Advanced argillic lithocaps in the Bolivian tin-silver belt

Abstract

Zones of advanced argillic alteration constituting lithocaps are commonplace in the shallow parts of porphyry copper systems. Similar lithocaps are also recognized in the shallowly eroded southern part of the Bolivian tin-silver belt, where mineralization typically is centred on felsic volcanic domes. A well-preserved lithocap at Potosí is dominated by vuggy residual quartz and contained the world's largest silver resource, whereas the basal remnant of a lithocap at Pulacayo is composed of barren quartz-alunite. Minor gold occurs in the structurally controlled roots of a lithocap at Tasna. The mineralization in these lithocaps is of high-sulphidation type and was generated in the epithermal environment. In contrast, the tin- and base metal-bearing massive sulphide veins and associated sericitic and quartz-tourmaline alteration that underlie the Bolivian lithocaps are typified by sulphides of low sulphidation state, but are dominantly mesothermal rather than epithermal in character. Magma chemistry is believed to account for the characterization of Bolivian lithocaps by silver-tin-antimony and porphyry copper lithocaps by gold-copper-arsenic. Low-grade, bulk-tonnage silver mineralization within Bolivian lithocaps and high-grade tin-bearing veins concealed beneath them both constitute attractive exploration objectives.