ZONING IN THE CARBONIFEROUS LOWER PERMIAN CRACOW EPITHERMAL VEIN SYSTEM, CENTRAL QUEENSLAND, AUSTRALIA

Four epithermal vein deposits (i.e. Dawn, Central Extended, Rose's Pri de and Klondyke) in the Cracow gold field, central Queensland were inv estigated in terms of paragenesis, mineralogy, vein textures, fluid in clusions and stable isotopes. The Cracow epithermal field is confined to an area approximately 6 by 5 kilometers. All the deposits are hoste d by the massive Camboon Andesite of Upper Carboniferous to Lower Perm ian age, occur as open-space vein fillings, and have similar paragenes is. However, significant variations in mineralogy, textures of quartz and adularia, and fluid geochemistry were found for a main mineralisat ion stage (Stage II) of each individual deposits. At Rose's Pride and Klondyke, base-metal sulphides are virtually absent, but significant a mounts of calcite and quartz with minor adularia are widely distribute d. Replacement textures are distinct, and mineralisation temperature i s less than 220 degrees C and salinity less than 0.2 wt.% The delta(18 )O values of quartz and calcite range from -2.65 to -2.06 parts per th ousand, and from -6.66 to -6.34 parts per thousand, respectively, and calculated delta(18)O(H2O) value is about -17 parts per thousand which represents a nearly unshifted palaeometeoric water. Gold mineralisati on is best developed at Central Extended among the studied deposits, w here patches rich in electrum are often observed in polished thin sect ions and where gold grades exceeding 10 g/t are frequently indicated b y assays. Base-metal sulphides are only present locally and rarely exc eed 5 volume percent of the vein samples. Quartz is the dominant gangu e mineral, but significant amounts of rhombic adularia and chlorite ar e widely distributed. Various primary and recrystallisation textures p ossibly inherited from silica gel are well developed and widespread. A t individual sites where crustiform bands developed from both walls of a fissure, temperatures could drop sharply from 275 degrees C to less than 220 degrees C. The ore-forming fluid at Central Extended, compar ed with that at Rose's Pride and Klondyke, was isotopically shifted fr om meteoric water with delta(18)O(H2O) value of -13.5 parts per thousa nd, calculated in equilibrium with quartz (delta(18)O values of -3.09 to -1.44 parts per thousand). The orebodies at Dawn are rich in base-m etal sulphides which are commonly coarse-grained and form up to 20 vol ume percent of the vein materials. Quartz is the predominant gangue mi neral, and commonly shows a coarse comb texture. The ore-forming fluid was 275 +/- 10 degrees C and low salinity (0.4 to 0.7 wt%). The delta (18)O values of quartz range from -3.97 to -3.22 parts per thousand, a nd calculated delta(18)O(H2O) value is about -12 parts per thousand, i ndicating large isotopic shifts from palaeo-meteoric water. A depth zo ning in typical boiling epithermal systems, corresponding to different fluid compositions, wall rock permeability and boiling behaviors, was invoked to explain different characteristics of these selected epithe rmal veins.