Abstract Scheelite (CaWO4) is considered a useful mineral for classifying ore deposit types because its trace element concentrations, especially rare earth elements (REEs), As, Eu, Mo, and Sr, can inform on paragenesis. In this study, this concept is tested by integrating trace elements, in situ Sr and Nd isotopes, and δ18O data from two quartz-scheelite vein swarms cutting Paleozoic plutons in New Zealand. In both cases, the scheelite has been inferred by previous researchers to be of magmatic-hydrothermal origin. However, the scheelite in both deposits exhibits highly variable inter- and intrasample trace element concentrations, which we interpret to relate to different substitution mechanisms and oxidation states of the mineralizing fluids. Moreover, age-corrected scheelite in situ 87Sr/86Sr and 143Nd/144Nd values diverge from those of the host granitoids despite some veins occurring entirely within the plutons. Quartz-scheelite oxygen isotope thermometry indicates equilibration at relatively low temperatures (~170°–280℃), and the calculated δ18OH2O values suggest mixing between W-bearing fluids and meteoric waters. The scheelite trace element chemistries, therefore, reflect a complex interplay between local element reservoirs and oxidation states. Since these processes appear to have operated at the deposit scale, scheelite-based trace element paragenesis discrimination models should be applied with caution. For example, a metamorphic W-bearing fluid may acquire a magmatic-like trace element signature through interaction with granitoids shortly before scheelite crystallization; conversely, a magmatic-hydrothermal W-bearing fluid may develop a nonmagmatic signature by reacting with the metamorphic country rocks.
Abstract The Cretaceous Zijinshan Au-Cu deposit (>300 metric tonnes Au, 2 Mt Cu) is hosted by the Zijinshan lithocap in the Zijinshan granite complex on the western margin of the coastal volcanic belt of South China. Hydrothermal alteration is zoned vertically over 1,200 m, starting from –200 m elevation with quartz-pyrophyllite and quartz-muscovite assemblages at depth, grading upward to quartz-dickite, quartz-alunite, massive quartz, and vuggy quartz alteration. Sulfides are also zoned vertically, with enargite mainly at depth (–200–600 m elevation), and digenite and covellite at shallower levels. The hypogene ores at Zijinshan are dominantly hosted in discordant breccias. Three main types are recognized based on field observations of breccia infill: (1) matrix-dominated breccias, (2) cement- and matrix-rich breccias, and (3) cement-dominated breccias. High-sulfidation-state mineral assemblages occur in the cements and altered clasts of these breccias, defining subfacies of cement-dominated and cement- and matrix-rich breccias, including quartz-pyrite-cemented (CM1), alunite- (CM2) and copper sulfide-cemented (CM3) breccias, and pyrite- and copper sulfide-cemented matrix-bearing (CMX1), and dickite-cemented matrix-bearing breccias (CMX2). Structural analyses show that the emplacement of matrix-dominated breccia dikes and cement-dominated and cement- and matrix-rich breccia veins were controlled by a series of syn-mineralization NW-trending normal oblique faults. The matrix-dominated dikes, and cement- and matrix-rich and cement-dominated breccia veins are interpreted as tectonic-hydrothermal breccias, although the relative influences of tectonic and hydraulic processes varied between breccia facies. A large (400 m diameter) matrix-dominated breccia body in the south of the deposit is interpreted as a diatreme. Matrix-dominated breccia dikes surround the diatreme and were emplaced prior to cement-dominated breccias, recording a transition from a predominance of attrition to dilation along the deposit-scale faults. Matrix-dominated breccias are common in the southern and central parts of the deposit, whereas cement-dominated breccias are located primarily in the north, with minimal superposition of the two breccia types. The cement-dominated breccias are generally associated with high copper grades. The spatial distribution of breccias implies that tectonic processes began with attrition along faults in the north, migrating to a predominance of dilation synchronous with main-stage mineralization in the deposit center. The complex sequence of brecciation at Zijinshan is intimately linked to the deformation history and was fundamental to the development of this giant high-sulfidation epithermal copper-gold deposit, controlling both the location and geometry of orebodies. The well-developed fracture mesh generated by faults at Zijinshan was critical to focusing fluid flow, resulting in a series of NW-trending mineralized veins and breccias, and facilitating the mineralization and formation of an extensive lithocap in an otherwise highly impermeable granite protolith.
