Evolution of clay mineral assemblages out of equilibrium in the Tolhuaca geothermal field, Southern Chile

Abstract

Clay minerals are alteration products in most of the active and fossil geothermal system. Many studies have considered both the sequential distribution from smectites to non-expandable di-or trioctahedral phyllosilicates as a function of the past or present thermal conditions. However, the fluid/rock ratio, the mechanism of alteration and the composition of the precursor seem to be important factors controlling the properties and nature of clay minerals.This study focuses on the clay minerals within the high-enthalpy Tolhuaca geothermal system (Southern Chile), strongly controlled by the nature of the Liquiñe-Ofqui Fault System and hosted in the Holocene Tolhuaca volcano. A continuous 1100 m depth drillcore (Tol-1) has been used to study hydrothermal alteration in this active geothermal system. This vertical sampling and subsequent XRD study offers the opportunity to elucidate how the clay mineral assemblages vary spatially as a function of intensity of hydrothermal alteration and how this alteration was mostly controlled by the structural domains defined. The well lithology is dominated by basaltic andesite lava flows and minor intercalated volcaniclastic layers. Three main structural-alteration domains have been defined based on detailed structural studies and hydrothermal assemblage: (1) argillic zone (0-300m), rich in clay minerals and Fe-oxides, consists of steeply dipping veins and normal faults; (2) subpropylitic zone (300-670m), with a pervasive alteration to clay minerals filling open spaces and discrete reverse faults; and (3) propylitic zone (670-1100m), where chlorite and epidote dominate in veins and minor fault-veins. Temperature obtained from fluid inclusions mostly fit with the present day in situ measured along the well.Concerning clay mineral assemblages, four main clay mineral alteration zones, from top to bottom, can be defined: zone I (20-160ºC) dominated by smectite and coincident with the argillic zone; zone II (160-220ºC) dominated by heterogeneous phyllosilicate assemblages (corrensite, smectite, chlorite and I-S R1); zone III (220-245ºC) dominated by chlorite, I-S R1 and I-S R3; and zone IV (245-270ºC) dominated by chlorite with absence of smectite.The vertical distribution of clay minerals shows a pattern of transformation from smectite at the top to illite at the bottom, with the progressive formation of corrensite and chlorite. Smectite is the dominat phyllosilicate in the argillic zone, where the temperature reach values around 160ºC. Subpropylitic zone is characterised by the occurrence of heterogeneous mineral assemblages. Clay mineral assemblage indicates that the mineral reactions took place far from the chemical equilibrium with nucleation of metastable phases, possibly representing various nucleation conditions (i.e. coexistence of smectite and I-S R1). These mineral assemblages could correspond to the partitioning of the geothermal system by different types and nature of fractures. In fact, the alteration proposed model for this geothermal system implies an early hydrothermal event that sealed the fractures and preserved it from further alteration processes. The hydrothermal alteration in propylitic zone appears to be temperature dependent. However, reaction of I-S R1 to R3 should occur between 150-190ºC, temperatures lower than the current ones. Therefore, the propylitic alteration occurred in a past geothermal event at lower temperature than the present geothermal fluids.