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.
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When the price of gold rose from about $200 (U.S.) an ounce in 1979 to nearly $700 an ounce by the end of the same year, the gold rush of the 1980s was under way. Gold production in the western world rose dramatically; from 1981 to 1986 production increased by 300 to 1,282 metric tons per year. Annual production may reach 1,500 to 1,600 metric tons by 1990 (Woodall, 1988). The major contributors to the increased stream of gold have been Australia, Canada, Brazil, and the United States together with other circum-Pacific countries. The increased price of gold and new methods of extraction have allowed many older deposits to be reopened, but the most important factor has been the high success level of exploration. This success has resulted in large part from the application of new genetic models and from the development of new exploration techniques.
There are hundreds of thousands of reported gold occurrences around the world. The majority are alluvial placers, but large numbers of bedrock occurrences have also been discovered. Most of these occurrences prove to be very small and are relatively unimportant in the overall world production level. Most mined gold has come from a small number of giant deposits, which were found by prospectors. It is becoming increasingly clear, however, that the discovery of giant deposits in the future will involve more than the sharp eyes and persistence of the old prospector. The use of sound geologic principles, and exploration programs based on those principles, is what the future holds. An example can be seen in the successful search for gold deposits in the South Pacific. There, exploration models have been based on principles developed in the study of modern geothermal systems. Giant deposits such as Lihir and Porgera have been the reward. Another example is the giant copper-gold-uranium deposit at Olympic Dam, South Australia, discovered beneath 300 m of cover using an exploration program based on models developed by Western Mining Corporation geologists for Zambian copper belt-type deposits.
Gold deposits are widely dispersed throughout many geologic settings and in virtually all kinds of rocks, but they do not seem to have formed at a uniform rate throughout geologic history. On the contrary, two very distinct metallogenic periods have been defined. The first is the Archean era, when most of the great deposits in greenstone belts were formed and the vast Witwatersrand basin deposits in