Silica hydrate preserved with δ¹⁸O-rich quartz in high-temperature hydrothermal quartz in the high sulfidation copper-gold deposit at El Indio, Chile

Quartz microcrystals from the El Indio Au-Ag-Cu deposit (Chile) preserve a rare glimpse into the high-temperature evolution of silica. Here, we show for the first time that aggregates of euhedral quartz microcrystals preserve cryptocrystalline cores that contain silica hydrates "opal" and moganite. We propose that these phases are metastable remnants of progressive dehydration from a precursor silica hydrate phase. Evidence for sequential dehydration to from silica hydrate to quartz (silica hydrate[U+232B]opal[U+232B]moganite[U+232B]quartz) is provided by SHRIMP ¹⁸O microanalytical data that show oscillatory isotopic zoning from 3.6‰ to 16.2‰ δ¹⁸O (±0.5‰) coupled with K and Al variations. We estimate that the precursor silica hydrate deposited between ~480-680°C and contained 32-63wt% H₂O. Silica hydrate is metastable with respect to quartz and forms during rapid deposition of silica at high silica supersaturation, a consequence of rapid expansion of magmatic fluid into the fracture array that hosts the El Indio copper-gold deposit.Modern understanding of ore-forming fluids in hydrothermal ore deposits is largely underpinned by the assumption that quartz and its included fluids faithfully record depositional conditions. The discovery of silica hydrate affects the paragenetic and geochemical interpretation of quartz and included fluids. Quartz matured from silica hydrate would record "pseudo-primary" fluid inclusions such that homogenization temperatures record retrograde rather than depositional conditions while δ¹⁸O data may bias fluid provenance interpretation within sub-volcanic systems.