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The composition of tourmaline in the San Rafael Sn-Cu lode, in southeastern Peru, provides an important record of the early evolution of the hydrothermal system that produced the world's richest tin deposit. Many forms, colors and compositions of tourmaline, ranging from dravite to schorl, are present in the deposit, but the late tourmaline that accompanied deposition of early cassiterite has an unusual dark green color, and exhibits a strong trend of enrichment in iron. Appearance of this tourmaline in the paragenesis coincided with a marked change in the vein style, refl ecting an opening of the vein system, and a dramatic change in the mineralogy of vein and alteration assemblages, evident from the precipitation of other iron-rich minerals (Fe-rich chlorite and cassiterite). This abrupt change in the plumbing of the hydrothermal system was associated with the introduction of dilute, relatively oxidizing, externally derived waters of meteoric origin that mixed with hot magmatic brines carrying high concentrations of dissolved tin and iron. The resulting sudden cooling, dilution, and oxidation of the ore fl uids created the conditions required for massive precipitation of cassiterite and formation of a very large, high-grade ore deposit.

The San Rafael tin-copper deposit in the Eastern Cordillera of the Peruvian Central Andes is the world's largest hydrothermal tin lode, with a total resource of about 1 million metric tons metal, at an average grade of 4.7 wt percent Sn. The mineralization is of the cassiterite-sulfide type and occurs in a vertically extensive veinbreccia system centered on a shallow-level, late Oligocene granitoid stock. The tin ores form cassiterite-quartzchlorite-bearing veins and breccias hosted by several large fault-jogs at depth in the lode. By contrast, the copper ores, which contain disseminated acicular cassiterite, are localized in the upper part of the system. Both ore types are associated with a very distinctive, strong chloritic alteration, which was preceded by intense sericitization, tourmalinization, and tourmaline veining. The δ 34 S values of the sulfides range between 2 and 6 per mil, and vary very little with location in the deposit. This indicates that the hydrothermal system was large, with a relatively homogeneous source of sulfur, likely of magmatic origin. This is confirmed by stability relationships of ore minerals, which indicate that the ore fluids were initially reducing. Microthermometric studies of fluid inclusions in cassiterite, quartz, tourmaline, and fluorite show that the fluids responsible for the early, barren stage were hot, hypersaline brines (380°-540°C, 34-62% NaCl equiv), whereas the ore-stage fluids had moderate to low salinity (0-21 wt % NaCl equiv), and were of moderate temperature (290°-380°C). In addition to the marked dilution of the ore fluids with evolution of the hydrothermal system, they became progressively more oxidizing, as inferred by the local association of minor hematite with cassiterite and the ubiquitous replacement of pyrrhotite by pyrite and marcasite. The δ 18 O values of the fluid decreased systematically with time, as indicated by the δ 18 O values of different generations of tourmaline, cassiterite, and quartz. This evolution was paralleled by an increase in the δD values of the fluid, inferred from the δD values of tourmaline and chlorite. This trend is consistent with mixing of the ore fluids with a cooler fluid that had substantially lower δ 18 O, and cannot be explained by fluid boiling. Based on structural evidence for an opening of the vein system and a transition from lithostatic to hydrostatic conditions at the onset of mineralization, we infer that ore deposition was caused by an influx of hot groundwater of meteoric origin which mixed repeatedly with tin-bearing magmatic brines. The oxidation, dilution, cooling, and acid neutralization of the ore fluids destabilized chloride complexes of tin and triggered the large-scale precipitation of cassiterite.

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