Jan Pašava

The Valeriano composite epithermal-porphyry Cu‒Au system in the El Indio-Maricunga metallogenic belt, Vicuña segment, consists of a shallow silicic lithocap with low-grade Au‒Ag ± Cu mineralization underlain by a large porphyry-Cu (± Mo, ± Au) deposit. A deep drillhole (VALD-D13-14) allows the systematic evaluation of bulk-rock Cu‒Mo isotope compositions over a vertical extent of ~ 1,900 m through the hydrothermal porphyry-epithermal system. Copper isotope values in the lower 1,000 m of the mineralized potassic and phyllic altered porphyry zone increase slightly upwards in the range of +0.2 to +0.4‰, and scatter in the upper phyllic and advanced argillic zone (δ ⁶⁵ Cu: ‒3.7‰ to +2.7‰). The near-surface low-Cu leached zone has very negative δ ⁶⁵ Cu values down to ‒6.5‰ and there are also some distinctly negative excursions in the deep potassic zone with subordinate advanced argillic alteration. Molybdenum isotope values are slightly negative in the potassic porphyry zone (δ ⁹⁸ Mo: ‒0.3 to ‒0.2‰), but increase over the phyllic zone to positive values of 0.2‒0.3‰, and strongly scatter at the transition to the high-sulfidation epithermal system. The Cu and Mo isotope patterns can be mainly attributed to Rayleigh fractionation between vapor and brine in the magmatic-hydrothermal porphyry system and to redox-controlled isotope shifts during supergene alteration in the epithermal system. Deviations from the general trend in some deep drillcore intercepts relate to short intervals of advanced argillic alteration and corresponding redox fluctuations with Cu enrichment.

We describe scheelite in five types of mineral deposit from the Bohemian Massif (greisen, orogenic Au, reduced intrusion-related gold systems (RIRGS), oxidized and reduced skarn) via cathodoluminescence (CL) imaging and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses. We document variability in the scheelite composition within single grains and/or deposit types that reflects the nature and evolution of the ore-forming fluids. We provide scheelite typical trace element characteristics and chondrite-normalized REE patterns for different deposit types, confirming that scheelite can be used as an indicator mineral for their exploration. For example, scheelite from greisen-related deposits is usually rich in Sn, Sr, Y and HREE with various ranges of median values in individual deposits (0.05–8.0 ppm, 232.4–567.1 ppm, 0.06–465.0 ppm and 0.06–632.5 ppm, respectively) and with typical chondrite-normalized REE patterns showing Eu negative or positive anomaly. Under UV light scheelite from Sn-W deposits displays variable colours from whitish via yellowish to bluish. In contrast to greisen-related deposits, scheelite from Au-orogenic association is usually relative rich in Pb (median range in individual deposits: 6.1–11.2 ppm), and relatively poor in Mg, Mo and Nb. Flat and bell-type chondrite-normalized REE patterns show a pronounced Eu positive anomaly. Under UV light it is predominantly homogenous and bluish. Scheelite from oxidized skarn is enriched in Mo, Nb and ΣLREE (median range in individual deposits: 311.4–7214.1 ppm, 1.3–214.9 ppm, and 1.1–1375.8 ppm, respectively), and scheelite from reduced skarns is characterized by the lowest median ranges of Nb (1.7–107.8 ppm), Y (0.69–41.1 ppm), Mn (0.5–17.0 ppm) and Zn (0.04–0.252 ppm) in individual deposits, when compared to all the groups studied. Scheelite from oxidized and reduced skarn deposits displays several types of chondrite-normalized pattern: (1) LREE-enriched with a positive or negative Eu anomaly, (2) Bell-shaped REE patterns with negative or without Eu anomaly, and (3) Flat REE patterns with a negative Eu anomaly. Scheelite from oxidized and reduced skarn deposits is commonly zoned and mostly bluish under UV light.

