Mineralium Deposita最新文献

High-grade (> 10 g/t) gold mineralization in orogenic gold deposits is of significant economic importance. Understanding the formation of such enriched ore zones is critical for gold exploration success. The world-class Jundee-Bogada gold camp in the Yilgarn Craton of Western Australia comprises both high-grade (avg. > 10 g/t, Jundee deposit) and low-grade (avg. < 3 g/t, Bogada prospect) lodes, despite shared host stratigraphy. The paragenetic framework established for the Jundee gold deposit suggests that the overall gold endowment developed over three deformation events. An early episode of low-grade gold mineralization is associated with colloform-crustiform veins that formed during extensional deformation (D_JB2A). A switch to transtensional deformation (D_JB2B) resulted in brecciation of the colloform-crustiform veins and coeval deposition of native gold. Late reverse faults record evidence for a third mineralization stage resulting from a NE-SW-directed shortening (D_JB3). Mineralization during this late stage was dominantly low-grade, with local occurrences of ultra-high-grade ore zones (> 100 g/t). Each event records transient changes in fluid chemistry during continued hydrothermal activity that spanned local deformation histories. We argue that at the Jundee gold deposit, protracted gold enrichment during three polyphased mineralization episodes resulted in the formation of high-grade gold ores. Whereas the complete metallogenic history is recorded at the Jundee deposit, gold within the Bogada prospect was introduced solely during the late contractional stage (D_JB3), resulting in a bulk low-grade endowment. We hypothesize that gold enrichment in high-grade orogenic gold deposits is a direct consequence of the spatial superimposition of protracted ore-forming events.

Phanerozoic orogenic gold deposits worldwide are commonly considered to be formed from metamorphic devolatilization of marine carbonaceous sedimentary rocks. Here we show that the Yindongpo gold deposit from the Qinling orogen (central China) is genetically associated with the metamorphism of volcanic rocks during the late Paleozoic orogeny, which involved the closure of the Shangdan ocean. Gold mineralization at Yindongpo is hosted in lower Paleozoic metavolcanic-sedimentary sequences and occurs mainly as lenticular to stratiform ore bodies that formed in three paragenetic stages represented by quartz-ankerite-pyrite (stage I), quartz-carbonate-sulfide (stage II) and quartz-calcite assemblages (stage III), respectively. Rutile grains coexisting with auriferous pyrite from stage II yield U–Pb ages of 395 ± 9 to 400 ± 13 Ma (2σ). Fluid inclusions in quartz of stages I and II are dominated by CO₂-rich (~ 10 mol%) aqueous fluids with low salinities (< 4.9 wt% NaCl equivalent) and total homogenization temperatures ranging from 241 to 352 ºC, whereas the values for H₂O-rich inclusions of stage III are 0.2 to 2.6 wt% NaCl equivalent and 151 to 164 °C. Based on secondary ion mass spectrometry analysis of oxygen isotopes of quartz (Qz-1 to Qz-4), the calculated δ¹⁸O_fluid values for the quartz-forming fluids are 1.3 to 7.0‰ in stage I, –3.1 to 6.6‰ in stage II, and –9.6 to –3.7‰ in stage III. These data indicate a metamorphic origin of ore fluids that underwent Rayleigh fractionation and incursion of meteoric water. The large variation in ⁴⁰Ar^*/⁴He ratios (1.7–30.0), caused by accumulation of radiogenic Ar^* and He loss within some pyrite samples, can be ascribed to regional metamorphism and deformation. Ore sulfides have sulfur (δ³⁴S_V-CDT = –2.1 to 3.3‰) and lead (²⁰⁶Pb/²⁰⁴Pb = 17.008–17.152, ²⁰⁷Pb/²⁰⁴Pb = 15.402–15.493, and ²⁰⁸Pb/²⁰⁴Pb = 38.254–38.564) isotopic compositions that are consistent with those of pyrite in the metavolcanic host rocks. Results presented here suggest that the ore fluids and, by inference, gold of the Yindongpo deposit were derived primarily from the volcanic sequences during regional metamorphism and deformation in response to the Early Devonian Qinling collisional orogeny. The Yindongpo deposit represents the first recognized Paleozoic orogenic gold deposit in the Qinling orogen, and thus has important implications for regional metallogeny and gold exploration.

