Wall-Rock Alteration, Victory Gold Mine, Kambalda, Western Australia: Processes and P-T-Xco2 Conditions of Metasomatism
M. E. Clark, D. M. Carmichael, C. J. Hodgson, M. Fu, 1989. "Wall-Rock Alteration, Victory Gold Mine, Kambalda, Western Australia: Processes and P-T-Xco2 Conditions of Metasomatism", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Gold mineralization at the Victory mine, Kambalda, is associated with discrete metasomatic alteration zones around quartz breccia zones, shear zones, and quartz vein arrays. The mineralogy, textures, and whole-rock chemistry of the wall-rock alteration zones are described for several different host rocks. Mineral assemblages at zone boundaries, calcite-dolomite geothermometry, and amphibole geobarometry have been used to estimate the temperature, pressure, and fluid composition associated with metasomatism. Fluid inclusion data have been used to estimate independently these conditions.
Wall-rock alteration zones extend up to 3 m from veins and breccias at the Victory mine. Textures indicate that the zoned wall-rock alteration and associated gold mineralization postdated regional metamorphism and outlasted retrograde carbonation.
Chemical variations across zoned alteration profiles indicate that alteration occurred at approximately constant volume. Outer alteration zones are characterized by the addition or loss of H2O, CO2, Na, and K whereas Al, Mg, Ca, Fe+2, and Fe+a were mobile in the inner alteration zones. Chemical changes and mineralogy of the alteration envelopes depended critically on the initial composition of the host rock which affected the resultant mineral assemblages.
Assuming that local equilibrium conditions existed at alteration zone boundaries, mineral compositions from microprobe data have been used to model equilibria in the system SiO2-Al2O3-MgO-CaO-K2O-H2O-CO2. The mineral equilibria together with calcite-dolomite geothermometry provide an estimate of 390° ± 40°C for metasomatism which is similar to a minimum temperature estimate of 370° + 30°C from fluid inclusion data. Mineral equilibria and fluid inclusion data suggest that pressure during metasomatism was approximately 1.7 to 2 kbars.
Fluid inclusion data indicate that metasomatism was associated with a homogenous H2O-CO2-NaCl fluid containing 19 to 36 wt percent CO2 CXCO2 = 0.1-0.2) and 8 to 9 equiv wt percent NaCl.
The data presented in this study indicate that metasomatism occurred at considerably lower temperatures and pressures than those estimated for peak metamorphic conditions at Victory. Thus after peak metamorphism, substantial uplift occurred before the hydrothermal emplacement of gold.