Mineralium Deposita最新文献

The Lindero deposit is located in the Puna plateau, northwest Argentina, at the southern end of the Central Volcanic Zone of the Central Andes. The high-K calc-alkaline dioritic composition of the subvolcanic intrusions, the shallow emplacement depth (< 1.5 km), and the gold-rich and copper-depleted mineralization style suggest that the Lindero deposit is a porphyry gold deposit. Porphyry gold deposits are scarce worldwide and the factors controlling their formation are still poorly known. Here we present a detailed study of fluid inclusions in order to characterize the mineralizing fluids that precipitated the Au mineralization at Lindero. Different types of fluid inclusions in quartz veins (A-type and banded quartz), which are associated with the K-silicate alteration, were analyzed using Raman spectroscopy, microthermometry, and LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry). Four inclusion types can be recognized in quartz veins: (i) Salt melt inclusions, which are characterized by a dense packing of daughter minerals (mainly Fe-chloride, sylvite, halite, anhydrite, and hematite), by a distorted vapor bubble, and by the lack of liquid phase; (ii) Halite-bearing inclusions which contain liquid, vapor, and halite; (iii) Two-phase aqueous inclusions that contain liquid and vapor; (iv) Vapor-rich inclusions containing only vapor. The inclusion types are related to different stages of hydrothermal evolution. Stage 1 is the main mineralization stage, characterized by vapor-rich inclusions coexisting with salt melt inclusions. Salt melt inclusions commonly show total homogenization temperature (Th_L) > 1000 °C. This Na-K-Fe-Cl-rich highly saline brine (~ 90 wt% NaCl eq.) was of magmatic origin and responsible for the Au mineralization. Two later stages involving cooler fluids (Th_L < 300 °C) and gradually lower salinities (from 36.1 to 0.2 wt% NaCl eq.) trapped by halite-bearing and two-phase aqueous inclusions during stages 2 and 3, respectively, correspond to a late magmatic-hydrothermal system, that is probably related to a deep supercritical fluid exsolution. Salt melt inclusions represent the most likely parental fluid of K-silicate alteration and associated Au mineralization at Lindero. This uncommon type of fluid must have played an important role in Au transport and precipitation in shallow porphyry gold deposits.

Tin (Sn) and tungsten (W) behave incompatibly in reduced magmatic systems and may become enriched in late highly-evolved melts. Nonetheless, Sn and W rarely concentrate in the same deposit. In deposits formed by Sn- and W-bearing granites, this separation may be due to the contrasting behavior of Sn and W during exsolution of a magmatic fluid or the scavenging of Sn by silicate minerals. We illustrate the separation of Sn and W for the world-class Zhuxi W skarn deposit (South China). Although tin orebodies have not yet been identified within the Zhuxi deposit, tiny (commonly < 20 μm) cassiterite grains are widespread within the endoskarn and the retrogressed exoskarn. We analyzed the W and Sn contents of the magmatic minerals biotite and ilmenite in ore-forming granites and the prograde anhydrous skarn minerals garnet, pyroxene and vesuvianite. Our data show that (i) magmatic ilmenite (65.5–79.1 ppm Sn; 8.7–14.3 ppm W) and biotite (109–120 ppm Sn; 1.3–6.3 ppm W) from biotite monzogranite strongly enrich Sn relative to W, implying that W partitioned more strongly into the magmatic fluids than Sn, (ii) there is 100 Kt non-recoverable Sn within the Zhuxi deposit in addition to the certified 3.44 Mt WO₃ reserves, and (iii) W is mainly hosted in scheelite, whereas Sn is dominantly sequestered in prograde skarn minerals, most importantly garnet (76–4086 ppm Sn, < 42 ppm W), pyroxene (3–103 ppm Sn, < 1 ppm W), and vesuvianite (43–361 ppm Sn, < 2 ppm W). The formation of secondary cassiterite requires the release of silicate-bound Sn by alteration of primary skarn minerals, which depends on the availability of magmatic or metamorphic fluids. Deep-seated granites such as those associated with the Zhuxi skarn deposit, which crystallized at 5 km to 12.6 km depth, do not release or mobilize copious amounts of fluid. Therefore, the Zhuxi deposit, like other deep-seated reduced skarn systems shows little alteration and most Sn remains in silicate minerals and is economically non-recoverable. Thus, reduced, deep-seated W skarn systems are unlikely to have associated Sn orebodies even if significant amounts of Sn are present.

