PACROFI VI - Electronic Program

Fluid Evolution in Shear-Zone Hosted Cu-Au Vein Deposits: Examples from the Cobar Region, Lachlan Fold Belt, Australia

P.K. Seccombe and Z.Jiang

Department of Geology, The University of Newcastle, Callaghan, New South Wales, Australia 2308

Metamorphic vein-style polymetallic gold and base-metal mineralization comprise a major resource in Early Devonian low-metamorphic grade turbidites and minor felsic metavolcanics in the Cobar region of the Paleozoic Lachlan Fold Belt, western New South Wales, Australia. One current producer (Peak mine) and one past producer (Chesney mine) collectively account for nearly 60 tonnes of gold in exploited+available gold-copper resources. Although syn-sedimentary (exhalative) and magmatic concepts have been promoted, current thinking on the origin of the Cobar ores favors a syntectonic concept, whereby structural focussing of metamorphic fluids sourced from Early Devonian basin sequences or older basement rocks has generated mineralization in shear zones at shallow crustal levels. Problems concerning Cobar geology and the development of a model for ore genesis include (i) the timing of deformation, which is variously ascribed to Early Devonian and/or Early Carboniferous events (ii) the general lack of igneous activity in the Cobar Basin and (iii) an inadequate geochemical data base to track the evolution of the vein systems. This paper mainly addresses the last problem by presenting the results of a fluid inclusion study aimed at defining the nature and evolution of mineralizing fluids in two major deposits at Cobar.

The fluid inclusion investigation was carried out on 94 doubly-polished sections prepared from selected mineral samples taken from underground and drill holes at Peak and Chesney. Fluid inclusions within the sections were documented on the basis of spatial distribution, size, shape, gas-liquid ratio, and the petrography and paragenesis of the mineral host. Heating and freezing experiments were performed using a Reynolds gas-flow stage mounted on a Leitz Laborlux-S microscope using a 32x objective lens, 12.5x oculars and JVC video monitor, which permits Th data to be collected on inclusions as small as 3 microns.

Several purified quartz and sulfide samples of crushed material in the size range 0.2 to 0.5 mm were also used for the bulk analysis of inclusion fluids. Analysis of inclusion fluids from sulfide minerals has proved especially important, since sulfide deposition is not accompanied by either quartz or calcite. Analyses were conducted by chromatographic, ion specific electrode, AAS and mass spectrometry techniques at the Department of Isotope Geology, Research Institute for Geology for Mineral Resources, Guilin, China. Temperatures calculated from Na/K ratios for inclusion fluids were used as a check on inclusion Th data. The laser Raman facility operated by the Australian Geological Survey Organisation, Canberra, was also used for analysis of the gas-phase from a variety of inclusion populations.

The Peak gold deposit

A total of seven paragenetic stages is defined at the Peak mine. Four of these stages involve deposition of vein quartz and give rise to four populations of primary fluid inclusions. Three intervening stages free of quartz deposition are also recognised (two of these stages involve gold and sulfide deposition; one is a late K-feldspar veining event). Heating/freezing data for fluid inclusions at Peak indicate that both Th and salinity increase during the first four paragenetic stages, corresponding to the introduction of Cu-Au, followed by Pb-Zn ores. Th data for primary fluid inclusions from Stage-1 vein quartz range from 232 to 317oC, with a mean of 270oC. Salinity measurements (Tm ice) ranging from -0.5 to -2.6oC for these inclusions correspond to fluids with low salinity (0.8 to 4.3 wt% NaCl equivalent). By contrast, secondary inclusions in recrystallized quartz grains of Stage-1 quartz, associated with the formation of overprinting sulfide veinlets in the ensuing major copper-gold event (Stage 2 of the paragenesis) have distinctly higher Th (ranging from 283 to 364oC, with a mean of 322oC) and lower ice-melting temperatures (Tm ice) ranging from -3.7 to -5.5oC, corresponding to fluids with a relatively high salinity (6.0 to 8.6 wt% NaCl equivalent).

Following the deposition of the copper-gold mineralization, fluids responsible for the third paragenetic stage in the evolution of the Peak vein-system display a broad range of Th (205 to 393oC). Boiling inclusion clusters were also observed in three samples taken from small, Stage-3 quartz veins, which have similar homogenization temperatures, but exhibit both gas-rich and liquid-rich groups of inclusions. Tm ice for Stage-3 primary inclusions range from -1.1 to -8.6oC which implies that salinities are moderately high (1.9 to 12.4 wt% NaCl equivalent).

