Showing papers in "Economic Geology in 2011"

A Carbonaceous Sedimentary Source-Rock Model for Carlin-Type and Orogenic Gold Deposits

Abstract: This paper presents evidence and arguments that carbonaceous sedimentary rocks were a source for Au and As in sediment-hosted orogenic and Carlin-type gold deposits and develops a corresponding genetic model. In this two-stage basin-scale model, gold and arsenic are introduced early into black shale and turbidite basins during sedimentation and diagenesis (stage 1) and concentrated to ore grades by later hydrothermal, structural, or magmatic processes (stage 2). In reduced continental margin basin settings, organic matter, sedimented under anoxic to euxinic conditions, immobilizes and concentrates gold, arsenic, and a range of trace elements (particularly V, Ni, Se, Ag, Zn, Mo, Cu, U) present in marine bottom waters, into fine-grained black mudstone and siltstone of slope and basin facies. During early diagenesis, gold and certain other trace elements (Ni, Se, Te, Ag, Mo, Cu, ±PGE) are preferentially partitioned into arsenian pyrite that grows in the muds. These processes produce regionally extensive black shale and turbidite sequences enriched in syngenetic gold and arsenic, commonly from 5 to 100 ppb Au and 10 to 200 ppm As. Rare organic- and sulfide-rich metalliferous black shales may contain up to 1 to 2 ppm Au and over 1,000 ppm As, present as refractory gold in arsenian pyrite and nanoparticles of free gold. During late diagenesis and early metamorphism (stage 2) the diagenetic arsenian pyrite is recrystallized to form coarser grained pyrite generations, and the organic matter is cooked to bitumen. Under higher grade metamorphism (lower greenschist facies and above) arsenian pyrite in carbonaceous shales is converted to pyrrhotite. These processes release gold, arsenic, sulfur and other elements (Sb, Te, Cu, Zn, Mo, Bi, Tl, and Pb) from the source rocks to become concentrated by hydrothermal processes, locally to produce gold ores, in structural sites such as fold hinge zones, shear or breccia zones within or above the black shale sequence. LA-ICP-MS analyses of diagenetic pyrite in carbonaceous sediments, both associated and not associated with gold deposits, suggests that invisible gold contents of greater than 250 ppb in diagenetic pyrite, are indicative of carbonaceous shale source rocks with the potential to produce economic gold deposits. Application of this sedimentary source-rock model enables a systematic exploration approach for sediment-hosted gold deposits, based on the distribution, composition and structure of carbonaceous shale sequences and their contained diagenetic pyrite.

HIGH Sr/Y ARC MAGMAS AND PORPHYRY Cu ± Mo ± Au DEPOSITS: JUST ADD WATER

Abstract: Porphyry Cu ± Mo ± Au deposits are commonly associated with intermediate composition arc-related igneous rocks with high Sr/Y (and La/Yb) ratios, and such rocks are equated by some researchers with melts derived from subducted oceanic crust undergoing eclogite facies metamorphism (“adakites”). However, similar geochemical characteristics are readily developed in normal asthenospheric mantle wedge-derived magmas by fractionation of amphibole and/or garnet, and/or by interaction with crustal materials during ascent through the upper plate lithosphere. While there is widespread evidence for such fractionation and contamination processes in porphyry magmas, there is little direct evidence for an origin by slab melting. The enhanced fertility of arc magmas relates primarily to their high water content, which is required for the formation of magmatic-hydrothermal systems upon shallow crustal emplacement. Magmatic water contents >4 wt percent H 2 O cause abundant amphibole (± garnet) fractionation but suppression of plagioclase crystallization at deep crustal levels, resulting in increasing Sr/Y and La/Yb ratios with differentiation, commonly into “adakitic” ranges, but not reflecting slab melting. Additional factors that affect arc magma fertility are relatively high oxidation states and sulfur content, but these are secondary to the requirement for sufficient water. Thus, arc magmatic suites with high Sr/Y ratios and evidence for the presence of hydrous phenocryst phases (hornblende and/or biotite) are indeed prospective for porphyry Cu ± Mo ± Au deposits, but only because these parameters indicate high magmatic water content.

Pyrite and Pyrrhotite Textures and Composition in Sediments, Laminated Quartz Veins, and Reefs at Bendigo Gold Mine, Australia: Insights for Ore Genesis

Abstract: The various types, textures, and compositional zoning of pyrite in the gold-bearing saddle reefs, quartz veins, and surrounding sedimentary rocks provide new information on the potential source and timing of gold and arsenic and related fluid processes responsible for mineralization at Bendigo. Nodular diagenetic pyrite in the black shale tops to sandstone turbidites is enriched in invisible gold and arsenic with mean values of 0.61 ppm Au, 1,300 ppm As, and Au/Ag <1, based on LA-ICPMS analyses. Other elements enriched in the diagenetic pyrite within the organic-rich shales are Mn, Zn, Mo, Cu, V, Ba, Ag, Cd, Tl, Co, Ni, Bi, Pb, and Te. In contrast, euhedral- and growth-zoned hydrothermal pyrite in the turbidites and bedding-parallel laminated quartz veins contains lower contents of most trace elements but has higher contents of invisible Au and As, especially on the outermost rim of the pyrite. The gold-rich pyrite rims generally become thicker (a few to hundreds of microns) in proximity to the gold-bearing saddle reefs. Pyrite in the reef commonly has the highest levels of invisible Au and As and the lowest levels of other trace elements. It is characterized by Au/Ag >1 and Au/Pb >0.01. In the deepest stratigraphic levels, below known productive gold reefs, diagenetic pyrite in the most carbonaceous shales has been replaced by pyrrhotite during metamorphism. LA-ICPMS analyses reveal that the disseminated pyrrhotite contains similar levels of Ni and Co to the diagenetic pyrite but is strongly depleted in As and Au. The spatial relationships between organic-rich shales, folded bedding-parallel laminated quartz veins, and gold-arsenic−bearing saddle reefs, combined with the consistent trends in the trace element composition of pyrite hosted by these three geological elements, is interpreted to indicate that the black shales were an initial source of Au and As, and the laminated quartz veins acted as the initial pathways for hydrothermal fluid flow carrying Au and As from the source shales to the saddle reefs. Maximum gold and arsenic input into the reefs, principally as free gold plus arsenopyrite, occurred late during deformation toward the end of the hydrothermal cycle and is expressed by the Au-As−rich rims to hydrothermal pyrite in the sedimentary host rocks, laminated quartz veins, and reefs. This corresponds with final fold lockup and the development of through-going fault arrays linking adjacent anticlines. The source of Au and As for this final, and most economically important, fluid-flow event is considered to be from carbonaceous shales deeper in the basin, where original gold-bearing diagenetic arsenian pyrite reacts with organic matter and is converted to pyrrhotite, with release of Au, As, and S to the metamorphic fluid.

Exploration Tools for Linked Porphyry and Epithermal Deposits: Example from the Mankayan Intrusion-Centered Cu-Au District, Luzon, Philippines

Abstract: The Mankayan mineral district of northern Luzon, Philippines, hosts several significant ore deposits and prospects of various types within an area of similar to 25 km(2), including the Far Southeast porphyry Cu-Au deposit, the Lepanto high sulfidation epithermal Cu-Au deposit, the Victoria intermediate sulfidation epithermal Au-Ag vein deposit, the Teresa epithermal Au-Ag vein deposit, the Guinaoang porphyry Cu-Au deposit, and the Buaki Old Palidan porphyry Cu-Au prospects, all having formed in a period of about 2 m.y., from similar to 3 Ma. The geologic units include (1) a basement composed of Late Cretaceous to middle Miocene metavolcanic rocks volcaniclastic rocks; (2) the Miocene 12 to 13 Ma tonalitic Bagon intrusive complex; (3) the Pliocene, similar to 2.2 to 1.8 Ma, Imbanguila dacite porphyry and pyroclastic rocks; and (4) postmineralization cover rocks, including die similar to 1.2 to 1.0 Ma Bato dacite porphyry and pyroclastic rocks and the similar to 0.02 Ma Lapangan tuff. Extensive advanced argillic alteration crops out for similar to 7 kin along the inconformity between the basement rocks and the Imbanguila dacite formation consists of quartz-alunite +/- pyrophyllite or diaspore, with local zones of silicic alteration and a halo of dickite +/- kaolinite. The alteration and its subhorizontal geometry indicate that it is a lithocap or coalesced lithocaps. The northwest-striking portion is similar to 4 km long and hosts the Lepanto enargite Au ore deposit, also controlled by the Lepanto fault. The Lepanto epithermal deposit is related to the underlying Far Southeast porphyry; the quartz-alunite alteration halo of Lepanto is contemporaneous with the similar to 1.4 Ma potassic alteration of the porphyry. There are also silicic-advanced argillic alteration patches similar to 600 m above the Far Southeast orebody at the present surface; these are interpreted to be perched alteration. There is no systematic mineralogical or textural zoning in the Lepanto lithocap that indicates direction to the intrusive source. Most surface samples of the lithocap contain less than 50 ppb Au, despite many being less than a few hundred meters from underground Cu-Au ore. This study found that several characteristics of the Lepanto lithocap change systematically with distance from the causative intrusion. The alunite absorption peak at similar to 1,480 nm in the short wavelength infrared (SWIR) spectrum shifts to higher wavelengths where the sample is closer to the intrusive center, due to higher Na and lower K content in the alunite; published experimental studies indicate that high Na/(Na + K) is related to higher formation temperature. High Ca alunite, including huangite, also occurs at locations proximal to the intrusive center. Alunite mineral composition analyzed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) indicates that the Pb content decreases toward the intrusive center, whereas Sr, La, Sr/Ph, and La/Pb increase markedly. Whole-rock compositions, using only nonmineralized (taken as Cu similar to 350 m above the upper extent of the veins. An airborne geophysics survey indicates that the Far Southeast orebody is associated with a wide zone of demagnetization due to extensive magnetite-destructive phyllic alteration. Such low magnetic anomalies on the margin of a large lithocap elsewhere may, deserve attention. The directional indicators and mineralization signatures found in this study have the potential to indicate direction to the intrusive center during exploration of similar porphyry-epithermal districts.