Porphyry-epithermal veins hosting Re-rich molybdenite and rheniite (ReS₂) from the Maronia Cu-Mo ± Re ± Au porphyry in Thrace, NE Greece, provide new insights into the hydrothermal processes causing extreme Re enrichment. Quartz trace element chemistry (Al/Ti, Ge/Ti), Ti-in-quartz thermometry, and cathodoluminescence imaging reveal multiple quartz generations in consecutive hydrothermal quartz-sulfide veins associated with potassic, sericitic, and argillic alteration. Fluid inclusions in different quartz generations indicate that phase separation and fluid cooling are the main ore-forming processes in the porphyry stage (~ 500 – 350 °C), whereas mixing of a vapor-rich fluid with metalliferous (e.g., Pb, Zn, Au) meteoric water forms the epithermal veins (~ 280 °C). These processes are recorded by trace element ratios in pyrite that are sensitive to changes in fluid temperature (Se/Te), fluid salinity (As/Sb, Co/As), and mixing between fluids of magmatic and meteoric origin (Se/Ge). Highly variable intra-grain δ³⁴S values in pyrite record S isotope fractionation during SO₂ disproportionation and phase separation, emphasizing the importance of in situ δ³⁴S analysis to unravel ore-forming processes. High δ³⁴S (~ 4.5‰) values of sulfides are indicative of low SO₄²⁻/H₂S fluid ratios buffered by the local host rocks and mixing of the magma-derived fluid with meteoric water. The formation of Re-rich molybdenite (~ 6600 ppm) is favored by cooling and reduction of a magma-derived, high-temperature (~400 °C), oxidized, and Re-rich fluid triggering efficient Re precipitation in early veins in the potassic alteration zone. The systematic temporal fluid evolution therefore reveals that coeval cooling and reduction of oxidized Re-rich fluids cause extreme Re enrichment at the Maronia porphyry system.

The Bushveld Complex in South Africa hosts the lion’s share of the world’s noble metal resources in platinum reefs – thin layers of silicate/chromite rocks containing platinum-rich sulphides. The reefs are widely attributed to multiple replenishments by ore-forming magmas that have been entering the evolving Bushveld chamber through numerous feeder conduits. The replenishment events are marked by regional and local disconformities/unconformities, significant isotopic shifts, and notable reversals in the whole-rock and mineral compositions. Surprisingly, however, so far no single feeder conduit for platinum reefs has been found despite extensive surface and underground mining for over a century. Feeder conduits appear entirely absent from the Bushveld Complex. This paradox has long been known but has never been specifically addressed. Here, we suggest that the absence of feeder channels is a natural consequence of the magma chamber replenishment through a cumulate pile. The fossilization of the feeder channels in the cumulate pile is likely impeded by two principal factors: (a) a cumulate pile is too hot to enable efficient cooling and crystallization of magma flowing through the channels, and (b) the channels are closed by an adjacent elastically deformable pile immediately after cessation of the magma emplacement. The feeding dykes are thus absent because there is little chance for the conduits to get preserved in a hot and deformable cumulate pile of layered intrusions.

