Groundwater-Gossan interaction and the genesis of the secondarysiderite rock at Las Cruces ore deposit (SW Spain)

Abstract

Part of the Las Cruces secondary siderite deposit has sparked an interest in the scientific communitybecause of its unique mineralogy. The original gossan formed by goethite and hematite has been replacedby a siderite and galena rock. We postulate that this rock can be formed by the interaction of iron oxideswith groundwater similar in composition to that of the present day. Hydrochemical and isotopic charac-teristics of groundwater support this hypothesis. The negative Eh values, the existence of H2S and the ten-dency toward high sulfate isotope values indicate a reducing groundwater condition. The highammonium, boron and iodine concentrations as well as the low d13C values of dissolved inorganic carbon(DIC) confirm the organic matter oxidation. The reductive dissolution of Pb-bearing goethite at theexpense of Dissolved Organic Carbon (DOC) leads to the precipitation of Fe-sulfides, galena and siderite.The formation of siderite from this process is confirmed by the low amount of dissolved Fe in groundwa-ter (<10 ppb), its low d13C values and thermodynamic calculations. One-dimensional reactive transportmodeling demonstrated that the present-day groundwater flux and chemical composition could formthe siderite rock in less than 1 Ma with no external supply of reactants. Sensitivity analyses revealed thatthe time of formation depends on the structure of the groundwater flux (spaced fractures or pervasive),the flow rate and especially the DOC concentration. In fact, calculations with the highest DOC measuredconcentration resulted in a mineral zonation: one zone formed by Fe-sulfides and other zone formed bysiderite, with galena in both zones. Reactive transport calculations and the similarity of its high d34S val-ues indicate that the sulfur of galena came from the current groundwater. Reactive transport calculationsand the similarity of its high d34S values indicate that the sulfur of galena came from the currentgroundwater.