Evidence for Magmatic-Hydrothermal Fluids and Ore-Forming Processes in Epithermal and Porphyry Deposits of the Baguio District, Philippines

Abstract: The Baguio district contains a diverse array of epithermal, porphyry and skarn deposits, together with a large, broadly strata bound, advanced argillic lithocap. Magmatism, mineralization, and alteration occurred in response to subduction of the South China Sea plate and the Scarborough Ridge beneath northern Luzon over the past 3 m.y. Rapid uplift and exhumation resulted in epithermal veins overprinting several porphyry Cu-Au deposits. Most of the epithermal Au-Ag deposits of the Baguio district (including Antamok and Acupan, the two largest Au deposits) are intermediate sulfidation state quartz-carbonate-adularia-illite-base metal sulfide veins that contain electrum and minor Au-Ag tellurides. In contrast, high sulfidation mineralization at Kelly includes enargite, tennantite, electrum, and precious metal tellurides and is associated with advanced argillic alteration. Although the mineralizing fluids that formed the porphyry and epithermal deposits had distinct temperatures and salinities, stable and radiogenic data provide evidence for direct magmatic contributions into each deposit type. The epithermal mineralizing fluids were dilute (generally, 600°C) hypersaline brines (30 to >70 wt % NaCl equiv) and low-density vapor. Sulfur isotope compositions of sulfides in the porphyry, skarn and intermediate sulfidation epithermal veins of the southern and central Baguio district are mostly between +1 and +6 per mil, consistent with a predominance of H2S in the mineralizing fluids (i.e., reducing conditions). In contrast, sulfides from the high sulfidation, porphyry, and intermediate sulfidation deposits located adjacent to the Baguio lithocap mostly have negative sulfur isotope values (−6.9 to +0.8‰), consistent with oxidizing (SO42−-predominant) mineralizing fluids. Intermediate sulfidation epithermal veins at Acupan have crosscut a well-mineralized porphyry Cu-Au stock-work at Ampucao. The two deposits cannot be distinguished on the basis of radiometric age determinations (Ampucao: 0.51 ± 0.26 Ma; Acupan: 0.65 ± 0.07 Ma), and are interpreted to be cogenetic, with telescoping of the two environments caused by the rapid uplift and exhumation associated with ridge subduction. Measured δ 34Ssulfide (+1.1 to +6.6‰), δ 34Ssulfate (+10.4 to +31.8‰) values and initial strontium ratios of anhydrite (0.70378–0.70385) are consistent with identical and predominantly magmatic sources of these components for the Ampucao porphyry and Acupan epithermal veins. Helium isotopes provide further evidence of mantle-derived components in the epithermal veins (R/Ra values of 6.0 and 6.7). Oxygen, deuterium, and carbon isotopes provide evidence for predominantly magmatic water at Ampucao and for hybrid magmatic-meteoric waters at Acupan that precipitated precious metals due to boiling. The proportion of magmatic water relative to meteoric water and precious metal grades both decreased with time during epithermal vein formation at Acupan. The common observation of cross-cutting relationships between porphyry and epithermal veins observed throughout the Baguio district imply that the evolution of porphyry-style to intermediate sulfidation-style mineralization was a common phenomenon in this region, and contributed significantly to its rich metal endowment.

The Pulang Porphyry Copper Deposit and Associated Felsic Intrusions in Yunnan Province, Southwest China

Abstract: The Pulang porphyry copper deposit recently discovered in northwestern Yunnan province, China, is located at the south end of the Triassic Yidun island arc. To date, 15 mineralized porphyry deposits have been defined in the Pulang area and the copper resource is estimated to exceed 10 Mt. The Pulang deposit, as currently defined, is made up of five ore-bearing porphyry deposits, covering an area of approximately 9 km2. Intermediate acidic porphyritic intrusions composed of quartz-diorite, monzodiorite, quartz-monzonite, and granodiorite are widespread in the Pulang area. The alteration zones identified with the porphyry deposits include silicic, potassium silicate, quartz-sericite, and propylitic zones. The porphyry deposits have hornfels at the contact with slate, sandstone, and andesite. Re-Os ages of molybdenite, and Ar/Ar and K-Ar dating of biotite indicate that the Pulang porphyry copper deposit formed during the Indosinian tectonic episode, with the main ore formation taking place from 216 to 213 Ma (Late Triassic, Norian); however, the whole process of hydrothermal activity, including overprinting, may have extended from 235 to 182.5 Ma.

Re-Os Age of Polymetallic Ni-Mo-PGE-Au Mineralization in Early Cambrian Black Shales of South China—A Reassessment

Abstract: The Early Cambrian Niutitang Formation in South China consists of a black shale sequence with a synsedimentary organic carbon-rich polymetallic sulfide layer with extreme metal enrichment locally mined as Ni-Mo-PGE-Au ore. We report a new composite Re-Os isochron age of the sulfide ore layer of 521 ± 5 Ma (n = 14) from three mine sites (Dazhuliushui and Maluhe in the Guizhou province and Sancha in the Hunan province) several hundred kilometers apart. This age corresponds to the biostratigraphic Tommotian age, and is in agreement with recent interpolated Pb-Pb and SHRIMP zircon U-Pb ages from an underlying tuff unit. The initial 187Os/188Os value of the sulfide ore is 0.87 ± 0.07, which is equal to the initial 187Os/188Os ratio of ~0.80 of the black shale host rock, consistent with the seawater origin of the metals under depositional conditions of very low clastic input.

In Situ Trace Element and Sulfur Isotope Analysis of Pyrite in a Paleoproterozoic Gold Placer Deposit, Pardo and Clement Townships, Ontario, Canada

Abstract: Extensive pyrite-bearing, auriferous cobble and boulder conglomerates are present in the basal 30 in of the Mississagi Formation in Pardo and Clement Townships, Ontario, Canada. The sedimentology of the conglomerates, combined with regional geology, indicates limited fluvial transport in a gravel bed braided river with local hyperconcentrated flows, with material derived from a highly restricted catchment area. Postdepositional overprinting of the conglomerates is related to the Penokean orogeny at 1.85 to 1.5 Ga and alkali metasomatism at similar to 1.7 Ga. Several pyrite varieties, including detrital and postdepositional recrystallized and altered grains, are present in the conglomerates. Detailed in situ laser ablation inductively coupled plasma-mass spectrometry (LA ICPMS) analysis of pyrite revealed that the gold in the deposit is intimately associated to large detrital pyrite grains, as "invisible" Au, with lesser amounts of free gold. Postdepositional pyrite and pyrite overgrowths have very low Au content. Elements such as Pb, Bi, Sb, Te, Ag, and Hg, together with Au were mobilized during hydrothermal alteration and dissolution of detrital pyrite grains. Small-scale transport and reprecipitation formed part of the postdepositional pyrite and free gold in immediate contact with postdepositional grains. In contrast, Ni, Co, and As were not mobilized. External fluid infiltration is negligible and is indicated only by minor, compositionally distinct, late sulfide veinlets crosscutting the conglomerates. Sulfur isotope analysis of detrital pyrite yields generally positive (delta(34)S values (0.97-9.26%0). The delta(34)S sulfur isotope composition of pyrite overgrowths and postdepositional grains overlaps the isotopic range of the detrital grains, suggesting a near-closed S system during postd.epositional processes. However, the detrital pyrite tends to have slightly negative Delta(33)S while the postdepositional and overgrowth pyrite are either neutral or slightly positive. The S isotope composition of the detrital pyrite is compatible with an origin of S as dissolved sulfate in an ocean under a low oxygen atmosphere. The potential source of the Au-bearing detrital pyrite appears to have been an, as yet, undiscovered Archean deposit located within 1 to 8 kin of the placer deposit.

