The Plutonic deposits in the current Georgetown model have previously been referred to as orogenic deposits because they are lodes formed in active shear zones during regional deformation and dewatering and without direct involvement of intrusions. This is a valid descriptive model, but it does not account for the Te-Bi geochemical signature of the deposits that is interpreted as magmatic. The Te-Bi geochemical signature is most prominent in the centres of the metal zoning pattern and less prominent in the peripheral As zone in the Georgetown region deposits and in the Early Devonian deposits at Woolgar and Charters Towers that are also interpreted to have formed in a peripheral part of the zoning pattern. The overall interpretation is that there is a more magmatic signature in the deepest and most central of the districts than at shallower and more peripheral positions where the contact metamorphic fluid is more prominent.
12.0 GEORGETOWN EXPLORATION POTENTIAL
The Kidston gold deposit was the biggest producer in the region (>5M oz) and is closely related to early Carboniferous sub-volcanic intrusives and mineralisation is hosted in a large (1km diameter) hydrothermal breccia pipe cut by sheeted veins. Many other intrusion-related gold (IRG) deposits lie marginal to and show genetic ties to the Early- Carboniferous Newcastle Range Volcanics and Lochaber and Bagstowe Igneous Complexes e.g. Christmas Hill, Mount Borium and Beverley.
Strategic Minerals have been exploring the Woolgar area since 1986 and through their persistence have managed to build up a significant resource. Extensive drilling has enabled a detailed understanding of the geochemical zonation within the deposits. This has proved important not just for vectoring towards the gold zones but also because the basemetal-rich portions of the deposit have more favourable metallurgy than the zones rich in arsenic. Historical treatment records have shown this phenomena to be applicable to other mine camps around Georgetown and Forsayth where galena bearing gold ores were reportedly easier to process than sphalerite and chalcopyrite ores e.g. Durham, Queenslander, Nil Desperandum, Havelock (Cameron, 1909). Applying this metal zoning technique to other camps in the Georgetown region may uncover ores that were previously thought to be refractory.
The Mount Hogan gold mine was the largest single producer in the Gilberton area (2530 kg). Mineralisation consists of a series of shallow dipping, stacked veins hosted in Proterozoic granite. Alteration at Mt Hogan has been dated at 400 Ma (Early Devonian) and the deposit is classed as Plutonic Mesothermal vein style. The high grade and flat-lying nature of the veins enabled Eltin Mining to construct a mill at Mount Hogan and extract 67,700 ounces of gold from two open cuts between 1992 and 1994. Alteration and veining has been mapped outside of the mine area and has been subject to limited drilling with some success. The Marquis (120kg?), Josephine (266.5 kg) and Jubilee Plunger (555 kg) gold mines (Forsayth) are three other early Devonian, Plutonic style lode deposits with flat-lying veins which, like Mount Hogan also warrant further exploration to better understand the controlling structures and identify mineralisation peripheral to the mine area.
The Cumberland Mine is the biggest individual, historical producer close to Georgetown, producing 1581 kg gold at an average grade of over an ounce /tonne (Jack, 1886). The deposit is hosted along a northeast striking early Devonian structure similar to the other deposits in the Camp. However, unlike the other deposits (Plutonic epizonal) the mineralisation is related to Permian dykes. Mine records show that the shape of the ore shoots was complex, controlled by jogs in the host structure and overprinting of early quartz vein material by gold- bearing sulphides. The mine reached a maximum depth of 310m and was only mined along strike for around 400m, and although the lode was recorded to have pinched out at depth the host structure was still present (Cameron, 1909). The nature of the ore suggest that other shoots probably exist along the controlling structure but this hasn’t been tested. Like the Cumberland Mine many other lode deposits in the Georgetown area are in dire need of modern exploration.
A cluster of +1 tonne (Au endowment), early Devonian lode style camps lie immediately south and west of Georgetown (Big Wonder, Durham, International) (Figure 3). Many of the deposits within these camps lie along major (20 km long) E-W trending structures e.g. Big Wonder Fault, Golden Bar Fault. Development of the mineralised lodes, veins and shoots occur at intervals along the entire length of these structures however, many have not been drill tested or mined below the level of oxidation.
Three large, Permo-Carboniferous porphyry systems, Phyllis-May, Mount Turner and Huonfels with anomalous gold, silver, base metal and molybdenum signatures lie west of Georgetown. The camps all possess extensive alteration systems developed in and around porphyry complexes. Mineralisation is primarily as disseminated sulphides but also occurs in fracture zones, veins, stockworks and breccia. To date, exploration has identified lode and linear zones of breccia hosting high silver and lead values (Three Musketeers, Cobar, Huonfels) with low grades of copper, gold and molybdenum. Some of the high grade silver prospects explored in the 1980’s should be reassessed as potential silver deposits.
The Agate Creek epithermal deposit (15,985 kg Au resource) is the best example of gold mineralisation related to early Permian volcanism. The mineralisation occurs as veins, stockwork and breccia hosted in rhyolite sills dated at 285Ma that cut Silurian (Robin Hood) granodiorite and Proterozoic metasediment. The younger volcanic rocks appear to be a more favourable host due to their brittle nature, similar to that observed at the Electric Light and Cumberland mines. Like Kidston, gold mineralisation at Agate Creek shows a close genetic link to Permo-Carboniferous intrusions, and should be included in any exploration strategy applied to the Georgetown region.
