Microthermometry and Geochemistry of Fluid Inclusions from the Tennant Creek Gold-Copper Deposits - Implications for Ore Deposition and Exploration

Gold-copper-bismuth mineralization in the Tennant Creek goldfield of the Northern Territory occurs in pipe-like, ellipsoidal, or lensoidal lodes of magnetite +/- hematite ironstones which are hosted in turbiditic sedimentary rocks of Proterozoic age. Fluid inclusion studies have revealed four major inclusion types in quartz associated with mineralized and barren ironstones at Tennant Creek; (1) liquid-vapour inclusions with low liquid/vapour ratios (Type I), (2) liquid-vapour inclusions with high liquid/vapour ratios or high vapour/liquid ratios and characteristic dark bubbles (Type II), (3) liquid-vapour-halite inclusions (Type III), and (4) liquid-vapour inclusions with variable liquid/vapour ratios (Type V). Type I inclusions are present in the barren ironstones and the unmineralized portions of fertile ironstones, whereas Types II and III inclusions are recognized in fertile ironstones. Trails of Types II and III inclusions cut trails of Type I inclusions. Type I fluid inclusions have homogenization temperatures of 100-degrees to 350-degrees-C with a mode at 200-degrees to 250-degrees-C. Type II inclusions in mineralized ironstones (e.g. Juno, White Devil, Eldorado, TC8 and Gecko K-44 deposits) have homogenization temperatures of 250-degrees-C to 600-degrees-C with a mode of 350-degrees-C. Type i fluid inclusions have a salinity range of 10 to 30 NaCl equiv. wt %. Salinity measurements on fluid inclusions in the mineralized zones gave a range of 10 to 50 NaCl equiv. wt % with a mode of 35 NaCl equiv. wt %. Fluid inclusion studies indicate that the Tennant Creek ironstones were formed from a relatively low temperature and moderately saline fluid, whereas gold and copper mineralization was deposited from later hydrothermal fluids of higher temperature and salinity. Gas analysis indicates the presence of N2 and CO2, with very minor CH4 in Types II inclusions but no N2 or CH4 gases in Type I inclusions. Microprobe analysis of the fluid inclusion decrepitates indicates that the inclusions from Tennant Creek contain sodium and calcium as dominant cations and potassium in a subordinate amount. The high temperatures (greater-than-or-equal-to 350-degrees-C), high salinities (greater-than-or-equal-to 35 NaCl equiv. wt. %) and cation composition of the Tennant Creek ore fluids suggest that the ore fluids were derived from upward migrating heated basinal brines, although contribution from a magmatic source cannot be ruled out. Close association of vapour-rich Type IIb and salt-rich Type III inclusions in the mineralized ironstones (e.g. Juno, White Devil, Eldorado, TC8 and Gecko K-44) indicates heterogeneous trapping of ore fluids. This heterogeneous trapping is interpreted to be due to unmixing (exsolution) of a gas-rich (e.g. N2) fluid during the upward migration of the metal bearing brines and/or due to degassing caused by reaction of oxidized ore fluids and host ironstones. Fluid inclusion data have important implications regarding the deposition of gold in the ironstones, and may have application in discriminating fertile from barren ironstones.