Geochemistry of precious metals and metalloids in silica sinter deposits from Puchuldiza, northern Chile

<p>Silica sinter are formed by precipitation from hydrothermal water as they discharge and cool at surface. They are composed mainly by silica, and are surface expressions of underlying geothermal systems and low sulphidation epithermal Au-Ag deposits (Rodgers et al.,2004). Sinter deposits can host significant amounts of metals such as Au, Ag, Hg and Cu, and metalloids such as As and Sb ( e.g.,Saunders,1990);hence, paleo-sinter are commonly used as paleo markers and vectors to mineralization of concealed mineral and geothermal<br>resource (Guido et al., 2002).</p> <p>Sinter deposits are texturally complex and exhibit different micro- to nano-morphologies and variable structural order, including silica phases as opal A, opal CT, opal C and microcrystalline quartz (Lynne et al., 2007). The formation of sinter begin with the silica nucleation and polymerization of Si(OH)4 in the geothermal fluid, promoting the colloidal particle growth and aggregation by Ostwald ripening<br>(Tobler and Benning, 2013). The transition from the amorphous opal A phase to crystalline quartz is commonly thought to occur as the result of variation in thermodynamic equilibrium conditions influenced by diagenetic processes (Rodgers et al., 2004; Lynne et al., 2007).<br>Previous studies in sinter deposits from geothermal systems in the Chilean Altiplano (e.g., El Tatio; Garcia-Valles et al., 2008; Nicolau et al.,<br>2014) have documented unique geochemical and textural characteristics. For example, mineralogical and textural observations suggest<br>that high-altitude environmental conditions may play an important role in silica precipitation rate, in particular, high evaporation rates, daily temperature fluctuations and lower boiling temperature (Nicolau et al., 2014). Furthermore, it has been suggested that dissolved species and metals may play an enhancing effect on silica precipitation rate (e.g; Tobler and Benning, 2013). However, very little information is currently available on the potential effect of high-altitude environmental conditions on silica precipitation and trace metal incorporation<br>in such systems.<br>This study aims to constrain the mineralogy, textures, and trace element geochemistry of silica sinter deposits from a high-altitude active geothermal field. This research is carried out at the Puchuldiza geothermal field in the Altiplano of northern Chile (Figure 1), a metal-rich geothermal system that has been explored for both high-enthalpy geothermal resources and epithermal gold deposits.</p> <p> </p

Geoquímica de Sinter Silíceo en el campo geotérmico de Puchuldiza

<p>Resumen Charla XIV Congreso geológico chileno.</p> <p>Los depósitos de sínter silíceo son rocas sedimentarias químicas ricas en sílice formadas por precipitación a partir de fluidos hidrotermales. Se asocian comúnmente a actividad geotermal y a mineralización epitermal de oro. A pesar de su importancia como marcadores en superficies de sistemas de alta entalpía y como vectores de mineralización aurífera, el contenido y especiación de metales (ej. Au, Hg) y metaloides (ej. As, Sb) en estos depósitos no se ha constreñido adecuadamente. En particular, se desconoce la relación entre el grado de cristalinidad o “madurez” de la sílice y la presencia/concentración de determinadas especies de metales y metaloides. En este estudio, se reportan observaciones mineralógicas por difracción de rayos X y análisis de elementos traza (LA-ICP-MS) en depósitos de sínter silíceo del campo geotérmico de Puchuldiza, en el Altiplano (4200 m s.n.m.), Región de Tarapacá. Se identifica mayoritariamente ópalo A, al igual que en El Tatio. Además, se reconoce que el Au y el Hg se ligan a fases maduras, mientras que el As tiende a asociarse a las fases inmaduras. Se sugiere preliminarmente un importante impacto de la morfología y grado de cristalinidad de la sílice en la captura de metales y metaloides en sistemas geotermales.</p