Figure 4: Proposed model for the genesis of the biogenic carbonate–sulphide rocks of the Las Cruces deposit.
From: Formation of recent Pb-Ag-Au mineralization by potential sub-surface microbial activity
(a) General idealized crosssection of the deposit. The SE–NW distance is ~300 m. (b) Cartoon based on the section depicted in a showing the proposed genetic model for the deposit. Carbonate–sulphide rocks (Red and Black rocks) formed in the supergene alteration zone (gossan) capping an older massive sulphide deposit. The porous gossan accumulated methane or light hydrocarbons that acted as electron donors for microbial reduction of aqueous sulphate transported by groundwater. This process destabilized goethite/haematite and oxidized minerals (beudantite, cerussite), leading to the formation of siderite, calcite, galena and silver-bearing sulphides, together with coarse-grained gold that has almost completely replaced the former gossan. Scale bar, ~50 m.