The Santa Rita Nickel sulfide deposit in the Fazenda Mirabela Intrusion, Bahia, Brazil: geology, sulfide geochemistry, and genesis

Abstract: The Santa Rita ore zone within the Fazenda Mirabela intrusion is an unusual example of high-tonnage magmatic Ni-Cu-PGE sulfide mineralization developed as a stratiform layer within the main cumulus sequence of a layered intrusion. The intrusion as a whole is about 4 × 2.5 km in outcrop dimensions and at least 3 km in original maximum stratigraphic thickness with an original boat-shaped morphology. The ore zone comprises a 50- to 200-m-thick interval containing up to 5 wt percent disseminated sulfides within a sequence of mostly unaltered harzburgites and orthopyroxenites. This orthopyroxene-dominated sequence lies between the dominantly olivine rich cumulates that comprise the lower half of the intrusion and the gabbroic cumulates that make up the upper half. Sulfide tenor variations and silicate host-rock compositions have been derived from a very large database of continuous assays of more than 500 diamond drill cores. Consistent patterns of sulfide tenor—gradually decreasing Ni from over 20 to about 10 wt percent, near-constant Cu and gradually decreasing PGEs with cyclically varying Pt/Pd ratios—are found within the Santa Rita ore zone across the entire intrusion and display relatively little lateral variability. The position of the sulfide mineralization envelope within the harzburgite-orthopyroxenite package migrates to higher stratigraphic levels from the northern to the southern margin of the intrusion. Maximum grades and thicknesses correlate with the maximum thickness of orthopyroxene-bearing cumulates, asymmetrically disposed toward the southern margin of the intrusion. Nickel tenors of the sulfides range from 15 to 25 wt percent, exceptionally high for rocks with relatively low values of MgO/(MgO + FeO) and are closer to the typical values observed in komatiite-hosted deposits, although Cu tenors (typically 5–8 wt %) are much higher than komatiite values. Sulfide PGE tenors range from less than 100 ppb to greater than 10 ppm of both Pt and Pd, with maximum values in S-poor rocks at the base of the sulfide-bearing interval. PGEs become significantly more depleted toward the top of the Santa Rita zone. We attribute these features, along with the localization of the sulfide-bearing interval at the contact between ultramafic and mafic cumulates, to mixing of an initially S-undersaturated, moderately Ni enriched resident magma with a relatively low Ni, PGE-depleted, significantly lower temperature replenishing magma charged with suspended sulfide liquid droplets.

Petrogenesis of Postcollisional Magmatism at Scheelite Dome, Yukon, Canada: Evidence for a Lithospheric Mantle Source for Magmas Associated with Intrusion-Related Gold Systems

Abstract: The type examples for the class of deposits termed intrusion-related gold systems occur in the Tombstone-Tungsten belt of Alaska and Yukon, on the eastern side of the Tintina gold province. In this part of the northern Cordillera, extensive mid-Cretaceous postcollisional plutonism took place following the accretion of exotic terranes to the continental margin. The most cratonward of the resulting plutonic belts comprises small isolated intrusive centers, with compositionally diverse, dominantly potassic rocks, as exemplified at Scheelite Dome, located in central Yukon. Similar to other spatially and temporally related intrusive centers, the Scheelite Dome intrusions are genetically associated with intrusion-related gold deposits. Intrusions have exceptional variability, ranging from volumetrically dominant clinopyroxene-bearing monzogranites, to calc-alkaline minettes and spessartites, with an intervening range of intermediate to felsic stocks and dikes, including leucominettes, quartz monzonites, quartz monzodiorites, and granodiorites. All rock types are potassic, are strongly enriched in LILEs and LREEs, and feature high LILE/HFSE ratios. Clinopyroxene is common to all rock types and ranges from salite in felsic rocks to high Mg augite and Cr-rich diopside in lamprophyres. Less common, calcic amphibole ranges from actinolitic hornblende to pargasite. The rocks have strongly radiogenic Sr (initial 87 Sr/ 86 Sr from 0.711-0.714) and Pb isotope ratios ( 206 Pb/ 204 Pb from 19.2-19.7), and negative initial ɛ Nd values (−8.06 to −11.26). Whole-rock major and trace element, radiogenic isotope, and mineralogical data suggest that the felsic to intermediate rocks were derived from mafic potassic magmas sourced from the lithospheric mantle via fractional crystallization and minor assimilation of metasedimentary crust. Mainly unmodified minettes and spessartites represent the most primitive and final phases emplaced. Metasomatic enrichments in the underlying lithospheric mantle are attributes of the ancient North American cratonic margin that appear to be essential prerequisites to this style of postcollisional magmatism and associated gold-rich fluid exsolution. This type of magmatic hydrothermal activity occurs in a very specific tectonic setting that typically sets intrusion-related gold deposits apart from orogenic gold deposits, which are synorogenic in timing and have no consistent direct relationship to such diverse and contemporaneous lithospheric mantle-derived magmas, although they too are commonly sited adjacent to lithospheric boundaries.

The Geochemistry of Carbonate-Replacement Pb-Zn-Ag Mineralization in the Lavrion District, Attica, Greece: Fluid Inclusion, Stable Isotope, and Rare Earth Element Studies

Abstract: Strata-bound carbonate-replacement Pb-Zn-Ag deposits in the Lavrion district, Greece, are spatially related to a late Miocene granodiorite intrusion (7–10 Ma) and various sills and dikes of mafic to felsic composition. The Plaka granodiorite contains porphyry molybdenum mineralization and is locally associated with a Ca-Fe skarn. Carbonate-replacement deposits occur predominantly in marbles (Upper and Lower Marble of the Basal unit), Kaesariani schists, and along a major detachment fault that separates the Basal unit from the Upper unit. Orebodies are mainly strata bound carbonate-replacement, although sulfides also occur in veins. The mineralogy of carbonate-replacement deposits is dominated by base metal sulfides and sulfosalts of Ag, Bi, Sn, Sb, As, and Pb, particularly at Plaka and Kamariza. Carbonates are intergrown with earlier formed sulfides and sulfosalts but are more abundant late in the paragenetic sequence with fluorite and barite. Fluid inclusion studies of sphalerite, fluorite, calcite, and quartz in carbonate-replacement deposits suggest that they were deposited from 132° to 365°C from CO2-poor, low- to high-salinity fluids (1–20 wt % NaCl equiv). Carbon and oxygen isotope compositions of calcite ( δ 13C = −15.6 to −1.5‰ and δ 18O = −9.2 to +17.3‰) intergrown with sulfides reflect variable exchange of the ore-bearing fluid with the Upper and Lower Marbles and proximity to the Plaka granodiorite. Post-Archean Australian Shale (PAAS)-normalized rare earth and yttrium patterns of the Upper and Lower Marbles, and calcite intergrown with sulfides show positive Eu and negative Ce anomalies as well as Y/Ho ratios between 40 and 80. Normalized rare earth and yttrium patterns of fluorite also have positive Eu and negative Ce anomalies. Such anomalies for both the carbonates and fluorite reflect the high pH or high f O2 conditions of the late-stage hydrothermal fluids and the likely derivation of calcium from marine carbonates (precursors of the Upper and Lower Marbles). The range of sulfur isotope compositions for sulfides ( δ 34S = −4.9 to +5.3‰, with one outlier of 9.4‰) in carbonate-replacement and vein deposits is due likely to a magmatic sulfur source with a contribution of reduced seawater sulfate. Sulfur isotope compositions of barite from carbonate-replacement range from δ 34S = 17.2 to 23.7 per mil and reflect Miocene seawater sulfate values. If a magmatic source of sulfur is assumed along with an average temperature of 250°C for the ore-forming fluids, as based on fluid inclusion studies, sulfides in carbonate-replacement deposits were deposited at values of log f O2 = −41 to −36 and a pH = 5.8 to 9.1. However, the range of sulfur isotope values does not rule out the possibility that sulfur in sulfides could have been produced by the reduction of seawater sulfate with no contribution from a magmatic source. The carbonate-replacement deposits resemble manto-type sulfide deposits in Mexico, central Colorado, South Korea, Nevada, and northern Greece.

Geology and Intrusion-Related Affinity of the Morila Gold Mine, Southeast Mali

Abstract: The ~8 Moz Morila gold mine, hosted within Paleoproterozoic Birimian volcano-sedimentary rocks of southeast Mali, is spatially and temporally associated with prolonged (2098–2065 Ma) arc magmatism during the late stages of the Eburnean orogeny. Visible gold at Morila is associated with variably deformed polymineralic veins containing native bismuth, maldonite, aurostibite, rare tellurobismuthite, and lollingite, suggesting a proximal intrusion-related source for this period of gold mineralization. This early formed mineralization is contained within a zone of hornblende hornfels contact metamorphism and is spatially associated with syn- to post-D2 emplacement of 2098 to 2091 Ma quartz-diorite, granodiorite, and leucogranite magmas. The occurrence of immiscible Au-Sb-Bi-Te blebs within sills or dikes associated with gold mineralization at the Morila deposit explicitly links granitic magmatism with gold mineralization This early intrusion-related gold system was over-printed by a younger post-D2 stage of hydrothermal alteration recorded by sulfidation along a north-north-east–trending zone characterized by disseminated idioblastic arsenopyrite porphyroblasts that contain polygonal gold blebs. Silicate alteration during this stage includes albitization of plagioclase and the growth of randomly distributed biotite and titanite, the latter typically surrounding ilmenite. Uranium-Pb dating of this generation of titanite yields a preliminary age for late-stage sulfidation of 2074 ± 14 Ma, which brackets mineralization to the interval 2098 ± 4 to 2074 ± 14 Ma. The geochemistry and isotope systematics of syn- to post-tectonic intermediate intrusions at the Morila deposit point to their derivation in a suprasubduction zone setting and emplacement into tectonically thickened crust. Based on these observations, it is suggested that the Morila gold deposit formed during late-stage collisional orogenesis involving the accretion of juvenile volcanic arc terranes against the Archean Man (Liberian) cratonic nucleus. This setting is analogous to younger Phanerozoic active continental margin settings which host the best-described examples of intrusion-related gold systems.

