Mineralium Deposita最新文献

Abstract

The Val-d’Or vein field (VVF), located in the southern Abitibi subprovince (Québec, Canada), is host to ~ 47 Moz gold and is therefore an example of a greenstone-hosted orogenic gold district. Gold is contained in quartz-tourmaline-carbonate veins that cut As-poor intermediate to mafic volcanic and intrusive rocks, including dioritic, granodioritic and gabbroic sills, dikes, stocks, and plutons. Five investigated orebodies (Goldex, Triangle, Plug #4, Pascalis Gold Trend, Beaufor) host gold in vein- and wallrock-hosted pyrite-rich sulfide aggregates (> 95 vol%) that show a porous core domain (Py1), with abundant inclusions of carbonate, silicate, and Fe-oxides up to several tens of µm in size. A homogeneous pyrite rim domain (Py2) surrounds Py1 and contains most of the gold as native gold and polymetallic (Au-Ag-Te-Bi) inclusions, primarily calaverite and petzite. The two pyrites show different Au and As contents (Py1 = Au ≤ 30 ppm; As ≤ 67 ppm; Py2 = Au ≤ 1250 ppm; As ≤ 550 ppm). Pyrite shows a ubiquitous shift in δ³⁴S values of up to + 3.0‰ from Py1 (δ³⁴S = − 0.4‰ to 5.8‰, n = 32) to Py2 (δ³⁴S = 0.0‰ to 6.3‰, n = 59) and records a small, slightly negative Δ³³S signature between – 0.20‰ and 0.01‰. The δ³⁴S shift suggests that removal of reduced sulfur species from auriferous hydrothermal fluids causes the formation of inclusion-hosted gold in Py2 by a decrease in the fluid sulfur fugacity (fS₂) through wallrock sulfidation of Fe-oxides. The shift also correlates with locally enriched Co and Ni concentrations in Py1 (< 1 wt%), compared to lower, oscillatory zoned concentrations (< 0.1 wt%) in Py2, respectively, indicating an overall decrease in fluid oxygen fugacity (fO₂). Contemporaneously, a decrease in fluid tellurium fugacity (fTe₂) drives polymetallic inclusion-hosted gold formation in Py2, initially as calaverite followed by increasingly Ag-bearing petzite and hessite. The multiple sulfur isotopes and trace element compositions recorded in pyrite in the VVF indicate that a homogeneous fluid reservoir introduced gold-sulfide complexes. Even if considered a localized process at the ore-shoot scale, fluid-wallrock sulfidation reactions can lead to a coupled decrease in fS₂, fO₂, and fTe₂ of auriferous hydrothermal fluids in a greenstone-hosted As-poor gold district.

This study presents an investigation of the distribution of trace elements within base metal sulfides from magmatic Ni-Cu-Co deposits from the Proterozoic anorthositic Espedalen Complex, Norway. The mineralisation occurs both as primary, undeformed sulfides and as deformed sulfides hosted within shear zones. Distinct deposits yield Ni tenors ranging from 3 to 10%, allowing the assessment of whether the metal tenor of a deposit is reflected in its sulfide composition. The results allow constraining geological processes spanning from the compositional traits of the parental melts to late-magmatic fluid interactions. Notably, the sulfides exhibit a relatively low concentration of chalcophile elements compared to other Ni-Cu-Co deposits worldwide, particularly platinum-group elements (PGE). This is because these deposits formed from PGE-depleted magmas. Elements compatible with monosulfide solid solution (MSS; Co, Rh, Ru, Ir, Os and Se) are predominantly hosted by sulfides, whereas a smaller proportion of incompatible elements (Pb, Cd, Ag, Bi, Zn, In, Tl, As, Sn and Mo) is also accommodated by sulfides. Our findings for sulfide composition support for the first time a positive correlation between Se concentrations in pentlandite and whole-rock Ni tenors for both Espedalen and magmatic Ni-Cu-Co sulfide deposits worldwide. This is because of the more efficient collection of both Ni and Se by an immiscible sulfide liquid under high R-factor regimes, combined with the fact that Se concentrations in pentlandite remain largely undisturbed during post-cumulus processes as opposed to other trace elements. Consequently, Se concentrations in pentlandite may serve as a proxy for metal enrichment in magmatic sulfide deposits.

