The hydrothermal fluid of Archæan lode-gold deposits at different metamorphic grades: compositional constraints from ore and wallrock alteration assemblages

Abstract

A low-salinity, mixed aqueous-carbonic fluid is common to all Archaean lode-gold deposits throughout the range of mineralising conditions from sub-greenschist to lower-granulite facies temperatures. Alteration assemblages and fluid-inclusion data give constraints on the fluid composition. Fluid XCO2 is 0.1–0.3 in typical greenschist-facies (mesothermal) deposits. At higher temperatures, the assemblages are consistent with formation from a fluid of similar composition, but slightly higher or lower XCO2 cannot be ruled out, and fluid-inclusion data indicate that CH4 may be an important component in ore fluids at these temperatures. Fluid pH is neutral or weakly alkaline at all conditions. A range of relative oxidation states of four orders of magnitude fO2 is indicated at any temperature, with deposits more oxidising relative to QFM at lower temperature. Sulphur contents of the fluids vary from ≈ 10 to 10−3.5m∑S, with a trend towards lower sulphur contents at lower temperatures. The relative concentrations of major cations in solution are similar at all conditions with Na ≫ K ≥ Ca, although Ca may be less abundant at low temperatures. The broad similarities in ore-fluid composition at all temperatures give support to ‘crustal-continuum’ models, in which Archaean lode-gold mineralisation involved either a single fluid moving through the middle and upper crust, or derivation of ore fluids by similar processes at different crustal levels. Many of the compositional differences between high- and lowtemperature ore-fluids may be attributed to evolution of deep-sourced hydrothermal solutions as they rise along structurally-controlled conduits. The constancy of major ore-fluid component concentration (e.g. CO2, Cl, ± K) suggests fluid-buffering and high fluid-rock ratios along fluid pathways. Fluid-buffered conditions can also explain the ore-fluid fO2-temperature relations; with equilibria between oxidised and reduced aqueous carbon or sulphur species controlling the oxidation state. In contrast, the concentrations of components present in lesser abundance in Archaean gold ore-fluids (e.g. S, Ca, H+) were probably controlled either by saturation of one or more mineral phases brought on by decreasing temperature, or were rock-buffered through fluid-rock reactions. Extrapolation to high temperatures of the K, Na and Ca contents of the gold-bearing fluids indicates that their composition is consistent with derivation from, or final equilibration with, rocks of intermediate-granitic composition, thus giving support to isotopic and geological arguments for ore-fluid source regions external to the greenstone belts. The fluid oxidation states are characteristic of a wide range of potential source rocks, including mantle-derived igneous rocks, calc-alkaline granitoids and magmas, and seaflooraltered metabasalts. Strongly oxidised magmatic sources or unusually oxidising source processes (e.g. CO2-streaming during granulitisation of the lower crust) are therefore not required in the genesis of Arch≸an lode-gold deposits.