Mineral Chemistry and In Situ Dating of Allanite, and Geochemistry of Its Host Rocks in the Bagjata Uranium Mine, Singhbhum Shear Zone, India—Implications for the Chemical Evolution of REE Mineralization and Mobilization

Abstract: The present study combines textures, compositions, and in situ dating of allanite and whole-rock geochemistry from the Bagjata uranium deposit in the Singhbhum shear zone, the most important uranium-producing belt in India, to investigate the chemical evolution of rare earth element (REE) mineralization. Allanite, volumetrically the most important sink of light rare earth elements (LREEs) in this deposit, occurs as disseminated grains, pockets, veins, and stringers in biotite schist and chlorite schist. Metasomatic addition of LREEs in the rock was associated with Ca-K-Fe ± B metasomatism, as evident from the vein mineralogy. The allanite-bearing metasomatized rocks are locally extremely enriched in REEs, reaching up to ~4.8 wt percent ∑REEs. The textures and compositions of allanite demonstrate that early REE-rich allanite was replaced by later, relatively less REE bearing allanite-epidote, and this alteration followed a substitution scheme of REE +3 + (Fe +2 , Mg +2 ) → Ca +2 + (Al, Fe +3 ). The presence of halite-bearing fluid inclusions in associated tourmaline, high Cl content of biotite, and other circumstantial evidence indicate that a high-salinity, Cl-rich fluid was likely responsible for initial LREE metasomatism. A later hydrothermal event altered preexisting allanite and removed REEs. The more oxidized nature of altered allanite and the presence of unidentified secondary REE minerals in association with sulfide might indicate the involvement of oxidized, sulfate-rich fluid in the alteration of allanite and the removal of REEs. Alternatively, fluoride complexing of REEs and selective removal of HREEs during this alteration are possible. In situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of allanite and monazite indicates that LREE metasomatism took place at ~1.88 Ga. The next hydrothermal event, which altered existing allanite, removed some REEs, and perhaps was associated with U (+HREE) addition, is dated at ~1.66 Ga. The youngest age determined from the thin rim of allanite is ~1.02 Ga. These three different ages, determined from allanite, overlap with the known thermotectonic events in and around the Singhbhum shear zone. Comparison between the calculated ∑REE content of allanite-rich rock prior to allanite alteration and whole-rock ∑REE composition of the same rock suggests that the addition of a large quantity of REEs was followed by significant REE removal. Strong variations in the modal abundances of the constituent minerals and in whole-rock compositions, including REEs, indicate open-system behavior. The whole-rock REE pattern suggests that decoupling between LREEs and HREEs might be possible during superimposed alteration. Therefore, petrogenetic modeling using an REE-based discrimination diagram and REE patterns should be done with caution for rocks that underwent multiple events of hydrothermal fluid flux.

The Sandstone-Hosted Beverley Uranium Deposit, Lake Frome Basin, South Australia: Mineralogy, Geochemistry, and a Time-Constrained Model for Its Genesis

Abstract: The sandstone-hosted Beverley uranium deposit is located in terrestrial sediments in the Lake Frome basin in the North Flinders Ranges, South Australia. The deposit is 13 km from the U-rich Mesoproterozoic basement of the Mount Painter inlier, which is being uplifted 100 to 200 m above the basin by neotectonic activity that probably initiated in the early Pliocene. The mineralization was deposited mainly in organic matter-poor Miocene lacustrine sands and partly in the underlying reductive strata comprising organic matter-rich clays and silts. The bulk of the mineralization consists of coffinite and/or uraninite nodules, growing around Co-rich pyrite with an S isotope composition ( δ 34 S = 1.0 ± 0.3‰), suggestive of an early diagenetic lacustrine origin. In contrast, authigenic sulfides in the bulk of the sediments have a negative S isotope signature ( δ 34 S ranges from −26.2 to −35.5‰), indicative of an origin via bacterially mediated sulfate reduction. Minor amounts of Zn-bearing native copper and native lead also support the presence of specific, reducing microenvironments in the ore zone. Small amounts of carnotite are associated with the coffinite ore and also occur beneath a paleosoil horizon overlying the uranium deposit. Provenance studies suggest that the host Miocene sediments were derived from the reworking of Early Cretaceous glacial or glaciolacustrine sediments ultimately derived from Paleozoic terranes in eastern Australia. In contrast, the overlying Pliocene strata were in part derived from the Mesoproterozoic basement inlier. Mass-balance and geochemical data confirm that granites of the Mount Painter domain were the ultimate source of U and REE at Beverley. U-Pb dating of coffinite and carnotite suggest that the U mineralization is Pliocene (6.7-3.4 Ma). The suitability of the Beverley deposit for efficient mining via in situ leaching, and hence its economic value, are determined by the nature of the hosting sand unit, which provides the permeability and low reactivity required for high fluid flow and low chemical consumption. These favorable sedimentologic and geometrical features result from a complex conjunction of factors, including deposition in lacustrine shore environment, reworking of angular sands of glacial origin, deep Pliocene weathering, and proximity to an active fault exposing extremely U rich rocks.

The evaluation of brine prospects and the requirement for modifications to filing standards

Abstract: The recent increase in demand for lithium has led to the development of new brine prospects, particularly in the Central Andes. The brines are hosted in closed basin aquifers of two types: mature, halite dominant, and immature, clastic dominant. The estimate of elemental resources in these salars depends on a detailed knowledge of aquifer geometry, porosity, and brine grade. The geometry of the aquifers can be evaluated by classical geophysical and drilling techniques, but because the resource is a fluid, with the attendant problems of in-aquifer mixing and reorganization, existing codes for filing resource and reserve estimates need modification. Total porosity is relatively straightforward to measure, but effective porosity and specific yield, which are required to estimate the resource, are more difficult. Recovery factors are low compared with most metalliferous and industrial mineral deposits due to reliance on pumping of the brine from wells for extraction. These and related issues lead us to believe that modifications to the existing standards for reporting mineral resources and reserves are required for these prospects.

Composition of Ore Fluid Inclusions from the Viburnum Trend, Southeast Missouri District, United States: Implications for Transport and Precipitation Mechanisms

Abstract: Fluid inclusions hosted by sphalerite and dolomite cement from the Mississippi Valley-type (MVT) deposits of the Viburnum Trend of the Southeast Missouri district were analyzed by microthermometry and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to determine the composition of mineralizing fluids and to seek insights into ore-forming processes. On the whole, fluid inclusions from throughout the paragenesis have compositions resembling deep sedimentary brines. A population of anomalously Pb-rich fluid inclusions with concentrations reaching 1,500 ppm was identified in some of the paragenetically early and main-stage sphalerite, which is interpreted to represent the influx of a Pb- and possibly overall metal-rich fluid into Southeast Missouri during the time of sulfide mineralization. This influx of Pb-rich fluid appears to have been intermittent, as at least one stage of dolomite cement that hosts Pb-poor fluid inclusions occurs between intervals of sphalerite that host Pb-rich fluid inclusions. If fluid flow into the district was principally from one direction, for example, from the Arkoma basin to the south, then the intermittency in high Pb concentrations could be explained by compositional changes in the source region or along the flow path. Alternatively, if fluid flow into the district was from two or more directions and fluid arriving from each direction had a different composition, then the intermittency in high Pb concentrations could be explained by transient variations in the relative flow rates from each direction. Lead concentrations on the order of thousands of parts per million make it unlikely that the Southeast Missouri ores were precipitated by any mechanism that involved simultaneous transport of metals and sulfide in the same fluid, as the concentrations of sulfide that could coexist with such high concentrations of Pb would be too low to allow the ores to be deposited within a geologically reasonable period of time, unless the pH was very low or groundwater flow rates were very high. Thus, some type of mixing scenario seems more likely involving either mixing of a metal- with a sulfide-rich fluid or mixing of a metal- and sulfate-rich fluid with a reductant present either in the host rock or in another fluid. Fluid inclusions hosted by sphalerite have K/Na and Ca/Mg ratios that partly resemble the compositions of both main-stage cuboctahedral galena and less abundant late-stage cubic galena, suggesting that sphalerite was precipitated from a mixture of the fluids that precipitated these two types of galena. However, the fact that aqueous Pb concentration does not correlate with these ratios or other indicators of main-stage mineralization, such as matrix Mn, Fe, Co, Cu, Ag, and Cd content in sphalerite, suggests that more than two fluids were involved in mixing.

