Concentrations of platinum-group elements (PGE), Re and Au in arsenian pyrite and millerite from Mo–Ni–PGE-Au black shales (Zunyi region, Guizhou Province, China): results from LA-ICPMS study

Abstract

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.