| Title | Multiple Generations of Silica in the Tolhuaca Geothermal System, Southern Chile, Constrained Using Sem-Based Cathodoluminescence Imaging and Fluid Inclusions Data |
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| Authors | B. Alvear, D. Morata, D. Moncada, M. Reich, A. Gómez |
| Year | 2016 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Andean Geothermal System, SEM-CL, mineral texture, fluid inclusions, boiling, flashing |
| Abstract | Imaging of silica minerals by scanning electron microscopy (SEM)-Cathodoluminescence (CL) reveals unique textures, and combined with fluid inclusion (FI) studies, these data can be used to constrain the evolution of geothermal systems. Here we present a SEM-CL and FI study of silica minerals that is aimed at increasing our understanding of the active Tolhuaca Geothermal System (TGS). SEM-CL observations in representative samples collected from a ~1000m long borehole reveal complex textural features and growth patterns in silica veins. Four silica generations were identified (G1-G4). The G1 group is characterized by colloform silica with bands of varying CL intensity. The G2 group shows jigsaw textures with bright-CL contrast, while euhedral quartz from the G3 group shows oscillatory zoning-CL and primary Fluid Inclusion Assemblages (FIAs) with liquid-rich inclusions and consistent liquid-to-vapour ratios. Finally, the G4 group is composed of massive silica with a homogeneous intermediate-CL contrast and isolated quartz crystals that are individually broken within the massive silica and form brecciated textures. These crystals show secondary FIAs that co-exist with liquid-rich and vapour-rich inclusions. FI microthermometric measurements from veins show a range of homogenization temperatures (Th) divided into two groups. Primary FIAs in group 3 in quartz homogenize from 170-293°C and the ice melting temperatures (Tm) range from -3.15 to -1.45°C. The G4 secondary FIAs show Th from 234 to 242°C and a Tm of -1.15°C. The recognized silica generations can be correlated with the different stages of evolution previously proposed by Sánchez-Alfaro et al. (2016) for the TGS: A stage-S1 characterized by flashing followed by boiling (S2), and a S3-stage dominated by non-boiling processes that preceded a final episode of brecciation of silica minerals and boiling (S4). The detailed SEM-CL characterization of silica paragenesis at the TGS is in agreement with previous studies of the TGS and confirms that these mineralogical studies provide useful information to refine conceptual models of geothermal systems. |