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Abstract

Gold, bismuth and copper mineralisation at Tennant Creek occurs
in transgressive magnetite- and hematite-rich lodes within the Carraman
Formation of the Lower Proterozoic Warramunga Group.
Rocks of the Warramunga Group are dominantly felsic greywackes
and shales with features indicative of turbidity current deposition. These
are interbedded with massive pyroclastic rocks, rhyolitic lavas, preconsolidation
slump breccias and minor lenses of banded iron formation.
The magnetite-hematite lodes (locally referred to as ironstones)
have an ellipsoidal to pipe-like shape commonly flattened in the direction
of the regional east-west cleavage. They are typically localised in
small anticlinal structures within the greywacke-shale turbidites adjacent
to thin lenses of hematite-chlorite-calcite bearing banded iron formations
or hematite-rich shales. A smaller number of mineralised ironstones have
replaced the preconsolidation slump breccia horizons within the felsic
sediment pile.
Silicate, oxide and carbonate gangue minerals within the lode
structures are grouped into a series of compositionally distinct zones
which exhibit sharp contacts against one another and the enclosing
country rocks , Massive magnetite (>80%) and Fe-Mg chlorite (<20%)
commonly constitute the core of the mineralised lode and are surrounded
by vari ous umbrella-shaped zones. These may be: talc-magnet.ite; dolomite;
chloritised sediments, as at J·uno Mine, or quartz-hematite; hematitemagnetite;
hematite-chlorite; chloritised sediments, as at Gecko Mine.
The magnesium content of the chlorites (dominantly ripidolites) increases
from the base, to the top, of the lode structures. The chemical and mineralogical characteristics of these zones
indicate contemporaneous formation and growth, resulting from the flow of
hydrothermal solutions which reacted with the host rocks and suffered
continual, and systematic, changes in chemistry. A zone of intense
chloritisation extends below each of the orebodies and constitutes what
is thought to be a channel of hydrothermal alteration.
This investigation deals with the structure, mineralogical
constitution, mineral zoning, textures, and origin of the lode rocks in
the three largest operating mines in the goldfield, namely the Juno, Gecko
and Warrego deposits.
At the Juno and Warrego mines, gold, bismuth and copper
mineralisation has been shown to occur in three overlapping zones within
the magnetite-rich lodes e Gold is concentrated at depth, and is overlain
above by an umbrella shaped zone rich in bismuth sulphosalts. Chalcopyrite
is concentrated at the top of the lode structures enveloping the
bismuth zone. At Gecko (anomaly 2), bismuth and copper show a similar
vertical zonation but gold is completely lacking. Within the bismuth zone
at Juno, the sulphur/selenium and bismuth/ lead ratios of the bismuth
sulphosalts increase from its inner edge (overlapping the gold zone) to
its outer edge (overlapping the copper zone).
The most common bismuth sulphosalt at Juno is junite, a new
mineral, unique to Juno, which has been shown by microprobe analysis to
have the formula Bi8PbJCu2 (S,Se) 16 , containing 3.8 to 11.6 wt.% selenium.
Junite is easily distinguished from other lead-bismuth sulphosalts by its
characteristic x-ray powder pattern. The second most abundant bismuth
sulphosalt has a composition close to Bi10Pbg(S,Se) 23 and may be equivalent
to the mineral wittite , previously reported from Falun, Sweden by
Johansson in 1924 . Other selenium bearing sulphosalts at Juno include
heyrovskyite and members of the aikinite-bismuthinite series.
Colloform textures occur throughout all the lode structures in the goldfield, strongly indicating that replacement of the sediment host
rocks to form the ironstone bodies was achieved by the processes of
dispersive metasomatism. Replacement of this type involves the gradual
hydrolysis
Mg 2+ 1' .n t h e
of the host sediments and permits ready exchange of Fe 2+ and
hydrothermal solutions with si4+, Al3+, Na+ and K+ in the
hydrolysed sediment matrix. Most of the magnetite in the central lode
zones replaced needle-shaped a - FeO(OH) and S - Fe203.H20 crystal forms
which probably developed from ageing of ferric hydroxide gels.
Thermodynamic considerations of gangue mineral stabilities in
hydrothermal solutions of the type which may have caused mineralisation
at Tennant Creek, suggests that the solutions were initially acidic in
nature and capable of 1 each1•. ng the I base catl.' ons I Fe 2+ and Mg 2+ (p 1 us ore
metals) from the Carraman sediments at depth. There is evidence of leaching
of this type in the lowest parts of the Juno hydrothermal channel.
The process of metasomatic lode formation was most probably initiated by
interaction of rising, chloride-rich, hydrothermal solutions with the
calcite bearing banded_ iron formation or with particularly porous horizons
containing abundant pore fluids (e.g., preconsolidation slump breccias)
leading to an increase in solution pH and f02 which caused the deposition
of amorphous, hydrated, ferric oxides.
Under the influence of increasing solution p~the gangue minerals
at Juno, were deposited in the order:- iron-rich chlorite at depth,
followed by hematite, magnetite, talc and dolomite, to form a well zoned
lode structur e . The progressive drop in f02 associated with the deposition
of magnetite probably resulted in the increase in sulphur/selenium and
sulphur/metal ratios of sulphides passing up the Juno lode structure, and
may have also lead to the zonal distribution of gold, bismuth and copper .
Sulphur isotope studies at Juno lend support to this proposal.
The
16o;18o and
12c;13c ratios in dolomites from the outer
envelope zone at The 32 S/34 S ratlio of syngenetic sulphides in the tuffaceous greywackes and vein sulphides in the hydrothermal channel,
below the orebody, supports a proposal that the sediments of the Lower
Carraman Formation provided the source for the sulphur. These sediments
also contain sufficient trace quantities of gold, bismuth and copper to
constitute a source for the ore components.
Connate wate.rs (pore water and interlayer water) released from
the argillaceous sediments in the vicinity of granitic and rhyolitic
porphyry intrusions provides the most probable source for the hydrothermal
solutions. Such solutions moved upwards continually leaching iron,
magnesium and ore elements from the sediments in their path and were
eventually channelled into low pressure anticlinal sites and deposited
their metal load in favourable structural-lithological traps.

Item Type:	Thesis - PhD
Authors/Creators:	Large, RR
Additional Information:	Copyright 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).
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