Supergene enrichment of precious metals by natural amalgamation in the Las Cruces weathering profile (Iberian Pyrite Belt, SW Spain)

Abstract

Natural Au–Ag–Hg alloys occur in the Las Cruces ore deposit, in the eastern part of the Iberian Pyrite Belt. They are mainly concentrated in the lower part of the gossan profile including a sheared black shale level where the gossan makes contact with a barren pyrite zone within the supergene Cu-rich mineralization.

Drill core analyses show a heterogeneous distribution of Au, Ag, and Hg within the weathering profile, with mean values of 5.1 ppm, 155 ppm, and 52 ppm, respectively. In general, the absolute tenures increase towards the bottom of the weathered profile. Mineralogical studies conducted on samples from the active mine workings indicate that Hg and precious metals occur mainly as Au–Ag–Hg alloys. These associations constitute the best potential resource for precious metals at the Las Cruces deposit.

This paper describes how this unusual precious metal enrichment is produced along the weathering profile by supergene processes. Combining paragenetic information, mineral chemistry and the data pertaining to the solubilities of Au, Ag, and Hg in a weathering profile, we suggest a two-stage genetic model for the formation of the Las Cruces Au–Ag–Hg mineralization: (1) release of Au, Ag, and Hg from the massive sulfide deposit by weathering processes during the gossan formation. At pH < 5.5 and Eh > 0.9 V conditions, Au, Ag and Hg are mobilized downward through the weathering profile as chloride complexes and fixed as elemental Au, halides, oxides, and sulfates; and (2) remobilization of Hg, Ag, and Au in the gossan after the deposit was buried beneath the Neogene carbonate-rich sedimentary cover. The buffering capacity of the percolating fluids due to their interaction with the carbonate-rich sedimentary pile leads to significant mineralogical and geochemical changes. At near-neutral conditions (pH = 6–7; Eh ≈ 0 V), Hg, Ag, and Au are newly-remobilized as thiosulfate, sulfate, and hydroxide complexes and newly-fixed by sorption during ferric hydroxide formation and as sulfates. Several cycles of dissolution–precipitation of Au, Ag, and Hg near the redox front occur by oscillations in the water table and changes in the pH–Eh conditions. The interaction of downward migrating fluids with high reductant lithologies (black shales and massive sulfides) seems to be responsible for the reduction of different complexes and for the precipitation of cinnabar, Ag-sulfides and sulfosalts as well as the precipitation of Au–Ag–Hg amalgams.