Gold behavior in supergene profiles under changing redox conditions : the example of the Las Cruces Deposit, Iberian Pyrite Belt

The Las Cruces deposit is in the eastern end of the Iberian Pyrite Belt (SW Spain). It is currently being mined by Cobre Las Cruces S.A. The main operation is focused on the supergene Cu-enriched zone (initial reserves of 17.6 Mt @ 6.2% Cu). An Au-Ag-Pb–rich gossan resource (3.6 Mt @ 3.3% Pb, 2.5 g/t Au, and 56.3 g/t Ag) occurs in the upper part of the deposit. The Au grade ranges from 0.01 ppm to >100 ppm, and occurs as three different Au ore types: (1) Au mineralization in the upper part of the gossan linked to Fe-oxides lithofacies, (2) Au concentration in the lower part of the gossan associated with leached black shales, and (3) Au ore in the cementation zone related to subvertical fractures. A hydroseparation device has been used to obtain heavy mineral concentrates from selected samples of different ore types. Reflected-light microscopy, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and electron probe microanalysis (EPMA) were used to study the separated Au particles. Significant differences between the defined ore types include the Au-bearing lithologies, mineral associations, textural features, particle sizes, morphologies, and fineness. Au-rich minerals include native Au, Au-Ag electrum, and Au-Ag-Hg amalgams. Gold-bearing mineral associations include Pb-oxihalides, Fe-oxides, galena, pyrite, cinnabar, and Ag-sulfosalts. The Au enrichment mechanism in the supergene profile involves (1) dissolution of Au from the primary sulfides as chloride-rich ionic complexes during the weathering of the deposit under subaerial exposure; dissolved Au is transported downward through the supergene profile under acidic and oxidized conditions; (2) destabilization of the Au complexes by Fe-controlled redox reactions; as a consequence, coarse-grained, high-fineness Au particles precipitated in association with Fe-oxyhydroxides. This resulted in secondary concentration in the upper gossan; and (3) after deposition of cover sediments took place a progressive change in the system conditions resulting in a later Au remobilization as hydroxidehalide, hydroxide, thiosulfate, and bisulfide complexes in the lowermost gossan and cementation zone. The main pathways for migration of enriched fluids to the cementation zone are secondary permeability zones linked to Alpine reactivated faults. Deposition of Au seems to be related to fluid interaction with reductant lithologies, including black shales and the primary sulfides.This research is a contribution to projects P-S Anoxia (CGL2011-30011) and Metodica (CGL2010-21956-C02-02), which are supported by the Spanish government. The authors thank Cobre Las Cruces S.A. for the field assistance and the ongoing collaboration. We are also grateful to Joel Brugger, Steve Hill, and Tim Baker for their constructive suggestions to improve the manuscript