Abstract Preservation of subseafloor replacement-style mineralization in the Wolverine volcanogenic massive sulfide (VMS) deposit illustrates the importance of ambient sedimentary environment and mud replacement in the formation of replacement-style VMS mineralization. Prior to deposit formation, framboidal pyrite, and likely bacterial sulfate reduction (BSR)-generated H2S in pore fluids, formed during diagenesis in host sediments in association with BSR of seawater sulfate. Diagenesis was succeeded by the first stage (stage 1) of low-temperature (<300°C) Zn-rich mineralization that utilized the preexisting framboids (and likely water and H2S in the sedimentary pore fluids) and involved both macro- and microscale replacement of mud and framboids and the deposition of a Zn-Pb-(Ag-Se-Sn)–rich assemblage dominated by sphalerite-galena; colloform, spongy, and euhedral pyrite; tetrahedrite; arsenopyrite; stannite; cassiterite; clausthalite; and naumannite. The stage 1 mineralization was replaced by a Cu-rich assemblage during stage 2 mineralization dominated by chalcopyrite, pyrrhotite, and rounded pyrite. This study interprets that Cu-rich fluids dissolved stage 1 Zn-rich sulfides, which resulted in remobilization of Zn into solution, precipitation of Cu-rich sulfides in their place, and subsequent reprecipitation of Zn at the stratigraphically overlying mud-sulfide interface. It is also interpreted that this zone-refining and Zn-redistribution process was important for the high Zn grades of the Wolverine deposit. In situ sulfur isotopes from various pyrite textural types, galena, pyrrhotite, and chalcopyrite show extreme variability in the diagenetic and stage 1 mineralization with δ34S values ranging from –27.4 to 12.1‰, whereas stage 2 pyrite, pyrrhotite, and chalcopyrite have restricted δ34S values ranging from 10.1 to 14.3‰. These δ34S variations reflect varying contributions of H2S derived from BSR, inheritance of BSR-derived sulfur from framboids during replacement, and hydrothermal H2S derived from thermochemical sulfate reduction (TSR) of seawater sulfate. The process of nucleation and replacement of VMS sulfides on diagenetic framboidal pyrite, mixing of VMS fluids with BSR-derived H2S in sedimentary pore fluids, and zone refining are likely important in many replacement-style VMS deposits and sediment-rich deposits globally (e.g., mafic-siliciclastic VMS, Irish-type and clastic-dominated Zn-Pb, and sediment-hosted Cu deposits).
Abstract Most models for the formation of magmatic Ni-Cu-platinum group element (PGE) sulfide deposits invoke high-temperature events (e.g., plumes) to sufficiently melt enough peridotite mantle to produce MgO- and Ni-Cu-PGE–rich magmas, and the deposits are thus intrinsically linked to the emplacement of large igneous provinces (LIPs) in mostly extensional tectonic regimes. However, many Ni-Cu deposits are hosted by intrusions unrelated to LIP events, having formed during compressional orogenic events. We present a new conceptual framework for global magmatic sulfide deposits based on chalcophile element geochemistry, which reflects distinct combinations of mantle source composition (anhydrous peridotite vs. hydrous peridotite/pyroxenite) and the temperature and/or volume of melting. Low volumes of melting of hydrous mantle at low temperatures (<1,200°C) produce alkalic ultramafic-hosted, Ni-poor, Cu-Au–dominant, low-temperature Cu (LT-Cu) deposits, displaying relative enrichment in Cu, Au, and Te over the other chalcophile elements. Greater volumes of melting of a hydrous source produce mafic-ultramafic-hosted low-temperature Ni (LT-Ni) deposits defined by distinctly PGE-poor chemistries. High-temperature (>1,200°C) melting of more anhydrous mantle peridotite forms both mafic-ultramafic-hosted high-temperature Ni (HT-Ni) deposits enriched in all chalcophile elements and layered mafic intrusion-hosted high-temperature PGE (HT-PGE) deposits, which display relative PGE enrichment. Very-high-temperature melting of peridotite mantle forms komatiite-hosted very-high-temperature Ni (VHT-Ni) deposits with chalcophile element profiles close to primitive mantle ratios. Temporally and spatially, all VHT and HT deposits are inextricably linked to LIPs throughout Earth’s history. The major HT-PGE deposits are all >2.0 Ga and hosted within cratons, the VHT-Ni deposits are all >1.8 Ga and hosted in komatiites, and the HT-Ni deposits are distributed throughout geologic time and are mostly located at cratonic margins. Conversely, LT deposits are all younger than 2.0 Ga, are almost never associated with LIPs, and have a more varied spatial distribution, but are more commonly located within orogenic belts. These secular changes in the nature of magmatic sulfide systems through time are a function of (1) a cooling mantle from the Archean to the present, (2) varying compositions of mantle as a product of increased mantle metasomatic processes through time, (3) the mineralogical deportment of chalcophile metals in mantle rocks, and (4) the variety of thermal triggers to melt such assemblages. We present a revised mineral system framework for the source and geodynamic controls on magmatic sulfides that takes into account all these criteria and allows enhanced targeting capability and prediction of the metal fertility of potential deposits.