The copper-nickel(-platinum-group element) sulfide resources of the Duluth Complex, Minnesota, USA, formed by assimilation of sulfur from the Virginia Formation black shale. In the normal black shale of the Virginia Formation, sulfur is mainly hosted in disseminated pyrite, whereas mm-scale pyrrhotite laminae dominate in the sulfur-rich Bedded Pyrrhotite Unit. The Bedded Pyrrhotite Unit was the main supply of sulfur in some of the magmatic sulfide deposits but its origin has not been studied in detail. Using Raman spectroscopy, we show that the carbonaceous material within the regionally metamorphosed normal black shale is graphitized biogenic material. The Bedded Pyrrhotite Unit contains pyrobitumen that represents residues of oil that accumulated to porous horizons, which formed due to dissolution of precursor sedimentary clasts. Replacement of the clasts by quartz and sulfides facilitated the formation of the pyrrhotite laminae of the Bedded Pyrrhotite Unit, which likely occurred during regional metamorphism.

The pyrite-bearing normal black shale experienced loss of H₂O, C_org, and sulfur during devolatilization caused by the Duluth Complex. The contact-metamorphosed Bedded Pyrrhotite Unit shows no systematic depletion of volatiles and is the most C_org and sulfur-rich part of the Virginia Formation. During devolatilization, sulfur was preserved because unlike pyrite, pyrrhotite was stable. Consequently, magmatic assimilation of sulfur from the Bedded Pyrrhotite Unit required partial melting. Retrograde hydration introduced H₂O, and possibly C_org, and sulfur, to the contact-metamorphosed Bedded Pyrrhotite Unit, which further affected the volatile budget. Our findings highlight why constraining diagenetic and regional metamorphic processes is important to understand magma-sediment interaction processes.

The S saturation and oxidation states of arc magmas are important factors in the formation of porphyry Cu–Au deposits. The Milin juvenile lower crustal cumulates (86.7–84.3 Ma) in the Gangdese provide insights into how sulfide saturation and oxidation states control porphyry mineralization. Zircons from the cumulates have low Ce⁴⁺/Ce³⁺ ratios (21–90) and reduced oxygen fugacities (ΔFMQ–1.8±0.5), which cannot be explained by fractional crystallization or crustal contamination, suggesting inheritance from a mantle source. Partial melting of the mantle under reduced conditions produced a sulfide-saturated primary arc magma with low chalcophile element contents owing to the residual sulfide in the mantle. The Milin lower crustal cumulates contain sulfides, indicating that the magma reached sulfide saturation in the early stages of magmatic differentiation. Based on our model, the primary arc magma before sulfide saturation contained 66.7 ppm Cu and 1.0 ppb Au. The residual magma after sulfide saturation in the lower crust contained 33–66 ppm Cu, 0.13–0.93 ppb Au; i.e., lower contents than those in arc basalts worldwide. Both these factors hindered the formation of Late Cretaceous large porphyry Cu–Au deposits in the Gangdese belt. Remelting of the Milin sulfide-rich cumulates can generate a Cu-rich andesitic magma only under high temperature and high-fO₂ conditions, and a melt with low Cu content under low temperature even high-fO₂ conditions. Thus, the temperature plays a crucial role in the remelting of the lower crust whether provide enough metals to match the Gangdese Miocene post-collisional porphyry Cu deposit.

