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Pegmont |
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Queensland, Qld, Australia |
| Main commodities:
Pb Zn Ag
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Super Porphyry Cu and Au
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The Pegmont Pb/Zn/Ag deposit is located in the Eastern Succession of the Mt Isa Block in north-west Queensland, Australia. It is some 25 km west of the Cannington mine, 110 km south of Cloncurry and approximately 200 km south-east of Mount Isa.
The gross sequences of the Mt Isa Block have been divided into three cycles of ensialic rifting by Beardsmore, etal., (1988). Each of these cycles of rifting is characterised by a threefold subdivision into distinct packages based on the dominant lithologies. These comprise from oldest to youngest, a lower package of felsic volcanics and associated volcaniclastics, the 'early rift' phase; followed by mafic volcanics and maturing coarse to fine grained clastic sediments, the 'maturing rift' stage; overlain by the final phase of fine grained, clastic and carbonate bearing sediments, the 'sag phase'.
These three rift sequences, each with its three phases, were deposited in a rift system whose axis moved progressively westward with time. The first and oldest rift, Rift Cycle 1, was centred on the Cloncurry-Selwyn Zone, and incorporated the 10 km thick Maronan Supergroup, commencing with the coarse terriginous and bi-modal volcanics of the Fullarton River Group, followed by the maturing clastic sediments and mafic volcanics of the Llewellyn Formation and Mt Norna Quartzite, capped by the sub-aqueous mafics, fine grained clastics and minor evaporites of the Toole Ck Volcanics. The close of Rift Cycle 1 is believed to have been due to a westward shift in the axis of rifting which heralded the commencement of Rift Cycle 2.
The host at Pegmont is the Mt Norna Quartzite which is regionally a 2000 m thick unit of thickly bedded feldspathic psammites, quartzites, poorly bedded pelites and amphibolite, with minor marble, calc-silicates and BIF's.
According to Overton (1983) the mineralised ironstone at Pegmont forms the only reliable (?) marker horizon within the Mt Norna Quartzite outcropping intermittently over a strike length of 4.4 km as a laminated amphibole-garnet-epidote-magnetite-sulphide-diopside rock. Locally the Mt Norna Quartzite comprises meta-arkose grading to biotite-muscovite schist, and intercalated micaceous quartzite and biotite-muscovite schist. There is a thin banded amphibolite several hundred metres into the footwall and sporadic pegmatite sills (Locsei 1977). The most common rock type is the 'meta-arkose' which contains 25 to 30% feldspar and 5% biotite, with concordant layers of mica schist. Pure quartzites are rare, being 0.1 to 1 m thick where present, although there are sheeted zones of quartz impregnation with associated muscovite mapped as quartzite.
The mineralised band has a gahnite-garnet selvage extending up to 17 m into the hangingwall and 5 m into the footwall, and averages 4 m in thickness. The sulphides, fresh below 50 m, are coarsely to finely banded dense pyrite and coarse (0.3 to 2 mm) grained galena and pale brown sphalerite, with some pyrrhotite, minor Ag and insignificant Ba, Cd and Cu. Two conspicuous bands of amber low Fe sphalerite from 1 to 12 cm thick stand out in the ore zone. The non-sulphide gangue comprises magnetite, gahnite, garnet and apatite, which are finely disseminated, and rarely flaky graphite, hornblende, quartz and possibly pyroxene. Where the ore fades out to the SE, it grades into pyrite, pyrrhotite and marmatite, with no galena. This change takes place over a short distance. The mineralised system is sulphur deficient. The mineralised band is strongly crumpled with little variation in thickness (including within the fold axes) or composition over a distance of 1 km.
The main mineralised body at Pegmont is enveloped by a large volume of late- to post-orogenic fracture controlled hydrothermal alteration that evolved in a cooling period from more than 500° to <300° C and in the presence of a highly saline Fe-Mn-Ca-K-Cl fluids which may have influenced the distribution of the Pb and Zn at Pegmont.
Resources have been stated as 11 Mt @ 8.4% Pb, 3.7% Zn, 11.5 g/t Ag (Williams, et. al. 1998).
The most recent source geological information used to prepare this decription was dated: 1998.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Williams P J, Pendergast W J, Dong G 1998 - Late orogenic alteration in the wall rocks of the Pegmont Pb-Zn deposit, Cloncurry district, Queensland, Australia: in Econ. Geol. v93 pp 1180-1189
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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