Showing papers in "Economic Geology in 2016"

Apatite Trace Element Compositions: A Robust New Tool for Mineral Exploration

Abstract: Apatites from the major types of mainly magmatic-hydrothermal mineral deposits (30 localities, mostly in British Columbia, Canada) together with apatites from carbonatites (29 intrusive complexes) and unmineralized rocks (11 localities) have been analyzed by electron microprobe and laser ablation-inductively coupled plasma mass spectrometry. Discriminant analysis using Mg, V, Mn, Sr, Y, La, Ce, Eu, Dy, Yb, Pb, Th, and U reveals that apatites from mineral deposits can be distinguished from apatites in carbonatites and unmineralized rocks. Apatites from mineral deposits are characterized by higher Ca and correspondingly lower total contents of trace elements that partition onto the Ca sites (rare earth elements (REEs), Y, Mn, Sr, Pb, Th, and U) than apatites from unmineralized rocks and carbonatites. Apatites from the different deposit types also have distinct trace element compositions that are readily discriminated by the discriminant functions. Apatites from worldwide carbonatites have the most fractionated REE distributions with light REE enrichment (Ce/Yb CN = 35–872), high V (1.6–1,466 ppm), Sr (1,840–22,498 ppm), Ba (1.8–275 ppm), and Nb (0.4–19 ppm) contents, the lowest W contents (0.05–0.55 ppm), and no significant Eu anomalies (Eu/Eu* = 0.9–1.2). Apatites from alkalic porphyry Cu-Au deposits in the North American Cordillera possess high V contents (2.5–337 ppm), whereas apatites from calc-alkaline porphyry Cu-Au and Cu-Mo deposits have high Mn contents (334–10,934 ppm) and typically large negative Eu anomalies (Eu/Eu* = 0.2–1.1). Apatites from iron oxide Cu-Au (IOCG) and related Kiruna-type iron oxide-apatite (IOA) deposits in Canada, China, and Mexico typically have large negative Eu anomalies (Eu/Eu* = 0.2–1.5) and low Mn contents (40–5,753 ppm). Apatites from orogenic Ni-Cu, porphyryrelated Cu-Au breccia, Au-Co skarn, Pb-Zn skarn, and Cu skarn deposits have relatively low abundances of impurity cations. This study demonstrates that detrital apatite grains collected during regional geochemical surveys are effective in identifying specific types of buried mineral deposits in glaciated terranes.

Relationships Between Gold and Pyrite at the Xincheng Gold Deposit, Jiaodong Peninsula, China: Implications for Gold Source and Deposition in a Brittle Epizonal Environment

Abstract: The Xincheng gold deposit, hosted by the Early Cretaceous 132 to 123 Ma Guojialing-type granitoids in northwest Jiaodong Peninsula, southeast North China craton, formed about 2 billion years later than regional metamorphism of the Archean Jiaodong basement rocks. The Xincheng deposit comprises mineralized zones with three types of hydrothermal pyrite associated with gold, tellurides, and a variety of sulfides: py1 as disseminated euhedral to subhedral grains in altered granitoids around quartz veins; py2 as subhedral grains with brittle cataclastic textures and fractures in quartz-pyrite veins; and py3 as subhedral, partially corroded crystals in sulfide-rich veins or veinlets. All three generations of pyrite are unzoned and have low trace element contents, including very low lattice-bound gold contents: (py1: 0.180 ppm; py2: 0.053 ppm; py3: 0.060 ppm). Given that there is 10 to 15% pyrite in the ore zone at Xincheng, its very low gold content indicates that it contributes <0.2% of gold to the 7.75 g/t gold in the orebody. Instead, over 99% of the gold is present as discrete electrum and/or gold (total range 0.02–59% silver) grains, which are largely sited in fractures at all scales in pyrite, other ore minerals, and quartz. Importantly, visible gold in py3 is also sited on solution-corroded pyrite grains. The pyrite textural and geochemical data indicate that it is impossible to derive the high gold-grade orebodies through local remobilization of originally lattice-bound gold in pyrite. Instead, the gold is interpreted to have been deposited through sulfidation reactions and phase separation of a H2O-CO2 ore fluid during progressive brittle cataclastic deformation associated with seismic activity and regional sinistral transtensional shear movement. This concomitant fluid infiltration and deformation caused episodic deposition and fracturing and corrosion of earlier formed pyrite and deposited visible gold in dilational cracks. The coupled development of the transtensional, rather than normal transpressional setting, and precipitation of gold within dilational veins and wall-rock alteration facilitated the deposition of visible gold and an exceptionally high gold tenor. All deposit characteristics indicate that the Xincheng gold deposit is a member of the epizonal orogenic deposit class.

Injection-Driven Swarm Seismicity and Permeability Enhancement: Implications for the Dynamics of Hydrothermal Ore Systems in High Fluid-Flux, Overpressured Faulting Regimes—An Invited Paper

Abstract: Many types of hydrothermal ore deposits form at overpressured conditions during high fluid flux through fault zones in the continental seismogenic regime. These include many orogenic gold deposits, some types of Fe oxide Cu-Au systems, and a variety of intrusion-related deposits, including some high-sulfidation epithermal systems. The internal structures of these deposits indicate formation while faults were active, in a regime involving episodic slip. Partial to complete sealing of fault zones by precipitation of hydrothermal minerals occurs between slip episodes. Such ore deposits occur typically within low displacement faults, or networks of such faults. Lodes seldom occupy areas much greater than 1 km 2 of fault surfaces, and net slip accumulated during ore formation is typically less than 150 m. Fluid injection experiments in low-permeability rocks, and seismicity styles in hydrothermally active settings, provide new insights about the dynamics of ore formation in high fluid-flux faults. In particular, they indicate that swarm seismicity is the characteristic response to injection of large volumes of overpressured fluids into intrinsically low-permeability rock. Injection-driven swarm seismicity and related permeability enhancement involves repeated sequences of thousands of ruptures, mostly with moment magnitude M w in the range −2 w 2 ; maximum cumulative slip is usually less than a few centimeters. Diffusion-like migration of a seismicity front away from the injection source at rates up to hundreds of meters per day is a key characteristic of injection-driven seismicity, and correlates with migration of a fluid pressure front along activated faults. By analogy with injection experiments and natural injection-driven swarm seismicity, the formation of fault-related ore deposits in overpressured, high fluid-flux regimes is interpreted to involve swarm seismicity, rather than mainshock-aftershock sequences. Recurrence intervals of swarms in contemporary injection-driven swarm sequences indicate that the total slip associated with formation of fault-hosted lodes can accumulate in periods as short as 104 to 105 years, during thousands of swarm episodes, and commonly involve >10 6 ruptures with −2 w 4 to 10 5 m 3 of fluid at injection rates of at least several tens of L.s −1 . Up to tens of kg Au are deposited per swarm. Cascades of thousands of microseismic ruptures and associated permeability enhancement, coseismic dilatancy, and fluid pressure changes during individual injection-driven swarm sequences create very dynamic hydrologic and reaction regimes. Ore formation involving transitory bursts of rapid flow, abrupt fluctuations in flow rates and fluid pressures, and possibly also fluid temperatures, is likely to promote severe chemical disequilibrium. This has implications for controls on ore deposition and grade distribution in many fault-related ore systems.

Hydrothermal Alteration Revealed by Apatite Luminescence and Chemistry: A Potential Indicator Mineral for Exploring Covered Porphyry Copper Deposits

Abstract: Apatite is a common resistate mineral occurring in a range of host rocks and ore-related hydrothermal alteration assemblages. Apatite in several porphyry copper deposits in British Columbia has a unique set of physical and compositional characteristics that can be used to evaluate the chemical conditions of magmas that formed the causative intrusions or associated hydrothermal alteration. Apatite under visible light and SEM shows no notable variations between unaltered and altered varieties but cathodoluminescence reveals significant differences. Apatite in unaltered rocks displays yellow, yellow-brown, and brown luminescence, whereas in K silicate-altered rocks apatite displays a characteristic green luminescence. The green-luminescent apatite replaces yellow- or brown-luminescent apatite and locally overgrows it. Apatite occurring with muscovite (i.e., phyllic)-altered rocks displays characteristic gray luminescence. The chemistry of apatite, as determined by electron microprobe and laser ICP-MS analyses, directly reflects its alteration and luminescence. The unaltered yellow-luminescent apatite has high concentrations of Mn (0.3–0.5 wt % MnO) and a high Mn/Fe ratio (>1), whereas the brown-luminescent apatite has low Mn, but higher concentrations of S and REE + Y. The green K silicate alteration-related luminescence is caused by lower Mn/Fe ratios (ca. 1) along with depletions of other trace elements such as Cl, S, and Na. Gray-luminescent apatite occurring with muscovite-altered rocks results from significant Mn loss ( The correlation between apatite texture, luminescence, and chemical composition with the type and intensity of porphyry alteration offers a potentially fast and effective method to utilize it as an indicator for porphyry mineralization in a range of exploration materials including soils, regoliths, and heavy mineral concentrates from glacial and fluvial materials.

