A comparative study of APS minerals of the Pacific Rim fold belts with special reference to south American argillaceous deposits

Abstract

Aluminum-phosphate-sulfate (APS) minerals are of particular interest to exploration geologists in search of nonmetal deposits emplaced in volcanic or subvolcanic environments. These minerals are often more sensitive to changes in the physicochemical conditions during mineralization than phyllosilicates and thus can shed some light on the evolution of argillaceous deposits that may be of supergene or hypogene origin. The APS-bearing argillaceous deposits under study are located in El Salvador, Peru, Chile and Indonesia and occur in volcanic and pyroclastic rocks of the circum-Pacific Rim fold belts. Some sediment-hosted kaolin deposits from Chile where APS minerals were not found and volcanic-hosted Sb–Au deposits from Bolivia, which contain massive alunite as the only alteration mineral, were also discussed to connect this research to neighboring realms such as sedimentary host rocks and metallic deposits. The mineral assemblages in volcanic and pyroclastic rocks consist of alunite, woodhouseite, and crandallite solid solution series with Fe disulfides, Fe oxide hydrates, 7, 10 and 14 Å sheet silicates, siliceous compounds, zunyite and topaz. The APS-bearing argillaceous deposits may be subdivided on the basis of their facies patterns (first-order-classification) and mineral zonation (second-order-classification). Facies patterns in the deposits vary with the distance of mineralization from the feeder channel system. A detailed study of these patterns provides an overview of temperature regime across the entire district of argillaceous mineralization. The APS-bearing argillaceous mineralization under consideration correspond to the following alteration patterns: advanced and intermediate argillic alteration, silicification, and sericitization, which is common to epithermal and porphyry-type ore deposits. The mineral zonation of APS mineral assemblages may be correlated with equivalent APS mineralization in ore deposits. The fourfold subdivision of APS mineralization reflects a decrease in the temperature of formation that controls the accommodation of bivalent (Sr 2+, Ba 2+) and trivalent (Ce 3+) cations in the lattice of APS minerals and is governed by the chemical composition of mineralizing fluids and preexisting element concentration in the volcanic host rocks.