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Abstract

The Mt Todd goldfield is located approximately 40 km northwest of the township of
Katherine, in the Northern Territory, Australia. The goldfield is host to several discrete ore
bodies that strike north-northeast within a broad northeasterly trending corridor of gold
mineralisation. The largest ore body occurs at the Batman mine and constitutes an identified
mineral resource of 92.9 million tonnes at 1.4 git Au.
The goldfield lies in the southern region of the Central Domain of the Pine Creek Inlier and is
hosted by a Palaeoproterozoic sequence of rocks termed the Burrell Creek Formation. The
formation is dominated by greywackes, siltstones, sandstones and shales that exhibit
sedimentary features akin to those of a river-dominant delta front to pro-delta environment.
The formation is conformably overlain by volcanodastic and volcanolithic sediments of the
Tallis Formation (-1890 Ma).
Three deformation events are recognised, viz. , 0 1, 0 2 and 0 3. The earliest deformation, 0 1,
is characterised by close to tight, northeast to northerly to northwesterly trending asymmetric
folds (F1), a continuous axial planar cleavage (S1), and numerous strike-slip faults. The
deformation is associated with the development of buck quartz veins and was preceded by
the emplacement of the Yenberrie Leucogranite of the Cullen Batholith (1835-1820 Ma)
which contact metamorphosed the sediments of the Burrell Creek Formation to hornblendehornfels
facies (H1 ) , with the development of cordierite porphyroblasts (type C1). D1 was coincident with peak regional metamorphism to greenschist facies.
0 2 is associated with westerly trending open folds (F2) , a spaced disjunctive to fracture
cleavage (S2) in transaction to the folds, and strike-slip and normal faults. The deformation is
associated with the development of quartz-tourmaline and gold-bearing quartz-sulphide
veins and lodes. It was preceded by the emplacement of the Tennysons Leucogranite
which contact metamorphosed the Yenberrie Leucogranite of the Cullen Batholith and the
sediments of the Burrell Creek Formation to hornblende-hornfels facies (H2), with the
development of cordierite porphyroblasts (type C2) .
D3 is characterised by the reactivation of strike-slip faults (mostly sinistral), a steeply dipping
Type S3-C type foliation, and mesoscopic en echelon folds (F3) that trend oblique to the
faults in a left stepping (sinistral) array. The faults offset 01 and 0 2 structures. Calcite-base
metal veins and epithermal style quartz veins may have formed during this deformation.