Abstract

The Zhazixi deposit hosted in sedimentary rocks is a major Sb-W deposit in South China. The mineral scheelite, which can be dated by the U-Pb method, commonly occurs in both tungsten (W)-dominated and antimony (Sb)-dominated ore veins of the deposit. Cathodoluminescence (CL) images reveal the presence of three distinct stages of scheelite (Sch-I, Sch-II and Sch-III) within the deposit. These three scheelites were dated using in-situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), yielding U-Pb ages of 158.1±5.4 Ma and 157.6 ± 4.9 Ma for Sch-I, 155 ± 11 Ma for Sch-II, and 151.3±6.1 Ma for Sch-III. These data suggest that the Zhazixi Sb-W deposit formed during the Late Jurassic (160–150 Ma) rather than the Late Triassic as previously suggested. Considering the temporal similarity with low-temperature hydrothermal Sb deposits in the Xiangzhong metallogenic province (XZMP), the formation of the Sb-W deposit may have followed a similar genetic model, where meteoric groundwater circulated to depth and extracted metals from fertile basement rocks through fluid-rock interaction, resulting in the generation of ore fluids. This study highlights that Late Jurassic low-temperature hydrothermal Sb-polymetallic mineralization in the XZMP is likely more extensive than previously perceived.

The origin of propylitic fluids in intermediate sulfidation mineralization has not been investigated in detail. Here, we present an extensive petrographic, geochemical, and isotopic (O-H-Sr) study of propylitic epidote, chlorite, and calcite from the Zhengguang intermediate sulfidation epithermal Au-Zn deposit, NE China. Propylitic minerals can be divided into three main types based on their different textural occurrences, namely interstitial cement of clasts of hydrothermal breccia, replacement of primary plagioclase or hornblende, and vein infill of cracks, with late, minor calcite as amygdules in vesicles of andesite representing a fourth textural occurrence. The H-O isotope compositions and mass balance calculations suggest that most propylitic epidote records a dominant (> 50%) contribution of magmatic fluids. The decrease of the average δ¹⁸({mathrm O}_{{mathrm H}_2mathrm O};)values equilibrated with different types of epidote (cement 6.8 ± 0.7‰, replacement 5.1 ± 1.1‰, vein 4.5 ± 1.4‰, 1 SD), and the decreasing content of high-temperature elements (e.g., Cu-Mo) from cement, through replacement to vein epidote and chlorite, collectively indicates a decreasing role of magmatic fluids. Replacement epidote and chlorite are enriched in Sr-Mn-Y-Sb, whereas replacement epidote and calcite record similar (⁸⁷Sr/⁸⁶Sr)_i values to the andesitic host rock, suggesting that replacement minerals inherit certain elements from plagioclase and hornblende, and the Sr isotope signature of the wall rocks. We highlight that propylitic alteration in epithermal deposits can involve significant proportions of magmatic fluids and texturally different alteration mineral types should be considered when using mineral isotopic or chemical compositions to track fluid sources or to vector towards the location of intrusive centers.

The world-class Huize deposit hosts significant germanium (Ge) resources in the Sichuan–Yunan–Guizhou (SYG) Mississippi Valley-type (MVT) Pb–Zn province of China. The distribution and enrichment mechanism of Ge is still poorly understood. In the main ore-forming stage of Huize, we identified six sphalerite colors from C1 (black) to C6 (white) in transmitted light. Two color sequences are confirmed, including C1 → C2 → C3 → C6 and C1 → C2 → C4 → C5 → C6. We used multiple analytical methods to reveal the Ge distribution and incorporation mechanism into sphalerite and the possible enrichment factors. Our results show that Ge occurs as argutite (GeO₂), and in the sphalerite crystal lattice, C1 and C3 sphalerite has up to 593 ppm Ge. Two substitution mechanisms, i.e., Ge⁴⁺ + □_(vacancy) → 2Zn²⁺ (e.g., C1 and C2) and Ge⁴⁺ + 2Cu⁺ → 3Zn²⁺ (e.g., C2, C3, C4, and C5), are inferred from the Huize sphalerite. They show different spatial structures of sphalerite and a weak shift of the white line observed by high-resolution X-ray absorption near-edge structure (XANES) spectroscopy. The trace-element composition of sphalerite suggests that reduced sulfur content of the ore-forming fluid contributes to Ge enrichment, followed by high temperature (> 300 °C).

