Feature<strong>shear</strong>ed granite samples from NuckullaHill prospects suggests that temperaturesranged from 350 to
Feature from leaching or desulphidation ofsulphide minerals from sediments orolder hydrothermal alteration,or by some combination of thesesources.Sulphur isotope geothermometryis not reported due to difficulties inestablishing isotopic equilibriumbetween mineral pairs. 34 S values range from –2.23 to3.19‰, with a mean of 0.09‰, and fallwithin the range of sulphides in mantlederivedmelts and intracontinentalgranitic rocks (Nielsen, 1979; Hoefs,1973).In summary, sulphur isotope resultsfrom pyrite within the <strong>Tunkillia</strong> oresystem are similar to, but form anarrower range than, isotopic values ofsome other similar-aged hydrothermalsystems on the Gawler Cratoninfluenced by Hiltaba Suite granitoids(Ferris, 2001). The H 2S values inferredare consistent with a magmatic originfor sulphur. This fits with the geologicalenvironment, since the <strong>Tunkillia</strong> <strong>Project</strong>mineralisation is <strong>hosted</strong> by granite, withno sediments observed in the immediatearea. However, the data do not precludean origin from a metamorphic sourcedominated by magmatic rocks. Otherdeposits on the Gawler Craton showmixing of a variety of sources, butall with a possible link to magmaticsources, though more rigorous analysisis required to fully constrain thisinterpretation.Physio-chemicalproperties of ore fluidThe mineralising fluid at <strong>Tunkillia</strong>was of moderate to high temperature(average trapping temperature of412°C), relatively low salinity (4.5) low salinity (less than seawater) moderate fO 2.These conditions are characteristic ofthe mineralising system at <strong>Tunkillia</strong>.The inferred trapping temperaturefor fluids related to <strong>gold</strong> precipitation at<strong>Tunkillia</strong> is 330–515°C, with an averageof 412°C. This is slightly lower thanmagmatic fluids related to porphyrycopper deposits (400–700ºC), and islower than the granite solidus underconditions of excess H 2O (e.g. Wyllie,1977). MacDonald (1983) reported thatthe lower temperature may be a result ofirreversible, adiabatic expansion of theTm (salinity) °CFrequency86420-20-15-10-5200 250 300 350Temperature°C200 250 300 350Homogenisation temperature202427_018202427_019Fig. 6 Tm (fi nal melting temperature in °C)versus temperature of homogenisation for<strong>Tunkillia</strong> samples.fluid, which can produce a temperaturedecrease of up to 300°C. Heat loss to theenvironment during transport away fromthe source granite will also reduce fluidtemperature. Within lode <strong>gold</strong> depositsof the Superior Province, Canada,fluid inclusion data suggest that thefluid responsible for <strong>gold</strong> precipitationwas of low salinity (1.0 g/cm 3 ), and homogenisationtemperatures ranged between 200and 400°C, clustering around 350°C(Colvine et al., 1988).Cassidy and Bennett (1993)summarised physio-chemical processesin which hydrothermal fluids precipitate<strong>gold</strong> within hydrothermal systems: decrease in temperature extensive mixing of fluids withcontrasting compositions fluid–wallrock interactions phase separation into immisciblefluids.The moderate temperature range of325–375°C would not result in major<strong>gold</strong> precipitation, and the lack of fluidinclusion data that suggest the presenceof two distinct fluid populationsdiscounts the first two processesoutlined above. The host rocks appearto have been extensively altered, and arepossibly more reduced than the HiltabaSuite granite. The fluid inclusion datasuggest that metamorphic fluid is apossibility because of the low salinityobserved at <strong>Tunkillia</strong>, but the data do notpreclude a mixing of magmatic and/ormetamorphic fluids.<strong>Tunkillia</strong> — magmaticversus metamorphicfluid source<strong>Tunkillia</strong> and other prospects withinthe <strong>Proterozoic</strong> <strong>gold</strong>-only provinceare almost certainly related to theMesoproterozoic Hiltaba Suitemagmatic–tectonic event, hence thequestion about how much influence didthe Hiltaba Suite have on these deposits.Cameron and Hattori (1987) reportedthat oxidised, CO 2-rich felsic magmasare the most likely source of <strong>gold</strong>-bearingoxidised fluids within several Archaean10MESA Journal 35 October 2004
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