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Geology and geochemistry of the Warrabarty carbonate-hosted Zn-PB prospect, Paterson Orogen, Western Australia

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Smith, SG (1996) Geology and geochemistry of the Warrabarty carbonate-hosted Zn-PB prospect, Paterson Orogen, Western Australia. PhD thesis, University of Tasmania.

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Abstract

Warrabarty is a sub-economic, carbonate-hosted Zn-Pb prospect in the Great Sandy Desert of
Western Australia. The prospect occurs beneath 80 to 150 m of younger cover rocks and is known
only from drilling. Mineralisation has been intersected over a strike length of approximately 2.5
km and has been tested by approximately 6 km of diamond drill core from 29 drill holes.
The prospect occurs within ?Meso- to Neoproterozoic, sub- to midgreenschist facies
metasediments of the Throssell Group of the Paterson Orogen. In addition to Warrabarty, the
Throssell Group hosts mineralisation at the Nifty copper deposit and the Maroochydore and
Rainbow copper prospects. The Warrabarty prospect is hosted by the upper Broadhurst
Formation of the Throssell Group, which has been affected by four deformations. Of these, D2 is
the most significant, producing northwest trending, gently plunging open to tight folds and a
variably developed northeast dipping cleavage. In the vicinity of the prospect the Broadhurst
Formation consists of a thick (> 400 m) sequence of carbonaceous dolostones (termed dark
dolostone) with lesser conformable, organic poor dolostones (termed dolostone A) and minor fine
grained limestones. Dolomitisation is interpreted to have occurred during late diagenesis in the
burial diagenetic regime.
A later generation of medium to coarse grained, light grey, unimodal, nonplanar dolostone,
termed dolostone B also occurs. Dolostone B forms interbeds, crosscutting zones and a >400 m
thick, partially crosscutting, massive unit in the southern part of the prospect area. Abundant
examples of dolostone B crosscutting bedding and pressure solution in earlier do los tones indicate a
later timing for dolostone B, and show that it formed by replacement of dark dolostone and
dolostone A Dolostone B is spatially associated with mineralisation and is interpreted to have
formed as an alteration feature immediately prior to and synchronous with mineralisation.
Warrabarty mineralisation occurs as breccias, veins and zones of dissemin.ated to massive
sulphide. Typical mineralisation is Zn-rkh and low grade (typically 3-6 wt % Zn + Pb), with
minor zones up to 40 wt% Zn + Pb; Cu, Ag and As values arc low. Sulphide mineralogy is simple
consisting of low Fe sphalerite, pyrite and galena with minor chalcopyrite and extremely rare
arsenopyrite, bornite and chalcocite. Gangue mineralogy consists of four stages of dolomite, minor
quarl:z, pyrobitumen and phlogopite. The mineralisation has been subdivided into two major
paragenetlc stages: grey stage and white stage. The grey stage predates the white stage and was
responsible for introduction of almost all Zn and Pb into the prospect. White stage
mineralisation remobilised Zn and Pb and introduced very minor Cu.
The most widespread mineralised breccia type at Warrabarty is typically clast supported and
consists of angular to rounded dolostone B clasts in a cement of sphalerite and dolomite, with minor pyrite ± galena. Brecciation and porosity creation was caused by a combination of
fracturing and carbonate dissolution, prior to mineralisation and dolostone B alteration. In these
breccias and most veins, sphalerite (± pyrite) was the earliest mineral deposited, forming rims
on breccia clasts and selvages to veins; galena occurs consistently later in the paragenesis than
sphalerite. In some breccias and veins, sphalerite has replaced clasts and waH rock, resulting in
higher Zn grades. Sphalerite is well zoned, has abundant solid pyrobitumen and carbonate
inclusions and contains dissolution surfaces which cannot be correlated between samples. The
minor Cu introduction that occurred during the white stage forms chalcopyrite disease of
sphalerite.
Grey stage mineralisation occurred after late diagenetic dolomitisation and bedding parallel
pressure solution, but is overprinted by 5:2 fabrics. Many white stage veins have fibrous crystal
