Metallogenesis and hydrothermal alteration at Cerro Rico, Bolivia
Cerro Rico is the world's largest silver deposit. Hypogene mineralisation is hosted by a sheeted, polymetallic vein system which cross-cuts a pervasively altered rhyodacite dome. The 13.8Ma dome was intruded along a regional dextral strike-slip fault. Subsequent movement focused stress within the dome, creating a closely-spaced fracture system, best described as an extensional duplex. Hydrothermal alteration shows features characteristic of both porphyry and epithermal deposits. Shallow acid-sulphate alteration, comprising sub-horizontal zones of advanced argillic alteration and residual vuggy silica, formed through the neutralisation of a supergene, acid-sulphate fluid derived from the atmospheric oxidation of hypogene H2S. Deep sericitisation which largely pre-dates the polymetallic vein mineralisation is the product of wall-rock reaction with magmatically-derived volatiles. Localised tourmalinisation resulted from the violent release of boron-rich fluids during early decompression events. A buried magma body is implied for the source of volatiles. Polymetallic vein mineralisation is vertically and laterally zoned from deep Sn-W-Bi-As-Cu assemblages to peripheral Pb-Zn-Sb-Ag. Hypogene silver occurs within complex Pb-Sb sulphosalts, tetrahedrite (freibergite), pyrargyrite and argentite. Mixing of reduced, near-neutral, hypogene Ag-bearing fluids and oxidised, supergene, acid-sulphate fluids caused the precipitation of 'bonanza' silver mineralisation within the acid-sulphate lithocap. Early pre-mineral fluids were hypersaline (34wt. %NaCl equivalent). Main stage ore fluids varied in temperature between 140 and 410°C with salinities averaging 11wt.% NaCl equivalent. Stable isotope data are consistent with a magmatic source both for the ore fluid and sulphur. Following collapse of the hydrothermal system, a fall in the palaeo-water table caused deep oxidation. Silver was released from hypogene sulphosalts and re-precipitated as disseminated acanthite and silver halides. Hypogene Ag2S remained stable in the weathering environment. The oxide zone, preserved by a semi-arid climate, currently constitutes a world-class silver orebody.