FLUCTUATIONS IN A MAGMATIC SULFUR ISOTOPE SIGNATURE FROM THE PINNACLES MINE, NEW-SOUTH-WALES, AUSTRALIA

Sulphur isotopic compositions of 29 sulphide samples from the Broken Hill-type Pinnacles Deposit, NSW, are found to cluster at 0 parts per thousand (mean -0.8 parts per thousand). The restricted range of the delta-S-34 values between -3.5 and +3.7 parts per thousand with a mean of -0.8 parts per thousand, is interpreted as reflecting partial oxidation of a dominantly magmatic sulphur source. delta-S-34 data for galena samples fall into two groups: (1) isotopically heavier galenas (range -0.7 to 0.0 parts per thousand; mean -0.4 parts per thousand) which come mainly from the footwall Zn lode and (2) isotopically lighter galenas (range -3.5 to -0.8 parts per thousand;, mean -2.2 parts per thousand) which are from the main Pb lode. Sphalerite, pyrrhotite and chalcopyrite have slightly heavier isotopic compositions (range -1.6 to +3.7 parts per thousand; mean +0.3 parts per thousand) but exhibit the same stratigraphic differentiation. These data are interpreted as representing fluctuating conditions at the site of ore deposition, in which upwelling hydrothermal fluids were subject to increasing fO2 and decreasing temperature with time.