Very low-grade secondary minerals as indicators of palaeo-hydrothermal systems in the Upper Cretaceous volcanic succession of Hannah Point, Livingston Island, Antarctica

Abstract

The Upper Cretaceous basic volcanic succession in Hannah Point, Livingston Island, Antarctica, presents a widespread occurrence of very low-grade secondary minerals. They occur filling amygdules, veins and veinlets, and replacing phenocrysts and groundmass/matrix. The paragenetic associations include minerals such as laumontite, heulandite, stilbite and clinoptilolite; mafic phyllosilicates corresponding to chlorite and smectite mixed layers (compositions ranging from 57% to 84% of chlorite), albite, calcite and minor celadonite. The mineral assemblages indicate, based on laboratory and field studies, these mineral paragenesis temperatures of 150200 C and pressures of 600-1.800 bars, which agrees with the calculated equilibrium temperatures of 160190 C, using chlorite geothermometry. These burial pressures, which were estimated from paragenesis, cannot be attained considering the present thickness of 500 m of the sequence, because at least 1 km of erosion is required to produce the mineral associations. Based on textural evidence, three successive stages are proposed to explain the genesis of the secondary minerals: (1) mafic phyllosilicates +/- celadonite, (2) zeolites and (3) calcite. The characteristics of these stages point to a regional burial metamorphism (stage 1) superimposed by hydro thermal alteration (stages 2 and 3). The mineral paragenetical evolution can be used as a proxy for the prospection of modem geothermal reservoirs by allowing the identification of hydrothermal alteration processes and burial metamorphism.