Linking mineral and fluid inclusion paragenetic studies: The Batman deposit, Mt. Todd (Yimuyn Manjerr)goldfield, Australia

Abstract

The quartz-sulphide vein system of the Batman deposit at the Mt. Todd Mine, Australia, formed during east-west extension of a north-northeasterly trending structure system, early in D2 during retrograde contact metamorphism associated with cooling of the Tennysons leucogranite. Four distinct fluid inclusion types have been identified in gold-bearing quartz-sulphide veins of the deposit: (Type I) low to moderate salinity aqueous primary inclusions (1 to 20 wt.% NaCl equivalent) with small amounts of CO2; (Type II) high salinity Na-Ca-Cl aqueous secondary inclusions (35 to 50 wt.% total NaCl-CaCl2), containing halite, carbonate and trona daughter minerals, and minor CO2-CH4 vapour; (Type III) liquid and vapour secondary fluid inclusions (28 to 34 or 17 to 18 wt.% NaCl equivalent) with CH4 vapour and discrete gold and bismuth grains and associated growth zones; (Type IV) vapour-rich CO2F(CH4) secondary inclusions with minor NaCl liquid (N 20 wt.% NaCl equivalent). A consistency in bulk fluid composition suggests a common parent fluid and a common parent source for the fluid inclusions. Previous paragenetic and textural studies have established a sequence in sulphide and silicate precipitation. The mineral paragenesis has been linked to the trapping chronology of Types I-IV fluid inclusions in order to constrain the fluid conditions during (micro)-tectonism and the mineralising process. The link is possible because two mineral phases exhibit a temporal genetic relationship with the formation of fluid inclusions: (1) Growth zones in simple open space quartz fill contain tourmaline crystals in intimate association with Type I liquid-vapour fluid inclusions; (2) Gold is intimately associated with bismuth in Type III vapour-liquid fluid inclusions, either as discrete inclusions within the trails, or wetting the inclusions. The progressive change in the composition of Types I-IV is consistent with the evolution of a retrograde contact metamorphic fluid in wall rock lithologies.