morphology and branching, tapered, sygmoidal shapes typical of syntectonic veins and are
interpreted to have formed during D2.
Primary fluid inclusions from grey stage sphalerite show a bimodal distribution of trapping
temperah1res; with an early low temperature population (165°C - 205°C) and a paragenetically
later higher temperature population (215°C - 245°C). These fluid inclusions have low first
melting temperatures indicative of complex CaCl2 (± Mg, K) bearing brines and salinity
estimates from final ice melting temperatures range from 22 - 25.5 wt % total salts. Grey stage
dolomite trapping temperatures (227°C - 276.5°C) are higher than sphalerite temperatures,
although first melting temperatures are similar and salinity estimates (14.5 - 26 wt % total
salts) are also comparable with sphalerite. White stage fluid inclusions associated with
chalcopyrite disease have trapping temperatures ranging from 300C to 400°C. First melting
temperatures for these fluid inclusions indicate CaClz bearing brines and salinities have been
estimated at 15 to 22 wt NaCl and 2.5 to 5.5 wt % CaClz.
Carbon and oxygen isotope data from least altered Throssell Group carbonates range from: 8l3c +
2.7 to+ 6.1 % and 8180 21.7 to 27.1 %. Dolostone B alteration and grey stage cements range
from: (8)13c + 0.4 to+ 6.8 %o and &18o from 21.7 to 28.4 %o and show almost complete overlap with
the least altered host rock field. White stage dolomite carbon and oxygen isotope compositions
range from o13C + 0.9 to + 4.5 %o and o18Q 16.7 to 27.5 % and define a relatively steep trend on
o18o- &13c plots which is most consistent with water-rock interaction.
Grey stage sulphide o34s compositions range from+ 1.5 <roo to+ 20.4 o/oo witi1 a distinct mode at 11
to 14 % and a sharp cut-off at approximately 14 %o. The data are interpreted to ret1ect mixing
between an isotopically light host rock sulphur source and an introduced, reduced sulphur source
of approximately +14 %o. The heavy, introduced sulphur was derived from Proterozoic seawater
(as connate brines or by dissolution of evaporites) shifted to lighter values by thermochemical sulphate reduction and mixing with biogenically reduced sulphur, far removed from the site of
mineralisation. White stage sulphide o34s ranges from - 6.9 to + 16.0 'X>O, with the lightest
values from chalcopyrite diseased sphalerite. The white stage is interpreted to have sourced
sulphur by remobilisation of grey stage heavy sulphur and from an introduced light sulphur
source which accompanied chalcopyrite formation. This light sulphur source has ()34g values
similar to the light sulphur signatures of the ThrosseU Group copper deposits.
Lead isotope ratios of galena from Warrabarty and the three Throssell Group copper deposits
define a linear trend on Pb-Pb plots, which implies that all deposits formed at approximately
the same time. Modelled compositions of possible source rocks indicates that lead was derived
from leaching of Throssell Group sedimentary rocks in the vicinity of the deposits. The lead
isotope compositions imply an age of approximately 840 Ma, consistent with a syn diagenetic
timing for mineralisation at Warrabarty and the copper deposits.
Thermodynamic considerations indicate that the Warrabarty grey stage fluid was capable of
transporting reduced sulphur together with metals and that sphalerite precipitation was caused
by increasing pH, resulting from dolomite dissolution. The Zn grade of any particular sample is
controlled by the coupled dolomite dissolution-sphalerite precipitation reaction. The increase in
temperature that occurred from sphalerite deposition to dolomite deposition may have resulted
in relatively short-lived sphalerite deposition, thereby contributing to the overall low grade of
the deposit. Consideration of Cu and Au solubility data indicates that the Throssell Group
white stage fluids were capable of carrying ore forming quantities of Cu and Au; therefore future
exploration in the district should consider Cu-Au targets. The Warrabarty grey stage
mineralisation shares many features with MVT and Irish-type .Zn-Pb deposits, although it
cannot be easily classified into either group.

Item Type: Thesis (PhD)
Date Deposited: 23 Jul 2012 03:34
Last Modified: 15 Sep 2017 01:06
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