Although the Georgetown area is best known for its gold mining history, during the 1970’s significant efforts were made exploring for uranium. Only one significant deposit was identified (Maureen) located in the far north of the study area (Hurtig et al, 2014).
Many of the Devonian age Plutonic lode deposits had base metal sulphides associated with the gold ore but rarely was the galena and chalcopyrite in sufficient concentrations to be worth treating. Some mines did however report small amounts of lead production e.g. Dry Hash, Queenslander, International and Monte Cristo Camps.
Alluvial tin, tungsten, tantalum and bismuth have been mined historically at a few locations near Percyvale Station and in the Grants Gully area near Western Creek. The minerals are understood to be shedding from Meso-Proterozoic pegmatites hosted in the metamorphics (Withnall, 1981) and can be found within the pegmatites as well in these areas.
Although the Georgetown, Forsayth and Gilberton regions host numerous deposits covering a range of mineralisation styles, the region has still suffered from a lack of modern, focussed and thorough exploration. Detailed studies around existing deposits aimed at understanding the metal zonation, structural controls on shoots and metallurgical studies on sulphide ores will unlock new resources and advance the discovery of new deposits.
13.0 CONCLUSIONS
The overall metallogenic model for the Georgetown region has three main components:
- The Early Devonian Plutonic Gold group forms a distinct corridor extending ~35km west of the Newcastle Range and ~140km from the younger cover in the north to the Gilberton Fault in the south. This corridor hosts the main group of gold deposits mined historically in three main centres at Georgetown, Forsayth and Gilberton. The deposits are mainly shear-hosted lodes in E and ESE trending faults. At each of the three centres there are distinct zones outward from hypozonal to mesozonal and epizonal level of emplacement and from geochemically from Bi-Te to Pb-Zn-Cu to As-Sb. This is interpreted as syn- to late-deformational mineralisation localised in active structures above stocks that emanate from an underlying Silurian – Early Devonian batholith.
- The Early Carboniferous Intrusion-Related Gold group is scattered occurrences in the Einasleigh Metamorphics along the eastern side of the Newcastle Range. The main occurrences, including the 5Moz Kidston gold deposit, are mesozonal-epizonal hydrothermal breccias and vein networks related to rhyolite plugs and dikes in the now exposed sub-volcanic periphery of the Newcastle Range Volcanics. The deposits have a distinct polymetallic geochemical signature with the best Au in As or Pb zones and a core of Mo-W. These are typical north Queensland IRGS with the same igneous- chemical signature as the Red Dome & Mungana deposits at Chillagoe.
- The Early Permian Intrusion-related and Epithermal Ag-Au group is in two distinct corridors adjacent to the Early Devonian corridor. One corridor NW of Georgetown extends NE from the Robertson Fault through the Phyllis May and Red Dam prospects towards Chillagoe and the other extends SE along the Robertson Fault from Greenhills prospect through the Agate Creek deposit to the Gilberton Fault. The deposits are Ag-rich polymetallic stockwork and breccia deposits centred on dioritic plugs as at Phyllis May and Ag-Au epithermal deposits associated with rhyolitic dikes adjacent to the volcanic centre as at Agate Creek. In several deposits including Mt Turner and Log creek there is also Au-Ag mineralisation associated with late Carboniferous sub-volcanic intrusions peripheral to the volcanic complexes. The distinctive intrusive and volcanic centres localised along the early Permian trans-tensional corridors have long been a target both here and around Mt Leyshon in Charters Towers.
The gold deposits are the predominant feature of the region but there are also: Paleo-Proterozoic (?) base metal deposits in Einasleigh Metamorphics for which we obtained an early Permian age that is difficult to explain; U-Mo-F deposits in the basal sedimentary portion of the early Carboniferous volcanic complexes; and the Nb-Ta +/- Li-W-Sn deposits mainly in Meso-Proterozoic pegmatites.
The most notable feature of the mining and exploration work in the region is the dearth of drilling, particularly into the sulfide lodes beneath the shallow oxide Au deposits in the recent mines. The metallurgical problems of the As-rich sulfide ores is appreciated, but the Pb-Zn zone in these deposits has better Au grade and better metallurgy as has been demonstrated in the resource definition in Big Reef South at Woolgar and in the recent sampling in the Gilberton District. The shallow dipping gold lodes at Mt Hogan and Jubilee Plunger have likewise only been tested at shallow level so the true extent of the mineralisation is not defined.
The IRGS deposits like Kidston and Mt Turner are distinctly metal zoned systems with the gold confined to a base metal zone beneath a gold-poor As zone. The known systems defined previously from hydrothermal features but only explored in the As zone like Beverley, Mt Borium or Ironhurst are thus of interest.
There are million ounce resources in both the early Permian (?) epithermal and Early Devonian mesothermal lodes in the Woolgar Inlier 60km SW of Gilberton. The now established link of these deposits to the Georgetown rather than the Lolworth Block demonstrates the potential beneath the Mesozoic cover in this region.
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