Porphyry and Epithermal Deposits and 40Ar/39Ar Geochronology of the Baguio District, Philippines

Abstract: The Baguio district is located in the Central Cordillera of northern Luzon, Philippines. It contains numerous mineralized porphyry copper-gold, epithermal gold-silver and skarn gold-lead-zinc deposits. The district is floored by Cretaceous-Eocene metavolcanic and metasedimentary rocks, which are overlain by marine to terrestrial sedimentary and volcanic rocks of early Miocene to Pliocene ages. Tertiary arc magmatism related to east-directed subduction of the South China Sea plate along the Manila Trench beneath northern Luzon produced a major batholith, the Central Cordillera Intrusive Complex, which defines the eastern boundary of the Baguio district. The intrusive complex was emplaced in the early Miocene based on 40Ar/39Ar age determinations for the Lucbuban gabbro (22.6 ± 0.5 Ma) and Virac granodiorite (20.23 ± 0.38 and 20.2 ± 0.7 Ma). Recent exploration has led to the discovery of several new porphyry copper-gold and skarn prospects in the western Baguio district. Mineralization was preceded by the intrusion of a suite of hornblende megacrystic andesite dikes in the central part of the district from 4.55 ± 0.15 to 3.45 ± 0.19 Ma. Porphyry copper-gold and skarn deposits at Black Mountain and Mexico, on the western side of the district, formed between 3.09 ± 0.15 and 2.81 ± 0.24 Ma. The Santo Tomas II Cu-Au-(Pd) porphyry deposit was emplaced into the central southern part of the district at about 1.5 Ma, based on secondary biotite 40Ar/39Ar ages of 1.48 ± 0.05 Ma and 1.47 ± 0.05 Ma. On the eastern side of the district, porphyry-style mineralization at the Hartwell and Ampucao porphyry Cu-Au prospects returned 40Ar/39Ar ages of 1.09 ± 0.10 and 0.51 ± 0.26 Ma, respectively. Quartz-carbonate-base metal sulfide style epithermal gold-silver veins are well-developed on the eastern side of the Baguio district, and crosscut porphyry-style mineralization at the Acupan, Baguio gold and Nugget Hill deposits. Epithermal veining is inferred to have occurred in the past million years, based on a reported K-Ar age determination of 0.65 ± 0.07 Ma from the Acupan gold mine. A large advanced argillic alteration zone (the Baguio lithocap) crops out in the northwest part of the district, and formed between 1.4 and 0.9 Ma, based on previous K-Ar dating. Pliocene-Pleistocene mineralization in the Baguio district was triggered by the east-directed subduction of the Scarborough Ridge. Ridge subduction caused the subduction angle along the Manila Trench beneath northern Luzon to decrease, which facilitated crustal thickening and exhumation. Ridge subduction also promoted the development of northwest-trending faults in the upper plate that interacted with major arc-parallel north- and arc-normal northeast-trending fault sets associated with the Philippine fault system. Fertile magmas were emplaced into transtensional strike-slip relay basins, resulting in the accumulation of more than 35 Moz of gold and several million tonnes of copper over approximately 3 m.y

Automated drill core logging using visible and near-infrared reflectance spectroscopy: a case study from the olympic dam iocg deposit, south australia

Abstract: Reflectance spectrometers with automated scanning capabilities can gather compositional information directly from the surface of drill core. To showcase the usefulness of this analytical technique, 300 m of drill core from the Olympic Dam iron oxide copper-gold (IOCG) deposit, South Australia, were scanned using HyLogger. The reflectance spectra (400–2,500 nm) were analyzed to identify hematite and phengite, which are the most important alteration minerals at Olympic Dam. The results were plotted as a function of depth to produce a log that accurately identified ore-bearing and barren rocks. The position of the most intense absorption feature between 850 and 970 nm was found to correspond to iron concentration, and the intensity of the most intense absorption feature between 2,190 and 2,230 nm was found to correspond to aluminum concentration. In addition, phengite located near the ore-bearing zone was found to contain more aluminum than phengite located in the barren rocks, and this difference in phengite mineral chemistry was observable in the reflectance spectra between 2,190 and 2,230 nm.

Subepithermal Au-Pd Mineralization Associated with an Alkalic Porphyry Cu-Au Deposit, Mount Milligan, Quesnel Terrane, British Columbia, Canada

Abstract: At the Mount Milligan Cu-Au porphyry deposit, Quesnel terrane, British Columbia, Canada, barren and weakly mineralized, late-stage hydrothermal veins occur in volcanic rocks adjacent to zones of Cu-Au porphyry mineralization, and have overprinted the porphyry-stage veins. The earliest of the late-stage hydrothermal veins are barren and consist of quartz ± pyrite ± carbonate ± chlorite ± tourmaline. These veins are similar to “transitional” to late-stage hydrothermal veins in other alkaline porphyry Cu-Au deposits, and we consider these to be the equivalent of transitional (post-porphyry, pre-epithermal) quartz-sericite-pyrite veins in calc-alkaline porphyry environments. A later generation of volumetrically minor, mineralized veins are composed of pyrite (Hg- and As-bearing) ± quartz ± carbonate ± chlorite and contain early electrum, arsenopyrite, tetra-hedrite-tennantite, platinum-group element (PGE) tellurides, galena, sphalerite, barite, and chalcopyrite as inclusions in pyrite, and a later assemblage of electrum, PGE tellurides, arsenides and antimonides, galena, sphalerite, chalcopyrite, and various Au-Ag-Te-Bi minerals in annealed fractures and open-space infillings in quartz and pyrite. Metal precipitation in these veins was temporally and spatially associated with the deposition and later recrystallization of pyrite. Primary fluid inclusions in quartz in the barren and weakly mineralized veins are two-phase (L+V), homogenize to liquid over a narrow range in T (~170°–270°C; n = 96, 12 veins), and show a wide range in salinity (4.2 wt % NaCl equiv to 28.7 wt % CaCl2 equiv) when all samples are considered. However, individual veins show narrow ranges in salinity and homogenization temperature. LA-ICP-MS analyses indicate that the fluids were highly enriched in As (to 2,260 ppm), Sb (to 230 ppm), B (to 5,400 ppm), Au (~1–2 ppm) and Pd (~0.5–1 ppm) but depleted in Cu ( 80 ppm) compared to typical porphyry-stage fluids. Metal ratios in the fluids overlap with bulk rock metal ratios in the mineralized veins. The inclusions are interpreted to contain a contracted magmatic vapor (produced by boiling) that lost Cu during the formation of porphyry stage veins at depth. Fluids show decreasing B, As, Sb, and increasing Sr, Ca, and salinity with time. Stable C, O, and H isotope analyses of vein minerals indicate that mixing of this magmatic fluid with meteoric water was not responsible for metal deposition. Rather, metal precipitation was possibly the result of mixing of the magmatic-derived fluid with a heated saline groundwater. The precious and accessory metal mineralogy of the hydrothermal veins is similar to that found in low- to intermediate-sulfidation epithermal systems. Fluid inclusion microthermometry and chlorite thermometry constrain the approximate formation conditions of the veins between ~200 and 1,500 bars and ~240° and 280°C. After the formation of the mineralized veins, circulation of low salinity, metal-depleted fluids occurred. These latest stage fluids may have formed by mixing of the saline magmatic fluid-groundwater hybrid with meteoric water. The results of this study suggest a genetic link between porphyry-stage events and the deposition of Au and PGE in late-stage veins in an alkalic igneous environment. Recognition of hydrothermal processes involving the transport of Au-PGE-As-Sb-Bi-Te-B-rich fluids in the “subepithermal” regimes implies that low-sulfidation epithermal Au deposits may have been present in the shallower parts of the magmatic-hydrothermal complex and that there is potential for the discovery of PGE-rich epithermal veins in less deeply exhumed terranes. On the other hand, the formation of high-grade, low-sulfidation epithermal Au-PGE deposits may be prohibited if porphyry-epithermal transitional fluids precipitate ore metals through mixing with groundwater prior to reaching the level where meteoric water mixing and epithermal boiling normally occur.

