Feldspar-brine alteration at high-temperatures

Abstract

A magmatic volatile phase (MVP) may be exsolved from a silicate melt through decompression during ascent and/or the crystallization of anhydrous mineral phases. MVPs are commonly chloride-rich fluids and are hypothesized to induce hydrothermal alteration in and around igneous intrusions. Feldspars are common alteration products in these systems with K-feldspar (potassic alteration zone) often found in areas directly above cupolas of the igneous intrusions. Albite-bearing alteration assemblages are found generally on the flanks of the intrusions, below the cupolas, or more distally. Mineral alteration reactions near the intrusion are likely influenced wholly or partially by a subcritical, vapor undersaturated high-salinity liquid phase (brine). Whereas the K-feldspar-MVP system is well studied, there is a dearth of data on albite-brine systems at elevated temperatures and pressures. This study addressed mineral phase stability and equilibrium brine compositions in the albite-andalusite-quartz-brine system (65 wt% NaCl equivalent; NaCl-HCl-H2O) at 600, 650, and 700 °C and 80 MPa. The stable mineral phases were ascertained through an examination of crystal morphology, optical microscopy, and by chemical compositions. The albite + andalusite + quartz + brine phase boundary was found to be at CNaClbrine/CHClbrine molality values of 87.5 ± 12.5, 67.5 ± 7.5, and 40 ± 10 at 600, 650, and 700 °C, respectively. Apparent equilibrium constants, not accounting for activity coefficients, for the reaction: albite + HCl = 0.5 andalusite + NaCl +2.5 quartz +0.5 H2O were calculated as 70 ± 10, 54 ± 6, and 32 ± 8 at 600, 650, and 700 °C, respectively. The alteration of albite to andalusite occurs at much lower HCl concentrations than is required to alter K-feldspar to andalusite for equivalent concentrations of chloride salts. Thus, HCl concentrations of brines within the potassic and/or sericitic alteration zones can be much higher than in albite-bearing alteration zones. The data presented in this study indicate that albite-rich mineral zones on the flanks of igneous intrusions are most likely produced by a low-acidity fluid with an elevated CNaClfluid/CKClfluid. Fluids that meet these specifications, and may be significant sources of sodic alteration, include non-magmatic fluids flowing towards the intrusion or a descending magmatic brine depleted in cations and metals by mineral alteration reactions near the cupola. Although the sodic alteration zone is unlikely to host significant concentrations of metals in porphyry ore systems, it can provide significant constraints on metasomatic reactions and fluid flow pathways. Sodic and sodic-calcic alteration sequences in iron-oxide‑copper‑gold deposits are likely produced by a low-acidity, saline fluid and, unlike porphyry systems, correlate with high metal contents which indicate that the precipitation mechanisms controlling ore distribution in these systems may not be as dependent on neutralization reactions as in potassic and sericitic alteration sequences.