Regional Metallogeny of Mo-Bearing Deposits in Northeastern China, with New Re-Os Dates of Porphyry Mo Deposits in the Northern Xilamulun District

Abstract: With many new discoveries of giant Mo deposits in the past decade, northeastern China has become the largest molybdenum mineralization region in China. The Xilamulun district is in the southern part of northeastern China and contains 25 Mo deposits and prospects, dominantly of porphyry type. These deposits are mostly along the E-striking Xilamulun fault. Re-Os dating of hydrothermal molybdenite from four deposits in the northern Xilamulun district revealed mineralization ages of 137 to 129 Ma (129.4 ± 3.4 Ma for Aolunhua, 135.3± 2.6 Ma for Shabutai, 136.4 ± 0.8 Ma for Haisugou, and 136.1 ± 6.6 Ma for Banlashan), in general agreement with the crystallization ages of their host granitoids. The compilation of existing data on Mo-bearing deposits in northeastern China, including the new data of this study, shows that Mesozoic Mo deposits in this region have a wide range of ages, from ~250 to 90 Ma. We propose that they are linked to three tectonic-magmatic events: (1) Triassic Mo deposits (250–220 Ma) are mainly distributed along the E-trending Xilamulun fault and are related to postcollisional crustal extension following the final closure of the Paleo-Asian ocean; (2) Jurassic Mo mineralization (200–140 Ma) displays a clear younging trend from southeast to northwest, coincides with the regional magmatism trend, and is interpreted to be related to the northwestward flat-slab subduction of the Paleo-Pacific plate beneath the Eurasian continent that started in the Early Jurassic (ca. 200 Ma); (3) Cretaceous Mo mineralization (140–90 Ma) shows a reversed migration trend from northwest to southeast, and can be explained by the coastward migration of slab rollback related to lower crust delamination, asthenospheric upwelling, and lithospheric thinning in eastern China. The spatial-temporal distribution of Mesozoic Mo mineralization is important for regional metallogeny and exploration. Recently numerous epithermal Au (Cu, Mo) deposits have been recognized in the southeast margin of northeastern China. According to this study, we predict that hidden porphyry/skarn systems could be located beneath these epithermal deposits.

Trace element signature of pyrite from the los colorados iron oxide-apatite (ioa) deposit, chile: a missing link between andean ioa and iron oxide copper-gold systems?

Abstract: Although studies have proposed that iron oxide-apatite (IOA) deposits may represent the deeper roots of some Andean iron oxide copper-gold (IOCG) systems, their genetic links remain obscure and controversial. A key question when considering an integrated genetic model is whether a magmatic-hydrothermal fluid that precipitates massive magnetite will continue transporting significant amounts of dissolved Fe, Cu, and Au after IOA precipitation. Here we provide new geochemical data for accessory pyrite from the Los Colorados IOA deposit in the Chilean iron belt that confirm the role of this sulfide as a relevant repository for economic metals in IOA deposits. Pyrite occurs at Los Colorados as disseminated grains and as veinlets associated with magnetite and actinolite that postdate the main igneous magnetite stage. Electron probe microanalysis (EPMA) data for pyrite show anomalously high Co and Ni concentrations (up ~3.9 and ~1.5 wt %, respectively) and relatively high As contents (100s of ppm to a maximum of ~2,000 ppm). When combined with results from secondary ion mass spectrometry (SIMS) spot analyses, pyrite data show significant amounts of Cu that range from sub-ppm values (~100 ppb) up to 1,000s of ppm, plus nonnegligible concentrations of Zn, Pb, Cd, Sb, Se, and Te (up to ~100 ppm). The highest contents of Cu measured (wt % level) most likely record the presence of Cu-bearing submicron-sized mineral inclusions. Contents of Au and Ag are up to ~1 and 10 ppm, respectively, with maximum concentrations that can rise up to ~800 ppm Au and ~300 ppm Ag due to the presence of submicron-sized inclusions. The high Co/Ni ratios of pyrite from Los Colorados are consistent with a magmatic-hydrothermal origin associated with a greater mafic affinity, compared to pyrite from porphyry Cu deposits. Furthermore, the geochemical signature of Los Colorados pyrite shares important similarities of composition and microtexture with the few published data for pyrite from IOCG deposits (e.g., Ernest Henry, Australia, and Manto Verde, Chile). These findings, combined with recent geochemical and isotopic studies that support an igneous origin for the dike-shaped magnetite orebodies at Los Colorados, point to a magmatic source of mafic to intermediate composition for the contained metals, and support the hypothesis that IOA systems can source Fe-Cu-Au-rich fluids. Based on experimental studies, these IOA-derived fluids may continue transporting significant amounts of metals to form IOCG mineralization at shallower levels in the crust.

Textures and In Situ Chemical and Isotopic Analyses of Pyrite, Huijiabao Trend, Youjiang Basin, China: Implications for Paragenesis and Source of Sulfur

Abstract: Many Carlin-like Au deposits occur within the late Paleozoic and Triassic Youjiang basin of southwest China. The Huijiabao trend in Guizhou Province contains over 300 metric tons (t; 10.6 Moz) of Au at an average grade of 7 to 18 g/t in a narrow corridor that is about 20 km long and 5 km wide. Petrographic and SEM studies of pyrite in barren host rocks and high-grade orebodies led to the recognition of four stages of pyrite. Py1 consists of fine-grained framboidal crystals in black mudstone. Py2 is comprised of coarser grained euhedralsubhedral clusters that are spatially related to organic matter. Py3 is coarse grained, euhedral, and occurs as overgrowths on Py1 and Py2. Py3’s porous texture, inclusion of randomly oriented detrital minerals, and association with quartz recrystallization suggest it was deformed during Late Triassic orogenesis with Py1 and Py2. Py4 generally occurs as rims on Py1 to Py3 and is intergrown with arsenopyrite. Sensitive high-resolution ion microprobe (SHRIMP) δ 34S analyses of each pyrite type and arsenopyrite show that Py1 is related to Py2 and that Py3 is related to Py4 and arsenopyrite. The S isotope compositions of Py1 (−7.5 to +5.9‰) and Py2 (−5.3 to +7.9‰) are bimodal, which suggests that H2S was generated by biogenic sulfate reduction in open marine and sulfate limited systems during sedimentation and/or diagenesis. The compositions of Py3 (−2.6 to +1.5‰), Py4 (−1.2 to +1.5‰), and arsenopyrite (−0.8 to +0.9‰) are homogeneous and have an intermediate range of values near 0‰ that suggest that H2S was derived either from average pyrite (0.2‰) in sedimentary rocks or from a concealed magmatic source. Laser ablation-inductively coupled plasma-mass spectrometer (LA–ICP–MS) trace element analyses (As, Ni, Co, Cu, Ag, Se, V) support different origins and show that Py3 and Py4 are ore related. The lower w (Co)/ w (Ni) and w (S)/ w (Se) ratios of Py1 and Py2 are consistent with formation during sedimentation or diagenesis, whereas the higher ratios of Py3, Py4, and arsenopyrite are consistent with a hydrothermal origin. The lower concentrations of Au in Py1 (0.23–2.5 ppm) and Py2 (0.06–12 ppm) show that little Au was added during sedimentation or diagenesis. The higher concentrations of Au in hydrothermal Py3 (1.1–110 ppm) and Py4 (0.34–810 ppm) indicate that most of the Au was introduced during subsequent hydrothermal fluid flow. The low Au contents of arsenopyrite (0.09–0.52 ppm) suggests they formed from Au-depleted fluids. The Au/As ratios of Py1 and Py2 are typical of diagenetic pyrite whereas Py3 and Py4 have ratios that approach those of ore-stage pyrite in Nevada Carlin-type deposits. The fracturing of Py3 and its cementation by Py4 suggests that ore fluid movement was associated with deformation. Published isochron ages on arsenopyrite (Re-Os ~200 Ma) and late calcite-realgar veinlets (Sm-Nd ~135 Ma) in the Huijiabao trend are older than mafic dikes (84 Ma) exposed ~20 km to the east. If the 200 and 135 Ma ages are valid, H2S and Au may be derived from a sedimentary source because igneous intrusions of this age have not been found. If these ages are not valid and the gold deposits are actually Late Cretaceous in age, then H2S and Au may be derived from a magmatic source. Additional geochronology and isotopic tracer studies are needed to resolve this uncertainty.

The Giant Xiarihamu Ni-Co Sulfide Deposit in the East Kunlun Orogenic Belt, Northern Tibet Plateau, China

Abstract: The Xiarihamu Ni-Co deposit, located in the East Kunlun orogenic belt, northern Tibet plateau, is the secondary largest Ni deposit in China and contains ~157 million metric tons (Mt) sulfide ores with average grades of 0.65 wt % Ni, 0.14 wt % Cu, and 0.013 wt % Co. The intrusion consists of gabbroic and ultramafic portions, the main orebody being hosted in the ultramafic portion. The zircons separated from the gabbronorite and websterite yield SHRIMP U-Pb ages of 405.5 ± 2.7 and 406.1 ± 2.7 Ma, indicating a genetic linkage with the Early Devonian regional magmatism (400–410 Ma). The main orebody includes two large ore pods comprising a pod of disseminated sulfides in the western portion grading into multiple ore sublayers to the east. The ore sublayers consist of net-textured and disseminated sulfides. The distribution and shape of the ore pods and sublayers of the orebody are conformable with the undulating roof and bottom of the ultramafic portion. The unusually high Ni/Cu ratios (~4–18) and extremely low tenors of platinum-group elements (PGE; <4 ppb Ir, <85 ppb Pt, and <115 ppb Pd) of the disseminated sulfides indicate a genetic relationship with low degrees of partial melting of a pyroxenite mantle source. The slightly higher Ir and Ru tenors suggest that the disseminated sulfides in the western portion of the main orebody were segregated from less evolved magma under higher mass ratios of silicate melt/sulfide liquid than the sulfides of the middle and eastern portions.

