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Geology of the Tanami gold mine, Northern Territory

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Tunks, AJ (1996) Geology of the Tanami gold mine, Northern Territory. PhD thesis, University of Tasmania.

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Abstract

The Tanami Gold Mine (TGM) is situated 600km NW of Alice Springs in the
Paleoproterozoic 'Granites-Tanami Inlier' of Northern Australia. The deposit, which was discovered in 1904 and has been mined intermittently since, is one of several gold-only deposits that occur in the inlier.
The host rocks to gold mineralisation at the TGM are a sequence of northwest-dipping, interbedded tholeiitic pillow-basalts and volcaniclastic-sedimentary rocks. The sediments were deposited by mass-flow mechanisms in a below wave-base subaqueous environment. The wholerock geochemistry of the basalts is similar to that of rift tholeiites, consistent with an intracontinental tectonic setting at the time of basalt eruption. The
presence of hematite and high-grade metamorphic detritus in the sedimentary lithologies is
also consistent with an intracontinental tectonic setting.
The deformational history of the Tanami area involved two sub-orthogonal episodes of
folding that generated northeast-trending Fl folds and northwest-trending F2 folds.
Interference between the two fold generations has created dome and basin interference
patterns. Illite-crystallinity measurements of sandstones and siltstones from the TGM
indic~te diagenetic temperatures, probably less than 250°C.
The metamorphic grade, intensity and number of deformations at the TGM are less than at
the 'Granites Gold Mine and elsewhere in the inlier. The host rocks to the mineralisation at
the TGM (Black Peak Formation) are therefore interpreted to be younger than at the
Granites (Ditjiedoonkuna Suite). Intracontinental rifting during the Palaeoproterozoic
Leichardt rifting event (1810-1740Ma) created a small rift basin into which the Black Peak
Formation was unconformably deposited onto a Archaean/Paleoproterozoic metamorphic
basement.
The Granites-Tanami Inlier is intruded by at least two distinct granite suites; the Mt
Winnecke Suite (1830-1815Ma) and the Gregory Suite (1800-1790Ma). The granites in
both suites are dominantly reduced, I-type granites that are enriched in incompatible
elements.
Gold mineralisation at the TGM is hosted within a complex sinistral wrench-fault array
and associated veins and alteration halos. The main mineralised faults trend approximately
NS and dip steeply east. Subsidiary structures trend at 030° and 070° and dip southeast.
Economic gold mineralisation occurs within quartz-carbonate veins and in the surrounding
sericite + quartz +pyrite± carbonate alteration halos. High-grade southeast-plunging,
oreshoots are present where the mineralised fault trends intersect. Detailed structural
studies indicate that the main mineralising event post-dated the bulk of F1 shortening, and
was synchronous with the emplacement of felsic dykes into the mine sequence. Stress inversion calculations, based on fault striation populations, have revealed that at the time
of the Au mineralising event, σ1 was sub-horizontal and SE-NW directed with σ2 subvertical.
This contrasts with the pre-mineralisation deformation which occurred under a
similarly directed σ1, but with σ3 sub-vertical. Flipping of the stress axes has allowed for
the formation of steeply-dipping faults that were effective fluid focussing zones during the
mineralisation event. A range of internally deformed vein textures and the presence of
crack-seal and extension veins are evidence for cyclic fault rupturing caused by variations
in the fluid pressure, shear stress and permeability of the fault zone.
Mass balance calculations undertaken on the sericitic alteration assemblages that are
spatially associated with Au mineralisation indicate addition of K, and volatiles (mainly
C02 and S), and leaching of Si and Na, which caused minor volume loss during the
metasomatic event. Fluid inclusion studies have revealed the presence of high-temperature
(300°C), low-salinity (5 wt.%) fluids with low C02 contents. Sulfur and oxygen isotope
data are consistent with a hybrid magmatic/contact-metamorphic origin for the ore-forming
solutions, which are inferred to be related to granite emplacement. During migration
through the footwall the fluids, partly re-equilibrated with wallrocks to acquire their
characteristic isotopic and geochemical compositions of δ34S≈12‰, δ18Ο≈10‰, K/Rb ≈
330.
Mineralising solutions were weakly.acidic (pH ≈ 5), reduced (S04≈/H2S ≈ 0.001) and
had a high ΣS content (0.006 molal); Gold was predominantly transported as AuHSO,
although Au(HS)2 may have also been important, particularly if the pH or ΣS was higher
than estimated. Gold deposition most likely occurred due to H1S loss associated with
sulfidation reactions as magnetite and hematite, present in the wall rocks, were altered to
pyrite. Phase separation occurring due to fluid pressure drops in dilational fault zones may
have also been locally important for gold deposition.
The ultimate source of gold at the TGM remains unclear. Two possibilities are suggested:
i) gold was magmatically sourced, and was partitioned into an exsolved magmatic-hydrothermal
fluid during magma crystallisation, or ii) gold was present as detrital gold in
the contact aureole of the granite and was scavenged and remobilised by magmatic and/or
contact metamorphic hydrothermal fluids. In the second scenario, Au mineralisation in the
underlying metamorphic basement may have provided a source of detrital gold in the
sedimentary lithologies of the Black Peak Formation.

Item Type: Thesis (PhD)
Keywords: Gold ores, Geology, Stratigraphic
Copyright Holders: The Author
Copyright Information:

Copyright 1996 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

3 folded maps in pocket at back of vol. Thesis (Ph.D.)--University of Tasmania, 1997. Includes bibliographical references

Date Deposited: 04 Feb 2015 23:23
Last Modified: 09 Jan 2017 23:25
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