During Stage-4, fluid inclusions from early vein quartz, which just precedes the major period of fracture filling, Stage-4 lead-zinc sulfide mineralization, have Th generally lower than 300oC and appear to be recrystallized adjacent to sulfide minerals. Later in Stage-4, quartz from the major lead-zinc veins is characterised by homogenization temperatures higher than 300 and up to 393oC. This higher Th is similar to those obtained from secondary inclusions in Stage-4 recrystallized quartz, which range from 302 to 343oC. A few three-phase inclusions from Stage-3 quartz which appears to have recrystallized during the Stage-4 Pb-Zn event, yield daughter mineral dissolution temperatures of 277 and 289oC, from which we infer that this ore-stage fluid has a composition of 36-37 wt% NaCl.

Following the deposition of most of the economic sulfide mineralization, fluids responsible for the Stage-5 vein assemblages at Peak are characterized by both a decrease in temperature and salinity. Th for fluid inclusions from Stage-5 vein quartz and calcite range from 190 to 355oC and Tm ice range from -1.6 to -4.0oC equivalent to salinities ranging from 2.7 to 6.4 wt% NaCl. Inclusion data are not available from Stage-6, which is characterized by a high-temperature K-feldspar veining event of limited extent (temperatures near 600oC are argued for this stage from oxygen isotope geothermometry). In the final post-ore event, primary inclusions in Stage-7 quartz display a further decline in homogenization temperature (ranging from 197 to 300oC with a mean of 233oC) and ice-melting temperature (ranging from -0.8 to -2.5oC), corresponding to a salinity ranging from 1.4 to 4.2 wt% NaCl equivalent.

Bulk analysis of inclusion fluids indicate a dramatic rise in the concentration of CO2 during the initial Cu-Au event (Stage-2) at Peak and the later Pb-Zn event (Stage-4). F- and Cl- levels are elevated in the fluids associated with the Pb-Zn event. By contrast, high CH4 contents (and low CO2/CH4 ratios) characterize intervening and later stages of barren quartz deposition. Gas-phase compositions are confirmed by microthermometric data and laser Raman spectroscopy on individual fluid inclusions.

The Chesney gold deposit

The Chesney deposit is one of a number of substantial Cu-Au resources located on a major shear-zone (the Great Chesney Fault) of at least 10km strike length and parallel to the system of shears at the Peak deposit. A total of seven paragenetic stages are recognised for the vein systems at Chesney. Evolution of the fluid system at Chesney is in contrast to that at Peak. Primary fluid inclusions from veins of the first four paragenetic stages are modified by subsequent external overpressure, from which we infer that quartz from Stages1-4 was emplaced prior to D2 (Early Carboniferous deformation). Additionally, Cu-Au ore deposition at Chesney represents the latest paragenetic event (Stage-7).

An general increase in both temperature and salinity is noted from early Stage-3 quartz to late, pre-ore Stage-5 quartz; thermometric data could not be collected for <3 micron, monophase-liquid inclusions in Stage-6 quartz. Most Th data for Stage-3 quartz are restricted to a temperature range from 179 to 258oC, with the exception of a single sample with a high Th range (243 to 317oC). Measurement of Tm for Stage-3 fluids gives a narrow range from -1.5 to -3.9oC, equivalent to a NaCl content ranging from 2.5 to 6.3 wt%.

A more homogeneous composition is inferred for Stage-4 inclusion fluids, which give a range in Th from 151 to 243oC and a Tm from -3.2 to -4.3oC, corresponding to a salinity from 5.2 to 6.9 wt% NaCl during deposition of the fourth-generation quartz. Data obtained from the Stage-5 quartz exhibit high homogenisation temperatures (285 to 381oC), low freezing point temperatures (-2.8 to -5.9oC) and corresponding high salinities (4.6 to 9.1 wt% NaCl). By comparison, Tm ranges from -4.2 to -4.8oC, equivalent to salinities from 6.7 to 7.6 wt% NaCl for the Stage-6 quartz.

High salinities and high temperatures appear to characterize each stage of sulfide deposition at Peak. Although direct data are lacking for the Cu-Au event at Chesney, high temperatures and moderate salinities are apparent in the paragenetic stages immediately preceding that event. Thermal and salinity cycles evident in the Cobar deposits are likely to be linked to reactivation on the major thrusts and transient supply of fluid from basin and basement lithologies.