Geochemistry of metals and metalloids in siliceous sinter deposits: Implications for elemental partitioning into silica phases

Abstract

Sinter deposits are formed by precipitation of silica from hydrothermal fluids that have reached the surface environment. They are commonly found around hot springs and represent surface expressions of underlying geothermal systems and/or low sulfidation epithermal gold-silver hydrothermal deposits. Several studies have reported ppm to weight percent concentrations of metals (e.g., Au, Ag, Cu) and metalloids (e.g., As, Sb, B) in sinters capping geothermal systems and epithermal gold-silver deposits. However, the relation between the maturity of the siliceous sinter and its metal enrichment remains unknown. Here we use geochemical and mineralogical data that link the silica crystallinity degree with trace metal and metalloid contents in sinter. In this paper, we provide in situ trace element data in metalrich silica sinter samples from the Puchuldiza geothermal field in the Altiplano of northern Chile that record the complete diagenetic sequence from non-crystalline opal A to microcrystalline quartz. Combined SEM, XRD and LA-ICP-MS data show that the concentration of metals and metalloids in sinters from Puchuldiza display a strong correspondence with silica crystallinity. While arsenic and boron are predominantly enriched in the more amorphous silica phases (opal A/CT), gold and silver show higher concentrations in the more crystalline phases (opal C/quartz). Silica structural, morphological and geochemical transformations from its initial precipitation to its final maturation after diagenesis are responsible for this differential enrichment. During the initial stages, gold and silver are incorporated into silica microspheres as cationic species and/or metal nanoparticles or colloids, while arsenic and boron incorporation is controlled by As-bearing accessory minerals and Fe-oxyhydroxides. As diagenesis progresses and the crystallinity of silica increases, diffusion-driven processes such as Ostwald ripening may progressively enrich gold and silver in the sinter, while metalloids are depleted owing to the low retention of arsenic by silica. These findings indicate that the diagenetic transitions of silica, defined by significant structural changes that involve generation of surface defects and the creation of reactive sites, may play an important role in elemental uptake by silica in near surface environments.