Giant mafic-ultramafic layered intrusions of Archaean-Proterozoic age are the fossilised remnants of huge injections of silicate magma in the Earth’s crust and are our most important repositories of platinum-group elements. Magmatic PGE-rich ore deposits, such as the Merensky Reef, are typically hosted in stratiform reefs at the contacts between ultramafic and feldspathic cumulates. The Merensky Reef is commonly characterised by coarse-grained and pegmatoidal textures that may provide important clues to its origin. We present textural and in situ geochemical data for Merensky pegmatoids at Styldrift Mine (Impala Bafokeng) in the Western Bushveld Complex of South Africa. This region is adjacent to an inferred magmatic feeder zone to the Bushveld. The Merensky pegmatoids are characterised by (i) amoeboid olivine inclusions in zoned orthopyroxene megacrysts with increasing molar Mg# of orthopyroxene towards olivine, (ii) fine-grained chains of orthopyroxene in compositional equilibrium with adjacent orthopyroxene megacrysts, (iii) increasing molar Mg# of orthopyroxene megacrysts and increasing molar An with decreasing ⁸⁷Sr/⁸⁶Sr_i (at 2.06 Ga) of plagioclase oikocrysts in pegmatoids laterally across a 10-km section distal to the feeder, and (iv) highly variable molar An and initial ⁸⁷Sr/⁸⁶Sr_i of interstitial plagioclase proximal to the feeder. We interpret the coarse-grained and pegmatoidal textures, their dissolution-reprecipitation features, and lateral chemical variations as the product of lateral melt infiltration and mixing in a crystal mush. We suggest that the platiniferous Merensky Reef was not formed at the base of a large melt-filled magma chamber but was instead the product of non-sequential magma emplacement that rejuvenated the crystal mush.

New inorganic and organic geochemical data from thucholite in the Upper Permian (Wuchiapingian) Kupferschiefer (T1) shale collected at the Polkowice-Sieroszowice Cu-Ag mine in Poland are presented. Thucholite, which forms spherical or granular clusters, appears scattered in the T1 dolomitic shale at the oxic-anoxic boundary occurring within the same shale member. The composition of thucholite concretions and the T1 shale differs by a higher content of U- and REE-enriched mineral phases within the thucholite concretions compared to the T1 shale, suggesting a different mineralising history. The differences also comprise higher N_tot, C_tot, H_tot, S_tot contents and higher C/N, C/S ratios in thucholite than in the T1 shale. The hydrocarbon composition of the thucholite and the surrounding T1 shale also varies. Both are dominated by polycyclic aromatic compounds and their phenyl derivatives. However, higher abundances of unsubstituted polycyclic aromatic hydrocarbons in the thucholite are indicative of its pyrogenic origin. Pyrolytic compounds such as benz[a]anthracene or benzo[a]pyrene are more typical of the thucholite than the T1 shale. Microscopic observations of the thucholite and its molecular composition suggest that it represents well-rounded small charcoal fragments. These charcoals were formed during low-temperature combustion, as confirmed by semifusinite reflectance values, indicating surface fire temperatures of about 400 °C, and the absence of the high-temperature pyrogenic polycyclic aromatic hydrocarbons. Charred detrital particles, likely the main source of insoluble organic matter in the thucholite, migrated to the sedimentary basin in the form of spherical carbonaceous particulates, which adsorbed uranium and REE in particular, which would further explain their different contents and sorption properties in the depositional environment. Finally, the difference in mineral content between thucholite and the T1 shale could also have been caused by microbes, which might have formed biofilms on mineral particles, and caused a change in the original mineral composition.

The Xianghualing large tin-polymetallic skarn deposit is located in the Nanling W-Sn metallogenic belt, South China, showing distinct spatial zoning of mineralization. From the contact between granite and carbonate rocks, the mineralization transitions from proximal skarn Sn ore to cassiterite-sulfide ore and more distal Pb–Zn-sulfide ore. This study reveals the fluid evolution and genetic links among these different ore types. The physical and chemical characteristics of fluid inclusions from each ore types indicate that the skarn Sn ore, cassiterite-sulfide ore, and Pb–Zn-sulfide ore all originated from the identical magmatic fluid exsolved from the Laiziling granite. Their formation, however, is controlled by diverse fluid evolutionary processes and host rock characteristics. The Sn–Pb-Zn-rich fluids were primarily derived from cooled and diluted magmatic brine, which is generated by boiling of initial single phase magmatic fluid. Mixing of magmatic brine with meteoric water is crucial to form skarn Sn ore. Redox reactions of aqueous Sn (II) complexes with As (III) species and/or minor CO₂ during short cooling period of ore-forming fluid is likely an effective mechanism to form high-grade cassiterite-sulfide ores, accompanied by favorable pH conditions maintained through interaction with carbonate host rocks. The later stage addition of meteoric water prompts the formation of Pb–Zn-sulfide ore. Comparing these findings with the characteristics of initial or pre-ore magmatic fluids in both mineralized and barren granitic systems indicates that high Sn content in the pre-ore fluids and the suitable fractional crystallization degree of the parent magma may determine high Sn mineralization potential in granitic magmatic-hydrothermal systems.