Abstract Nickel-Zn-Pt-Pd-Re–bearing hyper-enriched black shale (HEBS) occurs ≈3 to 30 m stratigraphically below the base of the Cardiac Creek Zn-Pb-Ag-Ba clastic-dominated (CD) deposit in the Late Devonian Gataga district, northeastern British Columbia. The HEBS horizon is ≈50 to 70 cm thick, stratiform and stratabound, and comprises disseminated sulfide mineralization within silty, calcareous shale. Bulk HEBS analyses have up to 0.9 wt % Ni, 2.7 wt % Zn, 110 ppb Pt, 126 ppb Pd, and >1 ppm Re. Sulfide mineral textures and crosscutting relationships suggest three stages of precipitation. Framboidal to microcrystalline pyrite (py1), very fine grained sphalerite (sp1), and very fine grained millerite (mlr1) are the earliest sulfides, all intergrown within the shale matrix and pyrobitumen (pyro1). Nodular pyrite (py2) forms anhedral masses on py1 or discrete nodules within the shale matrix. Subhedral to euhedral pyrite (py3), sphalerite (sp2), millerite (mlr2), gersdorffite, galena/clausthalite, chalcopyrite, and tennantite/tetrahedrite are the latest sulfides. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of sulfides (especially pyrite) within HEBS reveal a complex distribution of Re, Mo, and Pt within early phases (py1, mlr1-py1, and sp1). Later py2 and py3 have lower abundances of these metals, and sp2, mlr2, and galena/clausthalite contain no detectable amounts. Whole-rock Re-Os geochronology of an unmineralized shale 2.8 m stratigraphically above HEBS gives an isochron age of 380.2 ± 3.6 Ma and initial Os ratio (Osi) of 0.25 ± 0.04 (2σ), which is identical to contemporaneous seawater, indicating an undisturbed depositional age. Two HEBS samples yield ages of 362.1 ± 25.6 and 358.5 ± 5.4 Ma, with highly radiogenic Osi of 1.54 ± 1.78 and 1.03 ± 0.13, respectively, that are considerably younger than the overlying shale. Rhenium, Mo, and Pt enrichment in py1 suggests syngenetic to early diagenetic sulfide precipitation, whereas the radiogenic Osi values are likely the result of later (≈360 Ma) fluid flow that reset the primary age (380–390 Ma) of the HEBS during the formation of synsedimentary to early diagenetic hydrothermal, CD Zn-Pb deposits in the area. This work demonstrates that there can be spatial and temporal juxtaposition and preservation of polymetallic HEBS and hydrothermal Zn-Pb-Ag-Ba CD mineral systems in a common depositional setting. It also highlights the value of pyrite petrographic and geochemical investigations in resolving complex geologic problems.
Abstract The Coastal Cordillera of central-northern Chile hosts numerous stratabound (manto-type) Cu-(Ag) deposits, where sulfides are commonly associated with residual petroleum or pyrobitumen. El Soldado, the largest of these deposits in central Chile, exemplifies this relationship, raising questions about the provenance of metals and the role of hydrocarbons and hydrothermal fluids in ore formation. Although El Soldado is an important major source of Cu and Ag, the main sulfide phases—pyrite, chalcopyrite, bornite, and chalcocite—remain poorly constrained. Characterizing their trace element chemistry and compositional variability is key to better understanding the evolution of the deposit and the deportment of critical elements. To address this knowledge gap, we present a geochemical characterization of sulfides from El Soldado using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), in combination with electron probe microanalysis (EPMA) and micro-X-ray fluorescence (μXRF). Our results show that El Soldado sulfides—especially pyrite—contain a large variety of trace elements, including appreciable concentrations of Co, Ni, Pb, As, Ag, Mn, Mo, and Tl. Vanadium, Mn, Mo, and Tl can reach high concentrations in Cu-(Fe) sulfide phases (tens to hundreds of parts per million), suggesting a possible contribution from sedimentary sources in a basinal or marine environment. Microprobe analyses of pyrobitumen reveal high concentrations of S, Cl, Mn, Fe, Cu, and Ag, whereas μXRF maps show both homogeneous and irregular distribution of some of these elements at the grain scale (e.g., S and Cl). This suggests that the organic phase may have incorporated them through interaction with ore-forming hydrothermal fluids, acting as an important redox trap, and potentially providing certain components to the ore phases. Trace element ratios of Cu-(Fe) sulfides from El Soldado are compared with geochemical data from other ore deposits, including sediment-hosted copper deposits, iron oxide copper-gold (IOCG), iron oxide-apatite (IOA), and porphyry Cu systems. The Co/Ni (<1) and Mn/Fe (~0.00001–0.01) versus Mo (<100 ppm) relationships in pyrite, the In/Se ratio in chalcopyrite (~0.001–0.1), and the Ag/Bi ratio in bornite (~0.1–100) point to a potential sedimentary source for these metals in El Soldado, with possible contributions that include connate, basinal, marine, and meteoric fluids. Our data suggests that these geochemical signatures could help discriminate manto-type Cu mineralization, aiding in the exploration of new Cu resources in the Andean province and possibly elsewhere.
