San Rafael, Peru: geology and structure of the worlds richest tin lode

Stable Isotope Constraints on Ore Formation at the San Rafael Tin-Copper Deposit, Southeast Peru

“…Petrographic observations indicate that reopening of the early tourmaline veins was quite common, and that there were many episodes of brecciation. It is also apparent that some of the major tourmaline-bearing veins, which clearly follow the lode along strike and were responsible for establishing its initial structural framework, subsequently reopened during the ore stage (Mlynarczyk et al, 2003). The principal chemical changes associated with the strong sericite-tourmaline alteration were a nearly complete removal of alkalis and alkaliearth elements (Na, K, Ca, Ba, Rb, Sr, Cs, Li, and Cl) from the wall rock and a marked addition of B, Mg, and Fe, and possibly Sn, W, and In, though the latter could represent a late overprint (Mlynarczyk et al, 2009, submitted).…”

“…The pluton is a polyphase intrusion composed of coarse-to medium-grained K-feldspar megacrystic, cordierite-biotite-(±garnet-sillimanite) granite, leucogranite and granodiorite, with local minor enclaves of tonalite and quartz diorite. The magma was strongly peraluminous, S-type in character, and clearly formed due to the partial melting of metasediments (Mlynarczyk et al, 2003). However, it was not geochemically evolved, and the different igneous phases do not represent a fractional crystallization series (Dolejs et al, 2009, submitted).…”

“…The lode has an average strike of 330°, dips 40°to 75°NE, and is texturally quite complex, having undergone multiple episodes of vein reopening and brecciation. Mlynarczyk et al (2003), who provided a detailed description of the deposit geology, proposed that the large ore shoots at depth in the lode represent fault jogs formed during sinistral-normal strike-slip movement along the plane of the lode, which was synchronous with circulation of the ore fluids.…”

“…However, this hypothesis has not been properly tested because the contribution of meteoric water to hydrothermal systems depositing lode tin mineralization has been poorly documented. San Rafael, the world's richest primary Sn-Cu deposit (Kontak and Clark, 2002;Mlynarczyk et al, 2003), which is located in the high Andes of the Eastern Cordillera of southeastern Peru and is currently exploited by MINSUR S.A., is an ideal natural laboratory in which to investigate the various issues of lode tin genesis raised above. With a total resource of about 1 million metric tons (Mt) Sn, and an average grade of 4.7 wt percent Sn, the San Rafael lode, which has been exposed by mining activity over a vertical extent of more than 1,300 m, allows an unprecedented opportunity to advance our understanding of the formation of this important deposit type.…”

“…In addition, the deposit is very young (about 22-24 Ma; Clark et al, 1983;Kontak et al, 1987;Kontak and Clark, 2002) and, therefore, was not affected by any later geologic processes. Following a study of the deposit geology (Mlynarczyk et al, 2003) and its alteration (Mlynarczyk and WilliamsJones, 2006;Mlynarczyk et al, 2009, submitted), a comprehensive stable isotope (S, O, H, C) investigation of ore and gangue minerals and a preliminary fluid inclusion study were performed to evaluate the source of the fluids and metals, as well as the timing and the mechanism of ore deposition. Our data indicate that ore deposition was caused by an influx of hot groundwaters of meteoric origin and their mixing with the tin-bearing magmatic brines.…”