Abstract of Georgetown Inlier Synthesis

Record 2001/12

Compiled by Anthony Budd

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• Executive summary - geology
• Executive summary - metallogenic potential

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Executive summary - geology

The Georgetown Inlier in north Queensland occupies about 50 000 km² of the Cairns-Townsville hinterland. It consists largely of variably metamorphosed and deformed sedimentary and volcanic rocks of Palaeo- to Mesoproterozoic age, intruded by Mesoproterozoic granitoids. The eastern margin is in faulted contact with the Palaeozoic Hodgkinson and Broken River provinces of the Tasman Orogen. The western and central parts are variably overlain by scattered remnants of Mesozoic sedimentary rocks, and the eastern part by Cainozoic basalt (Bain et al. 1990; Bain and Draper 1997).

Withnall (1997) has recognised several structural units in the Georgetown 'Region', including the Etheridge Province and the Croydon Province:

The Palaeoproterozoic Etheridge Province has been divided into the Forsayth and Yambo Subprovinces (Withnall 1997). The Forsayth Subprovince includes the Etheridge and Langlovale Groups, McDevitt Metamorphics, various mafic intrusive rocks and Mesoproterozoic granites of the Forsayth and Forest Home Supersuites. The metasedimentary sequence was deposited in an intracratonic rift setting between about 1700 Ma and at least as late as 1650 Ma. It underwent a major metamorphism and deformational event at about 1550 Ma, at which time most of the S-type granites were emplaced (Forsayth, Forest Home, Lighthouse suites). The Palaeoproterozoic Yambo Subprovince represented by the Dargalong Metamorphic Group occurs in the northern part of the region. The rocks there may be slightly younger than at least the lower part of the Etheridge Group, and were possibly deposited after 1640 Ma. Isotopic dating indicates major granite emplacement at about 1580 Ma, and metamorphism at about 1575 Ma.

A Mesoproterozoic 'cover sequence' in the west, assigned to the Croydon Province, comprises the Croydon Volcanic Group and related granites of the Esmeralda Supersuite. They were emplaced at about 1550 Ma, probably at the close of the main deformation event in the Etheridge Province. They are overlain by the Inorunie Group. All of these rocks were previously assigned to the Croydon subprovince of the Georgetown Province (Withnall et al. 1980).

Four suites or supersuites of Proterozoic granites and several ungrouped granites are known in the Georgetown Inlier (Champion 1991). They are the Forest Home Supersuite, Esmeralda Supersuite, Forsayth Supersuite, the Lighthouse Suite, and ungrouped granites in the Yambo Subprovince.

The Forsayth Supersuite includes the Aurora, Delany, Forsayth, Goldsmith, Mistletoe, Ropewalk and Welfern Granites. The Mywyn and Mount Hogan Granites, and the Fig Tree Hill Complex also appear to be part of this Supersuite. Members of this supersuite mostly comprise light to dark grey, biotite granite and granodiorites. Alkali feldspar megacrysts and muscovite are relatively common constituents. Cordierite is reported in the Mistletoe Granite, and metasedimentary xenoliths are common particularly in the Mistletoe, Ropewalk and Forsayth Granites. Black and McCulloch (1990) dated the Mistletoe and Forsayth Granites at 1550 ± 6 Ma and 1544 ± 7 Ma respectively (U-Pb zircon). This is the 'type' Proterozoic Australian restite S-type suite.

The Forest Home Supersuite consists mostly of grey biotite trondhjemite, dated at 1550 ± 50 Ma (Black & Holmes, cited in Withnall et al. 1988). This supersuite is low in K₂O but high in Na₂O, and is Sr-undepleted, Y-depleted. Granites of this type are uncommon in the Australian Proterozoic, and are the only ones that show evidence of garnet in the source region, indicating a mantle component.