Rare earth elements, including yttrium (REYs), are the most sought-after among the so-called critical or strategic elements for the low-carbon energy industry. In the past decade, karst bauxite deposits have been targeted as potential nonconventional sources of these elements. The karst bauxite deposits from the Mercedes-Aceitillar mining district, classified as Fe-rich bauxites and bauxites sensu stricto, contain anomalously high REY contents oscillating between 514 and 28,787 ppm (median of 1,489 ppm), making them the most REY-enriched karst bauxites globally. Most of the samples studied are enriched in light rare earth elements (La-Nd; up to 7,449 ppm) and Y (up to 14,830 ppm). However, the most REY-enriched bauxites yield particularly high middle rare earth element (Sm-Gd; up to 4,579 ppm) and heavy rare earth element (Tb-Lu; up to 6,163 ppm) contents. In addition, the studied bauxites contain significant amounts of other critical metals, such as Sc and Ga (median of 62 and 39 ppm, respectively). The mineralogy consists predominantly of Al oxyhydroxides (mostly gibbsite), with variable amounts of Fe oxyhydroxides and kaolinite. The REY mineralogy in samples with high REY contents (>3,000 ppm) consists mainly of REY-bearing phosphates (e.g., monazite, xenotime, rhabdophane, churchite) and/or carbonates (bastnäsite group). In samples with moderate REY contents (<3,000 ppm), the main REY-bearing minerals are Al oxyhydroxides, onto which the REYs are most likely adsorbed. These findings indicate that the potential extraction of REYs and other critical metals, such as Sc and Ga, in the studied bauxites from the Mercedes-Aceitillar mining district could add, on average, an ~20% surplus to the revenue obtained from Al2O3 exploitation. The unprecedented REY contents discovered in these karst bauxites from the Dominican Republic present a unique opportunity to unravel critical metal concentration mechanisms in supergene environments and to develop new geochemical models for nonconventional REY deposits.
The Archean Mooshla Intrusive Complex is a polyphase magmatic body that is spatially, and possibly genetically, associated with numerous world-class Au-rich volcanogenic massive sulfide deposits and epizonal subseafloor intrusion-related (or porphyry-epithermal-style) Au-(Cu) vein systems in the Abitibi greenstone belt, Canada. To elucidate any genetic links between deposits and the Mooshla Intrusive Complex, mineralized samples from two intrusion-related Au-Cu vein deposits (Doyon and Grand Duc deposits) have been characterized using a combination of detailed petrography and trace element and sulfur isotope analyses of sulfide phases. The pyrite trace element and S isotope systematics indicate that numerous processes were operating to produce the different ore assemblages at Doyon and Grand Duc. However, the main Au precipitation mechanism for both the Doyon and lower-grade vein systems at Grand Duc is interpreted to be influenced by wall-rock sulfidation. Pyrite from high-grade veins at Grand Duc display significantly different trace element and S34S patterns compared to those at Doyon and lower grade veins at Grand Duc. Pyrite from high-grade veins at Grand Duc also have elevated concentrations of epithermal-suite elements (i.e., Au-Ag-As-Te-Bi-Sb-Se) along pyrite rims and, combined with a drop in S34S from the metal-poor core to the metal-rich rim in individual pyrite grains, suggest gold precipitation resulted from a magmatic-hydrothermal fluid boiling event. Although pyrite records a fluid boiling event, gold and tellurobismuthite occur synchronous with chlorite alteration, indicating gold and tellurides were likely a result of Au remobilization from pyrite during regional metamorphism. Although the trace element and ore mineral characteristics have been influenced by regional metamorphism, the S34S compositions and patterns have retained their magmatic signature and importantly the Q33S values are near zero within and between individual pyrite grains. Therefore, the combination of S34S and Q33S indicates a singular source of sulfur and rules out non-magmatic sulfur sources. Even though these deposits have been variably modified during regional metamorphism, the combination of methods applied here can identify primary magmatic-hydrothermal processes responsible for the development of these Au-rich deposits, potentially linking them to causative magmatism.
