Part of the Las Cruces secondary siderite deposit has sparked an interest in the scientific community because of its unique mineralogy. The original gossan formed by goethite and hematite has been replaced by a siderite and galena rock. We postulate that this rock can be formed by the interaction of iron oxides with groundwater similar in composition to that of the present day. Hydrochemical and isotopic characteristics of groundwater support this hypothesis. The negative Eh values, the existence of H2S and the tendency toward high sulfate isotope values indicate a reducing groundwater condition. The high ammonium, boron and iodine concentrations as well as the low δ13C values of dissolved inorganic carbon (DIC) confirm the organic matter oxidation. The reductive dissolution of Pb-bearing goethite at the expense 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 groundwater (<10ppb), its low δ13C values and thermodynamic calculations. One-dimensional reactive transport modeling demonstrated that the present-day groundwater flux and chemical composition could form the siderite rock in less than 1Ma with no external supply of reactants. Sensitivity analyses revealed that the 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 measured concentration resulted in a mineral zonation: one zone formed by Fe-sulfides and other zone formed by siderite, with galena in both zones. Reactive transport calculations and the similarity of its high δ34S values indicate that the sulfur of galena came from the current groundwater. Reactive transport calculations and the similarity of its high δ34S values indicate that the sulfur of galena came from the current groundwater.