The Lautenthal sphalerite-galena vein deposit is part of the world-class Upper Harz Pb-Zn-Ag district in the Harz uplift block of the Paleozoic Variscan fold belt in Germany. Its sphalerite-dominated mineral association was studied using bulk-ore chemistry, electron probe microanalysis, and laser ablation-ICP-mass spectrometry. Gallium and locally In are the main high-tech-relevant trace elements hosted by sphalerite, with up to 150 ppm Ga and up to 380 ppm In in hand-picked sphalerite samples (mean In/Zn, 0.70 × 10⁻³). Ore concentrates (≤ 50 kg) contain up to 65 ppm Ga and up to 109 ppm In (mean In/Zn, 0.36 × 10⁻³). Accessory Fe-Co-rich gersdorffite-1 occurs in the earlier quartz-sulfide ore stage and Sb-rich gersdorffite-2 in the later carbonate-sulfide stage. Enrichment patterns of In are either defined by overprinting textures in the Fe-richer sphalerite-1 of the earlier stage, or relate to primary growth zoning in Fe-poor sphalerite-2 of the later stage. Using the sphalerite geothermometer GGIMFis, formation temperatures (median) of sphalerite-1 were estimated at ~ 230 °C for the Lautenthal orebody and at ~ 175 °C for the Bromberg orebody, which may indicate lateral T-zonation for the earlier ore stage. Sphalerite-2 data indicate formation temperatures of ~ 185 °C (median). Copper-bearing brines of the carbonate-sulfide stage with assumed temperatures of ~ 250 °C initiated replacement of In-poor sphalerite-1 by chalcopyrite and remobilization of Zn and trace elements. Indium-rich sphalerite-2 occurs associated with calcite and fine-grained galena. A direct spatial or temporal link of ore formation to a magmatic-hydrothermal system is unlikely, which contrasts to In-rich epithermal and tin-polymetallic vein deposits worldwide.

The Bohemian Massif hosts significant hydrothermal U-deposits associated with shear zones in the high-grade metamorphic basement. But there is a lack of evidence of a genetic link between mineralization and U-fertile igneous rocks. This contribution provides constraints on the major U source of the vein-type U-deposits, the timing of ore formation and the metallogenetic model. The anomalous trace element signatures of the low-temperature hydrothermal deposits (high Zr, Y, Nb, Ti, ∑REE) and their close spatial relation with ultrapotassic rocks of the durbachite series point to a HFSE and REE enriched source rock. The durbachites have high U content (13.4–21.5 ppm) mainly stored in magmatic uraninite and other refractory minerals (e.g., thorite, zircon, allanite) that became metamict over a time interval sufficient to release U from their crystal structure, as suggested by the time gap between emplacement of the durbachites (EMP uraninite U–Pb age ~ 338 Ma) and hydrothermal activity (SIMS uranium ore U–Pb age ~ 270 Ma). Airborne radiometric data show highly variable Th/U ratios (1.5–6.0), likely reflecting a combination between (1) crystallization of magmatic uraninite, (2) hydrothermal alteration, and (3) leaching and mobilization of U along NW–SE-trending fault zones, manifested by elevated Th/U values in the radiometric map. The presence of rare magmatic uraninite in durbachites suggests almost complete uraninite dissolution; EMP imaging coupled with LA-ICP-MS analyses of refractory accessory phases revealed extensive mobilization of U together with HFSE and REE, providing direct evidence for metal leaching via fluid-driven alteration of radiation-damaged U-rich minerals. The large-scale HFSE and REE mobilization, demonstrated by the unusual trace element signatures of the U-deposits, was likely caused by low-temperature (270–300 °C), highly alkaline aqueous solutions containing F-, P-, and K-dominated complexing ligands. The first SIMS U–Pb age of 270.8 ± 7.5 Ma obtained so far for U-mineralization from the Bohemian Massif revealed a main Permian U mineralizing event, related to crustal extension, exhumation of the crystalline basement, and basin formation, as recorded by U–Pb apatite dates (280–290 Ma) and AFT thermal history models of the durbachites. The Permo-Carboniferous sedimentary cover probably represented a source of oxidized basinal brines infiltrating the basement-hosted durbachite plutons and triggering massive metal leaching. The interaction between basin-derived brines and durbachites resulted in significant modification of the chemical composition of the hydrothermal system (K and F release during biotite chloritization, P liberation through monazite alteration), leading to the formation of ore-bearing fluids responsible for the metallogenesis of the basement-hosted unconformity-related U-deposits in shear zones in the Bohemian Massif.