Alteration Facies Linkages Among Iron Oxide Copper-Gold, Iron Oxide-Apatite, and Affiliated Deposits in the Great Bear Magmatic Zone, Northwest Territories, Canada.

Abstract: High-temperature metasomatism driven by ascent of voluminous, saline fluid columns in the upper crust plays a major role in the genesis of iron oxide-alkali alteration ore systems but fundamental questions remain on genetic linkages among iron oxide copper-gold (IOCG), iron oxide-apatite (IOA), albitite-hosted uranium, and skarn deposits that they produce. Excellent surface exposures of such systems in the Great Bear magmatic zone of northernwestern Canada record the depth to paleosurface, prograde evolution of iron oxide-alkali alteration facies, and mineralization. Across the belt, albitite corridors that are tens of kilometers in length record the earliest reactions between highly saline fluids and host rocks along fault zones and subvolcanic intrusions. Pervasive albitization partitioned metals from the host rocks into the ascending fluid column, leaving behind structurally weakened corridors of porous albitite. These corridors were cut, replaced, and overprinted by amphibole- and magnetite-bearing, calcic-iron alteration assemblages. In extreme cases, the discharge of calcium, iron, and specialized metals formed iron oxide-apatite deposits (±vanadium ± rare earth elements) while recharging the outgoing fluids in sodium, potassium, and base and precious metals. As temperatures declined and fluid chemistry evolved through fluid-rock reactions, the formation of potassic-iron alteration assemblages, breccias, and sulfides resulted in magnetite- and hematite-group IOCG mineralization. Within carbonate units, skarns formed prior to, are replaced by, and evolved to calcic-iron alteration facies. Skarns can locally host base metal mineralization. Tectonically uplifted albitite breccias replaced by potassic-iron alteration assemblages became a preferential host for uranium mineralization. The results of this study also illustrate that permutations and cyclical build-up of alteration products can arise from a combination of faulting, differential uplift, and renewed magmatism. Framed within an alteration-facies deposit model, alteration zones and mineral occurrences play a pivotal role in predicting the mineral potential of iron oxide and alkali-altered systems at district to deposit scales.

Hydrothermal Transport of Ag, Au, Cu, Pb, Te, Zn, and Other Metals and Metalloids in New Zealand Geothermal Systems: Spatial Patterns, Fluid-Mineral Equilibria, and Implications for Epithermal Mineralization

Abstract: In order to analyze the concentrations of Ag, As, Au, Cd, Cu, Hg, Mn, Mo, Ni, Pb, Se, Sb, Te, Tl, W, and Zn in deep hydrothermal chloride waters at 195° to 320°C, and in shallow boiled chloride waters at 160° to 230°C, we sampled wells drilled to 3-km depth in geothermal systems of the North Island, New Zealand. Six of the systems are located in a segment of volcanic arc in the central Taupo Volcanic Zone, and the seventh system is associated with an intraplate mafic (felsic) volcanic center. The concentrations of metals range widely from 0.1 to >1,000 μ g/kg with a large degree of intersystem variability. Some of the largest contrasts in Au, Ag, Pb, and Te concentrations are observed in the two nearest systems, Rotokawa and Wairakei, which are only 10 km apart. The correlations between metals are poor, except for Ag-Au-Pb-Te, and As-Sb. The correlations between metals, Cl, and H2S are also poor, with the exception of Rotokawa where the highest concentrations of Ag, Au, Cu, and Te correlate with the highest concentration of aqueous H2S. Speciation calculations indicate that the dominant aqueous species of Ag, Au, Cu, Pb, and Zn involve HS− complexes. The calculations also show that the states of saturation range from undersaturated conditions for acanthite, arsenopyrite, and gold to oversaturated conditions for chalcopyrite, sphalerite, and tellurides. Notably, the Au- and Ag-transporting capacities of the deep chloride waters are much larger than the measured aqueous concentrations. These results suggest that fluid-mineral equilibria and the concentrations of ligands exert weak influence on metal concentrations at the temperatures and depths of sampling. The complex trends in hydrothermal metal concentrations strongly suggest that the deep-seated sources of metals, comprising magmatic intrusions, deep country rock, and their related fluids, limit the hydrothermal supplies of metals. Between the geothermal systems, hydrothermal fluxes of Ag (6–8,000 kg/y), Au (0.9–66 kg/y), Cu (30–23,500 kg/y), and Te (2–10,400 kg/y) are variable. The highest concentrations and fluxes of Ag, Au, and Te in Rotokawa and Mokai are attributed to direct fluid inputs from intrusions of andesitic and basaltic magmas, respectively. Compared to their deep counterparts, boiled chloride waters are strongly depleted in Ag, Au, Cd, Cu, Pb, and Te, because these metals deposit in sharp response to gas loss and cooling in the well. By comparison, the As, Mn, Mo, Ni, Sb, Tl, and Zn concentrations are measurably less depleted in boiled waters, making them available to form metal anomalies at shallow depths and in the peripheral parts of the epithermal environment. Periods of strong metal flux through the geothermal system combined with deep boiling favor epithermal ore formation and the development of large precious metal deposits. However, even moderate metal fluxes can produce Au and Ag mineralization as long as the duration of focused fluid flow and boiling can be sustained. There is no evidence that the compositions of deep magmatic intrusions, mafic or intermediate, limit the ore-forming potential of a geothermal system.

Trace Element Content of Pyrite from the Kapai Slate, St. Ives Gold District, Western Australia

Abstract: The Kapai Slate is a continuous, pyrite-rich carbonaceous shale horizon within the St. Ives Au district that is spatially related to high-grade Au mineralization. In situ laser ablation-inductively coupled mass spectrometry (LA-ICPMS) trace element analyses, in situ sensitive high resolution ion microprobe, stable isotope (SHRIMP-SI) S isotope analyses, and optical microscopy pyrite texture analyses were used to examine the different pyrite types in the Kapai Slate and Au deposits. These data were also used to confirm that the trace element signature of sedimentary pyrite can be preserved in rocks that underwent upper to mid-greenschist facies metamorphism and significant hydrothermal overprint. The data were further utilized to gain a more detailed understanding of the ocean conditions during deposition of the Kapai Slate and determine whether some of the Au and S in the St. Ives district could have been sourced from the Kapai Slate. Seven different types of pyrite were identified: fine-grained sedimentary pyrite (Py 1 ), nodular sedimentary pyrite (Py 2 ), remobilized sedimentary pyrite (Py 3 ), coarse-grained, inclusions poor late pyrite (Py 4 ), inclusion-rich magnetite series pyrite (Py 5 ), ore stage pyrite (Py 6 ), and pyrite associated with the mafic units (Py 7 ). Each type of pyrite was found to have distinctive trace element compositions and S isotope signatures. The results of the LA-ICPMS analyses provide evidence for early trace element enrichment in the Kapai Slate sedimentary pyrite (median values of 158 ppm Ni, 387 ppm Co, 82 ppm Cu, 727 ppm As, 1.91 ppm Mo, 13 ppm Se, 0.25 ppm Au, 7.72 ppm Te and 3.36 ppm Ag for Py 1 and 223 ppm Ni, 158 ppm Co, 99 ppm Cu, 856 ppm As, 1.27 ppm Mo, 10.2 ppm Se, 0.57 ppm Au, 10.09 ppm Te, and 6.62 ppm Ag for Py 2 ). Concentrations of Ni and Co are low, relative to other late Archean sedimentary pyrite (median of 813 and 465 ppm, respectively) and Mo levels are near that of the euxinic shales of the similar-aged Jeerinah Formation in the Hamersley Basin, Western Australia. These data suggest that the Kapai Slate was deposited in an anoxic to euxinic basin with relatively low biological productivity. The Δ 33 S and δ 34 S signatures of the sedimentary pyrite suggest two different sources of S. Positive δ 34 S and negative Δ 33 S signatures indicate bacterial reduction of SO 4 2− from seawater, whereas positive δ 34 S and positive Δ 33 S signatures indicate an elemental S 8 source, indicating the pyrite formed later during diagenesis. This S isotope signature is consistent with a transition between a near-sediment environment to a more distal environment source. Analyses of the ore-phase pyrite yield weakly positive Δ 33 S values. This suggests there was a minor contribution of sedimentary S to the more significant oxidized ore-forming fluids, which is consistent with a small contribution of Au from a sedimentary source. Approximations of the degree of sedimentary pyrite destruction in the pyrrhotite/pyrite dominated zones and pyrrhotite/magnetite/pyrite zones of the northern part of the St. Ives district were used to calculate the amount of Au released from the early sedimentary pyrite. The calculation suggests that a minor, though possibly locally significant, amount of Au could have been sourced from the Kapai Slate.