Geology of the Didipio Region and Genesis of the Dinkidi Alkalic Porphyry Cu-Au Deposit and Related Pegmatites, Northern Luzon, Philippines

Abstract: The Dinkidi Cu-Au porphyry deposit, northern Luzon, Philippines, formed at the tip of the southward-propagating Cagayan Valley basin in a late Oligocene to early Miocene back-arc setting. The 110 Mt ore deposit is characterized by elevated hypogene grades (avg 1.2 g/t Au and 0.5% Cu) and is one of only a few alkalic porphyry deposits to have been discovered outside of British Columbia and eastern Australia. Dinkidi is hosted by the Didipio intrusive complex, which intruded calc-alkalic to shoshonitic volcanic rocks of the Mamparang and Upper Mamparang formations in the late Oligocene. An early, composite diorite-monzodiorite pluton was intruded by the Surong monzonite and the Dinkidi stock. The initial phases of the Cu-Au mineralized Dinkidi stock were the equigranular biotite-amphibole–bearing Tunja monzonite and the Balut dike—a thin, variably textured and strongly mineralized clinopyroxene syenite pegmatite dike. The youngest phases of the Dinkidi stock are a plagioclase-phyric monzonite-syenite (Quan porphyry) and the Bufu syenite, a crystal-crowded leucocratic quartz syenite. Postmineralization andesite dikes cut the intrusive complex. Emplacement of the Tunja monzonite was temporally and spatially associated with pervasive stage 1 biotitemagnetite- K silicate alteration, which primarily affected the diorite-monzodiorite pluton. Emplacement of the diopside-phyric Balut dike produced the stage 2 calc-potassic diopside-actinolite-K feldspar-bornite vein stockwork and a calc-potassic alteration assemblage typical of silica-undersaturated alkalic porphyry deposits. Stage 2 lacks quartz, contains high gold grades (2–8 g/t Au) and its sulfides have a34S values of –3.5 to –0.7 per mil. The stage 2 calc-potassic assemblage is inferred to have formed at temperatures in excess of 600°C from an oxidized (sulfate-predominant) Na-K-Ca-Fe–rich brine. Intrusion of the quartz-saturated Quan porphyry and Bufu syenite led to the formation of the stage 3 quartzillite- calcite-chalcopyrite stockwork vein and alteration assemblage. The quartz stockwork hosts most of the lower-grade (1–2 g/t Au) mineralization at Dinkidi and is typical of silica-saturated alkalic porphyry systems. A coarse-grained assemblage of quartz-actinolite-perthite (the Bugoy pegmatite) formed as an apophysis on the Bufu syenite and was subsequently brecciated by faulting late in stage 3. The stage 3 quartz stockwork was emplaced at high temperatures (mostly >600°C) from a quartz-saturated, oxidized (sulfate-predominant) Na- K-Fe brine (>68 wt % NaCl equiv) that contained up to 0.6 wt percent Cu and 4 wt percent Fe. Cooling to ~430°C and sulfate reduction by wall-rock interaction led to the precipitation of stage 3 sulfides with a34S values of –4.2 to –0.2 per mil in the quartz stockwork. The quartz-bearing assemblage formed at paleodepths of 2.9 to 3.5 km. Periods of quartz growth from overpressured brines were interrupted episodically by brittle failure events that caused the system to depressurize to near-hydrostatic conditions, triggering vapor generation via boiling. Mineralization was followed by intermediate argillic and high-level advanced argillic alteration (stage 4), and by late-stage, fault-related zeolite-calcite alteration and veins (stage 5). The hydrothermal mineral assemblages at Dinkidi reflect the composition and degree of fractionation of the associated intrusions. Extensive fractionation in a silica-undersaturated dioritic magma chamber is interpreted to have ultimately caused quartz saturation and the development of the late-stage syenite intrusions and related quartz stockwork mineralization. The calcic, silica-undersaturated pegmatitic Balut dike, which is associated with the calc-potassic stockwork, does not fit this fractionation trend and is interpreted to have formed by interaction between the late-stage syenitic melt and a comagmatic mafic melt that underplated the siliceous magma chamber prior to formation of the Balut dike. A reversion to fractionation-dominated magmatic processes in the silicic magma chamber then led to the intrusion of the quartz-saturated Quan porphyry and Bufu syenite. Ultimately, the residual mafic melt was emplaced as a series of late-stage andesite dikes.

A New Geochronological Framework for Mineralization and Alteration in the Selwyn-Mount Dore Corridor, Eastern Fold Belt, Mount Isa Inlier, Australia: Genetic Implications for Iron Oxide Copper-Gold Deposits

Abstract: New Re-Os in molybdenite and U-Pb in titanite and zircon age data have been used to discern several discrete alteration, mineralization, and igneous events that occurred in iron oxide-copper-gold (IOCG) deposits in the Selwyn-Mount Dore corridor of the Proterozoic Mount Isa inlier. Two distinct sodic-calcic alteration events that occurred prior to mineralization have been recognized. This suggests that sodic-calcic alteration may be a fundamental precursor to IOCG mineralization as it mobilizes large amounts of metals, including Fe, K, and Cu. Sodic-calcic alteration at Starra was early synmetamorphic (Isan orogeny) at 1594 ± 8 Ma. Some ironstone-hosted mineralization at Starra may have formed during this event, but an Re-Os molybdenite age of 1568 ± 7 Ma suggests that mineralization also occurred during a (late) metamorphic tectonic event. These ages demonstrate that the Starra system is unrelated to igneous activity; a metamorphic fluid source is proposed. Titanite from a preore alteration assemblage at Mount Elliott yields a U-Pb age of 1530 ± 11 Ma which is within error of the ~1515 Ma Re-Os molybdenite ages from both Mount Elliott and the adjacent SWAN deposit. A magmatic-related origin for the Mount Elliott and SWAN deposits is favored given that the titanite and main-stage molybdenite ages are similar to the emplacement ages for the nearby Squirrel Hills Granite. Molybdenite-bearing calcite veins that crosscut main-stage IOCG mineralization at SWAN have been dated at ~1355 Ma, indicating that this paragenetic stage is unrelated to the Squirrel Hills Granite. U-Pb SHRIMP zircon analyses of volumetrically minor trachyandesitic dikes from Mount Elliott and SWAN reveal relatively young crystallization ages of 1119 ± 15 and 1096 ± 10 Ma, therefore they are not a suitable fluid or metal source for these deposits. Two Re-Os molybdenite analyses from Mount Dore (1503 ± 5 and 1508 ± 5 Ma) are similar to the ~1515 Ma ages from Mount Elliott and SWAN emplacement age of and the Mount Dore Granite. However, two other Re-Os dates (1497 ± 6 and 1501 ± 5 Ma) demonstrate that parts of the Mount Dore system may be slightly younger than Mount Elliott and SWAN. This younger timing is consistent with relative timing criteria that demonstrate mineralization postdates the crystallization of the Mount Dore Granite. Molybdenite from Lady Ella gives an age of 1487 ± 5 Ma. Mineralization at Mount Dore and Lady Ella is probably related to a late-stage, evolved magmatic fluid that generated more potassic alteration. The revised geochronological framework for alteration and mineralization in the Cloncurry district partially explains the fundamental differences in geologic characteristics between IOCG deposits in the district and clearly demonstrates that not all of the IOCG occurrences are related to the intrusion of the voluminous Williams-Naraku batholith. Instead metal-rich reservoirs were generated under the Mount Isa inlier by subduction along the southern margin of the North Australia craton; these reservoirs have been periodically tapped during tectonothermal events, including metamorphism during the Isan orogeny and hot spot activity that generated extensive A-type magmatism that have contributed to the significant metal enrichment across the Mount Isa inlier.

Geochemistry of Tertiary Igneous Rocks of Northern Luzon, Philippines: Evidence for a Back-Arc Setting for Alkalic Porphyry Copper-Gold Deposits and a Case for Slab Roll-Back?

Abstract: Oligocene to early Miocene volcanic rocks are preserved in the Central Cordillera range and Cagayan Valley of northern Luzon, Philippines. Basaltic and andesitic rocks of the Pugo Formation in the Baguio district of the Central Cordillera were intruded by the ~27 to 20 Ma calc-alkaline Central Cordillera Diorite Complex. In the southern Cagayan Valley the subalkaline to alkaline late Oligocene Mamparang Formation overlies the Cretaceous Caraballo Formation and has been intruded by the Didipio Igneous Complex, the Cordon Syenite Complex, and the Palali batholith. The Didipio complex comprises an early suite of diorites, which were intruded by the strongly mineralized stocks of the Dinkidi Cu-Au porphyry deposit. Whole-rock geochemical data for intrusive and extrusive rocks of the Baguio district range from low K calc-alkaline to shoshonitic basalts to dacites with rare earth element (REE) and high field strength element (HFSE) characteristics of suprasubduction zone magmas and are all interpreted to have been sourced from the same parent melt. Samples from Didipio display higher alkali contents but similar trace element characteristics. New age dates for the Didipio area range from 25.7 to 24.8 Ma. The potassic magmas of the Cagayan Valley are interpreted to have formed in a back-arc coeval to the mainarc sequence that is preserved in the Baguio Miocene rocks. This contradicts earlier models, which invoke an early Miocene arc reversal in the northern Luzon archipelago with the switch from early westward subduction to later eastward subduction attributed to a variety of causes. The lack of a single compelling trigger for arc reversal combined with the coeval emplacement of arc magmas in the west and back-arc magmas in the east in northern Luzon is best interpreted as the result of eastward subduction since the late Oligocene. The presence of ~20 Ma adakitic magmas in the Baguio district may indicate that flattening of the downgoing slab resulted in a hiatus in magmatism and termination of back-arc rifting.

