Magmatic salt melt and vapor: Extreme fluids forming porphyry gold deposits in shallow subvolcanic settings

Abstract

The recently discovered Biely Vrch deposit in the Western Carpathian magmatic arc is the most extreme example of a porphyry gold deposit, being practically free of copper, molybdenum or any other sulfide minerals. Microanalytical data on fluid inclusions in quartz veinlets, including a characteristic type of banded veinlets, show that this deposit formed from nearly anhydrous Fe-K-Na-Cl salt melts containing ~10 ppm Au, coexisting with hydrous vapor of very low density. This exceptional fluid evolution required an Fe-rich dioritic source magma that was emplaced at shallow subvolcanic depth (<3.5 km), directly exsolving a hypersaline liquid and magmatic vapor at high temperature (~850 °C). During ascent to the level of the porphyry intrusion (0.5-1 km), fluid expansion at high temperature but low pressure led to halite precipitation and further water loss to the vapor, generating an increasingly Fe-K-rich salt melt that transported high concentrations of Au but negligible Cu into the fractured porphyry stock. The low sulfur fugacity resulting from fluid expansion suppressed precipitation of sulfide, explaining the gold-only enrichment in this globallyrecurring but rather rare type of gold ore.

Mineralogy, provenance, and diagenesis of a potassic basaltic sandstone on Mars: CheMin X‐ray diffraction of the Windjana sample (Kimberley area, Gale Crater)

“…At least one rock in Gale Crater (named Preble) shows possible evidence of potassic metasomatism—it includes an area rich in Na, K, and Si that is interpreted as a veinlet rich in alkali feldspar and silica [ Sautter et al , ]. On Earth, many varieties of metasomatic rock are enriched in potassium and thus alkali feldspar (Table ), in part because potassium is readily soluble in aqueous and salt‐rich fluids, such as those that can form in groundwater or be exsolved from crystallizing magmas [ Beratan , ; Kodera et al , ; Sillitoe , ]. The examples of Table are from the terrestrial experience of rocks enriched in potassium and alkali feldspar, but it is not clear that these rock types (and their parent processes) are possible or present on Mars.…”

LA-ICP-MS analysis of fluid inclusions: contamination effects challenging micro-analysis of elements close to their detection limit

“…[29][30][31][32] Gold can also be present at µg•g -1 levels in natural fluids but it is generally close to the detection limit. 3,[33][34][35][36] Even though Au is expected to be enriched in S-rich vapor 3 37 , the reduced amount of an analyte in low-density vapor inclusions produces shorter signals, which are subject to larger uncertainties regarding representative quantification by sequential sampling of the signal 18,20 . Counting statistics is the major source of uncertainty for quantification of elements close to the analytical detection limit, requiring Poisson statistics 18,38,39 rather than a Gaussian approach 11 .…”

High‐temperature gold‐copper extraction with chloride flux in lava tubes of Tolbachik volcano (Kamchatka)

“…Furthermore, exceptionally high solubility of gold in anhydrous chloride melt at oxidized conditions at high temperature (~850°C) in the presence of silica has been recorded in natural quartz‐hosted inclusions (~10 ppm Au, Biely Vrch porphyry gold deposit, Koděra, Heinrich, Wälle, & Lexa, ) and laboratory experiments (Shmulovich, Bukhtiyarov, & Persikov, ). Importantly, octahedral crystals (Figure a,b) and mesh‐like aggregates of gold (Figure e) in chlorides from the Tolbachik lava tubes are indistinguishable from the experimental gold formed upon crystallization of molten salt (Figure f; Shmulovich et al, ).…”