Lithogeochemical Vectors for Hydrothermal Processes in the Strange Lake Peralkaline Granitic REE-Zr-Nb Deposit

Abstract: Extreme enrichment and postmagmatic hydrothermal mobilization of the rare earth elements (REE), Zr, and Nb have been reported for a number of anorogenic peralkaline intrusions, including the world-class REE-Zr-Nb deposit at Strange Lake, Quebec, Canada. Establishing lithogeochemical vectors for these types of deposits is a challenging task because the effects of hydrothermal processes on element distribution are poorly known and the relationships of alteration types to mineralization stages have not been well documented. Here, we present results of a detailed mineralogical and geochemical investigation involving a dataset of over 500 mineral and bulk-rock analyses of a northeast-southwest section through the potential ore zone at Strange Lake. Based on these data, we develop a model that explains the role of hydrothermal processes in concentrating metals in peralkaline granitic systems and identify lithogeochemical vectors for their exploration. The B zone, located along the northwestern margin of the Strange Lake pluton, contains a lens-shaped, pegmatite-rich domain comprising subhorizontal sheets of pegmatites hosted by granites with a total indicated resource of 278 million tonnes (Mt) grading 0.93 wt % total rare earth oxides (TREO), of which 39% are heavy rare earth elements (HREE). Within this resource, there is an enriched zone containing 20 Mt of ore grading 1.44 wt % TREO, of which 50% are HREE. The pegmatites are characterized by a core enriched in quartz, fluorite, and light rare earth elements (LREE) fluorocarbonates, and a granitic border enriched in zirconosilicates and granitic minerals. The pegmatite sheets and surrounding granites evolved in three essential stages: a magmatic stage (I), a near-neutral hydrothermal stage involving their interaction with NaCl-bearing orthomagmatic fluids (II), and an acidic hydrothermal stage (III, comprising high-[IIIa] and low-temperature [IIIb] substages) that resulted from their interaction with pegmatite-sourced HCl-HF–bearing fluids. Stage IIIa led to pseudomorphic mineral replacement reactions (e.g., Na-Ca exchange during replacement of zirconosilicates) and formation of an aegirinization/hematization halo around the pegmatites. In contrast, stage IIIb, which was responsible for the hydrothermal mobilization of Zr and REE, is manifested by fluorite and quartz veins, zircon spherules, gadolinite-group minerals, gittinsite, ferriallanite-(Ce), and a pervasive replacement of the granite by these minerals. The distribution of REE, Zr, Nb, and Ti was controlled by the competition between hydrothermal fluids and the stability of primary REE-F-(CO 2 ) minerals (e.g., bastnasite-(Ce) host to LREE), zirconosilicates (i.e., Na zirconosilicates and zircon host to HREE and Zr), and Nb-Ti minerals (i.e., pyrochlore host to Nb and narsarsukite host to Ti), and the stability of secondary LREE silicates (i.e., ferriallanite-(Ce)), HREE silicates (i.e., gadolinite-(Y)), zirconosilicates (i.e., gittinsite and zircon), and Nb-Ti minerals (i.e., titanite and pyrochlore). Lithogeochemical vectors were identified to distinguish between the high-temperature acidic alteration (IIIa), using CaO/Na 2 O (indicator of Ca metasomatism) and Fe 2 O 3 /Na 2 O ratios (indicator of aegirinization/hematization), and the low-temperature acidic alteration (IIIb), using the CaO/Al 2 O 3 ratio (indicator of Ca-F metasomatism). Bulk-rock compositional data show that alteration was accompanied by an enrichment in heavy rare earth oxides (HREO) and ZrO 2 at the deposit scale, whereas there was no selective enrichment in the light rare earth oxides (LREO). A 2-D geochemical model of the deposit indicates that the LREO are more dispersed, whereas HREO and ZrO 2 are selectively distributed. These variations in LREE/HREE are also reflected in the mineral chemistry, especially in hydrothermal zircon crystals showing an unusual LREE enrichment and HREE depletion, contrasting with pseudomorphs, which are enriched in HREE. Hydrothermal ferriallanite-(Ce) and gadolinite-group minerals also show a clear trend of REE depletion with Ca enrichment. Controlling factors for the hydrothermal mobilization of LREE, HREE, and Zr were temperature, pH, and the availability of fluoride ions (F − ) in the fluid for the dissolution of zircon, and chloride ions (Cl − ) for the complexation of the REE. The study of rare hydrothermal minerals in conjunction with field observations and the evaluation of variations in bulk-rock composition allowed us to develop a new model for the hydrothermal evolution stage of Strange Lake.

The Nature and Origin of the REE Mineralization in the Wicheeda Carbonatite, British Columbia, Canada

Abstract: The Wicheeda carbonatite is a deformed plug or sill that hosts relatively high grade light rare earth elements (LREE) mineralization in the British Columbia alkaline province. It was emplaced within metasedimentary rocks belonging to the Kechika Group, which have been altered to potassic fenite near the intrusion and sodic fenite at greater distances from it. The intrusion comprises a ferroan dolomite carbonatite core, which passes gradationally outward into calcite carbonatite. The potentially economic REE mineralization is hosted by the dolomite carbonatite. Three types of dolomite have been recognized. Dolomite 1 constitutes the bulk of the dolomite carbonatite, dolomite 2 replaced dolomite 1 near veins and vugs, and dolomite 3 occurs in veins and vugs together with the REE mineralization. Carbon and oxygen isotope ratios indicate that the calcite carbonatite crystallized from a magma of mantle origin, that dolomite 1 is of primary igneous origin, that dolomite 2 has a largely igneous signature with a small hydrothermal component, and that dolomite 3 is of hydrothermal origin. The REE minerals comprise REE fluorocarbonates, ancylite-(Ce), and monazite-(Ce). In addition to dolomite 3, they occur with barite, molybdenite, pyrite, and thorite. Minor concentrations of niobium are present as magmatic pyrochlore in the calcite carbonatite. A model is proposed in which crystallization of calcite carbonatite preceded that of dolomite carbonatite. During crystallization of the latter, an aqueous-carbonic fluid was exsolved, which mobilized the REE as chloride complexes into vugs and fractures in the dolomite carbonatite, where they precipitated mainly in response to the increase in pH that accompanied fluid-rock interaction and, in the case of the REE fluorocarbonates, decreasing temperature. These fluids altered the host metasedimentary rock to potassic fenite adjacent to the carbonatite and, distal to it, they mixed with formational waters to produce sodic fenite.

Mineralogy, chemistry, and fluid-aided evolution of the Pea Ridge Fe oxide-(Y + REE) deposit, southeast Missouri, USA

Abstract: The Kiruna-type Pea Ridge iron oxide-apatite (IOA) deposit is hosted by a sequence of 1.47 Ga rhyolite tuffs of the St. Francois Mountains, southeast Missouri, USA. It consists of a series of altered zones composed mainly of amphibole, magnetite, hematite, and quartz, together with the presence of several rare earth element (Y + REE)-rich breccia pipes. In many cases, the fluorapatite within these zones is rich in inclusions of monazite, iron oxide, and quartz inclusions, plus minor xenotime. Monazite and minor xenotime are also found intergrown as inclusions in the fluorapatite, as well as in surrounding recrystallized magnetite and hematite in the magnetite ore. Monazite and xenotime typically occur as inclusions within both oxides. Monazite-(Ce) and xenotime-(Y) are both relatively poor (<2 wt %) in ThO2 and UO2. No significant compositional differences exist in the (Y + REE) chemistry between monazite and xenotime inclusions in fluorapatite compared to grains intergrown with magnetite and hematite, suggesting that these two REE-rich minerals are cogenetic. Monazite-xenotime geothermometry and geochronology of monazite inclusions in fluorapatite provide evidence that formation/remobilization of the (Y + REE) phosphates took place at ca. 50° to 400°C, approximately 5 to 10 m.y. after emplacement of the main iron oxide-phosphate orebody. Evidence from field relationships and fluid inclusion chemistry, together with the massive recrystallization and remobilization of fluorapatite, monazite, xenotime, and iron oxides at Pea Ridge, suggest a subvolcanic origin coupled with a strong metasomatic reworking of the IOA deposit.

Geochronological, Petrological, and Geochemical Constraints on Ni-Cu Sulfide Mineralization in the Poyi Ultramafic-Troctolitic Intrusion in the Northeast Rim of the Tarim Craton, Western China

Abstract: The Poyi magmatic Ni-Cu sulfide deposit is situated in the Beishan fold belt in the northeastern rim of the Tarim craton. Many Permian magmatic Ni-Cu sulfide deposits, such as those in East Tianshan, are present in the adjacent Central Asian orogenic belt to the north. The Poyi deposit is hosted in a small dike-like ultramafic-troctolitic body that was emplaced into a much larger gabbroic intrusion. Our new zircon U-Pb isotope data reveal that these two intrusions formed ~6 Ma apart. The ultramafic-troctolitic intrusion was emplaced at 269.9 ± 1.7 Ma, whereas the gabbroic intrusion was emplaced at 276.1 ± 1.9 Ma. The results show that the Poyi deposit is the youngest among the major magmatic Ni-Cu sulfide deposits (≥0.2 Mt Ni) of Permian ages in the Beishan-Tianshan region. Sulfide mineralization in the Poyi deposit occurs as steeply dipping disseminated sulfide lenses mostly associated with wehrlites in the center of the dike. Olivine from the Poyi ultramafic rocks has Fo content up to 91 mol %, which is similar to the lower limit of mantle olivine and the most primitive within the Permian Beishan-Tianshan nickel belt. Like other magmatic sulfide deposits in this belt, olivine from the Poyi deposit is depleted in Ca (<1,000 ppm). The estimated parental magma for the Poyi most primitive ultramafic rocks contains 15 ± 2 wt % MgO. Cotectic olivine-sulfide segregation from the Poyi magma is inferred from systematic variation of Fo-Ni contents in olivine from some sulfide-barren ultramafic rock samples, and supported by the occurrence of sulfide droplets as small inclusions in olivine (Fo90 mol %) in these rocks. The involvement of multiple pulses of sulfide-charged magma with different compositions is indicated by the abrupt change of olivine Fo content with depth and the presence of olivine with similar Fo contents but dramatically different Ni abundances in the different parts of the deposit. The (87Sr/86Sr)i ratios and ɛ Nd(total) of the Poyi ultramafic rocks and troctolites range from 0.7042 to 0.7052 and from 4.9 to 6.0, respectively, which are close to the isotope compositions of depleted mantle and within the ranges of major magmatic Ni-Cu sulfide deposits of Permian ages in East Tianshan. No more than 5 wt % of bulk crustal contamination is required to explain the variations of Sr-Nd isotopes in the Poyi ultramafic-troctolitic intrusion. The abundances of incompatible trace elements in whole rocks and clinopyroxene crystals indicate very weak light REE enrichments coupled by significant negative Nb-Ta anomalies in the parental melts. Bulk sulfides in the Poyi deposit are characterized by positive correlations between any pair of platinum-group elements (PGE), indicating that PGE tenor variations in the deposit are mainly controlled by variable R-factors (magma/sulfide mass ratios). The estimated initial concentrations of PGE in the parental magma for the Poyi deposit are almost two orders of magnitude lower than the abundances of PGE in some continental picrites. Given that the parental magma for the Poyi deposit is as primitive as a primary mantle-derived magma, the depletion of PGE in the Poyi deposit is most likely due to previous sulfide segregation at depth. Based on these observations, we conclude that sulfide saturation in the Poyi PGE-depleted, Mg-rich magma was triggered by addition of crustal sulfur during magma ascent and that the Poyi deposit was a dynamic conduit used by multiple pulses of olivine- and sulfide-charged magma.