The Esmeralda Supersuite comprises the Esmeralda, Nonda, Mooremount, Little Bird, Macartneys, Olsens, Dregger and Bimba Granites, and is comagmatic with the Croydon Volcanic Group (CVG). These granitoids comprise granites and monzogranites with lesser granodiorites. A feature of the granites is the presence of locally abundant graphitic inclusions. The Supersuite is felsic, fractionated, reduced to oxidised, weakly peraluminous to peraluminous, and hydrothermally altered in parts. Traditionally, this Supersuite has been classified as a S-type granite, mostly because of its high Aluminium Saturation Index (ASI), but also because of the presence of garnet, muscovite and graphite inclusions. This project questions the traditional view, and raises the possibility that it is an I-type with genetic associations with the Croydon Goldfields. The factors in contention are outlined below:

• Quite a few of the samples have ASI between 1 and 1.1 (ie are weakly peraluminous), and show what may be described as an increasing trend with increasing SiO₂. This trend is seen in other strongly fractionated Australian Proterozoic I-type granites (such as the Kokatha suite of the Hiltaba Supersuite). However, this trend is also common to S-types.
• Hornblende was found in two samples of the Esmeralda Granite by Sheraton and Labonne (1978), which they considered to be relict primary hornblende. However, Champion (1991) considers that this hornblende is a replacement.
• The Supersuite contains both reduced and oxidised components. S-type granites are almost always reduced. Three possibilities exist to explain the oxidised/reduced nature of the Supersuite. The favoured explanation is that an oxidised magma partly assimilated the locally-abundant graphitic sediments, and became more reduced in the locations where assimilation occurred. (Further evidence of the oxidised nature of the Supersuite is the relatively high magnetic remanence and susceptibility (Idnurm, pers. comm.), which indicates the presence of magnetite.) Another explanation is that the Supersuite may have been derived from several sources, resulting in the oxidised and reduced granites and volcanics. The third possibility is that it is a very rare oxidised S-type.

More detailed mapping and petrology of the Supersuite is needed to unravel its exact nature. An I-type nature is favoured here because most metallogenic models favour I-types, and this Supersuite is interesting enough to warrant further attention.

The Lighthouse Suite is relatively small, outcropping along the eastern margin of the Forsayth Granite near Georgetown. It is felsic, and veins and pegmatites are common, suggesting that the granite may be fractionated (despite its narrow silica range). This granite shows an affinity to the Silurian 'I-type' granites in the region (Champion 1991). It has not been dated, and may be younger than Proterozoic.

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Executive summary - metallogenic potential

Of all the suites in the Georgetown Inlier, the Esmeralda Supersuite is considered to have the greatest potential. This suite is spatially related to the Sn deposits of the Stanhills and Mount Cassiterite areas and to the Croydon goldfield, although there is considerable debate about the age of the Au mineralisation. Bain et al. (1990) suggested that Au-bearing quartz vein deposits such as the Golden Gate lode in the Croydon goldfield are likely to be Proterozoic, as they are associated with extensive areas of hydrothermal alteration that appear to be related to Croydon Group volcanism. Denaro et al. (1997) also support the Proterozoic age and state that the nature of the gold lodes suggests that ore deposition was post-magmatic into fractures and faults within the consolidated Croydon Volcanic Group and Esmeralda Granite. The lodes have been classified as plutonic veins by Morrison and Beams (1995).

In contrast, some workers have interpreted these deposits as Palaeozoic in age, based on the structural setting and K-Ar ages of alteration of the Esmeralda Granite and sericite alteration (e.g., Lawrie et al. 1998). Henderson (1989) considered these ages to be a Carboniferous to Permian thermal event superimposed on earlier Proterozoic mineralisation and alteration.

Disseminated replacement gold deposits (Carlin-style) occur within thick predominantly pelitic units containing highly pyritic, carbonaceous and locally calcareous mudstone, siltstone and sandstone (Candlow and Lane Creek Formations of the Etheridge Group) (Denaro et al. 1997) within 3 km of the Illewana, Dregger and Bimba Granites.

Numerous tin deposits are thought to be associated with the Esmeralda Supersuite, including the Stanhills, Comet and Mount Cassiterite deposits (Denaro et al. 1997).The Comet deposit has been Pb-dated at ~1530 Ma (Carr, pers comm).

If the Au mineralisation is shown to be Palaeozoic then a possible model is that significant Au was disseminated in the pyrite-bearing members of the Esmeralda Supersuite during crystallisation and was then remobilised in a later hydrothermal event. During crystallisation of the pyrite-bearing phases of the Esmeralda Supersuite, Au was possibly incorporated into an intermediate solid solution (iss, Cygan and Candela 1995) forming a protore, which may then have been remobilised during the Palaeozoic hydrothermal event. This model may be tested by microprobing the gold content of pyrite in the gold-bearing granites and volcanics.

The Forsayth, Lighthouse and Forest Home Suites/Supersuites are not considered to have any significant mineralisation potential.