Petrographic and geochemical evidence for hydrothermal evolution of the North Deposit, Mt Tom Price, Western Australia

High-grade iron mineralisation (>65%Fe) in the North Deposit occurs as an E-W trending synclinal sheet within banded iron formation (BIF) of the Early Proterozoic Dales Gorge Member and consists of martite-microplaty hematite ore. Three hypogene alteration zones between unmineralised BIF and high-grade iron ore are observed: (1) distal magnetite-siderite-iron silicate, (2) intermediate hematite-ankerite-magnetite, and (3) proximal martite-microplaty hematite-apatite alteration zones. Fluid inclusions trapped in ankerite within ankerite-hematite veins in the hematite-ankerite-magnetite alteration zone revealed mostly H2O-CaCl2 pseudosecondary and secondary inclusions with salinities of 23.9 +/- 11.5 (1sigma, n = 38) and 24.4 +/- 11.5 (1sigma, n = 66) eq.wt.% CaCl2, respectively. Pseudosecondary inclusions homogenised at 253 +/- 59.9degreesC (1sigma, n = 34) and secondary inclusions at 117 +/- 10.0degreesC (1sigma, n = 66). The decrepitation of pseudosecondary inclusions above 350degreesC suggests that their trapping temperatures are likely to be higher (i.e. 400degreesC). Hypogene siderite and ankerite from magnetite-siderite-iron silicate and hematite-ankerite-magnetite alteration zones have similar oxygen isotope compositions, but increasingly enriched carbon isotopes from magnetite-siderite-iron silicate alteration (-8.8 +/- 0.7%, 1sigma, n = 17) to hematite-ankerite-magnetite alteration zones (-4.9 +/- 2.2%., 1sigma, n = 17) when compared to the dolomite in the Wittenoom Formation (0.9 +/- 0.7%, 1sigma, n = 15) that underlies the deposit. A two-stage hydrothermal-supergene model is proposed for the formation of the North Deposit. Early 1a hypogene alteration involved the upward movement of hydrothermal, CaCl2-rich brines (150-250degreesC), likely from the carbonate-rich Wittenoom Formation (delta(13)C signature of 0.9 +/- 10.7%, 1sigma, n = 15), within large-scale folds of the Dales Gorge Member. Fluid rock reactions transformed unmineralised BIF to magnetite siderite-iron silicate BIF, with subsequent desilicification of the chert bands. Stage 1b hypogene alteration is characterised by an increase in temperature (possibly to 400degreesC), depleted delta(13)C signature of -4.9 +/- 2.2% (1sigma, n = 17), and the formation of hematite-ankerite-magnetite alteration and finally the crystallisation of microplaty hematite. Late Stage 1c hypogene alteration involved the interaction of low temperature (similar to120degreesC) basinal brines with the hematite-ankerite-magnetite hydrothermal assemblage leaving a porous martite-microplaty hematite-apatite mineral assemblage. Stage 2 supergene enrichment in the Tertiary resulted in the removal of residual ankerite and apatite and the weathering of the shale bands to clay.