Nature and Origin of Ore-Forming Fluids in the Ravenswood Gold System, North Queensland: Evidence from Fluid Inclusions and Stable Isotopes

The Ravenswood vein array/stockwork deposits (total endowment similar to 4Moz @ 1.6g/t), and the nearby Mt Wright breccia-hosted deposit (total resource similar to 1Moz @ 3.3g/t), are both Permo-Carboniferous intrusion-related systems, located in North Queensland, Australia. The Ravenswood system includes both the recently exhausted large tonnage Sarsfield-Nolans deposit and the historic, smaller, higher grade Buck Reef West-Sunset mines. Gold mineralisation in both areas is associated with quartz and sulphides developed within altered Paleozoic intrusive rocks of the Ravenswood Batholith. Analysis of ore stage fluid inclusions from the three deposits indicate distinct temperatures for ore deposition, mostly between 200 and 300 degrees at Buck Reef West, from 300 to 400 degrees C at Sarsfield and about 400 to 550 degrees C at Mt Wright. Salinities for the fluid inclusions from the different deposits are similar and mostly between 8 and 12 wt% NaCl equivalent. The delta S-34 of sulphides indicate that the sulphur in the ore fluids had a variable composition with higher delta S-34 at geologically distinct Mt Wright compared to the Ravenswood veins. Collectively, stable isotopes data are consistent with a predominantly magmatic source for the ore forming fluids at the Ravenswood deposits, although there is also evidence of involvement of surface waters in the mineralizing systems at Sarsfield and Buck Reef West.