Abstract

Report of a meeting of the Mineral Deposits Studies Group held on 15–16 December 1986, at the University of Southampton. The meeting included a thematic session on fluid inclusions which is reported elsewhere in this volume.

Six presentations encompassed epithermal or volcanic-hosted precious-metal deposits and mineralized breccias. The Tambo deposit, near El Indio in Chile, was described by Bernard as a typical hot spring system with bedded sulphates, sulphate sinters, explosion breccias and native sulphur o sulphur overlying hydrothermal breccias and veins. Gold occurred as relatively coarse grains in the silicified wallrocks and also finely dispersed throughout barite. Mixing of ascending auriferous fluids with near-surface acid-sulphate waters was postulated to explain the mineral paragenesis. A similar mixing model was presented by Redwood to account for the disseminated and vein-type Au–Ag–Cu–Pb–Zn mineralization of the Altiplano in Bolivia. The mineralization was associated with silicic flow-dome complexes, and high-K calc-alkaline and shoshonitic magmatism; oxygen and hydrogen isotope data indicated an important magmatic input to the hydrothermal system.

A silver-dominated vein-type mineralization comprising silver sulphosalts, native silver, argentite, minor sulphides, calcite, and baryte at Caracoles, northern Chile, was described by Fontbote, Schmidt & Flint, who emphasized the localization of high-grade ore at the intrusive contact between a porphyritic granite and bituminous siltstones and limestones. The role of decompressive shock and hydraulic decrepitation in the generation of hydrothermal breccias and lodes was reviewed by Halls & Allman-Ward. Mineralized hydromagmatic breccias of Hercynian age were identified as the source of the gold