Banded iron formations (BIFs) are chemical sediments that reflect the composition of the seawater from which they were deposited. Therefore, they provide a key part of the evidence for the modern scientific understanding of paleoenvironmental conditions in Archean and Paleoproterozoic times. Although BIFs have been extensively studied, many aspects (e.g., specific mechanisms controlling iron (Fe) and silicon (Si) precipitations) of their origin still remain enigmatic because of the lack of modern analogues. In China, abundant BIFs occur throughout within the late Neoarchean volcanic and sedimentary succession and therefore are the principal source of Fe for the Chinese steel industry. Here, we examine the ~ 2.53 Ga Qidashan BIF, one of the most extensive BIFs in China, by conducting a detailed petrographic and multi-proxy investigation to well constrain its formation mechanism. The BIF consists mainly of magnetite and quartz with lesser amounts of calcite and various types of silicate minerals, of which the content of Al-rich minerals (i.e., chlorite) is rare, coupled with a low abundance of detrital geochemical indicators (e.g., Al and Ti), suggesting that the BIF is relatively pure with insignificant terrigenous contamination. A wide range of Nd isotope compositions and shale-normalized patterns and specific anomalies of rare earth elements, especially highly positive Eu anomalies, indicate that the BIF precipitated from seawater imprinted by high-temperature hydrothermal fluids. Furthermore, there is a significantly negative correlation between Nd isotope values and total Fe contents of the BIF. This suggests that such enhanced hydrothermal activity provided vast volumes of dissolved Fe(II) necessary for the formation of the BIF via alteration of ancient continental crust. In addition, the Qidashan BIF was deposited under pervasively anoxic conditions, as revealed by the absence of shale-normalized Ce anomalies and the presence of consistently positive Fe isotope values. Hence, anoxygenic photosynthesis is the most plausible mechanism responsible for Fe(II) oxidation. Given that Fe─Si bonding has a strong impact on Si isotope fractionation, the formation of primary Fe(III) oxyhydroxides should have exerted a first-order control on the negative Si isotope signatures observed in the studied BIF samples. It is also noted that the BIF possesses a variation of negative Si isotope values, further implying that diagenetic dissolution and reprecipitation of silica took place after primary Si precipitation associated with Fe.

Orogenic gold systems are arguably the most variable mineral system globally in terms of an extreme range of depositional depths, corresponding P–T conditions and wallrock alteration assemblages, structural controls and styles, and element associations. This diversity has ignited controversy on genetic models for the two decades since orogenic gold became a widely accepted term. From the diverse genetic models proposed, the two groups of fluid-source models that meet most genetic constraints are the following: (1) deposition from crustal fluids via metamorphic devolatilization at the amphibolite-greenschist transition, or potentially even deeper under specific tectonic conditions, and (2) deposition from sub-crustal fluids either by direct devolatilization of subducted oceanic crust and overlying sediment wedge or of previously metasomatized and fertilized mantle lithosphere. Both models normally postulate gold deposition within a geodynamic system that evolves from extension through compression into syn-gold transpression. Crustal metamorphic models normally invoke subduction-driven geodynamic systems that involve advection of crustal metamorphic fluids up crustal-scale faults. In contrast, sub-crustal devolatilization models involve subduction-related processes as both geodynamic drivers and gold sources with fault-controlled fluid conduits extending to below the Moho. The overall lack of orogenic gold and other subduction-related mineral systems during the unique Boring Billion (1.8–0.8 Ga) period provides an important constraint on this genetic debate. Boring Billion orogens had varying geodynamic drivers, asthenosphere upwelling, and low-P metamorphic terranes with crustal-scale faults, all parameters consistent with formation of orogenic gold systems, during subduction-independent accordion-type tectonics. The absence of orogenic gold during the Boring Billion provides critical evidence against the crustal metamorphic model and furthers the sub-crustal model which requires subduction as both the geodynamic driver and auriferous fluid source.

Magmatic Ni–sulfide ore deposits are generally associated with basaltic to komatiitic igneous rocks that originate by partial melting of the mantle, which is usually modelled as a uniform four-phase peridotite. Existing models accept that the key metal contributors to mantle melts are olivine (Ni) and sulfide (Cu, platinum group elements (PGEs) and minor Ni). However, melting in the mantle commonly begins in volumetrically minor mantle assemblages such as hydrous pyroxenites that occur as veins in the peridotite mantle, which are rich in the hydrous minerals phlogopite, amphibole and apatite. The contribution of hydrous pyroxenites to the metal endowment of mantle melts may have been underestimated or overlooked in the past, partly because evidence of their input is partially erased as melting intensifies to involve peridotite.

