Fluid-wall rock interaction in an Archean hydrothermal gold deposit; a thermodynamic model for the Hunt Mine, Kambalda

Abstract

Gold mineralization at the Hunt mine, Kambalda, is situated in a 20-m-thick, steeply dipping shear zone within a sequence of rather uniform metabasalts. Zonal alteration of the host rocks occurred around fractures which acted as conduits during gold mineralization. Limited major element mobility ensured assemblages of low variance over most of the alteration zone, providing a suitable environment for the application of thermodynamics in conjunction with detailed petrographic, geochemical (electron microprobe analysis of mineral compositions), and fluid inclusion studies (microthermometry).Equilibrium equations involving fluid components and alteration minerals are used to determine conditions during the alteration event, and in particular, activity gradients of components. Profiles across the schist zone show increasing f (sub H 2 S) around auriferous quartz veins (log f (sub H 2 S) = -0.6) but constant f (sub O 2 ) (log f (sub O 2 ) = -29.7). These results are near the magnetite-pyrrhotite-pyrite triple point for the inferred temperature of 350 degrees C and explain the pyrite-pyrrhotite-magnetite zonation sequence outward from veins. The profile of X (sub (CO 2 )) is very close to the value of 0.25 determined independently from fluid inclusions. The K (super +) /H (super +) profile shows increases adjacent to auriferous veins corresponding to the zone of biotite stability. Using a value for fluid salinity of 2 equiv wt percent NaCl, pH was calculated to be around 6.9.The solubility of gold in solution was estimated for the calculated conditions: for the complex Au(HS) (super -) 2 the maximum solubility is around 0.5 mg kg (super -1) (i.e., 0.5 ppm), for Au(Cl) (super -) 2 it is close to 10 (super -6) mg kg (super -1) , indicating the efficacy of sulfide transport of gold, compared to chloride transport, under these conditions. The calculated decrease in f (sub H 2 S) away from the auriferous quartz veins results in an 80 percent (or greater) decrease in gold solubility, suggesting hat the sulfidation of Fe-rich wall rocks is a geologically reasonable and very efficient mechanism for precipitating gold from solution at elevated temperatures.