The Las Cruces deposit, Iberian Pyrite Belt, Spain

Abstract

The Las Cruces deposit is located at the eastern margin of the Iberian Pyrite Belt (IPB), beneath the Neogene-Quaternary deposits of the Guadalquivir Basin. These sediments covered and 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, including both the gossan and the enrichment zone. In that sense, the present study provides new data on the IPB ore deposits and their post-Palaeozoic evolution.

The primary mineralization consists of massive and semi-massive polymetallic sulfides overlaying a cupriferous and pyritic stockwork. The supergene enrichment extends from the primary zone to gossan and consists of a thick cementation zone characterized by intense replacement of part of the primary sulfides by Cu-rich supergene sulfides.

At present, the only economic resource of Las Cruces is the cementation zone, with initial reserves of 17.6 Mt @ 6.2% Cu, but the orebody also includes significant contents of Cu and Zn within the primary mineralization, and Au, Ag, and Pb in the gossan.

Mineralogical and geochemical data confirm that the primary mineralization at Las Cruces resembles other IPB deposits in terms of distribution and evolution, however the gossan and cementation zone show an uncommon evolution for this kind of supergene profile. The major singularity that makes the Las Cruces supergene mineralisation noticeably different from others in the IPB or elsewhere resides in its unusual mineralogical composition, mostly comprised of newly-formed siderite, calcite, Fe-sulfides and galena.

The genetic model proposed for the Las Cruces deposit includes three main stages: (1) ore genesis and evolution of primary mineralization, which seems to be comparable to other IPB deposits; (2) genesis of Cu-rich secondary mineralization and gossan by weathering, after Miocene exhumation, of the upper part of the massive sulfide deposit, under oxidizing and acidic conditions; and (3) late evolution of supergene profile 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.