Thermodynamic model of ore-forming processes in a submarine island-arc hydrothermal system

Abstract

A thermodynamic model suggested for ore-forming processes in a hydrothermal system (HS) in an island arc is based on the technique suggested earlier in [1] for simulating ore-forming hydrothermal systems in mid-oceanic ridges. This technique make use of the principle of flow-through multistep reactor and encompasses (a) the region where hydrothermal solutions are generated when seawater interacts with rocks (descending convection branch); (b) the region where material is transported with the solution at decreasing pressure (feeder channel); and (c) the region where the ore material is deposited (orebody). Hydrothermal systems in island arcs exhibit the following distinctive features taken into account in the model: (1) the composition of the host crustal rocks (rocks of mafic-acid composition instead of basalt and serpentinite) and (2) possible significant involvement of magmatic gases in the feeding of the hydrothermal system. The naturally occurring prototype of the simulated system is the hydrothermal system in the caldera of a submarine volcano in an island arc. The model is simulated in a number of variants in which the hydrothermal fluid is exogenic (heated seawater convecting through hot volcanic rocks), magmatic, or mixed (magmatic plus exogenic) is involved. The simulations were carried out using the HCh version 4.3 [2] program package for the multisystem H-O-K-Na-Ca-Mg-Fe-Al-Si-C-S-Cl-Cu-Zn-Pb-As-Sb-Ag-Au at temperatures of 25–370°C and pressures of 10–500 bar. The multisystem included 88 possible solid phases and aqueous solution with 95 species. The thermodynamic properties of compounds were calculated using the UNITHERM databank. The model is underlain by the principle of multiwave flow-through multistep reactor (MFTMR) with a starting rock/water (R/W) ratio of 1: 1. As progressively more solution portions passed through the rocks, the participation of fresh rock in the interaction accordingly diminished because the rock material was gradually exhausted in the system. The magmatic fluid had a composition selected based on data on fumaroles at Kudryavyi volcano [3] with a correction for the degassing pressure. The evolution of ore deposition was simulated in compliance with the scheme described in [4], which was implemented using the technology of “openness from above” [3]. The model was simulated with various compositions of the host rocks (basalts, andesites, dacites, and rhyolites) and the origin of the fluid (magmatic fluid alone, seawater alone, and variable proportions of both). Our simulation results indicate that the metallogeny (relative enrichment in Pb, As, Sb, or Ag) of island-arc ore deposits is controlled by the abundances of metals in the host rocks predominant in the hydrothermal system. The mineralogy and geochemistry of ores generated in arc hydrothermal systems are predetermined by the effective transport of metalloids (S, As, and Sb) that have a high migration capacity in these systems. Magmatic gases introduced in the hydrothermal systems play dualistic roles in the ore-forming processes. If the hydrothermal fluid in a hydrothermal system is dominated by magmatic components, deposits of native sulfur are formed, and the precipitation of base metal is thereby suppressed because of the high acidity of the generated hydrothermal solutions. The involvement of magmatic gases in an amount of a few percent in a hydrothermal system enhances the overall oregenerating potential of the system in terms of sulfide ores.