The Las Cuevas deposit, Almaden district (Spain): an unusual case of deep-seated advanced argillic alteration related to mercury mineralization

The Hercynian mercury mineralization of Las Cuevas is hosted by a highly folded and sheared sequence of basalts, intrusive breccias, slates, psamitic rocks and quartzites. The mineral paragenesis is simple and consists of cinnabar, native mercury and pyrite. Hydrothermal alteration can be divided into 'proximal' and 'distal' with respect to the mineralized bodies. The proximal alteration (less than or equal to 1.3 m wide) consists of quartz-pyrophyllite-kaolinite, quartz-pyrophyllite-(kaolinite)-(illite), and quartz-illite-(pyrophyllite)-(kaolinite). The distal alteration (similar to 100 m wide) consists of (quartz)-illite-chlorite-(pyrophyllite), or rectorite-(chlorite). These assemblages overprint an earlier, regional alteration consisting of quartz-chlorite-albite-carbonates (+/- ankerite, +/- siderite, +/- magnesite, +/- calcite). The mercury deposit of Las Cuevas can be regarded as an unusual combination of mercury deposition and advanced argillic alteration within a relatively deep environment (greater than or equal to 1.8 km).