Exploration applications of copper isotopes in the supergene environment: A case study of the bayugo porphyry copper-gold deposit, Southern Philippines

Abstract: Copper isotope compositions of secondary phases from the supergene profile of the Bayugo porphyry copper-gold deposit (Philippines) show significant isotopic fractionation relative to hypogene sulfide phases and reveal systematic patterns of isotopic enrichment in the leached cap iron oxides and supergene copper sulfides. Thirty-nine iron oxide, copper oxide, and copper sulfide minerals yielded δ 65 Cu values between −3.4 and +6.9 per mil. Samples of hypogene sulfides have a narrow range of values near zero per mil (−0.6 to +1.0‰). In contrast, samples of supergene sulfides (chalcocite and djurleite) collected from the exotic and enriched copper zones define a trend of decreasing δ 65 Cu from the enriched source area and proximal exotic zone (>3‰) downgradient to the distal portions ( These findings are significant, because within exotic copper zones the copper isotope values of supergene sulfides can provide an indication both of proximity and direction to the source area. The technique could be applied in the search for unrecognized porphyry centers upgradient from headless exotic copper occurrences, by exploring for the supergene copper phases with the highest δ 65 Cu values. Independently, copper isotope ratios of iron oxides in surface samples or fossil leached caps might be used to rank prospects and to focus drilling in areas with the greatest potential for mature enrichment profiles, by mapping the distribution of isotopically enriched leached cap iron oxides. The leached cap screening and targeting tool could enhance the iron oxide mapping techniques traditionally applied in leached cap interpretation and exploration.

Igneous Geochemistry of Mineralized Rocks of the Baguio District, Philippines: Implications for Tectonic Evolution and the Genesis of Porphyry-Style Mineralization

Abstract: The Baguio district of the Philippines is one of the world’s premier mineral provinces, containing >35 million ounces (Moz) of gold and 2.7 million metric tons (Mt) of copper in epithermal, porphyry, and skarn deposits that formed in the last 3.5 m.y. Pliocene and Pleistocene magmatic rocks of the Baguio district that are spatially and temporally associated with mineralization can be broadly subdivided into an intermediate to felsic suite of mineralized low to medium K intrusions, some of which have adakitic affinities and a suite of mafic to intermediate, medium K to shoshonitic hornblende-phyric dikes. The geochemical and isotopic characteristics of the dikes are consistent with primitive mantle-derived melts that underwent minimal crustal contamination as they ascended through the arc crust. In contrast, the intermediate to felsic suite has been contaminated by young arc crust, suggesting ponding and fractionation within shallow-crustal magma chambers. The Philippine arc has formed in a complex tectonic environment and is currently sandwiched between two active subduction zones. Eastward-directed subduction of the Scarborough Ridge along the Manila trench is currently associated with flattening of the downgoing slab. The formation of the Mafic dike complex is broadly coeval with the onset of subduction of the Scarborough Ridge and slab flattening. The extinct Scarborough Ridge would have been younger than the downgoing plate and consequently more susceptible to melting. These melts can account for the isotopic recharge of the Pliocene subarc mantle as well as the generation of the primitive melts and adakitic rocks found within the Baguio district. The interaction between primitive mafic melts and the more felsic calc-alkaline rocks has

Seismic Architecture of the Archean North American Mantle and Its Relationship to Diamondiferous Kimberlite Fields

Abstract: The seismic architecture of the average Archean-only mantle beneath exposed and subsurface Archean crust of the United States and Canada is presented here in three dimensions for the first time, using a high lateral resolution Rayleigh wave phase velocity model of the upper mantle (30- to 250-km depth). The morphology of the cratonic coherent mantle is compared with other regional and local geophysical models, geologic interpretations, and published xenolith barometric studies. In particular, the kimberlite magma source regions at the Lithosphere-Asthenosphere Boundary (LAB) inferred from xenolith data are consistent with the bottom topography of the Archean seismic mantle signature. The characteristic fast seismic response found beneath much of the exposed Archean crust is also found in Canada beneath some covered terranes, sedimentary basins, and Proterozoic mobile belts. The northeastern and northwestern parts of the Superior craton host, with the central Hearne craton, the deepest mantle roots of North America (225- to 240-km depth). However, the southern portion of the Superior craton is characterized by an east-west channel that is 30 percent slower in seismic velocity than its northern counterpart. This contrasting seismic signature correlates with the location of the southernmost Neoarchean greenstone belts and to their plume-driven subduction zones. The scar in the mantle produced by this early tectonothermal event has been reused by widespread and sporadic carbonatite and kimberlite magmatic events spanning from the Early Proterozoic to the Cretaceous, and as a consequence, the diamond stability field has been partially to totally overprinted. Almost all diamondiferous kimberlites in Canada are located vertically over an interval of 160- to 200-km depth in areas of steep slopes surrounding deep (180–240 km), relatively small, and flat-bottomed Paleo-Mesoarchean cratonic keels.

Geology, Mineralization, and Fluid Inclusion Characteristics of the Kensu W-Mo Skarn and Mo-W-Cu-Au Alkalic Porphyry Deposit, Tien Shan, Kyrgyzstan

Abstract: The Kensu deposit is located within a large metallogenic belt of W-Mo, Cu-Mo, W-Au, Au, and Pb-Zn deposits along the Late Paleozoic active continental margin of Tien Shan. The deposit is related to a small Late Carboniferous to Early Permian multiphase gabbro-monzonite(syenite)-granite pluton representing an alkaline potassic (shoshonitic) suite. The igneous rocks are related to a deep-rooted magmatic source but the pluton emplacement corresponds to shallow (“porphyry”) levels. The deposit represents an example of a complex W-rich alkalic magmatic-hydrothermal system that complements the typical mineral deposits related to shoshonitic igneous suites. It contains large bodies of W-Mo-skarn of the oxidized type, with abundant andradite garnet, scapolite, K-Na feldspars, and Fe oxides (magnetite, hematite). The skarns are overprinted and surrounded by extensive halos of stockwork-disseminated (porphyry-style) Mo, W-Mo, W-Cu, Pb-Zn, and Au-W mineralization with propylitic (chlorite-amphibole-epidote–dominated) and phyllic (quartz-sericite-carbonate-sulfide) alteration assemblages. Consistent with the oxidized nature of the major mineral assemblages, the deposit may represent the “oxidized lithophile” W-Mo-Cu-Au metallogenic type. The hydrothermal stages alternate with magmatic phases, and fluid inclusion data show the predominance of high-temperature (~550°–450°C), high-pressure (1,200–700 bars) and high-salinity (60–45 wt % NaCl equiv) magmatic-hydrothermal fluid at prograde and early retrograde skarn stages, with its possible direct exsolution from crystallizing magma. The general trend of cooling and dilution was complicated by the influx of higher-temperature and higher-salinity fluid that correlates to the intrusion of quartz monzonite, and the initial stages of its degassing and cooling. This more advanced phase of magmatic differentiation likely corresponded to the enrichment of residual melt and related magmatic fluids in W and Mo. Intense fluid boiling and phase separation triggered the deposition of molybdoscheelite in skarns that formed at T = >625°–500°C and P = 510–300 bars and continued to a nonboiling fluid at T = >420°–370°C and P = 1,150–750 bars. Mineralization in propylitic alteration zones was formed by highly to moderately saline nonboiling fluids. Late quartz-sericite-carbonate-sulfide stage involved both high to low salinity aqueous fluids and CO2-rich fluids, under decreasing temperature (from >400°C to 200°–300°C) and varying pressure (from ~1,200 bars to 750–600 bars).

Carbonic fluid overprints in volcanogenic massive sulfide deposits: examples from the kelan volcanosedimentary basin, altaides, china

Abstract: The Tiemurte and the Dadonggou Zn-Pb-(Cu) volcanogenic massive sulfide (VMS) deposits, situated in the Devonian Kelan volcanosedmentary basin of the south margin of the Chinese Altaides, were metamorphosed and overprinted by metamorphic sulfide-quartz veins during Early Carboniferous to Early Permian. Two mineralizing periods of ore mineral growth can be identified: (1) disseminated, banded, and massive sulfide ores related to a primary depositional sea-floor volcanic-hydrothermal activity; and (2) the foliated sulfide-quartz veins (Q1) related to synorogenic metamorphism and late chalcopyrite-bearing quartz veins (Q2) cutting the schist related to a younger metamorphic overprinting event. Carbonic (CO 2 -CH 4 -N 2 ) fluid inclusions are ubiquitous in Q1 and Q2 veins. A few carbonic fluid inclusions may be primary and some may be pseudosecondary, whereas the vast number of carbonic fluid inclusions are secondary, representing later events. A microthermometry study shows that primary carbonic fluid inclusions in Q1 and Q2 have Tm CO2 ranging from −64.5° to about −59.4°C with Th CO2 = −13.4° to about +18.6°C. The secondary carbonic fluid inclusions exhibit two behaviors when cooling and heating: the Tm CO2 of the first group (L CO2 ) ranges from −63.3° to −57.7°C, and that of the second group (L CO2-CH4-N2 ) ranges from −83.4° to −61°C. The second group of carbonic fluids has much higher CH 4 and/or N 2 proportions than the first group. The trapping temperatures for the carbonic inclusions have been estimated to be 243.1° to 412.1°C (for Tiemurte) and 216° to ~430°C (for Dadonggou) on the basis of some L CO2 -L H2O inclusions associated with carbonic inclusions, and the trapping pressures have been estimated to be 120 to ~340 MPa, which are consistent with deformation P-T conditions of quartz, and slightly less than the P-T conditions of the biotite and garnet metamorphic zones. These abundant carbonic inclusions at the Tiemurte and the Dadonggou deposits were not a part of a volcanogenic ore-producing system but represent a much younger event, possibly having originated from a synorogenic metamorphism which may have contributed to orogenic gold.

