Constraints on the genesis of lode-style tin mineralization: Evidence from the San Rafael tin -copper deposit, Peru

Fluid evolution at the Peruvian San Rafael deposit, which represents the world's largest hydrothermal tin lode was marked by two contrasting episodes. The early stage, characterized by intense sericitic and tourmaline alteration was barren and produced numerous tourmaline-quartz veins, which are sealed and reflect largely lithostatic conditions of formation. A subsequent reopening of the vein-breccia system initiated the ore stage, which was characterized by a distinctive chloritic alteration and an open fracture-filling vein style. The first part of the ore stage was dominated by tin deposition and resulted in the formation of abundant cassiterite-quartz-chlorite-bearing veins and breccias, at depth in the lode. This was followed by copper deposition (with subordinate tin), which concentrated sulfide ores in the upper part of the vein system. The waning stages of hydrothermal activity superimposed late, barren quartz veins on earlier mineralization.;Microthermometric studies of fluid inclusions in cassiterite, quartz, tourmaline and fluorite showed that the fluids responsible for the early, barren stage were hot, hypersaline magmatic brines (>340-535°C, 34-62 wt.% NaCl eq.), whereas the ore stage fluids had a moderate temperature and salinity (>230-360°C, 0-21 wt.% NaCl eq.). The local association of minor hematite with cassiterite, and the ubiquitous replacement of pyrrhotite by pyrite, marcasite and hematite, indicate that the ore fluids became progressively more oxidizing. In addition, the stable isotope compositions of tourmaline, cassiterite, wolframite, quartz, chlorite, calcite, and siderite reveal that the early fluid had a magmatic signature, but its delta18O value markedly decreased with time. These trends are consistent with mixing of the ore fluids with a cooler, dilute fluid, enriched in the light oxygen isotope, and cannot be explained by boiling or simple cooling. Fluid-rock interaction was not a control on tin deposition either, as chloritic alteration of granitic rocks increases the fluid acidity, and this in turn increases tin solubility.;I infer that the creation of several large fault jogs at depth in the lode, synchronous with hydrothermal activity at the San Rafael igneous center, focused an influx of heated meteoric waters, which mixed with tin- and iron-rich magmatic brines. The oxidation, dilution, cooling and acid neutralization of the ore fluids destabilized chloride complexes of tin and iron, and triggered the massive precipitation of cassiterite with chlorite, in localized, high-grade ore shoots, as opposed to a dissemination of tin in the chloritic alteration.;The formation of the San Rafael deposit took place in Late Oligocene, at the onset of a major metallogenic episode in the Inner Arc of the Central Andes. We propose that Tertiary tin-tungsten(-silver) mineralization in this province (and possibly some of the earlier metallogenic episodes) was produced by a recurrent compressional interaction between the Farallon/Nazca oceanic plate and the South American continent. Periodic "collisions" between the tectonic plates were focused on this segment of the Andean orogen and generated in its back arc voluminous peraluminous magmas, which evolved by fractional crystallization and exsolved the fluids responsible for the rich Sn-W(-Ag) mineralization.;The delta34S values of sulfide minerals from San Rafael range between +2 and +6 ‰, suggesting a large-scale hydrothermal system, supplied by a relatively homogenous, presumably magmatic source of sulfur. Stability relationships of the ore minerals indicate that the ore fluids were reducing, which is consistent with their capacity to transport significant quantities of tin.