Significance of phyllosilicate mineralogy and mineral chemistry in the epithermal environment. Insights from the Palai-Islica Au-Cu deposit (Almería, SE Spain)

Abstract

X-ray diffraction, optical and electronic microscopy (scanning and transmitted), electron microprobe and Fourier transform infra red spectroscopy has been used to study phyllosilicates in the Palai-Islica Au-Cu epithermal, volcanic-hosted deposit, in order to link phyllosilicate mineralogy and mineral chemistry to ore genesis. Thus, different phyllosilicate assemblages characterize two types of mineralization, and related hydrothermal alteration. Chlorite and mica appear in polymetallic quartz veins with sulfides, and in the related chloritic and sericitic hydrothermal alteration. These minerals have notable textural and chemical differences (i.e. Fe/(Fe+Mg), Si and Al in chlorite and illitic and phengitic components in mica) amongst veins and altered rocks, revealing different genetic conditions. These chemical features also distinguish propylitic and regional, non ore-related, low-temperature alteration. Hot hydrothermal fluids of near-neutral pH are responsible for vein mineralization and alteration. Illite, interstratified illite-smectite, kaolinite and pyrophyllite are characteristic, with a distribution pattern by zones, for the intermediate argillic and advanced argillic alteration around areas of silicification. In the latter, native gold appears associated to interstratified illite-smectite, suggesting a relatively low temperature formation. Hot, low-pH fluids are responsible for this mineralization and alteration assemblage. The present study contributes to epithermal models showing the co-existence of two different alteration styles in the same hydrothermal system.