The Val-d'Or gold district (Abitibi greenstone belt) hosts atypical Cu-Au occurrences associated with the Archean East Sullivan stock, located north of the Cadillac-Larder Lake fault zone. This study integrates geologic, geochemical, isotopic, and Re-Os geochronological data to characterize this intrusion-related system and its association with orogenic gold mineralization. The East Sullivan stock is a highly oxidized, water-rich, fertile, porphyritic, monzonitic-syenitic sanukitoid intrusion that crystallized at ca. 2684 Ma (U-Pb zircon). At least two mineralization styles are associated with the East Sullivan stock: (1) porphyry-style Cu-Mo-Au mineralization hosted within the East Sullivan stock, partly associated with a vuggy endoskarn facies; molybdenite from this style yields an Re-Os age of ca. 2695 +/- 12 Ma, constraining the timing of porphyry mineralization; and (2) extensive Cu-Au-Ag exoskarn mineralization hosted in adjacent volcanic rocks of the Heva Formation. The exoskarn developed through a garnet-dominated prograde stage, followed by a calc-silicate-dominated retrograde stage that is associated with the main Cu-Au-Ag mineralization. We suggest that the East Sullivan stock is an Archean porphyry skarn system linked to sanukitoid magmatism that formed similar to 40 m.y. before the main orogenic gold events (similar to 2643 Ma) in the district. These temporally distinct magmatic-hydrothermal and orogenic systems highlight the protracted, multistage metallogenic evolution of this major Archean gold camp.
The J-M reef of the Stillwater Complex is characterized by a high Pd/Pt ratio (mean similar to 3.8 with a standard error of 0.03) with a homogeneous geospatial distribution at the deposit scale. In this contribution, we demonstrate that the Pd/Pt ratio of the reef is the product of equilibration of an immiscible sulfide liquid with a silicate melt rich in Pd relative to Pt. Despite the high tenors of the J-M reef sulfides (avg 2,700 ppm Pt and 770 ppm Pt), numerical modeling shows that the parental melts did not have extraordinary Pd and Pt concentrations. Instead, the initial composition of a plausible parental silicate melt can have Pd and Pt contents well within the expected range of a normal, mantle-derived partial melt (i.e., similar to 10-20 ppb for both Pd and Pt with Pd/Pt of similar to 1). The relative differences in the partitioning behavior of Pt and Pd between sulfide liquid and silicate melt are unlikely to produce a consistent Pd/Pt ratio across a wide range of silicate melt to sulfide liquid mass ratios (i.e., R factors). Instead, the pre-emplacement fractionation of Pt alloy from S-undersaturated silicate magma accounts for the homogeneous and high Pd/Pt ratio of the J-M reef. We show that batch equilibration of sulfide liquid with silicate melt can produce the high Pd/Pt ratios of the reef if the partition coefficients between sulfide liquid and silicate melt for Pd and Pt are extremely high (>10(6)). In an alternative model, Pd enrichment could be achieved by sulfide upgrading in resident footwall mush even if the partition coefficients between sulfide liquid and silicate melt are relatively small (between 10(4) and 10(6)) because the instantaneous mass ratio of silicate melt to sulfide liquid is small (R approximate to 100-700), so the partitioning behavior of Pt and Pd has little impact on the composition of sulfide liquid.
Structural and paleogeographic controls on sediment-hosted Au deposits are investigated using the giant Zhaishang Au deposit in the west Qinling orogen as an example. Two distinct types of mineralization are identified. The first is fault-controlled mineralization within Devonian Fe-rich calcareous siltstone, deposited in a platform-edge environment. The second is stratabound-replacement mineralization within Permian carbonaceous mudstone. This mudstone, which contains up to 1.46% organic carbon, was deposited in a low-energy, deep-sea environment. Geophysical sections demonstrate that the two types of Au mineralization are bounded by a WNW-ESE-trending fault called F5, which extends to a depth of at least 1 km with secondary mineralized structures converging into it. New structural mapping reveals that deposit-scale, NW-SE-trending locked-up folds and thrust faults developed under north-northeast-south-southwest compression at ca. 279 to 275 Ma according to the sericite Ar-Ar dating. In situ U-Pb dating of Au-related apatite from mineralized veinlets indicates that the WNW-to E-trending reverse faulting, stratabound replacement, and Au mineralization took place at ca. 220 Ma. The fault-controlled Au in the Devonian beds comprises polymetallic sulfides associated with decarbonation with both native gold and invisible Au within pyrite. In contrast, the stratabound-replacement Au orebodies in the Permian beds comprise pyrite and arsenopyrite hosting invisible Au associated with quartz alteration. The contrasting mineralization styles can be attributed to variations in host rocks and their mechanical competency determined by distinct paleogeographic settings and different pathways for fluid-rock interaction.