Post-collisional porphyry Cu deposits are genetically related to the magmas generated by partial melting of sulfide-bearing lithosphere fertilized by subduction components. The ore-forming magmas are suggested to be enriched in chalcophile elements compared to the barren magmas. However, the chalcophile element contents in the post-collisional magmas and its role in controlling the porphyry ore formation remain unclear. Platinum-group element (PGE) geochemistry has been used as a proxy for Cu and Au. In this study, we report PGE concentrations of representative post-collisional ore-associated and barren suites in the eastern Tethyan metallogenic domain. The ore-associated suites have moderate Pd and Pt contents ranging from ~ 0.05 to 0.5 ppb, which are comparable to those associated with giant porphyry systems in continental arc settings. In contrast, most of the barren suites have systematically lower Pd and Pt concentrations below ~ 0.1 and 0.05 ppb, respectively. Numerical models show that the ore-forming magmas, derived from partial melting of subduction-modified lithospheric mantle, have precipitated a small amount of sulfide phases during magma differentiation, leading to the moderate depletion of Pd and Pt in the ore-associated suites. Although the sulfide segregation has depleted highly chalcophile element contents, the ore-forming magmas contain sufficient Cu to form porphyry Cu deposits. This contrasts with the barren suites, which mainly originated from partial melting of the lower crust and contain about five times lower Cu contents, unfavorable for porphyry Cu mineralization. We suggest that moderate chalcophile element contents in the ore-associated magmas have increased the porphyry ore-forming potential in the eastern Tethyan domain.

The stratiform and vein-hosted Kapunda Cu deposit in South Australia contains a saprolitized hydrothermal vein with 12.37 wt.% total rare earth oxide (TREO). The vein was analyzed by X-ray diffraction, scanning electron microscopy, synchrotron-based X-ray fluorescence microscopy and electron backscatter diffraction to understand the controls that govern high-grade REE accumulation during periods of intense weathering. Petrological assessments indicate the transformation of an apatite-calcite-aluminosilicate-bearing protolith to a supergene assemblage of Fe-oxides, kaolinite and REE-phosphate minerals that include rhabdophane-(Ce), monazite-(Ce) and florencite-(Ce). This transformation was facilitated by progressive acidification of the weathering fluid, which is indicated by: 1) the increasing crystallinity of authigenic Fe-oxides and kaolinite, which led to REE desorption; 2) the textural evolution and increase in grain size of authigenic REE-phosphates from nanoscopic crystallites, to acicular needles, to micro-scale hexagonal prisms; 3) the late dissolution of REE-phosphates; and 4) the replacement of goethite by jarosite, whose sulfate component originated from the oxidation and weathering of proximal sulfide minerals. Alongside the depletion of pH-buffering carbonate minerals that are indicated by the preservation of calcite menisci, this sulfide dissolution also facilitated acid generation. Results illustrate how highly acidic weathering fluids might facilitate either REE mobilization or REE accumulation in regolith. High-grade REE accumulation under acidic supergene conditions is prioritized when the host-rock contains a significant source of depositional ligands (i.e., phosphate in the form of apatite) that can be readily leached during intense weathering. Exploration companies should therefore assay routinely for REEs in any heavily weathered phosphatic rock, due to the observed efficiency of phosphate minerals as geochemical traps for REE accumulation.