Early Cambrian Black Shale-Hosted Mo-Ni and V Mineralization on the Rifted Margin of the Yangtze Platform, China: Reconnaissance Chromium Isotope Data and a Refined Metallogenic Model

Abstract: The transgressive basal unit of the Early Cambrian black-shale sequence along the rifted margin of the southeastern Yangtze platform hosts a wide spectrum of marine sedimentary rocks distributed over more than a length of 1,000 km. A few-centimeter-thick sulfide-rich black shale units have spectacular metal tenors (Mo and Ni in the percent range, PGE + Au around 1 ppm) and consist of submillimeter-scale laminated sulfide and carbonaceous material and centimeter-sized pebble-like rip-up clasts of Mo-S-C compounds, pyrite, and Ni-rich polymetallic sulfides in carbonaceous and phosphate-rich matrix. The δ 53/52 Cr authigenic values of Mo-Ni sulfide-rich black shale samples from the Zunyi mining district (Guizhou province), as well as from the Sancha district (Hunan province), 400 km northeast, have a mean of 0.96 ± 0.22‰ ( n = 8), while V-rich black shale from both districts has a mean of 1.34 ± 0.46‰ ( n = 5). These data indicate significantly positively fractionated values compared to igneous silicate Earth. The Cr isotope values of the studied shales compare with recent findings of positively fractionated δ 53/52 Cr values in Late Neoproterozoic-Phanerozoic marine carbonates and shale/mudstones and attest for the operation of an intensified oxidative surface Cr cycle from at least around ~0.75 Gyr ago. We propose that the major change in Cr cycling around the Precambrian-Cambrian boundary was caused by a significant rise of atmospheric oxygen levels. The Cr isotope data confirm earlier conclusions from Mo and Os isotopes, which indicate a seawater metal source with ultimate metal supply by oxidative weathering of continental crust. In particular, covariation trends of Re-Os and of platinum-group elements display a distinct seawater signature, and a degree of metal enrichment which is an extension of the normal black shale pattern. The Mo-Ni sulfide-rich black shale units are probably produced by winnowing of subaquatic hardground deposited under euxinic conditions. The same stratigraphic level also hosts anoxic/suboxic V-rich black shale (V in illite) and huge phosphorite deposits in oxic/suboxic settings, bedded barite, as well as 10s of meter-thick sapropelic alginite (combustible shale). We show that the extreme metal enrichment of the Mo-Ni sulfide-rich black shale can be understood as a combination of redox cycling and bottom-water/sediment-interface scavenging under euxinic conditions (Mo, noble metals), and of oxidation (remineralization) of organic matter settling from the photic zone, with sulfide fixation of Ni and other biophile elements under denitrifying and sulfate-reducing conditions. High biological activity (as in coastal upwelling settings with high nutrient supply), very low clastic accumulation rate (as in protected basins), and low organic matter deposition (high rate of remineralization) in stratified oxic-suboxic-euxinic basins are requirements for advanced metal enrichment in black shales. A special bathymetric condition of slope deposition with intrabasinal olistostrome-like mass movement and gravitational winnowing explains both the peculiar ore textures and the further metal enrichment to ore grade in the Early Cambrian rock units on the Yangtze platform. The Mo-Ni sulfide-rich sediments can be regarded as the euxinic variant of the marine hydrogenous ore deposit spectrum, where ferromanganese nodules/crusts represent the oxic variant of extreme fractionation from seawater.

Stable isotope and fluid inclusion constraints on the source and evolution of ore fluids in the Hongniu-Hongshan Cu skarn deposit, Yunnan Province, China

Abstract: The Hongniu-Hongshan Cu skarn deposit (77.8 Mt at 1.8% Cu) is located in the central part of the Zhongdian porphyry and skarn Cu belt in southwestern China. Skarn and orebodies occur mainly between the different units of the Upper Triassic Qugasi Formation or within altered limestone adjacent to Late Cretaceous intrusions (78–76 Ma). Three main paragenetic stages of skarn formation and ore deposition have been recognized on the basis of petrographic observations: (1) pre-ore-stage hornfels with diopside (Di87–72Hd12–7), small-scale endoskarn with reddish grossular (Adr22–57Gr78–43), diopside (Di83–92Hd7–15), vesuvianite, and abundant exoskarn with red-brown andradite (Adr75–98Gr2–22), sahlite (Di28–41Hd58–71), and wollastonite; (2) syn-ore-stage retrograde minerals, sulfides (pyrite, chalcopyrite, pyrrhotite, molybdenite, galena, and sphalerite), quartz, and calcite; and (3) post-ore-stage calcite veins. Sulfur isotope values of sulfides are relatively high, with an average δ 34S = 4.9‰ (n = 40), suggesting that the ore-forming fluid was magmatic and that the sulfides precipitated from a relatively reducing ore fluid. The coexistence of silicate melt and primary fluid inclusions in quartz phenocrysts of the mineralization-related quartz monzonite porphyry indicates the simultaneous entrapment of fluid and melt, and records the process of the aqueous fluid exsolving from the crystallizing melt. The initial single-phase fluid has a salinity of 8.8 to 12.7 wt % NaCl equiv and homogenization temperatures of 566° to 650°C, corresponding to pressures of 680 to 940 bar and lithostatic depth of 2.5 to 3.5 km. The primary fluid inclusions in the pre-ore-stage garnet and pyroxene composed of coeval vapor-rich (V type) and halite-bearing (S-I and S-II types containing sylvite) inclusions (32–>79 total wt % salts) share similar homogenization temperatures (450°–550°C), indicative of the occurrence of fluid unmixing under lithostatic pressures of ~550 to 780 bar (>2.0-km depth). Primary fluid inclusions trapped in syn-ore quartz, calcite, and epidote show the common development of S-type inclusions (~37.3 wt % NaCl equiv) with coexisting V-type, liquid-rich (L type), and CO2-bearing (C-I type) inclusions, all of which have homogenization temperatures of 300° to 400°C and trapping pressures of 100 to 400 bar (~1.5-km depth). Brine inclusions homogenized by halite dissolution after vapor disappearance in both the pre- and syn-ore stages are interpreted to have been trapped under overpressured conditions (>1,520 bar). Oxygen isotope analyses were conducted on garnet, wollastonite, epidote, quartz, and calcite. The pre-ore-stage garnet and wollastonite have δ 18Ofluid values of 5.6 to 8.1‰, whereas the syn-ore-stage epidote, quartz, and calcite have more variable δ 18Ofluid values in the range of 3.9 to 17.5‰. The δ 18Ofluid values of the post-ore-stage vein calcite (15.2–21.3‰) are much higher than both the pre- and syn-ore stages. The vapor phase of inclusions contains H2S, CH4, and C2H6 in the syn-ore stages. All these observations reveal that (1) the formation of the Cu skarn deposit was dominated by a magmatic hydrothermal system, (2) multiple fluid pulses contributed to the formation of the pre- and syn-ore-stage skarn minerals and sulfides, and (3) the increase in pH due to the neutralization of the acidic fluid could be the main factor controlling the large-scale ore deposition in Hongniu-Hongshan.