Advanced argillic lithocaps, defined by zoned residual quartz with silicification, alunite, pyrophyllite, dickite, and kaolinite, are recognized above and/or alongside five major iron oxide-apatite (IOA) deposits in the iron belt of coastal northern Chile. The IOA deposits are centered on massive magnetite bodies formed along the transtensional Atacama fault system during Lower Cretaceous dioritic magmatism. Subsequent erosional degradation reduced the size of the lithocaps as well as removed those that are thought to have formerly overlain other magnetite deposits in the belt. The IOA lithocaps closely resemble those documented from the shallow parts of porphyry copper and subvolcanic tin systems and must have formed by condensation and absorption of large volumes of magmatic vapor into groundwater aquifers. The presence of these lithocaps has important genetic implications: They require involvement of magmatic vapor and not solely hypersaline brine in IOA genesis; imply formation of the massive magnetite bodies at paleodepths of less than approximately 4 km; and render it unlikely that IOA deposits transition upward to either iron oxide-copper-gold (IOCG) deposits or pegmatitic breccias and veins. There are also important exploration implications: IOA deposits elsewhere in the iron belt could be concealed beneath lithocaps, which themselves could contain elements of economic value.
The Miocene Moss low-sulfidation epithermal deposit is a significant precious metal producer within the Oatman district in northwest Arizona. Elevated precious metal grades occur in quartz-calcite veins and breccias containing dark-gray bands that host ore minerals, including native gold, acanthite, and silver sulfosalts. The ore minerals form dendritic aggregates hosted by a matrix of quartz that formed through maturation from a noncrystalline silica precursor. The transformation of the thermodynamically unstable, noncrystalline precursor to quartz has progressed to completion, resulting in the development of characteristic quartz textures. These textures include mosaic quartz characterized by interpenetrating grain boundaries and flamboyant quartz showing radiating arrays of inclusions. Prismatic quartz grains transected by remnant recrystallization fronts containing abundant inclusions are common. The ore-bearing bands occur adjacent to bands that contain bladed calcite. The textures of the calcite and the surrounding quartz are inconsistent with calcite having formed in open spaces. Similar to the ore mineral dendrites, the textural evidence suggests that the calcite blades grew in a gel-like silica matrix. This type of calcite is texturally distinct from lattice-bladed calcite, in which polyhedral cavities between the blades are filled with late drusy quartz that grew in open space. Microthermometric investigations suggest that calcite deposition occurred at similar to 265 degrees C approximately 600 m below the paleosurface. It is proposed that ore-mineral formation and calcite growth in the noncrystalline silica precursor took place during short-lived episodes of fluid flashing at far-from-equilibrium conditions. The amount of vapor generated during flashing presumably played a key control on mineral precipitation and growth.
The multimillion-ounce Fenelon gold deposit is a postvolcanic pre-Timiskaming deposit located in the northwestern part of the Neoarchean Abitibi greenstone belt, Canada. Two stages of epigenetic gold mineralization are recognized: an early stage (M1) characterized by quartz veins with native gold and molybdenite, and a later stage (M2) with sulfide-rich mineralization accompanied by native gold and Bi-Ag tellurides. Both occur within high-strain zones along lithological contacts and foliation planes. Multiple sulfur isotope analyses of M2 pyrrhotite, chalcopyrite, arsenopyrite, and pyrite yielded near-zero 033S values and positive 634S values (+2.0%c avg), contrasting with those for diagenetic pyrrhotite in the sedimentary host rocks (033S = +0.45%c avg, 634S = +0.6%c avg). Some sedimentary-rock-hosted mineralization exhibits elevated 033S values (up to +0.54%c) and lower 634S values (down to +0.90%c), consistent with local dissolution and reprecipitation of sedimentary sulfide. The sulfur isotope signature is that of fluids exsolved from crystallizing intrusions or metamorphic devolatilization of igneous rocks. A progressive increase in 634S values is observed throughout the paragenesis, including within concentrically zoned M2 pyrite, where 634S values increase from cores (+1.7%c avg) to rims (+3.6%c avg), accompanying inclusions of native gold and pyrrhotite. This trend is consistent with a progressive decrease in fluid fO2, likely due to interaction of the fluid with graphite in the sedimentary host rocks. These results show that the Fenelon deposit is either an intrusion-related deposit or, less likely, an early orogenic deposit formed from metamorphic fluids derived from igneous rocks. The results also underline the importance of fluid reduction by carbonaceous sedimentary rocks in gold mineralization.
