Abstract

Central and northern Chile hosts several well-known epithermal precious-metal deposits associated with kaolinite-alunite-quartz alteration, but only in the Combarbalá district (Coquimbo region) is the altered rock extracted for ornamental purposes. This ornamental rock, called combarbalita or combarbalite, is mined from pockets and veins hosted by altered, sub-horizontal, pyroclastic sedimentary strata of Cretaceous age that crop out around the town of Combarbalá. A study has been undertaken to define the conditions required for combarbalite formation and to evaluate, by analogy with Cu-Au and Cu-Pb deposits in adjacent districts, the significance of silicified bodies within the advanced argillic alteration as possible indicators of metalliferous mineral concentrations in the underlying propylitic andesites. Field observation, structural analysis, microscopy, X-ray diffraction and chemical analysis were the principal methods used. The pyroclastic sediments that host combarbalite are crosscut by cylindrical, massive silica bodies interpreted as vents and considered to be the principal conduits for hydrothermal fluids. The silica bodies contain Cu and Pb minerals, have anomalous contents of As, Sb, Hg, Ag and Au and are cut by barite veins. Regional structural control for the silicic and advanced argillic alteration was exerted by faults oriented N 60° W, obliquely to the present north-south graben structure in which the district is located. Malachite and cerussite mineralization roughly follows these north-west-trending fractures, whereas fracture-bound chrysocolla, pyrite and chalcopyrite are found in the underlying propylitic alteration. Sulphide mineral concentrations are likely to be located close to the intersection of the vent structures with the propylitic alteration. As to the existing mining activity, the best, massive varieties of combarbalite are developed in conglomerates, where very fine-grained mineralogy gives the rock workability and preserves its attractive original textures.