The age of emplacement of the leucogranites, D1 and D2 is constrained by the age of
emplacement of the Cullen Batholith at 1835-1820 Ma. D1 and D2 are correlated with
deformation during the Maud Creek Event (-1850 Ma), while D3 is correlated with
deformation during the Shoobridge Event (-1780 Ma).
Mineralisation in the goldfield is associated with quartz-sulphide veins and lodes that
comprise pyrrhotite, chalcopyrite, arsenopyrite, pyrite, marcasite, loellingite, with minor
galena, sphalerite, bismuth-sulphosalts, ISS CuFeS minerals, and gold. Gold is associated
with bismuth and CH4-rich hypersaline fluid inclusions (29-33 wt% NaCl eq) in trails which
cross-cut early quartz and sulphide. The general vein/lode alteration assemblage comprises
quartz, biotite, muscovite, chlorite, sericite, rutile, accessory tourmaline, chalcopyrite,
pyrrhotite, pyrite or marcasite.
The quartz-sulphide veins and lodes are typified by extension concomitant to hydrothermal
fracturing and/or normal faulting. Across the goldfield, the veins and lodes exhibit a
remarkable similarity with respect to strike geometry, morphology, vein forming process and
isotopic composition. They are located within the thermal aureole of the Tennysons
Leucogranite and strike north-northeast within a corridor that trends northeasterly above a
basement strike-slip fault. They are preferentially hosted in competent rock types such as
greywackes and siltstones. Detailed structural and petrographic analyses has indicated that
the veins and lodes were generated at the same time by hydrothermal activity during
retrograde (contact) metamorphism associated with cooling of the Tennysons Leucogranite,
and early in D2, prior to the development of the regional S2 fabric.
Integrated petrologic, isotopic (sulphur and oxygen) and fluid inclusion studies suggest that
the hydrothermal fluids were chiefly magmatic-metamorphic in origin. The fluids were
enriched in metal and sulphur derived either as magmatic metal or sulphur (from the
Tennysons Leucogranite), or scavenged during fluid-rock reactions from sources in the
sedimentary pile, the Yenberrie Leucogranite, or from sulphide-bearing greisens in the
Yenberrie Leucogranite. Fluid evolution was controlled by retrograde reactions which
accompanied rehydration of the country rock and T ennysons Leucogranite, at low fluid-rock
ratios.
The hydrothermal fluid was reduced in nature and exhibited a tendency to lower fO2 and fS2
concomitant to an overall decrease in temperature and salinity for a given pH. δ34S values of
4.5%o to 13.3%0 indicate that the source for sulphur was mixed (magmatic-sedimentary).
Initial precipitation of crack-seal fibre quartz from a silica-saturated brine occurred during early
retrograde metamorphism by progressive fracturing and sealing of the host rocks. The subsequent precipitation of quartz and tourmaline coincided with a significant vein dilation
event and accompanied decompressive effervescence and boiling of a surface-derived
hydrothermal fluid at a minimum pressure of 180-225 bars or a minimum depth of 1.8 to 2.6
kilometres. Fluid salinity ranged from 1 to 20 wt% NaCl eq: this fluid interacted with a mixed
magmatic-metamorphic fluid.
The precipitation of a silicate-sulphide-carbonate assemblage accompanied an influx of hot
(~ 400°C), acidic (pH ~ 4.0 at 330°C), hypersaline brine (30-50% NaCl-CaCl2). This metal and
sulphur enriched brine was mixed magmatic-metamorphic in origin. The principal cause of
metal deposition was fluid unmixing at a shallow crustal level (minimum depth of 1.8 to 2.6
km) during a decrease in temperature from approximately 370°C to 240°C, a decrease in
salinity from 30-50% NaCl-CaCl2 to 29-33 wt% NaCl eq, and vertical transport of the brine
along joints, fractures and faults. Gold precipitation occurred late in the development of the
silicate-sulphide-carbonate assemblage from a hot (~ 250°C), acidic (pH of 2.3-4.9 at 250°C),
hypersaline brine (29-33 wt% NaCl eq), and accompanied the precipitation of bismuth and
the entrapment of CH4-rich fluid inclusions.
A genetic model for mineralisation in the goldfield invokes sinistral reactivation of a northeasterly
trending basement strike-slip fault under the influence of effective tensile stresses
caused by cooling of the Tennysons Leucogranite. The reactivation of the fault caused
brittle failure in the upper crust and/or dilation of existing north-northeasterly trending faults,
fractures and joints in competent rock types. The generation of dilatant structures, coupled
with a sudden reduction in pressure facilitated channelization of fluid flow into the upper
crust, probably away from convection cells juxtaposed about the cooling leucogranite.
Rising fluids decompressed causing phase separation with mineral precipitation. Throttling
of the conduit or fluid pathways resulted in over-pressuring of the fluid, this giving way to
hydrothermal fracturing and an enhanced permeability.

Item Type:	Thesis - PhD
Authors/Creators:	Hein, KAA
Keywords:	Gold ores, Geochemistry
Copyright Holders:	The Author
Copyright Information:	Copyright 1994 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:	Library has additional copy on microfiche. 3 v. in case. Vol. 2 contains appendices A-D; vol. 3 appendices E-F. Thesis (Ph.D.)--University of Tasmania, 1995. Includes bibliographical references
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