The Archean Goldex deposit, located in the Val-d'Or gold camp in the southern Abitibi greenstone belt, Canada, comprises two contrasting styles of gold mineralization that result from two distinct ore-forming events that happened >= 25 m.y. apart. The first is an early (pre-2685 Ma), pre-D-3 event forming the volcanic-hosted South Zones vein system in the immediate structural footwall (south side) of the Goldex quartz diorite. The second is a late, ca. 2660 to 2640 Ma syn-D-3 event forming an orogenic vein system within the 2686.3 +/- 1.2 Ma quartz diorite. The South Zones consist of deformed smoky quartz veins and diffuse domains of intense silicification with variable amounts of disseminated pyrrhotite-pyrite +/- sphalerite-chalcopyrite native gold, bordered by a metamorphosed assemblage of pervasive biotite-quartz-pyrrhotite +/- amphibole-epidote alteration. Many characteristics of the South Zones, such as folding, boudinage, and dynamic recrystallization, indicate that they predate the main phase of regional deformation (D-3) shortening and peak metamorphism. In contrast, the bulk of the Goldex deposit consists of a network of steeply and shallowly S-dipping quartz-tourmaline-carbonate-pyrite vein stockworks and breccias with proximal albite-pyrite alteration. The South Zones deposit is part of a group of early structurally controlled gold deposits that are spatially associated with a WNW-trending structural corridor (referred to as the Marbenite-Norbenite corridor) that originated prior to D-3, which plausibly represents a significant D-1 to D-2 fault zone. The juxtaposition of these two temporally and geologically distinct gold deposits (i.e., pre-D-3 vs. syn-D-3) attests to the longevity of the Marbenite-Norbenite structural corridor and highlights the importance of early structures as gold metallotects and multiple mineralizing events in the formation of world-class gold camps in polydeformed terranes.
The Xiadian orogenic gold deposit, located within the giant Jiaodong gold province, is remarkable for hosting three orebody groups with distinct structural, alteration, and mineralization characteristics within the footwall of the Zhao-Ping fault zone. The No. 1 orebody group, containing 75% of the gold resources, is situated within the immediate footwall. The No. 2 and No. 3 orebody groups are located progressively farther from the main fault within the footwall, with correspondingly fewer gold resources. This deposit geometry provides a rare opportunity to investigate the critical factors controlling deposit architecture and mineralization processes. Integrated macroscopic geologic mapping and in situ analysis of pyrite textures, trace elements, and sulfur isotope ratios indicate that each orebody group formed under different structural-hydrodynamic regimes from a single fluid source. The No. 1 orebody group represents a fault core, where quasicontinuous fluid flow under low differential stress formed large tonnage and disseminated ores. The No. 2 orebody group, interpreted as a damage zone, experienced episodic pressure fluctuations and fluid immiscibility, leading to discontinuous gold mineralization and stockwork ores. In contrast, the No. 3 orebody group experienced highly localized, intense fluid pulses through extensional-shear fractures, resulting in higher gold grades but limited resources. This study highlights the importance of coupling macroscopic and microscopic parameters to understand the structural and hydrodynamic factors that control the spatial distribution of gold mineralization in and adjacent to fault zones. This research establishes a predictive model for gold exploration in similar structural settings.
The banded iron formation (BIF) of the Saksaganska Formation within the Kryvyi Rih belt in Ukraine hosts one of the largest Fe ore resources in Europe. The multistage genesis of Fe ores involves gas-bearing fluids of complex and unresolved origins. This study aims to identify sources of these gases and their generation mechanisms within evolving fluid systems, which were responsible for Fe ore formation, upgrading, and alteration, as this is crucial for refining ore genetic models. Trace element geochemistry and stable C-N isotopes of fluid inclusion gases are used to determine multiple generations of iron oxides and associated gangue minerals, reflecting mineral transformations that fostered the development of large-scale gas-bearing fluid systems during high-temperature metamorphic and metasomatic-hydrothermal reactions. The C-N isotope compositions of quartz-hosted fluid inclusion gases from various sites within the Kryvyi Rih belt record high carbon fluxes and progressive evolution of CO2( +/- N2-CH4)-bearing fluid systems during the late Paleoproterozoic. The dynamic large-scale fluid migration encompassing multiple fluid flow pulses played a pivotal role in the upgrading of Fe quartzites into high-grade hypogene Fe ores of the Saksagansk type. The delta 13C(CO2) and delta 15N values of fluid inclusion gases hosted in quartz within schists, ferruginous horizons, and shear zones suggest different carbon sources, including graphite-rich schists, metamorphic decarbonation reactions, devolatilization of greenstone belt sequences, metasomatic-hydrothermal decomposition of Fe carbonates, and igneous sources. Unique trace element compositions observed in specular hematite and magnetite indicate selective alteration by Nb-Ta-rare earth element + Y (REY)-bearing hydrothermal fluids likely originating from a deep alkaline magmatic source.