Regional Geologic and Petrologic Framework for Iron Oxide ± Apatite ± Rare Earth Element and Iron Oxide Copper-Gold Deposits of the Mesoproterozoic St. Francois Mountains Terrane, Southeast Missouri, USA

Abstract: This paper provides an overview on the genesis of Mesoproterozoic igneous rocks and associated iron oxide ± apatite (IOA) ± rare earth element, iron oxide-copper-gold (IOCG), and iron-rich sedimentary deposits in the St. Francois Mountains terrane of southeast Missouri, USA. The St. Francois Mountains terrane lies along the southeastern margin of Laurentia as part of the eastern granite-rhyolite province. The province formed during two major pulses of igneous activity: (1) an older early Mesoproterozoic (ca. 1.50–1.44 Ga) episode of volcanism and granite plutonism, and (2) a younger middle Mesoproterozoic (ca. 1.33–1.30 Ga) episode of bimodal gabbro and granite plutonism. The volcanic rocks are predominantly high-silica rhyolite pyroclastic flows, volcanogenic breccias, and associated volcanogenic sediments with lesser amounts of basaltic to andesitic volcanic and associated subvolcanic intrusive rocks. The iron oxide deposits are all hosted in the early Mesoproterozoic volcanic and volcaniclastic sequences. Previous studies have characterized the St. Francois Mountains terrane as a classic, A-type within-plate granitic terrane. However, our new whole-rock geochemical data indicate that the felsic volcanic rocks are effusive derivatives from multicomponent source types, having compositional similarities to A-type within-plate granites as well as to S- and I-type granites generated in an arc setting. In addition, the volcanic-hosted IOA and IOCG deposits occur within bimodal volcanic sequences, some of which have volcanic arc geochemical affinities, suggesting an extensional tectonic setting during volcanism prior to emplacement of the ore-forming systems. The Missouri iron orebodies are magmatic-related hydrothermal deposits that, when considered in aggregate, display a vertical zonation from high-temperature, magmatic ± hydrothermal IOA deposits emplaced at moderate depths (~1–2 km), to magnetite-dominant IOA veins and IOCG deposits emplaced at shallow subvolcanic depths. The shallowest parts of these systems include near-surface, iron oxide-only replacement deposits, surficial epithermal sediment-hosted replacement deposits, synsedimentary ironstone deposits, and Mn-rich exhalite deposits. Alteration associated with the IOA and IOCG mineralizing systems of the host volcanic rocks dominantly produced potassic with lesser amounts of calcic- and sodic-rich mineral assemblages. No deposits are known to be hosted in granite, implying that the mineralizing systems were operative during a relatively short, postvolcanic period yet prior to intrusion of the granitoids. Companion studies in this special issue on mineral chemistry, stable isotopes, and iron isotopes suggest that the magnetite within the IOA deposits formed from high-temperature fluids of magmatic or magmatic-hydrothermal origin. However, the data do not discriminate between a magmatic-hydrothermal source fluid exsolved from an Fe-rich immiscible liquid or an Fe-rich silicate magma. Mineral chemical, fluid inclusion, and stable isotope data from these new studies record the effects of metasomatic fluids that interacted with crustal reservoirs such as volcanic rocks or seawater.

Iron and Oxygen Isotope Signatures of the Pea Ridge and Pilot Knob Magnetite-Apatite Deposits, Southeast Missouri, USA

Abstract: New O and Fe stable isotope ratios are reported for magnetite samples from high-grade massive magnetite of the Mesoproterozoic Pea Ridge and Pilot Knob magnetite-apatite ore deposits and these results are compared with data for other iron oxide-apatite deposits to shed light on the origin of the southeast Missouri deposits. The δ 18 O values of magnetite from Pea Ridge ( n = 12) and Pilot Knob ( n = 3) range from 1.0 to 7.0 and 3.3 to 6.7‰, respectively. The δ 56 Fe values of magnetite from Pea Ridge ( n = 10) and Pilot Knob ( n = 6) are 0.03 to 0.35 and 0.06 to 0.27‰, respectively. These δ 18 O and the δ 56 Fe values suggest that magnetite crystallized from a silicate melt (typical igneous δ 56 Fe ranges 0.06–0.49‰) and grew in equilibrium with a magmatic-hydrothermal aqueous fluid. We propose that the δ 18 O and δ 56 Fe data for the Pea Ridge and Pilot Knob magnetite-apatite deposits are consistent with the flotation model recently proposed by Knipping et al. (2015a), which invokes flotation of a magmatic magnetite-fluid suspension and offers a plausible explanation for the igneous (i.e., up to ~15.9 wt % TiO 2 in magnetite) and hydrothermal features of the deposits.

Short-Wave Infrared Spectral and Geochemical Characteristics of Hydrothermal Alteration at the Archean Izok Lake Zn-Cu-Pb-Ag Volcanogenic Massive Sulfide Deposit, Nunavut, Canada: Application in Exploration Target Vectoring

Abstract: Short-wave infrared (SWIR) field and laboratory spectra were used to identify and characterize hydrothermal alteration mineral chemical variability in host rocks proximal to the Izok Lake Zn-Cu-Pb-Ag volcanogenic massive sulfide (VMS) deposit in Nunavut, Canada. The deposit is hosted within a sequence of predominantly felsic pyroclastic rocks of Archean age that are regionally metamorphosed to amphibolite facies. These rocks are characterized by the muscovite-biotite-sillimanite and chlorite-biotite-cordierite mineral suite. Proximal to the deposit, white micas are Al rich to potassic muscovite, and the biotite and chlorite are Mg rich. In areas that are immediately outboard of the proximal alteration zones, rocks are altered to Al-poor muscovite and intermediate to Fe-rich biotite and chlorite. Outboard of this, distal areas are characterized by potassic muscovite and Mg-rich biotite and chlorite. The white micas and biotite/chlorite group minerals display considerable variation in their Al-OH and Fe-OH absorption feature wavelength positions. The variations in muscovite and biotite/chlorite compositions (as determined by the Al-OH and Fe-OH absorption feature wavelength positions) broadly correlate with changes in hydrothermal alteration intensity, as measured by the Ishikawa and chlorite-carbonate-pyrite alteration indices. Our findings suggest that the large-scale alteration intensity trends of the Izok Lake study area can be elucidated using the spectral properties of the hydrothermal alteration minerals.

Synsedimentary, Diagenetic, and Metamorphic Pyrite, Pyrrhotite, and Marcasite at the Homestake BIF-Hosted Gold Deposit, South Dakota, USA: Insights on Au-As Ore Genesis from Textural and LA-ICP-MS Trace Element Studies

Abstract: The Paleoproterozoic Homestake deposit, northern Black Hills, South Dakota, is the largest banded iron-formation (BIF)-hosted gold deposit in the world and one of the largest single gold deposits globally (~1,300 t Au mined at an average grade of 8.4 g/t). The origin of the deposit has remained in dispute from its discovery, with views broadly falling into two groups: (1) the gold and associated elements were externally sourced and deposited in a suitable structural and/or chemical trap (e.g., iron formation), and (2) gold was indigenous to the host iron formation, from which it was remobilized into dilatant structural zones during deformation and metamorphism. In recent times, most workers have favored the former model, appealing to the broad synchroneity of Paleoproterozoic metamorphism, felsic magmatic activity, and gold event timing. LA-ICP-MS analysis of synsedimentary to early diagenetic sulfides at Homestake and surrounding area indicates that the Au in the orebodies may have originally had a significant syngenetic component. In particular, LA-ICP-MS mapping of multistage pyrites from carbonaceous shale in the upper Poorman Formation shows that Au and a suite of trace elements (Co, Ni, Cu, Zn, Se, Mo, Ag, Sb, Te, Au, Hg, Tl, Pb, and Bi), similar to those in diagenetic pyrite from around the globe, are contained in anhedral, sponge-textured cores and nodules surrounded by relatively trace element barren, Au-poor, euhedral pyrite rims. Furthermore, interelement ratios (e.g., Co/Ni, Ni/Ag) of the trace element-rich "spongy" and nodular pyrites are also similar to those of typical diagenetic pyrite. LA-ICP-MS imaging of pyrrhotite in the same rocks reveals multistage growth in this mineral as well, with the earlier generation having higher concentrations of Ag, Sb, Pb, Tl, and Bi than its presumed metamorphic counterpart. Imaging of marcasite in the shales of the upper Poorman Formation demonstrates this mineral’s high abundance of W and Tl relative to all generations of pyrite and pyrrhotite. Mass-balance calculations indicate that the volume of upper Poorman Formation in the mine area (approx. 12 km3) could potentially yield ~4,500 t of Au, a value greater than the total mined resource by more than a factor of three. These geochemical data and calculations suggest that a significant portion of the gold at Homestake may have been sourced from the relatively thick carbonaceous and sulfidic black shale facies in the stratigraphic footwall to the host iron formation, in a manner similar to other sediment-hosted gold districts.

Sulfide Replacement Processes Revealed by Textural and LA-ICP-MS Trace Element Analyses: Example from the Early Mineralization Stages at Cerro de Pasco, Peru

Abstract: The large Cerro de Pasco Cordilleran base metal deposit in central Peru is the result of three successive mineralizing stages comprising both low- and high-sulfidation mineral associations: (A) several pyrrhotite pipes grading outward to sphalerite and galena replacement bodies, (B) a massive, funnel-shaped pyrite-quartz replacement orebody, and (C) E-W–trending Cu-Ag-(Au-Zn-Pb) enargite-pyrite veins and well-zoned Zn-Pb-(Bi-Ag-Cu) carbonate-replacement orebodies. This superposition of hydrothermal events leads to complex replacement textures and crosscutting relationships. A detailed study of the textures and mineral composition of the up to 15-m-wide replacement front existing between the pyrrhotite pipes and the pyrite-quartz body allows for clarification of the relative chronology of the hydrothermal events. The results show that, in contrast to previous interpretations, the emplacement of the pyrrhotite pipes and their Zn-Pb mineralized rims precedes that of the pyrite-quartz body. The replacement textures affecting pyrrhotite and arsenopyrite and the nature of the newly formed minerals have been used as a qualitative way to track the evolution of fS2, fO2, and pH of the mineralizing fluids. Two steps of pyrrhotite replacement have been recorded. The first one takes place under moderate acidity and relatively reduced to moderately oxidized conditions and is marked by replacement of pyrrhotite by euhedral nonporous pyrite. The second step occurs under more acidic and oxidized conditions and is characterized by replacement of pyrrhotite by porous marcasite and replacement of arsenopyrite by pyrite. Subsequently, marcasite is partly replaced by fine-grained euhedral nonporous pyrite. LA-ICP-MS trace element analyses of the replaced pyrrhotite and arsenopyrite and of the newly formed marcasite and pyrite support dissolution-reprecipitation as the main mechanism for replacement. Positive correlations between some of the elements (e.g., Pb-Sb, Pb-Ag) are indicative of the possible presence of nanoscale solid inclusions as main carriers for those elements; however, coupled substitutions and incorporation of some of the elements at a ppm level into the pyrite and marcasite structures cannot be excluded. The obtained As, Sb, Pb, and Bi values in pyrite are systematically higher than published data of pyrite in epithermal and porphyry systems. Nature and trace element content of the newly formed minerals yield information on the physicochemical conditions during their precipitation, the initial trace element content of replaced minerals, and the subsequently dissolved neighboring phases. The results show that the metal concentration of the fluid is locally influenced by the composition of the dissolved minerals. This study leads to a simpler interpretation of the fluid evolution than previously proposed, with a progressive increase of fS2, fO2, and pH as a result of decreasing wall-rock buffering during the three successive mineralizing stages at Cerro de Pasco.

