THE AGE AND THERMAL HISTORY OF CERRO-RICO-DE-POTOSI, BOLIVIA

Cerro Rico de Potosi, Bolivia, is the world's largest silver deposit a nd has been mined since the sixteenth century for silver, and for tin and zinc during the twentieth century, together with by-product copper and lead. The deposit consists primarily of veins that cut an altered igneous body that we interpret to be a dacitic volcanic dome and its underlying tuff ring and explosion breccia. The deposit is composition ally and thermally zoned, having a core of cassiterite, wolframite, bi s-muthinite, and arsenopyrite surrounded by a peripheral, lower-temper ature mineral assemblage consisting principally of sphalerite, galena, lead sulfosalt, and silver minerals. The low-temperature assemblage a lso was superimposed on the high-temperature assemblage in response to cooling of the main hydrothermal system. Both the dacite dome and the ore fluids were derived from a larger magmatic/hydrothermal source at depth. The dome was repeatedly fractured by recurrent movement on the fault system that guided its initial emplacement. The dome was extrud ed at 13.8 +/- 0.2 Ma (2 sigma), based on U-Th-Pb dating of zircon. Mi neralization and alteration occurred within about 0.3 my of dome empla cement, as indicated by a Ar-40/Ar-39 date of 13.76 +/- 0.10 Ma (1 sig ma) for sericite from the pervasive quartz-sericite-pyrite alteration associated with the main-stage, high-temperature, mineralization. The last thermal event able to reset zircon fission tracks occurred no lat er than 12.5 +/- 1.1 Ma (1 sigma), as indicated by fission-tract datin g. Minor sericite, and magmatic-steam alunite veins, were episodically formed around 11 Ma and between 8.3 and 5.7 Ma; the younger episodes occurring at the time of extensional fracturing at Cerro Rico and wide spread volcanism in the adjacent Los Frailes volcanic field. None of t hese younger events appear to be significant thermal/mineralizing even ts; the exceptionally flat thermal release pattern of Ar-39 from seric ite and the results of the fission-tract dating of zircon show that no ne of the younger events was hot enough, and lasted long enough, to ca use significant loss of Ar or annealing of zircon fission tracks. U-Th -Pb dating of zircon cores indicates a Precambrian progenitor for some zircons, and REE analyses of dated samples of hydrothermally altered dacite show the presence of a prominent positive Eu anomaly, which con strains interpretations of the origin and evolution of the magmatic/hy drothermal system.