Abstract

Tin production from the central and southern parts of the Bolivian tin province has been derived largely from lode systems associated with high-level, late Tertiary stocks of intermediate composition. This paper proposes that several of these stocks, including those at the major Llallagua, Potosi, Oruro, and Chorolque mineralized centers, geological characteristics of which are summarized, contain stockwork, disseminated, and breccia-filling cassiterite mineralization. The deposits are designated as porphyry tin deposits, a name considered to possess the same status as porphyry copper and molybdenum deposits.The geological features of the deposits possess similarities with those of typical porphyry copper deposits. The stocks, commonly 1 to 2 km 2 in area and more than 1 km in known vertical extent, were largely passively emplaced and are pervasively sericite altered. Locally sericitic alteration grades outward to propylitic alteration, and at Potosi grades upward to silicification generated by hydrolytic leaching. A quartz-tourmaline core occurs in hydrothermal breccia at Chorolque. Pyrite halos are recognizable at Chorolque and Llallagua. The cassiterite is present as part of sericitic and quartz-tourmaline assemblages. Large volumes of rock contain grades of 0.2 to 0.3 percent Sn. Several pulses of hydrothermal breccia span the period of alteration-mineralization, and breccia carries important tin ore at Chorolque and Llallagua. The stocks are thought to have been overlain by stratovolcanoes at their time of emplacement; coeval volcanics are preserved at Oruro and Chorolque, and perhaps also at Potosi. A mineralized volcanic edifice remains at Chocaya and is believed to overlie a subjacent stock.The Bolivian porphyry tin deposits differ from many porphyry copper deposits: in not being susceptible to supergene enrichment; in not possessing known potassium silicate alteration; in being associated with stocks that have the form of inverted cones rather than upright cylinders; and in being cut by swarms of later vein deposits.An orthomagmatic model is accepted for the genesis of the porphyry tin deposits, with intrusion, brecciation, alteration, and porphyry-type and vein mineralization comprising a single, extended sequence of cogenetic events. The formation of a porphyry tin deposit is believed to be one of the last events at a volcanic center.

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