El Gossan de Las Crucesgeología, mineralogía y mecanismos de concentración de metales preciosos

Resumo

The Las Cruces VMS deposit is located at the eastern corner of the Iberian Pyrite Belt (IPB), beneath the Neogene-Quaternary sediments of the Guadalquivir Basin. These sediments preserved the supergene profile somewhat after its generation until its recent discovering. For this reason, the Las Cruces ore deposits, together with Lagoa Salgada in Portugal, are the only known deposits in the IPB that preserve the entire weathering profile. The deposit is currently mined by CLC S.A., being the main operation focused on the supergene Cu-enriched zone (initial reserves of 17.6 Mt @ 6.2% Cu). In addition, the deposit also includes important resources in the gossan cap and in the massive sulfide orebody. The present study provides new data on the IPB deposits and their post-Palaeozoic evolution. In this sense, mineralógica! and geochemical data confirm that the primary mineralization at Las Cruces resembles other IPB deposits, however the supergene profile show an uncommon evolution. The major singularity that makes the Las Cruces supergene mineralization noticeably different from others in the IPB or elsewhere resides in its unusual mineralogical composition. For the Las Cruces gossan, a complex mineral assemblage has been reported composed by carbonates, Fe- sulfides, galena, stibnite, Pb-Sb sulfosalts, Ag-Hg-Sb sulfides and sulfosalts, Ag-Au-Hg amalgams, and Bi-Pb sulfides and sulfosalts. Remains of the former oxidized assemblage appear as relicts comprised of hematite and goethite. Regarding the precious metals content, drill core analyses show a heterogeneous distribution within the supergene profile, with mean values of 5.1 ppm of gold and 155 ppm of silver, being the absolute tenures increased towards the bottom of the weathered profile. The Au grade ranges from 0.01 ppm to >100 ppm, and occurs as three different Au ore types: (1) Au concentration in the lower part of the gossan associated with leached black shale horizons, where the gossan makes contact with a barren pyrite zone within the supergene Cu-rich mineralization; (2) Au mineralization in the upper part of the gossan linked to Fe-oxihydroxide lithofacies; and (3) Au ore in the cementation zone related to subvertical faults. A three-stage genetic model has been proposed for the Las Cruces deposit: (1) Ore genesis and evolution of primary mineralization, which seems to be similar to other IPB deposits. (2) Genesis of Cu-rich secondary mineralization and Fe-oxyhydroxides-bearing gossan by weathering under oxidizing and acidic conditions of the upper part of the massive sulfide deposit, after Miocene exhumation. (3) Late evolution of the Las Cruces weathering profile occurs below the carbonated-rich sediments of the Guadalquivir Basin, with the subsequent change in the redox conditions due to water-rock interaction between the supergene profile and basinal fluids. In this way, the Fe-oxyhydroxides formed during the early gossan stage, were replaced by carbonates and sulfides suggesting that the early gossan was modified under changeable physico-chemical conditions. All these suggest the basinal fluid-gossan interaction, the equilibration of the fluid with the carbonated sediments and the subsequent gossan carbonatization and sulfidation led the Fe-carbonates and Pb-Sb-sulfides and sulfosalts.