Magmatic vapor contraction and the transport of gold from the porphyry environment to epithermal ore deposits

Abstract

Figure 1. Phase stabilities in NaCl ؉ H 2 O fluid system, emphasizing two-phase coexistence surface (vapor ؉ brine; gridded by isopleths and isotherms; Driesner and Heinrich, 2003). Critical curve (heavy gray line at crest of two-phase surface) swings from high salinities at high pressure and temperature (P, T ) across to lower salinities at low P and T. Hot vapor-like fluid (V) of low to medium salinityoriginating from slightly above two-phase surface or coexisting with brine (B) along that surface-can be cooled and contracted to an aqueous liquid (L) without heterogeneous phase change provided that, during this stage of cooling, P remains above two-phase surface (arrows in 7% NaCl plane). Backface shows projection of critical curve (thick gray), halite saturation envelope (thin line), and one P-T path, including isobaric cooling interval (long arrow segment) modeled in Figure 2. ABSTRACTFluid-phase stability relations combined with thermodynamic modeling using fluid-inclusion analyses and new gold-solubility experiments lead to an integrated geological interpretation linking epithermal gold mineralization and porphyry-style ore formation to the cooling of hydrous magma chambers. The essential chemical requirement for gold transport to low temperatures is an initial excess of sulfide over Fe in the magmatic fluid, which is best achieved by condensing out Fe-rich brine from a buoyant, low-to medium-salinity vapor enriched in volatile S. This vapor can contract directly to an aqueous liquid, by cooling at elevated pressure above the critical curve of the salt-water fluid system. Physical and chemical conditions are matched when magmatic fluid is released through a gradually downward-retracting interface of crystallizing magma beneath a porphyry stock, predicting the consistent zoning and overprinting relations of alteration and mineralization observed in magmatic hydrothermal systems.

Formation and Aggregation of Gold (Electrum) Nanoparticles in Epithermal Ores

“…We envision that this "transporting solution" was a low-density magmatic fluid like those that have been described as separating from more saline, higher density fluids in the "porphyry environment" [54,55]. These nanoparticle-bearing, low-density magmatic fluids would have been enriched in such volatile elements as Au, Ag, Cu, Se, As, etc.…”

Subsurface hydrothermal processes and the bioenergetics of chemolithoautotrophy at the shallow-sea vents off Panarea Island (Italy)

“…5 reviewed extensively (e.g., see Alt, 1995;Berndt et al, 1988;Bischoff and Rosenbauer, 1985;Fournier, 2007;Foustoukos and Seyfried, 2007;German and Von Damm, 2003;Hanningtion et al, 2005;Hedenquist and Lowenstern, 1994;Heinrich et al, 2004;Henley and Ellis, 1983;Seyfried and Mottl, 1982;Tivey, 2007). The final compositions are a function of many factors including fluid-mineral equilibrium, phase separation, magmatic inputs, mineral precipitation, and mixing.…”

“…Recent evidence suggests that supercritical fluids occurring near a magma chamber can ‗compact' without crossing the two-phase solvus (i.e., do not boil), thus creating very high salinity fluids crucial for development of high sulfide epithermal mineral deposits (Hedenquist and Lowenstern, 1994;Heinrich et al, 2004). However, subcritical phase separation below the critical point of seawater occurs as a result of decompression during upflow, wherever the 2-phase boundary is intercepted, producing ~3.2 wt% NaCl liquid coexisting with a low-salinity vapor (Berndt et al, 2001;Foustoukos and Seyfried, 2007;Seyfried et al, 2003).…”

Indosinian isotope ages of plutons and deposits in southwestern Miaoershan-Yuechengling, northeastern Guangxi and implications on Indosinian mineralization in South China

“…Most of deposits around the Miaoershan-Yuechenling complex pluton have W-Sn (Mo) element association and high temperature alteration such as greisenization, suggesting they are related to magmatic hydrothermal activities. Magmatic hydrothermal deposits are genetic related to an aqueous volatile phase exsolved from silicate melts [32,[38][39][40][41][42][43][44][45][46] or fluids in equilibrium with silicate melts. If the silicate melts underwent mineralization processes during the ascending and crystallization of the melts, the aqueous volatile phase resolved from the melts should have reacted with the adjacent country rocks.…”