Figure 2 Mount Turner Camp. Example of GIS data available for the Georgetown Metallogenic Study area i.e. geology, structure, dyke – lode – vein orientations, mineral occurrence locations etc

METALLOGENIC STUDY OF THE GEORGETOWN, FORSAYTH AND GILBERTON REGION Part1

August 19, 2019

PLATE 3. Polymict matrix support breccia with a complex history of formation. At least four phases of pre-breccia and post-breccia quartz veining of variable texture. Fine, dark coloured, silica – pyrite breccia cement. Specimen of ore from the Cumberland Mine, Georgetown (Sample #231594; 4.46 g/t Au, 24.3 g/t Ag).

METALLOGENIC STUDY OF THE GEORGETOWN, FORSAYTH AND GILBERTON REGIONS-Part 3

August 19, 2019

METALLOGENIC STUDY OF THE GEORGETOWN, FORSAYTH AND GILBERTON REGIONS Part 2.

Published by Simon at August 19, 2019

3.0 GEORGETOWN MINE ENDOWMENTS and HISTORICAL GOLD PRODUCTION

Figure 3 shows the gold endowment for each camp in the Georgetown region. The production, remaining resources and style of deposit are listed in Table 3.1. Interestingly, the three mineralisation styles identified in this study (Plutonic, Intrusion Related, Epithermal) are represented in the top three deposits by endowment (Table 11.1). By far the biggest producer in the region was the Kidston gold mine which operated from 1985 to 2001 and produced 145 tonnes of gold and an overall endowment in excess of 5Moz (158 tonnes) (Figure 3).

Table 3.1 : Georgetown region, summary of mine production history, metal endowment, style and age of mineralisation.

Figure 3 Map of Georgetown, Forsayth and Gilberton region showing location of historical gold mines and gold endowments.

After Kidston, of the eleven other camps that have an endowment greater than 1 tonne of gold metal, seven are Early Devonian Plutonic lode or vein style deposits, two are Early Permian intrusion- related epi-mesozonal vein or stockwork deposits and two are epithermal deposits (Table 3.1). The Woolgar mesozonal and epithermal (Lost World) deposits which are separated spatially and temporally have the second and third largest endowment (37.3 & 21.9 tonnes respectively) but only a comparatively small production history (979.94 kg from the mesozonal deposits only; Denaro et al, 2001).

The Agate Creek epithermal deposit currently hosts the fourth largest mineral endowment in the Georgetown region (15,985 kg Au) and 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 Mount Hogan gold mine was the largest single producer in the Gilberton area (2530 kg). 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. 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.

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 were 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 Electric Light (1325 kg) and Red Dam (2997 kg) gold deposits were mined by Deutsch -Rohstoff in 2011. Pits were only excavated to the base of oxidation (10 – 15 metres) as test work showed the fresh sulphide ores to be refractory and contain high levels of arsenic. Significant high grade (>10 g/t) resources still exist below the level of oxidation. Further metallurgical studies of the sulphide ore are warranted to determine if modern processing and treatment methods can successfully recover the gold from what is currently thought to be refractory ore. Additional drilling at depth and along strike may also assist with metal zonation patterns within the mineralised structure to determine if less refractory ores are present.

Based on population estimates and uncertain production reports by the Mining Warden, Withnall (1981) approximated that 4000 kg of alluvial gold was produced from the Gilberton region between 1869-1873 and 1876-1881. More recent alluvial production from the immediate area around Gilberton by Portman Ltd (1985-1987) was reported as 112 kg. Sandhurst Mining extracted 100kg of alluvial gold from the Percy and Gilbert Rivers between 1987 and 1989. Alluvial gold has been mined sporadically at Western Creek, 20 km NW of Forsayth by ERO Georgetown Gold Operations P/L and at Mosquito Creek, 10kms west of Forsayth.

4.0 GEOLOGY

The Georgetown region lies in the western part of the Forsayth Subprovince of the Proterozoic Etheridge Province of the North Australian Craton (Jell, 2013). Proterozoic rocks were extensively intruded by Silurian to Early Devonian granitoid batholiths of the Pama Igneous Association and dominantly felsic Carboniferous to early Permian intrusive and extrusive complexes of the Kennedy Igneous Association. Parts of the region, particularly in the north-west and south-west, are covered by Jurassic to Cretaceous clastic sedimentary rocks and Cainozoic sediments and (locally) basalts (Figures 4 & 5).