The Punta del Cobre district near Copiapó is a center of iron oxide-copper–gold (IOCG) mineralization spatially and temporally associated with regional sodic-calcic hydrothermal alteration, the Atacama fault system (AFS), and two phases of Early Cretaceous magmatism. Here, we investigate the spatiotemporal and geochemical relationships between magmatism, ductile deformation, and hydrothermal alteration along the ~ 200 to 300-m-thick steeply NW-dipping Sierra Chicharra shear zone, interpreted to be the major strand of the AFS. Mylonitic fabrics and oblique sinistral-reverse kinematic indicators together record coaxial flattening in a transpressional regime. Deformation on the AFS took place before, during, and after intrusion of the synkinematic Sierra Chicharra quartz diorite of the Coastal Cordillera arc at ~ 122 Ma and terminated before intrusion of the unstrained ~ 114 Ma Sierra Atacama diorite of the Copiapó batholith. Geochemical data show that the Copiapó batholith was more mafic and more K-rich than the calc-alkaline Coastal Cordillera arc. This time period thus overlaps IOCG mineralization in the Punta del Cobre district (~ 120 to 110 Ma). Multiple phases of sodic-calcic alteration in and around the AFS shear zone are recognized. Textures of altered rock in the shear zone show both synkinematic assemblages and post-kinematic hydrothermal oligoclase. A ~ 775-m-long andradite vein that cuts the shear zone formed broadly at the end of magmatism in the district (~ 95 Ma). Oxygen isotope ratios from the vein indicate that hydrothermal fluids were likely magmatically derived. Together, this work shows the AFS-related shear zone and nearby IOCG mineralization developed in a regional transpressional regime produced by SE-directed oblique convergence across a NE-striking shear zone. IOCG-related magmatic-hydrothermal fluids exploited this transcrustal shear zone to produce multiple episodes of regional sodic-calcic alteration formed from fluids exsolved from magmas or driven by the heat of the Coastal Cordillera arc and Copiapó batholith.

Determining the association of Pb–Zn(-Ag) mineralization with granite is crucial for understanding metallogeny and identifying exploration targets. The genesis of Pb–Zn-Ag-Sb deposits and their genetic association with Sb(-Au) deposits and granite-associated Sn-W deposits in the Tethys Himalaya of southern Tibet, China, remains controversial. Our comprehensive study of in situ element compositions and sulfur isotopes of sulfides, together with in situ quartz oxygen isotopes for the Zhaxikang Pb–Zn-Ag-Sb deposit, sheds light on this issue. LA-ICP-MS analyses of early sulfides in manganosiderite veins, coupled with C-O isotopes of manganosiderite, indicate that the early fluids were enriched in Pb, Zn, Ag, Sb, Sn, and Cu, originating from magmatic fluids mixing with meteoric water. The early formed sulfides underwent fluid-mediated remobilization and dissolution, releasing many metallic elements (e.g., Pb, Zn, and Ag) into later As-Sb-rich fluids. These elements reprecipitated as Fe-poor sphalerite, As-rich pyrite, and abundant Sb-Pb sulfosalts with minor Ag-bearing minerals. Oxygen isotopes of quartz indicate that the later fluids were derived from pulsed releases of magmatic fluids mixing with meteoric water. In situ sulfur isotopes of three generations of pyrite indicate that early Pb–Zn(-Ag) sulfide precipitation was linked to magmatic sulfur, whereas precipitation of the later sulfosalts and stibnite involved external sulfur with relatively lower sulfur isotopes compared with early mineralization. We argue that Pb–Zn-Ag-Sb deposits in the Tethys Himalaya resulted from two distinct mineralization pulses. The early Pb–Zn(-Ag) mineralization was associated with crustal magmatic rocks (e.g., leucogranite), followed by the overprinting of later Sb-rich magmatic fluids. Notably, the later magmatic fluids responsible for Zhaxikang Pb–Zn-Ag-Sb mineralization were also associated with the regional Sb(-Au) deposits in the Tethys Himalaya.