Basinal Brines at the Origin of the Imiter Ag-Hg Deposit (Anti-Atlas, Morocco): Evidence from LA-ICP-MS Data on Fluid Inclusions, Halogen Signatures, and Stable Isotopes (H, C, O)

Abstract: The Imiter Ag-Hg deposit is located in the Precambrian volcano-sedimentary formations of the Saghro inlier (eastern part of the Anti-Atlas Mountains, Morocco). The orebodies consist of northeast-southwest to east-west veins and lenses hosted by Cryogenian black shales and gray-wackes and Neoproterozoic conglomerates, and are controlled by an east-west fault network, the so-called Imiter fault. Mineralogical and paleo-fluid geochemistry investigations (microthermometry, Raman spectroscopy, LA-ICP-MS on individual inclusions, bulk crush-leach analyses, and stable isotope data (O, H)) show that the main Ag ore stage is related to circulation of deep-basinal sedimentary brines (Na-K-(Mg) (salinity = 16.7 to ≥26 wt % NaCl equiv, molar Cl/Br = 330, δ 18 O = 2.15–2.35‰ SMOW , and δ D = −53.8 to −65.5‰ SMOW ), at temperatures of about 180° to 220°C and hydrostatic pressures. The main driving mechanism for silver ore deposition is the dilution of ore-bearing brines by a low-salinity meteoric fluid containing a low-density volatile component (N 2 > CH 4 > CO 2 ), T h = 180° to 220°C, δ 18 O = −1.4‰ SMOW , and δ D of about −28.2‰ SMOW . Silver content of the brines ranges from 2 to 30 mmol/kg solution (up to 3,200 ppm Ag, avg Ag concentration about 900 ppm), whereas the maximum Ag content found in dilute waters is about 0.4 mmol/kg solution (40 ppm). The ore-forming model proposed for the Imiter deposit is (1) Ag extraction from the basement by the penetration of deep-basinal brines, and (2) deposition in a structural trap through fluid mixing with recharge fluids. This model is comparable to that described worldwide for the origin of Pb-Zn, F, Ba, and U deposits near unconformities between basement and sedimentary basins. Similarities among the major Ag deposits from the Anti-Atlas (Imiter, Zgounder, Bou Azzer) strongly suggest that they resulted from a unique event, likely related to the opening of the Atlantic Ocean. The silver ores are superimposed on the same lineament as a preexisting uneconomic Pan-African Co-Ni-As system linked to magmatic intrusions, but Ag ores have no genetic relationship with them.

On the Feasibility of Imaging Carbonatite-Hosted Rare Earth Element Deposits Using Remote Sensing

Abstract: Rare earth elements (REEs) generate characteristic absorption features in visible to shortwave infrared (VNIR-SWIR) reflectance spectra. Neodymium (Nd) has among the most prominent absorption features of the REEs and thus represents a key pathfinder element for the REEs as a whole. Given that the world’s largest REE deposits are associated with carbonatites, we present spectral, petrographic, and geochemical data from a predominantly carbonatitic suite of rocks that we use to assess the feasibility of imaging REE deposits using remote sensing. Samples were selected to cover a wide range of extents and styles of REE mineralization, and encompass calcio-, ferro- and magnesio-carbonatites. REE ores from the Bayan Obo (China) and Mountain Pass (United States) mines, as well as REE-rich alkaline rocks from the Motzfeldt and Ilimaussaq intrusions in Greenland, were also included in the sample suite. The depth and area of Nd absorption features in spectra collected under laboratory conditions correlate positively with the Nd content of whole-rock samples. The wavelength of Nd absorption features is predominantly independent of sample lithology and mineralogy. Correlations are most reliable for the two absorption features centered at ~744 and ~802 nm that can be observed in samples containing as little as ~1,000 ppm Nd. By convolving laboratory spectra to the spectral response functions of a variety of remote sensing instruments we demonstrate that hyperspectral instruments with capabilities equivalent to the operational Airborne Visible-Infrared Imaging Spectrometer (AVIRIS) and planned Environmental Mapping and Analysis Program (EnMAP) systems have the spectral resolutions necessary to detect Nd absorption features, especially in high-grade samples with economically relevant REE accumulations (Nd > 30,000 ppm). Adding synthetic noise to convolved spectra indicates that correlations between Nd absorption area and whole-rock Nd content only remain robust when spectra have signal-to-noise ratios in excess of ~250:1. Although atmospheric interferences are modest across the wavelength intervals relevant for Nd detection, most REE-rich outcrops are too small to be detectable using satellite-based platforms with >30-m spatial resolutions. However, our results indicate that Nd absorption features should be identifiable in high-quality, airborne, hyperspectral datasets collected at meter-scale spatial resolutions. Future deployment of hyperspectral instruments on unmanned aerial vehicles could enable REE grade to be mapped at the centimeter scale across whole deposits.

An Improved Approach to Characterize Potash-Bearing Evaporite Deposits, Evidenced in North Yorkshire, United Kingdom

Abstract: Traditionally, potash mineral deposits have been characterized using downhole geophysical logging in tandem with geochemical analysis of core samples to establish the critical potassium (% K2O) content. These techniques have been employed in a recent exploration study of the Permian evaporite succession of North Yorkshire, United Kingdom, but the characterization of these complex deposits has been led by mineralogical analysis, using quantitative X-ray diffraction (QXRD). The novel QXRD approach provides data on K content with the level of confidence needed for reliable reporting of resources and also identifies and quantifies more precisely the nature of the K-bearing minerals. Errors have also been identified when employing traditional geochemical approaches for this deposit, which would have resulted in underestimated potash grades. QXRD analysis has consistently identified polyhalite (K2Ca2Mg(SO4)4·2(H2O) in the Fordon (Evaporite) Formation and sylvite (KCl) in the Boulby Potash and Sneaton Potash members as the principal K-bearing host minerals in North Yorkshire. However, other K hosts, including kalistrontite (K2Sr(SO4)2) a first recorded occurrence in the UK, and a range of boron-bearing minerals have also been detected. Application of the QXRD-led characterization program across the evaporitic basin has helped to produce a descriptive, empirical model for the deposits, including the polyhalite-bearing Shelf and Basin seams and two, newly discovered sylvite-bearing bittern salt horizons, the Pasture Beck and Gough seams. The characterization program has enabled a polyhalite mineral inventory in excess of 2.5 billion metric tons (Bt) to be identified, suggesting that this region possesses the world’s largest known resource of polyhalite. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed.

New Constraints on the Timing of Host-Rock Emplacement, Hydrothermal Alteration, and Iron Oxide-Apatite Mineralization in the Kiruna District, Norrbotten, Sweden

Abstract: High spatial resolution zircon U-Pb geochronological data obtained directly on the Kiirunavaara iron oxide-apatite (IOA) deposit, related orebodies, and host rocks provide new constraints on the timing of mineralization in these deposits. These data raise new arguments in the debate of a magmatic versus hydrothermal/metasomatic genesis of these major (2,500 Mt, 30–70 wt % Fe) Paleoproterozoic deposits. The main orebody at Kiirunavaara contains Ti-poor magnetite and minor (0.05–5 wt % P) apatite, located between a trachyandesite footwall and a rhyodacite hanging wall, which also hosts smaller orebodies (Nukutus, Rektorn, and Tuolluvaara). The pervasive Na and K metasomatism in the host rock is documented by whole-rock geochemical data and cathodoluminescence (CL) microscopy. Zircon U-Pb data for the metavolcanic rocks in the footwall and hanging wall cluster between 1884 ± 4 and 1880 ± 3 Ma. In the footwall, a syenite-aplite system yields ages of 1880 ± 7 and 1881 ± 4 Ma; a granite pluton exposed underground has an age of 1874 ± 4 Ma. Zircons in two ore samples, never directly dated before this study, yield ages of 1877 ± 4 and 1874 ± 7 Ma. Brecciation at the contacts between the ore and host rocks, the tight age at ca. 1880 Ma for most volcanic and plutonic rocks in the footwall and hanging wall, and the marginally younger age for ore at ca. 1877 to 74 Ma, matching the age of the spatially related granite pluton, suggest a magmatic-hydrothermal emplacement model for the Kiruna area IOA ores.

