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 t he Northern Territory occurs in pipe-like, ellipsoidal, or lensoidal l odes of magnetite +/- hematite ironstones which are hosted in turbidit ic sedimentary rocks of Proterozoic age. Fluid inclusion studies have revealed four major inclusion types in quartz associated with minerali zed and barren ironstones at Tennant Creek; (1) liquid-vapour inclusio ns with low liquid/vapour ratios (Type I), (2) liquid-vapour inclusion s with high liquid/vapour ratios or high vapour/liquid ratios and char acteristic dark bubbles (Type II), (3) liquid-vapour-halite inclusions (Type III), and (4) liquid-vapour inclusions with variable liquid/vap our ratios (Type V). Type I inclusions are present in the barren irons tones and the unmineralized portions of fertile ironstones, whereas Ty pes 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 inc lusions in mineralized ironstones (e.g. Juno, White Devil, Eldorado, T C8 and Gecko K-44 deposits) have homogenization temperatures of 250-de grees-C to 600-degrees-C with a mode of 350-degrees-C. Type i fluid in clusions 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 %. Flui d inclusion studies indicate that the Tennant Creek ironstones were fo rmed from a relatively low temperature and moderately saline fluid, wh ereas gold and copper mineralization was deposited from later hydrothe rmal fluids of higher temperature and salinity. Gas analysis indicates the presence of N2 and CO2, with very minor CH4 in Types II inclusion s but no N2 or CH4 gases in Type I inclusions. Microprobe analysis of the fluid inclusion decrepitates indicates that the inclusions from Te nnant Creek contain sodium and calcium as dominant cations and potassi um in a subordinate amount. The high temperatures (greater-than-or-equ al-to 350-degrees-C), high salinities (greater-than-or-equal-to 35 NaC l 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 b e ruled out. Close association of vapour-rich Type IIb and salt-rich T ype III inclusions in the mineralized ironstones (e.g. Juno, White Dev il, Eldorado, TC8 and Gecko K-44) indicates heterogeneous trapping of ore fluids. This heterogeneous trapping is interpreted to be due to un mixing (exsolution) of a gas-rich (e.g. N2) fluid during the upward mi gration of the metal bearing brines and/or due to degassing caused by reaction of oxidized ore fluids and host ironstones. Fluid inclusion d ata have important implications regarding the deposition of gold in th e ironstones, and may have application in discriminating fertile from barren ironstones.