Ore fluids of the Tanami region  

Authors / CoAuthors

Mernagh, T.P. | Wygralak, A.S.

Abstract

Fluid inclusion studies of the major deposits and prospects in the Tanami region indicate that they formed over a range of temperatures and depths. Gold mineralisation is structurally controlled with the highest grades occurring in brittle, dilational structures or at the intersection of quartz veins and anticlines. Mineralisation occurs in a number of different rock assemblages including amphibolite facies sediments, greenschist facies sediments, intercalated units of basalt and sediments, and dolerite intrusives, but in all cases there is a close relationship between mineralisation and carbonaceous shales or iron-rich lithologies. The ore-bearing fluids appear to range in composition but are generally moderate to low-salinity fluids (<13 wt% NaCl eq), which contain CO2 to a greater or lesser extent. The different gas compositions of the fluids result from fluid-rock interaction leading to generation of CH4 or N2 depending on whether the fluids have interacted with mafic or sedimentary sequences. Microthermometric and laser Raman microprobe data indicate that the Groundrush deposit formed at the greatest temperatures and depths (260 ? 430 ?C and up to 11 km), from moderate salinity fluids (?13.9 wt.% NaCl eq.) that were enriched in CH4 ? CO2. The Granites deposit formed at temperatures ranging from 220 ? 312 ?C and depths between 2.3 and 9.8 km. The ore-fluids contained CO2 ? N2 ? CH4 and were of low to moderate salinity (0 ? 14 wt.% NaCl eq.). The Coyote prospect formed over the range 290 ? 360 ?C and at depths between 3.4 and 7.5 km. This deposit contained CO2-rich inclusion (some with graphite crystals), CH4-rich inclusions (also some with graphite crystals), and CO2 ? N2-bearing inclusions, all with salinities below 6.7 wt.% NaCl eq. The Callie deposit formed at temperatures ranging from 254 ? 326 ?C and depths between 2.3 and 6.0 km. The ore-forming fluids contained CO2 ? N2 and had salinities ?13 wt.% NaCl eq., but there is also evidence for isothermal mixing with higher salinity fluids. Deposits in the region of the Tanami Gold Mine formed at temperatures ranging from 260 ? 380 ?C and depths between 1.5 and 5.6 km. Low to moderate salinity (?13 wt.% NaCl eq.),CO2-bearing inclusions were only observed in quartz veins from 2 of the open-cut pits but these inclusions are believed to represent the mineralising fluids in this region. The quartz veins from these deposits typically contain ubiquitous 2-phase, aqueous inclusions with evidence for isothermal mixing between high- and low-salinity fluids at temperatures between 100 and 200 ?C. Hydrogen, oxygen, and carbon isotopic data from fluid inclusions and mineralised quartz veins indicate that the source of the mineralising fluids was either a metamorphic or mixed magmatic/metamorphic fluid. However, as most deposits in the Tanami region appear to post-date the main period of magmatic activity, a metamorphic origin for the mineralising fluids appears the most likely. Gold mineralisation in the Tanami region resulted from fluid/rock interaction and reduction of the ore-bearing fluids by Fe-rich rocks or graphitic sediments. However, at higher crustal levels, boiling/effervescence due to pressure fluctuations may have enhanced gold deposition. There is also evidence for fluid mixing which, particularly for the deposits of the Tanami Gold Mine, occurs at lower temperatures than those estimated for mineralisation, indicating that it is a late stage process with little effect on the main stage of mineralisation.

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nonGeographicDataset

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49606

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• External PublicationScientific Journal Paper

( Theme )

• economic geology

( Theme )

• geochemistry

• AU-NTAU-WA

Australian and New Zealand Standard Research Classification (ANZSRC)

• Earth Sciences

• Published_Internal

Publication Date

2003-01-01T00:00:00

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Australian Government Security ClassificationSystem    

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Creative Commons Attribution 4.0 International Licence    

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geoscientificInformation

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[-21.0, -18.0, 128.0, 131.0]

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