Early Cambrian black shales on the Yangtze Platform host a regionally distributed highly metalliferous sulfide-rich carbonaceous unit which has been the subject of extensive debate. This marker unit, with a few centimeters or tens of centimeters in thickness, displays extreme enrichment in Mo and Ni (wt% range), and in a broad spectrum of other metals such as As, Au, PGE, Re, Cu, Zn, Cd, Ag, Sb, Se, Tl, and Hg, and occurs discontinuously along the western passive margin of the Yangtze Platform. It grades laterally in stratigraphically equivalent meter-thick vanadium-rich shale and tens-of-meter-thick sapropelite (combustible shale). New Cu and Zn isotope data, combined with existing Cd, Cr, Ni, Mo, Hg, and Se isotope and other chemical data, allow to attempt an integrated view on the formation of this intriguing unit of hyper-enriched metalliferous black shale. The authigenic Cu enrichment in the 1000-ppm range has produced no or little Cu isotope fractionation (0.03 ± 0.26 ‰ δ⁶⁵Cu) beyond the lithogenic background. Heavy zinc and cadmium isotope enrichment in the sulfidic samples (1.11 ± 0.18 ‰ δ⁶⁶Zn, 0.31 ± 0.10 ‰ δ¹¹⁴Cd) is controlled by sulfide fractionation and contrasts with V-rich and barren shale (0.60 ± 0.18 ‰ δ⁶⁶Zn, 0.00 ± 0.14 ‰ δ¹¹⁴Cd). The distinctly negative Ni isotope composition of the metalliferous unit (−0.84 ± 0.05 ‰ δ⁶⁰Ni) with Ni in the percent range has been interpreted as due to hydrothermal activity related to the leaching of mafic rocks and their sulfides. Sorption processes (Fe-oxyhydroxides) and redox cycling in the water column and the bottom sediment with microbial activity could be an alternative interpretation. The extreme metal enrichment can be understood as due to a process chain, from high biological productivity in the oxic photic zone to sulfate reduction in the deeper sulfidic water column and upper sediment layer. Key to the metal enrichment seems to be extremely low clastic sedimentation and advanced carbon destruction by anaerobic oxidation. Hydrothermal input of basinal brines along the rifted margin of the Yangtze Platform was likely a part of this scenario.

Lower Cambrian Mo–Ni sulphidic black shales from the Huangjiawan mine (Guizhou Province, south China) have anomalous platinum-group element (PGE) concentrations (up to ~1 ppm in total). We used LA-ICPMS to study the distribution of PGE in pyrite and Ni-sulphide (millerite) and FE-SEM/EDS for determination of As in pyrite. A sulphide concentrate was produced by innovative hydroseparation techniques from one representative sample, which contained 162 ppb Pt, 309 ppb Pd, 12.2 ppb Ru, 11.3 ppb Rh, 1.5 ppb Ir, 11 212 ppb Re and 343 ppb Au. Mineralogical analysis revealed that pyrite forms ~12 vol%, which corresponds to a calculated ~18.4 wt% of all mineral phases in mineralized black shale. We found that pyrite contains on average (144 analyses) 0.10 ppm Pt, 0.11 ppm Re and 1.40 ppm Au (Ru, Rh, Pd, Os and Ir were below detection limit). It also contains from ~0.5 to ~1.8 wt% As and can be therefore classified as arsenian pyrite. Millerite (77 analyses) showed PGE, Re and Au values below detection limit. We suggest that pyrite represents a dominant Au carrier, containing between 64 and 83% Au of the total Au mineralised rock budget. Conversely, pyrite does not bear any significant amount of Re and Pt, contributing up to ~0.2% and ~12.5% to their whole rock budgets, respectively. Time resolved LA-ICPMS spectra in pyrite indicate that Pt, Re and Au behave as typical lattice-bound elements, with only Re locally forming micro-inclusions. Arsenic is heterogeneously distributed in pyrite and the Au/As ratio (much lower than 0.02) is in support of Au to be structurally bound in solid solution.