This study assesses the potential of chlorite, white mica, and garnet chemistry, coupled with hyperspectral data, as vectoring tools in metamorphosed volcanic-hosted massive sulfide (VHMS) deposits. Samples were collected from the King Zn deposit of the Eastern Goldfields superterrane, Yilgarn craton, which was metamorphosed to the amphibolite facies. In situ chlorite-white mica chemical trends match the shifting positions of the 2,200W and 2,250W absorption features obtained by short-wave infrared (SWIR) spectroscopy. These include a shift from muscovite to Na muscovite in the felsic footwall approaching mineralization, to phengite in the hanging wall, and mostly mixed Fe-Mg chlorite compositions with shifts to Mg-rich chlorite in the Mg-metasomatized felsic footwall. Fluorine (up to 2,500 ppm) and Mn (up to 1.1 wt %) contents of chlorite increase systematically through the footwall of the deposit toward the massive sulfide but drop sharply in the hanging wall-a trend mirrored in F contents in white mica (up to 2,500 ppm) and Mn in garnet (up to 17 wt %). These variations are attributed to premetamorphic footwall metasomatism and can trace hydrothermal up-flow zones in metamorphosed VHMS systems. Thermal infrared (TIR) signatures (11,100W and 11,300DW) correlate negatively with almandine and positively with spessartine contents in garnet, while rare earth element (REE) profiles of garnet further aid in the assessment of VHMS prospectivity. Positive Eu anomalies and downward-dipping heavy rare earth element (HREE) profiles in garnet enriched in the almandine end member distinguish mineralization-related garnet from those of barren metamorphic or igneous origins. Several elements, including Sr-Ba-Rb contents in white mica, Al in chlorite, and Ca-Mn with positive Eu anomalies in garnet, also differentiate footwall from least altered hanging-wall units. This study highlights the effectiveness of integrating chlorite, white mica, and garnet chemistry with hyperspectral data for regional VHMS exploration in high-grade metamorphic terranes.
The central-southern Youjiang basin in southeastern Yunnan hosts numerous Carlin-type gold deposits, such as the pyroclastic-hosted Bieli and the siliciclastic-hosted Bielidong deposits. In both deposits, gold occurs as lattice-bound Au+ in sulfide minerals of different paragenetic stages: sedimentary framboidal pyrite (Py-S), diagenetic dissolution-reprecipitation porous pyrite (Py-DR), hydrothermal early ore pyrite (Py1), main ore pyrite/arsenopyrite (Py2/Apy2), and late ore pyrite (Py3). The textures show there were multiple pulses of fluid flow related to the formation of the deposits. Sedimentary and diagenetic pyrite has negligible Au (<1 ppm) and As (<0.5 wt %) contents and extremely negative delta S-34 (-49.7 to -21.4 parts per thousand) values, which differ markedly from early ore Py1 (Au = 0.05-4.20 ppm; As = 0.32-4.16 wt %; delta S-34 = -3.54 to 10.5 parts per thousand), main ore sulfide minerals (Py2 and Apy2; mean Au = 9.28 and 39.2 ppm, mean As = 3.16 and 44.6 wt %, mean delta S-34 = 4.68 and 12.1 parts per thousand), and late-stage Py3 (Au = 2.27-27.4 ppm; As = 7.80-9.49 wt %; delta S-34 = 8.62-13.5 parts per thousand). The main ore sulfide minerals display positive delta S-34 values (4.68-12.1 parts per thousand) and pronounced mass-independent sulfur isotope signatures (Delta S-33 = -2.80 to +8.16 parts per thousand). The chemical composition and the delta S-34 and Delta S-33 values of ore sulfides indicate that the fluids were derived from a sedimentary source, most probably the Precambrian metamorphic rocks beneath the Phanerozoic basin. Both deposits formed by the same process, and the fluids came from the same source. Differences between the deposits are largely related to the local host rocks. The mobilization of the fluids seems to be the far-field effect of tectonic processes at the margins of the South China block.
Alteration halos in wall rocks are potential indicators of rare element pegmatites; however, halos associated with niobium-yttrium-fluorine (NYF) pegmatites are poorly documented. It is also unclear whether such halos can survive subsequent orogenic overprinting. In this contribution, we examine the wall rocks of Paleoproterozoic rare element pegmatites that underwent amphibolite facies metamorphism during the Scandinavian Caledonian orogeny. These pegmatites represent the largest known intraplutonic NYF-type bodies globally. Radiometric and geochemical profiles of the surrounding granitic gneiss wall rocks reveal similar to 2- to 15-m-wide dispersion halos marked by radiometric anomalies and enrichment in rare and radioactive elements, including Cs, Li, F, Sn, Ta, U, Th, Nb, Rb, and Tl. These elements are carried by Caledonian metamorphic minerals but originated from the interaction between Paleoproterozoic pegmatite-derived fluids and the granitic protolith. Remarkably, the Caledonian profiles preserve the geochemical signatures of the original Paleoproterozoic pegmatite halos. Our findings demonstrate that the geochemistry of alteration halos of rare element pegmatites can persist through high-grade metamorphic overprinting. These halos can be traced using radiometry or bulk and mineral geochemistry, offering a promising vectoring tool for exploring ancient NYF-type pegmatites in younger orogenic belts.