Feldspathic lherzolite and harzburgite are reported here for the first time in the southern Lac des Iles Complex; an ~ 2.69-Ga arcuate mafic intrusion that hosts world-class Pd mineralization within varitextured and brecciated gabbronoritic rocks. The olivine-bearing rocks (Mg#_75.9–80.8) are medium- to coarse-grained, weakly to strongly serpentinized, and bordered by variably altered norite. They possess relatively high Al₂O₃ contents (4.8–10.3 wt.%), pronounced negative Nb/Nb* (0.07–0.25) values, flat to shallow negatively sloping REE profiles (La/Yb_N 1.3–4.4), and variable Eu/Eu* (0.4–1.6) values. Weakly altered samples comprise subhedral olivine (Fo_78.6–81.8) with polymineralic melt inclusions and peritectic orthopyroxene rims, cumulus orthopyroxene, sub-poikilitic clinopyroxene, as well as plastically deformed and clustered plagioclase crystals. With increasing degrees of alteration, olivine is variably serpentinized or pseudomorphically replaced by an assemblage of talc, carbonate, magnetite, and Fe-sulfides. Sparsely disseminated pentlandite-chalcopyrite-pyrrhotite (± sphalerite) blebs with platinum-group minerals (zvyagintsevite, kotulskite, and sperrylite) are rare and commonly partially replaced by magnetite. Nickel concentrations are primarily controlled by olivine (1900–4200 ppm Ni), as supported by a positive correlation between whole-rock MgO and Ni contents. Sulfur, Cu, Pd, and Pt show positive correlations and Pd/Pt values range from 2.6 to 6.7. The whole-rock and mineral compositions can be replicated through the modeling of batch crystallization of a hydrous andesitic magma that has interacted with antecedent feldspathic cumulates. The parent magma was likely at or close to sulfide saturation upon emplacement and may have co-existed with a volatile-rich phase. The Lac des Iles Complex may serve as a type example of Archean continental arc-related magmatic sulfide deposits, fed by fertile andesitic parent magmas formed through the differentiation of primitive sub-arc mantle melts in the juvenile crust.

One of the largest chromium deposits on Earth occurs in the Rustenburg Layered Suite (RLS) of the Bushveld Complex as laterally continuous chromitite layers. None of the hypotheses proposed for the origin of the chromitites can explain both the abundance of Cr in the RLS and the unusual enrichment in Cr and V over Ni, relative to typical depleted mantle values. This study investigates the possibility that the layering and chromitite formation are consequences of the entrainment of source components into the magmas that formed the RLS. Thermodynamic modelling results reveal a wedge-shaped domain in pressure-temperature space in the subcratonic mantle within which Cr-bearing orthopyroxene forms as a peritectic product of incongruent melting. Entrainment of this orthopyroxene produces magmas that crystallise peritectic olivine and chromite on ascent, due to the consumption of orthopyroxene by melt. The chromite- and olivine-bearing magmas intrude as sills and can produce chromite and dunite layers by density separation. This model, which interprets the RLS Sr-isotopic composition to reflect prior mantle metasomatism by crustal fluids (ideally ancient and of low volume), readily explains the formation of chromitite layers from relatively thin sills, as well as the very high ratios of Cr and V to other compatible elements relative to typical mantle compositions. The special circumstances required to produce the RLS chromitites do not relate to some oddity of repetitive crustal assimilation or magma compositions that allow chromite-only saturation. Rather, they relate to speed of melting and magma extraction which enabled peritectic orthopyroxene entrainment to the magmas.

Several lines of evidence, including hydrous melt inclusions and unusually Cl-rich apatite, have been used to suggest that the reappearance of olivine and PGE-sulfide of the J-M Reef in the Stillwater Complex, Montana, is due to fluid infiltration and hydration melting. This study builds upon the hydration melting model using the programs MELTS and PELE with Stillwater bulk rock compositions for the original protolith. Cl-bearing phases are not modeled by MELTS and thus simple oxide mixtures of either a pure H₂O or a H₂O + Na₂O “faux brine” are added to norite, gabbronorite, and melanorite protoliths at 1050 °C at 2 kbar pressure, conditions for which the nominally “dry” protolith is > 95% solid. Incongruent hydration melting results in up to 37% olivine produced in the melanorite. The olivine Fo content is a function of the partial melt retained on cooling, and ranges between 76 and 86, overlapping the natural range of olivine compositions observed in the rocks. Modeling with the PELE program, which includes a silicate liquid Cl component, sulfur species, and a more complex C-O–H-S fluid, suggests that, for CO₂-rich fluids, fluid metal concentrations on the order of 25 ppm Pt, 75 ppm Pd, 0.03 wt.% Cu, and 0.20 wt.% Ni at a fluid/rock mass ratio of ~ 0.25 are needed to account for the observed ore grades. Sulfide and ore metals are readily remobilized for more H₂O-rich fluids, consistent with heterogeneous distribution of sulfide and regionally variable ore grades.