Rare Earth Deposits of the Murmansk Region, Russia—A Review

Abstract: This paper reviews the available information on the geology, mineralogy, and resources of the significant rare earth element (REE) deposits and occurrences in the Murmansk Region, northwest Russia. The region has one of the largest endowments of REE in the world, primarily the light REE (LREE); however, most of the deposits are of potential economic interest for the REE, only as by-products of other mining activity, because of the relatively low REE grade. The measured and indicated REE2O3 resources of all deposits in the region total 22.4, and 36.2 million tonnes, respectively. The most important resources occur in (1) the currently mined Khibiny titanite-apatite deposits, and (2) the Lovozero loparite-eudialyte deposit. The Kovdor baddeleyite-apatite-magnetite deposit is a potentially important resource of scandium. These deposits all have polymetallic ores, i.e., REE would be a by-product of P, Ti, and Al mining at Khibiny, Fe, Zr, Ta, and Nb mining at Lovozero, and Fe and Ti mining at Afrikanda. The Keivy block has potential for heavy REE exploitation in the peralkaline granite-hosted Yumperuaiv and Large Pedestal Zr-REE deposits and the nepheline syenite-hosted Sakharyok Zr-REE deposit. With the exception of the Afrikanda perovskite-magnetite deposit (LREE in perovskite) and the Kovdor baddeleyite-apatite-magnetite deposit (scandium in baddelyite), carbonatite-bearing complexes of the Murmansk Region appear to have limited potential for REE by-products. The sound transport, energy, and mining infrastructure of the region are important factors that will help ensure future production of the REE.

The Giant Mascot Ni-Cu-PGE Deposit, British Columbia: Mineralized Conduits in a Convergent Margin Tectonic Setting

Abstract: The Giant Mascot Ni-Cu-PGE deposit is the only past-producing nickel mine (1958–1974) in the Canadian Cordillera of British Columbia, Canada, with ~4.2 Mt of ore grading 0.77% Ni and 0.34% Cu (Ni/Cu = 2.3). The deposit is part of an emerging class of convergent margin Ni-Cu-PGE sulfide deposits where the host rocks are characterized by abundant orthopyroxene and magmatic hornblende. The Giant Mascot intrusion is a crudely elliptical, 3- × 2-km plug composed of ultramafic arc cumulates (olivine-orthopyroxene, hornblende-clinopyroxene) that intruded the Late Cretaceous Spuzzum dioritic pluton. The ores are contained within subvertical pipe-like, lensoid, and tabular bodies ( n = 28), interpreted to represent mineralized conduits and consist of pyrrhotite, pentlandite (+argentopentlandite), chalcopyrite, minor pyrite, troilite, and Pt-Pd-Ni bismuthotellurides. The sulfides have moderate tenors (3–14 wt % Ni, 0.1–17.1 wt % Cu, 84 ppb to 5 ppm total PGE), and variations in PGE concentrations distinguish two geographic mineralized zones (western and eastern). Sulfur isotopes for sulfides span a restricted range of δ 34 S values (−3.4 to −1.3‰) and overlap with analyses from locally pyritiferous Settler schist (−5.4 to −1.2‰). Olivine chemistry reveals a wide range of both Fo (80–89 mol %) and Ni contents (336–3,859 ppm) and provides evidence that Ni contents of olivine were locally upgraded during equilibration with sulfide liquid. Sulfide saturation in the Giant Mascot parental magma(s) was triggered in response to (1) reduction of an oxidized, mantle-derived arc magma, producing ores that formed at an oxidation state of ~ ΔQFM +1; (2) addition of external sulfur and silica by assimilation of Settler schist and Spuzzum diorites; and (3) fractional crystallization of olivine ± orthopyroxene. The presence of high-tenor sulfides in the Giant Mascot deposit indicates that orthomagmatic Ni-Cu-PGE deposits in convergent margin tectonic settings may be of significance to future global exploration.

Mineral Thermometry and Fluid Inclusion Studies of the Pea Ridge Iron Oxide-Apatite–Rare Earth Element Deposit, Mesoproterozoic St. Francois Mountains Terrane, Southeast Missouri, USA

Abstract: Mineral thermometry and fluid inclusion studies were conducted on variably altered and mineralized samples from the Mesoproterozoic Pea Ridge iron oxide-apatite (IOA)-rare earth element (REE) deposit in order to constrain P-T conditions, fluid chemistry, and the source of salt and volatiles during early magnetite and later REE mineralization. Scanning electron microscopy (SEM)-cathodoluminescence and SEM-backscatter electron images show that quartz and rutile precipitated before, during, and after magnetite and REE mineral growth. Ti-in-quartz and Zr-in-rutile equilibration temperatures range from ≤350° to 750°C in the amphibole, magnetite, hematite, and silicified zones where T increased during magnetite and quartz growth and dropped precipitously after fracturing and brecciation. Late drusy quartz cements within a REE-rich breccia pipe record the lowest T (≤315°–400°C). Liquid-, vapor-rich, and hypersaline (±hematite, calcite) fluid inclusions are common and liquid CO 2 is present locally. Salinities define three populations: saline (10–27 wt % NaCl equiv), hypersaline (34–>60 wt % NaCl equiv), and dilute (0–10 wt % NaCl equiv ). The wide range of eutectic melting temperatures (−67° to −19°C) suggests that saline inclusions trapped variable proportions of a CaCl-MgCl-FeCl-bearing fluid end member and an NaCl-KCl fluid end member. Homogenization temperatures and pressures of these saline inclusions suggest they were trapped when fluids unmixed into brine and vapor at T The Na/Cl, Na/K, and Cl/Br ratios of fluid inclusion extracts provide evidence for mixtures of magmatic hydrothermal fluids and evaporated seawater. Extracts from magnetite, hematite, and pyrite plot in the magmatic-hydrothermal field, indicating that Fe was derived from a magmatic source. Their enrichments in Mg and Ca are consistent with a mafic magmatic source. The positive correlation between Na/Mg and Na/Ca ratios may be due to halite saturation or albitization of igneous rocks. Extracts from barite in the REE-rich breccia pipes are enriched in Na and Br and plot near the seawater evaporation trend. He is highly enriched relative to Ne and Ar in fluid inclusion extracts, which precludes air as a source of He. Although the He is mostly of crustal origin, pyrite with a 3 He/ 4 He (R/R A ) of 0.1 contains up to 12% mantle He. Many extracts have low 20 Ne/ 22 Ne ratios due to nucleogenic production of 22 Ne in high F/O minerals such as fluorapatite or F biotite. The arrays of data for 3 He/ 4 He (R/R A ) and 22 Ne/ 20 Ne suggest that volatiles were derived from two sources, a moderate F mafic magma containing mantle He and a high F silicic magma with crustal He. Together with other evidence cited in this report, these data (1) support a magmatic hydrothermal origin for the Mesoproterozoic magnetite-apatite deposit with ore fluids derived from a concealed mafic to intermediate-composition intrusion, (2) suggest that the REE minerals in breccia pipes were either derived from apatite or precipitated in response to decompression and cooling during breccia pipe formation, (3) provide evidence for the influx of basinal brine, magmatic fluids from granitic intrusions, and meteoric water after breccia pipe formation, and (4) show that Pea Ridge was relatively unaffected by the late Paleozoic Mississippi Valley-type (MVT) Pb-Zn system in overlying Cambrian sedimentary rocks.

Characterization of Gold Mineralization in the Northern Cariboo Gold District, British Columbia, Canada, Through Integration of Compositional Studies of Lode and Detrital Gold with Historical Placer Production: A Template for Evaluation of Orogenic Gold Districts

Abstract: There is a strong association between regions containing orogenic gold mineralization and exploitation of placer gold, although, in many cases, the nature of the source mineralization for these placer deposits remains unclear. This study describes a novel approach to evaluating the economic potential of in situ orogenic gold mineralization through characterization of both lode and placer gold mineralogy, followed by synthesis of this information with records of both mineral occurrences and historical placer mining. The northern Cariboo Gold District in east-central British Columbia, Canada, was chosen as a location for the study because of the gold endowment of the area (1.2 million ounces [Moz] of lode gold and between 0.5 and 3 Moz. of placer gold) and the information available from both placer and lode gold mining. Compositional analysis of 533 gold grains from 21 lode localities and 1,914 gold grains from 30 placer localities from throughout the Cariboo Gold District has identified four main compositional types in terms of their alloy compositions and associated suite of mineral inclusions revealed in polished section. A distinctive low (4–7%)-Ag gold that exhibits a strong Bi association in the mineral inclusion suite is geographically limited to the Wells area, where it is recorded in both lode mineralization and its placer expression. Regionally pervasive mineralization yields gold of binary Au-Ag alloy and a simple inclusion suite of sulfides and sulfarsenides. Gold in most large placers in trunk drainages was derived from multiple (mostly small) occurrences of this type. The nature of compositional variation between gold grains liberated from hypogene ore has informed the history of episodic mineralization and suggests multiple stages of gold emplacement at some localities, while others are dominated by gold deposited in a single stage. The new information from gold grain analysis has been considered in the context of other information. Classification of a placer as either allocthonous or autocthonous both informs interpretation of compositional characteristics of the detrital gold grains and provides information on distance to the hypogene source. Mineral inclusion assemblages observed in sample populations of placer gold grains have been correlated with reports of hypogene vein mineralogy described in mineral occurrence records to clarify the geographical extent of specific mineralization types. The compositional range of alloys of different gold types has been compared to historical records of gold production and gold fineness (Au-Ag ratio) to reconstruct the size and distribution of hypogene sources prior to erosion. Synthesis of gold compositional studies with other publicly accessible data sets provides a new generic approach capable of evaluating the most attractive targets for future exploration and highlighting compositional signatures of placer gold which relate to undiscovered in situ sources.