Abstract

The Bolivian tin belt extends for about 800 km down the Eastern Cordillera of the Central Andes. In the northern part the ore deposits are mainly tin-tungsten veins which are associated with granite-granodiorite batholiths. Previous work has shown that the batholiths can be divided into a northern group (Cordillera Real) which gives Mesozoic K-Ar ages and a southern group (Illimani, Quimsa Cruz, and Santa Vera Cruz) which gives lower Miocene ages.In central and southern Bolivia the tin deposits are complex vein systems associated with near-surface stocks and subvolcanic eruptive complexes. The ages of these fall into two groups. A northern group, extending as far south as the Kari-Kari batholith near Potosi, ranges in age from 23 to 20 m.y. Several of the major tin deposits of Bolivia, including the Catavi deposit at Llallagua, fall within this group, which is apparently part of a larger lower Miocene igneous province that includes the plutons of Illimani and the Cordilleras Quimsa Cruz and Santa Vera Cruz. A southern group includes the Cerro Rico stock at Potosi and the mineralized eruptive complexes of the Quechisla region which give upper Miocene ages ranging from 17 to 12 m.y. There are also extensive post-mineral ignimbrite sheets which overlie rocks of both the above groups.The data show that igneous activity within the tin belt took place as a series of relatively discrete pulses, with a general southward shift with time. The north to south zonation of the metals contained in the ore deposits, from tungsten in the north, through tin, to base metals with tin in the south, is not consistently dependent on the age of the associated igneous rocks and appears to be primarily related to the depth at which the deposits were formed and hence to the regional erosion level, rather than to changes in the geochemistry of the magmas with time or space. The Miocene-Pliocene igneous rocks of the tin belt fit rather well into the consistent pattern of Andean magmatic evolution which is becoming recognizable throughout the Central Andes as a whole.

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