The Nebo-Babel Ni-Cu-PGE sulfide deposit (West Musgrave Block, Australia): Pt. 2. Constraints on parental magma and processes, with implications for mineral exploration

Abstract: The Nebo-Babel Ni-Cu-platinum-group element (PGE) sulfide deposit (West Musgrave, Australia) is hosted in a gabbronorite chonolith emplaced into sulfur-free country-rock orthogneiss at ca. 1068 Ma. Five different types of mafic dikes are found in the area or are spatially associated with the Nebo-Babel intrusion. The mafic dikes are divided into three groups based on petrography, whole-rock major, trace, and PGE chemistry, Sm-Nd isotope ratios, and silicate mineral chemistry: (1) low Ti basalts (NB-1, NB-2, and NB-3); (2) high Ti basalts (NB-4); and (3) alkali basalts (NB-5). Recent age constraints on similar magma suites intruding in the west Musgrave indicate the low and high Ti basalts are coeval with the Nebo-Babel intrusion and hence can potentially be equivalent to its parental magma composition(s). Based on this, we use whole-rock major, trace, and PGE chemistry, Sm-Nd isotope data, silicate mineral chemistry, and geochemical modeling to constrain the petrogenesis of each mafic dike type and its potential relationships with the Nebo-Babel intrusion. Our results indicate that crustal contamination did not play a major role in the generation or evolution of the different magma series. The geochemical variations observed are rather interpreted to reflect different mantle source compositions and different degrees of partial melting. The low Ti basalts exhibit subduction-related geochemical signatures and are interpreted to have been generated by 5 to 10 percent partial melting of a hydrous spinel-bearing mantle. The melting is inferred to have been triggered as a result of mantle plume impingement in an area of the mantle that has previously been metasomatized. The high Ti basalts are interpreted to have formed by mixing of the sublithospheric mantle with deeper asthenospheric mantle melts from the mantle plume. The decompressional melting of the plume head led to the formation of alkali basalts (characterized by typical ocean island basalt (OIB)-like compositions) generated by 4 to 5 percent of partial melting of a garnet-bearing lherzolite. Our results show that the Nebo-Babel intrusion and its associated Ni-Cu-PGE sulfide mineralization formed between the intrusion of the low- and high Ti basalts and may have originated from either mixing between these two magma types, or as a result of continuous change in the melting conditions between these two magmas types. The spatial analysis of the repartition of the different magma suites allowed highlighting zones where the two magma types are spatially associated and where Ni-Cu-PGE sulfide anomalies were discovered, suggesting that magma mixing may be a key factor in the ore genesis at Nebo-Babel. This observation is strengthened by the modeling of the sulfur concentrations at sulfide saturation, which indicates that the assimilation of country-rock orthogneiss by the mixed magma caused a drop in sulfur solubility which in turn led to sulfide saturation in the early stages of the mixing and/or assimilation and potentially to the formation of the Nebo-Babel Ni-Cu-PGE sulfide deposit. Calculated metal concentrations of the parental magma from which the Ni-Cu-PGE mineralization formed are: 165 ppm Ni, 0.15 ppb Ir, 0.30 ppb Ru, 0.22 ppb Rh, 3.40 ppb Pt, 3.30 ppb Pd, and 150 ppm Cu. These metal concentrations are similar to those obtained from the low Ti basalt compositions after they experienced a small amount (<0.001%) of sulfide segregation prior to their final emplacement.

Application of the 190Pt-186Os Isotope System to Dating Platinum Mineralization and Ophiolite Formation: An Example from the Meratus Mountains, Borneo

Abstract: The formation age of platinum-group minerals (PGM) in placer deposits has traditionally been difficult to constrain. We have applied the Pt-Os and Re-Os isotope systems to this problem by analyzing a suite of PGM from a placer deposit in southeastern Borneo that are derived, by mechanical processes, from chromitites of the Meratus ophiolite. Published subduction and emplacement ages and biostratigraphy of pelagic sediments of the ophiolite sequence define a minimum age for genesis at a spreading ridge. However, igneous components of the ophiolite have previously been undateable. Alluvial PGM grains (n = 260) from the Pontyn River, which drains the Meratus Mountains, were analyzed by laser ablation-multicollector-inductively-coupled mass spectrometry (LA-MC-ICPMS). Re-Os data do not show any isochronous relationship. Despite a significant range in 187Os/188Os (0.122–0.141), 187Re/188Os values show a very narrow range (0.000005–0.002980). In contrast, the PGM have a wide range in both 186Os/188Os (0.119801–0.120315) and 190Pt/188Os (<0.00001–1.493430), yielding a precise Pt-Os isochron age of 197.8 ± 8.1 Ma (2 σ ). This age fits well with published age constraints for this ophiolite and we argue that it dates the crystallization of the PGM. Previous studies have shown that the Pontyn PGM are derived from ophiolitic chromitite; therefore, the PGM Pt-Os isochron age also provides the first absolute age constraint for the genesis of igneous rocks of the Meratus ophiolite. These results highlight the potential of the Pt-Os geochronometer as a tool for dating the crystallization age of PGM found in placer deposits, for dating primary platinum mineralization in general, and for use in ophiolite geochronology.

Hydrothermal Alteration and Veins at the Epithermal Au-Ag Deposits and Prospects of the Waitekauri Area, Hauraki Goldfield, New Zealand

Abstract: The Waitekauri area of the Hauraki goldfield, New Zealand, contains several adularia-sericite epithermal Au-Ag deposits and prospects From west to east, the area contains the Sovereign, Jubilee, Scimitar, Scotia, Teutonic, and Jasper Creek deposits and prospects, which are hosted by andesitic and dacitic flows, breccias, and localized pyroclastic and air fall deposits Drill core reveals spatial and temporal zonation of alteration and vein minerals along a 3-km-long composite cross section through the area Most host rocks are intensely altered, with 100 percent of the igneous minerals replaced by hydrothermal minerals, although the alteration intensity becomes more variable and weaker toward the east Alteration minerals include quartz, adularia, albite, chlorite, pyrite, illite, interstratified illite-smectite, smectite, calcite, hematite, and minor epidote Many of these minerals have zoned distributions; adularia is widespread at Sovereign, but is restricted to shallow levels at both Scotia and Jasper Creek Albite occurs in a discrete zone below adularia at Scotia, and minor epidote is restricted to Sovereign and Jubilee Illite occurs throughout Sovereign and Jubilee and at the western margin of Scotia and Scimitar, where it grades eastward into interstratified illite-smectite and smectite at Teutonic and Jasper Creek Veins are typically less than 10 cm wide, but have diverse mineralogy with zoned distributions Quartz veins dominate at Sovereign and Jubilee, whereas calcite veins are more abundant at Scotia, Scimitar, and Jasper Creek Laumontite occurs at Scotia and locally at Scimitar, whereas veins of clinoptilolite and mordenite ± calcite occur at Jasper Creek and stilbite veins occur at Teutonic Fluid inclusions in quartz and calcite homogenized between 132° and 310°C and trapped a dilute solution with an apparent salinity of less than 26 wt percent NaCl equiv Homogenization temperatures are highest at Sovereign (avg 241°C), Jubilee (avg 239°C), and Scimitar (avg 236°C), lower at Scotia (avg 204°C) and lowest at Teutonic (avg 168 °C) and Jasper Creek (avg 162°C) Estimated positions of the paleowater table above Sovereign, Jubilee, Scimitar, Scotia, Jasper Creekg and Teutonic relative to present elevations was at least 690, 750, 575, 450, 225, and 150 m above sea level, respectively; the deposits and prospects, therefore, span a 600-m vertical interval Individual deposits and prospects have undergoen at least 35 to more than 455 m of erosion with the greatest erosion to the west Alteration intensity, alteration and vein mineral distributions, and fluid inclusion microthermometry are interpreted to indicate that Sovereign and Jubilee formed at relatively high temperatures, whereas Teutonic and Jasper Creek formed at relatively cooler temperatures Several hydrologic reconstructions are possible, including (1) a single hydrothermal system with an inclined water table and significant lateral outflow to the east, or 2) a single low-relief hydrothermal system with a flat-lying water table that has subsequently been displaced by postmineral faults or tilted approximately 10° to the east Regardless of the preferred reconstruction, the Sovereign and Jubilee deposits appear to have formed in the main zone of fluid upflow, whereas the Teutonic and Jasper Creek prospects appear to have formed toward the margin Moreover, the greatest erosion has occurred at the Jubilee and Sovereign deposits (~300–400 m), and these may represent the roots of a more extensive vein network that has largely been eroded