Hydrothermal alteration zonation and fluid chemistry of the Southern Ridge and North deposits at Mt Tom Price

Abstract

The Mt Tom Price deposit is a world-class high-grade hematite deposit in the Hamersley Province of Western Australia with an original resource of 900 Mt of almost pure hematite, averaging 63·9 wt-%Fe. Petrological and geochemical studies at both the Southern Ridge and the North deposit at Mt Tom Price have identified three hypogene alteration zones between unmineralised banded iron formation (BIF) and high-grade iron ore: distal magnetite-siderite-stilpnomelane; intermediate hematite-magnetite-ankerite-talc-chlorite; and proximal martite-microplaty hematite-magnetite-apatite alteration zones. Fluid inclusions trapped in siderite within the distal magnetite-siderite-stilpnomelane alteration zone at the North deposit revealed primary high salinity (25·5 eq.wt-%NaCl–CaCl2) inclusions that homogenised between 107 and 142°C into liquid. Fluid inclusions trapped in ankerite within ankerite-microplaty hematite veins in the intermediate hematite-ankerite-magnetite-talc-chlorite alteration zone at the North deposit revealed mostly H2O–CaCl2 pseudosecondary and secondary inclusions with salinities of 22·4–25·4 equivalent wt-%CaCl2 and 22·9–25·9 equivalent wt-%CaCl2 respectively. Pseudosecondary inclusions homogenised between 153 and 449°C and secondary inclusions homogenised between 103 and 157°C. Fluid inclusions trapped in apatite within talc-microplaty hematite veins in the intermediate hematite-magnetite-talc-chlorite-apatite alteration zone at Southern Ridge revealed that primary, medium salinity (7·0–9·0 equivalent wt-%NaCl) inclusions homogenised between 181 and 257°C and primary high salinity (22·8–25·9 equivalent wt-%CaCl2) fluid inclusions homogenised between 118 and 257°C into liquid. Microthermometric analysis of quartz from quartz-hematite veins from the Southern Batter Fault, Southern Ridge shows a complex fluid inclusion history. Primary fluid inclusions consist of: low and high salinity H2O–NaCI inclusions trapped at temperatures of approximately 140–230°C; vapour-rich inclusions of unknown compositions; and 'complex' salt-rich (Ca, Mg, K and Na inclusions). Secondary inclusions consist of medium-salinity fluid inclusions trapped at temperatures of about 140–280°C. A two stage hydrothermal model is proposed for the formation of both the Southern Ridge and North deposits. Early stage 1a hypogene alteration involved the release of hydrothermal NaCl-CaCl2-rich (25·5 equivalent wt-%) basinal brines (110–150°C) from the underlying Wittenoom Formation and directed upward along normal faults and focused within the silica-rich rocks of the Dales Gorge Member, by the shales of the underlying Mt McRae Shale Member and overlying Whaleback Formation. Within the Dales Gorge Member hydrothermal basinal brines migrated laterally within large-scale folds with permeability controlled by shale bands and the NW trending dolerite dyke sets. Fluid rock reactions transformed unmineralised BIF to magnetite-siderite-iron silicate BIF, with subsequent desilification of the chert bands. Stage 1b hypogene involved an increase in temperature of the hydrothermal, CaCl2-rich saline (24 equivalent wt-%) basinal brines (250–300°C) resulting in formation of hematite-ankerite-magnetite- talc-chlorite alteration and the crystallisation of microplaty hematite. Late stage 1c hypogene alteration involved the interaction of low-temperature (∼120°C) basinal brines with the hematite-ankerite-magnetite mineral assemblage. At the Southern Ridge deposit this process was very intense (i.e. with a high fluid flux), therefore resulting in the almost total removal of ankerite and resulting in the increased porosity of the ore. Stage 2 supergene enrichment, during the Tertiary, resulted in the removal of residual ankerite and apatite and the weathering of the shale bands to clay.