerro Yanacocha (9.9 Ma), and Yanacocha Lake (8.4 Ma). Most of the gold (>47 Moz) was deposited at Yanacocha during intrusion of the late Coriwachay Dacite. These late dacites are volumetrically smallest (~2% of the total volume of erupted magma) and are temporally associated with stage 3 to 5 alunites. Stage 3 alunite (11.0–10.7 Ma) developed along a northeast trend for 9 km that includes the gold deposits of Corimayo, San Jose, Carachugo, and Maqui Maqui. The deeper Kupfertal Cu-Au porphyry has an age of 10.7 Ma on hydrothermal biotite and underlies zones of stage 4 and 5 quartz-alunite alternation that are 0.8 and 1.5 m.y. younger, respectively. Stage 4 alunite ranges in age from 10.2 to 10.3 Ma at the Tapado and Chaquicocha Sur gold deposits to Cerro Sugares east of the Maqui Maqui deposit. Stage 4 also includes a younger alunite age of 9.9 Ma from the San Jose gold deposit. Stage 5 alunite ranges from ~9.3 to ~8.2 Ma at Cerro Yanacocha, the largest gold deposit in the district. All these deposits contain massive and vuggy quartz, quartz-alunite, and quartz-pyrophyllite alteration associated with pyrite±enargite-tennantite-covellite. Magmatism in the Yanacocha district lasted for ~11 m.y. The Yanacocha Volcanics spanned the last ~6 m.y. of this period and were associated with long-lived magmatic-hydrothermal activity and episodic gold mineralization. The Yanacocha calc-alkaline suite was oxidized, water and sulfate rich, and evolved from early pyroxene>hornblende andesite to late titanite-bearing dacite and minor rhyolite. Several dacites contain populations of both high- and low-aluminum hornblendes that crystallized in the middle and upper crust, respectively. The variation of Mg, Ti, P, Sr, and Ba contents in these rocks is consistent with a complex magmatic origin via both cooling, fractional crystallization, periodic recharge of deeply derived hydrous basaltic or andesitic melts, and mixing with silicic melts derived by crustal melting. Low eruption rates, high phenocryst contents of the volcanic rocks, and widespread hydrothermal alteration are consistent with the hypothesis that most of the magmas at Yanacocha crystallized in shallow chambers as granitoids that passively degassed ore fluids. The compositional diversity of the volcanic rocks together with an extended magmatic-hydrothermal history reflect complex magmatic processes that were optimum for producing the world-class gold deposits at Yanacocha.

Broad Synchroneity of Three Gold Mineralization Styles in the Kalgoorlie Gold Field: SHRIMP, U-Pb, and 40Ar/39Ar Geochronological Evidence

Abstract: There has been a long-standing controversy regarding the timing and number of gold mineralization events at Kalgoorlie. Uranium-Pb dating of zircons' and hydrothermal monazite and xenotime, as well as Ar-40/Ar-39 analysis of metasomatic fuchsite and white mica. are used to date pre- to synore dikes. alteration, and orebodies in order to resolve this issue. The majority of gold mineralization at Kalgoorlie, including ductile-brittle Fimiston-, brittle-ductile Oroya- and brittle Mount Charlotte-style gold, are different expressions of a complex mineralizing system that was active at broadly the same time at ca. 2.64 Ga. Gold mineralization was thus deposited in both ductile and brittle structures at approximately the same crustal level at broadly the same time, limier similar-PT conditions. This giant ore system formed after ea. 2.69 Ga basic magmatism, intrusion of the Golden Mile Dolerite sill at 2680 ± 9 Ma, and intrusion or calc-alkaline feldspar-quartz porphyry dikes at 2670 ± 5 Ma. Gold mineralization was broadly coeval with lamprophyre dike intrusion at 2642 ± 6 Ma and overlapped the waning stages of hornblende and albite-bearing porphyry dike emplacement at 2650 ± 6 Ma and regional metamorphism. Subsequent brittle deformation in the Kalgoorlie gold held way, accompanied by hydrothermal activity that may have led to some late gold mineralization or remobilization in extensional quartz vein arrays in the Golden Mile between about 2.61 and 2.60 Ca. This late hydrothermal activity and associated brittle deformation marks the last event to significantly affect the rocks it Kalgoorlie and may be related to uplift lift and final cooling of the terrane. Despite this late event, the geometry of the Kalgoorlie gold field and its contained lode systems has remained essentially the same since the time of gold mineralization.