Here, we compile new results from experiments and natural rocks which demonstrate that the hydrous minerals such as phlogopite, amphiboles and apatite all have high partition coefficients for Ni (3–20) and may be important repositories for Ni in mantle sources of igneous rocks. This implies that hydrous minerals hosted in metasomatic mantle lithologies such as hydrous pyroxenites may be important contributors to some magmatic Ni–sulfide ore systems. Hydrous pyroxenites contain hydrous minerals in large modal abundances up to 30–40 vol% in addition to clinopyroxene and a few vol% of oxide phases, such as rutile and ilmenite. These mantle lithologies are commonly associated with cratonic and continental regions, where low-temperature, low-degree volatile-rich melts commonly modify lithospheric peridotite mantle, depositing variable hydrous pyroxenites.

The lower melting temperatures of hydrous minerals in hydrous pyroxenite lithologies also means that the generation of magmatic ore deposits may not require a major thermal perturbation such as a plume, as the melting temperatures of hydrous pyroxenites lie around 300–350 °C lower than dry peridotites. Partial melts of hydrous pyroxenite are more voluminous at low temperatures than melts of peridotite would be. Furthermore, it is argued in the following that they would contain similar or even higher concentrations of Ni. Thus, predictive exploration models should consider domains of the lithospheric mantle where hydrous pyroxenites may be localised and concentrated, as they may have been episodically melted throughout the long-lived geological evolution of cratonic blocks, yielding Ni-rich melts that may be hosted in conduits of varying size and geometry at various crustal levels.

Attempts to geochemically distinguish between metamorphic-hydrothermal systems that form orogenic gold deposits and both reduced and oxidized magmatic-hydrothermal systems using isotopes or metal associations have proven ambiguous, particularly for orogenic gold and reduced intrusion-related gold systems. The absence of conclusive geochemical discriminators and the overlap in geologic characteristics have led to gold deposit models being potentially incorrectly applied, which in turn negatively affect regional mineral exploration and mine planning. In this study, in situ electron microprobe geochemical analyses of hydrothermal monazite and xenotime crystals associated with different types of gold-bearing deposits are shown to be effective geochemical discriminators. There are notable differences in mineral chemistry such as rare earth element (REE) profiles, total light REE, Dy, Er, Pr, Y, Nd/Sm, and La/Sm that distinguish monazite precipitated from metamorphic-hydrothermal fluids that form orogenic gold deposits and those precipitated from magmatic-hydrothermal fluids that form both porphyry Cu-Mo-Au and reduced intrusion-related gold deposits. Notable differences in overall xenotime abundances and concentrations of heavy REEs, Ca, and Sc are distinctive between the different deposit classes for xenotime. The origin of the controversially classified Pogo gold deposit, Tintina gold province, Alaska, which has been characterized as both a reduced intrusion-related and an orogenic gold deposit, is tested based upon the noted chemical differences associated with these hydrothermal phosphates. The findings of this study have implications for exploration and mine development in the Tintina gold province and other areas that contain deposits that are controversially classified as either orogenic or as magmatic-hydrothermal gold deposits.

Silver is probably the closest isotopic proxy to track monoisotopic gold and has been shown to have great potential to yield new information on the origin and enrichment processes of gold. This study describes the development of a tailored analytical protocol for accurate analysis of Ag isotopes and provides the first Ag isotope data for the Paleoproterozoic Rajapalot Au-Co deposit, Finnish Lapland. Six native Au samples yield ε¹⁰⁹Ag values (relative to NIST SRM 978a) from −6.8 to +2.1 and are within the range of Ag isotopic compositions reported for native Au samples. The mean of the analyzed Au samples is ε¹⁰⁹Ag = −3.8 ± 1.7 (2SD) with most of the samples with negative ε¹⁰⁹Ag values (−6.7 to −2.0); one sample has a positive ε¹⁰⁹Ag value of +2.1 ± 0.5. Silver isotope fractionation in the Rajapalot Au deposit was likely associated with physicochemical processes related to deposition and/or re-mobilization of the ore rather than with source region inheritance. It is suggested that redox reactions involving Ag⁺ ↔ Ag⁰ phase change primarily account for the isotopic differences within the deposit. Our results also suggest that the Rajapalot Au-Co deposit was formed via multistage ore-forming processes and/or that the primary ore was re-mobilized, which caused isotope fractionation along fluid pathways. Silver isotope variation within a deposit may mark a fractional crystallization trend with the lightest isotopic composition representing the earliest precipitate. Hence, Ag isotopes show potential as an isotopic vectoring tool in search of Au-enriched domains.