Igneous geology and the evolution of hydrothermal systems in some sub-volcanic tin deposits of Bolivia
Authors: Grant J. N. , Halls C. , Avila W. , Avila G. 
Published: May 2008

Synopsis

The tin deposits of the southern part of the Cordillera Oriental of Bolivia consist of complex vein systems and pervasive disseminated mineralization within zones of strong hydrothermal alteration in, or at the margins of, eruptive centres of late Tertiary age. At some centres erosion has removed the volcanic superstructure completely and only intrusive rocks are preserved (Llallagua, San Pablo de Morococala). At others, both the intrusives and coeval volcanics are preserved (Chorolque, Oruro, Potosi), whereas in some cases only the partially eroded volcanic superstructure is exposed and no unequivocally intrusive rocks are known (Chocaya, Tatasi).

In general, the geological relationships at all these deposits suggest that the mineralization is the product of hydrothermal systems generated in the inner, deeper regions of terrestrial strato-volcanoes.

The igneous rocks most closely associated with mineralization are strongly altered quartz porphyries. Their original composition was probably rhyodacite. Where stocks are preserved, they are complex bodies, often conical in vertical section, narrowing with depth. They show evidence of several phases of intrusion, igneous brecciation and explosive hydrothermal brecciation, which pre-date the formation of the major vein systems.

Silicate alteration assemblages include quartz-tourmaline, tourmaline-sericite and sericite-clay, which often show rough concentric zonation in that order outward from the centre. An outermost aureole of propylitic alteration is sometimes present where there are remnants of the volcanic superstructure. Alteration is pervasive and not controlled by the major veins. Low-grade cassiterite mineralization is dispersed throughout the inner zones.

Fluid inclusion studies at Chorolque show that the hydrothermal system was initiated by the separation of a highly saline brine, or melt, of complex chemistry, which formed at temperatures above 500°C. The pervasive quartz-tourmaline alteration, initial fracturing of the igneous rocks of the vent and the hornfelsing of the adjacent sedimentary rocks were accomplished while temperatures fell to below 450°C and intermittent boiling took place. Widespread disseminated cassiterite was deposited during this phase. The earliest stages of growth of the major quartz-cassiterite veins took place at temperatures of around 400°C, from a fluid the composition of which was similar to that of the earliest fluid, though substantially diluted (about 40 wt % NaCl equivalent) Vein growth continued while temperatures fell to below 200°C and salinity decreased. Most cassiterite deposition seems to have taken place in the temperature range 300–250°C and may have been accompanied by a major decrease in the salinity of the fluids.

The data obtained from the fluid inclusion studies, taken in conjunction with the gross geometry of the mineralization, suggest that hydrothermal processes at the volcanic centres were initially controlled by failure of the balance between confining lithostatic pressure and the pressure of the hydrous fluid residuum in the differentiated parts of the rhyodacite magma, leading to pervasive hydraulic fracturing, brecciation and alteration, of which the early generation of cassiterite mineralization was an integral part. The upper levels of the mineralized volcanic structures appear to have stabilized following the hydrothermal exhaustion of the apical portions of the sub-volcanic stocks. The focus of hydrothermal activity must then have been relocated at greater depths from which mineralization was controlled by the interplay of tectonic stresses in the volcanic infrastructure and surrounding basement and the build-up of hydrothermal fluid pressure in parallel vein-fault systems.

This concept of a bimodal style of mineralization related to a retreating hydrothermal focus can lead to a clearer understanding of the xenothermal or ‘telescoped’ type of mineral and temperature zonation in sub-volcanic systems. Temperature data obtained from fluid inclusion studies show, however, that mineral paragenesis is not an unambiguous criterion of temperature zonation and that the two have to be considered independently before a combined synthesis becomes meaningful.