4.1 PROTEROZOIC METAMORPHIC AND INTRUSIVE ROCKS

Proterozoic rocks are dominated by the Paleoproterozoic Etheridge Group. The Etheridge Group includes compositionally diverse metamorphic rocks, subdivided into numerous individual units – Einasleigh Metamorphics, Cassidy Creek Metamorphics, Juntala Metamorphics, Bernecker Creek Formation, the Robertson River Subgroup (Daniel Creek Formation, Dead Horse Metabasalt, Corbett and Lane Creek formations), Upper Etheridge Group (Towneley, Hellman and Candlow formations, Lungdon River Mudstone) (Withnall, 1984; Bain and Draper, 1997; Jell, 2013).

The Einasleigh Metamorphics, dominant in the south and east of the region, are characterised by layered biotite and calc-silicate gneisses, with common amphibolites and migmatites. The Juntala Metamorphics, composed of mica schists (locally graphitic) and minor quartzites, grade into and are faulted against the Einasleigh Metamorphics in the south of the region. The Cassidy Creek Metamorphics, compositionally similar to the Juntala Metamorphics, are structurally juxtaposed against the Einasleigh Metamorphics in the east of the region (north-east of Einasleigh). The Bernecker Creek Formation consists predominantly of calcareous to dolomitic fine-grained sandstones, siltstones and mudstones, grading to the east into calcareous mica schists and quartzites with calc-silicate minerals and finally – calc-silicate gneisses, compositionally similar to parts of the Einasleigh Metamorphics into which the Bernecker Creek Formation proably grades.

The Robertson River Subgroup is mostly composed of originally fine-grained meta-sediments (calcareous siltstones, calcareous or carbonaceous mudstones, minor sandstones), interlayered with dominant meta-basalts with common pillow-lava textures in the Dead Horse Metabasalt. The stratigraphically higher Upper Etheridge Group is dominated by siltstones and mudstones (locally calcareous or carbonaceous), minor sandstones and rare thin limestones. The Langdon River Mudstone, the uppermost unit of the Etheridge Group, consists of laminated carbonaceous pyritic mudstones.

Depositional age of the Etheridge Group is constrained by U-Pb geochronology of magmatic and detrital zirons. The lower part of the Etheridge Group (including the Einasleigh Metamorphics, the Bernecker Creek Formation and the Robertson River Subgroup) was deposited between 1700 and 1660 Ma (Jell, 2013). The minimum age of the top of the Etheridge Group is constrained at ~1630 Ma by the maximum depositional ages (U-Pb on detrital zircons, Neumann and Kositcin, 2011) of the Langdon River Mudstone (1629 ± 12 Ma) and the Langlovale Group (1625 ± 5 Ma), which unconformably overlies the Etheridge Group along the western margin of the Etheridge Province (Withnall, 1984; Jell, 2013).

The Etheridge Group was intruded by extensive Mesoproterozoic predominantly S-type granitoids, mostly forming the Forsayth Batholith. The main individual plutons include the Aurora, Delaney, Forsayth, Goldsmiths, Mount Turner, Bowler Creek, Mistletoe, Ropewalk, Welfern, Mywyn, Mount Hogan and Lighthouse granites, the Forest Home and Talbot Creek trondhjemites and the Brandy Hot Granodiorite. U-Pb zircon geochronology indicates the major emplacement age of the Mesoproterozoic granitoids across the region at ~1550-1560 Ma (Black and Withnall, 1993; Bain and Draper, 1997; Neumann and Kositcin, 2011; Jell, 2013). The presence in the region of relatively common zircon cores in zoned magmatic zircons from Mesoproterozoic and Palaeozoic granites and sedimentary rocks with measured ages of ~1580 Ma (Murgulov et al., 2007; Kositcin and Bultitude, 2015; Kositcin et al., 2015) indicates an earlier event of magma emplacement or partial melting.

Figure 4: Georgetown Region Stratigraphic Column, modified from Jell, 2013.

Figure 5 Simplified geology of the Georgetown region with major structures only. — Figure 5: Simplified geology of the Georgetown region with major structures only.

The Georgetown region experienced multiple phases of regional metamorphism and deformation in the Proterozoic. Metamorphic grades in the Etheridge Group generally increase from the west to the east and range from the lower greenschist to granulite facies. The main regional high-grade metamorphic and deformational event affecting the region accompanied the emplacement of the Mesoproterozoic granitoids (~1550-1560 Ma), but one or two older deformational events pre-dated the magmatism in the period between 1590 and 1620 Ma (Jell, 2013).

4.2 SILURIAN – DEVONIAN INTRUSIVE ROCKS

Extensive plutonic dominantly I-type granitoids of the Siluro-Devonian Pama Igneous Association were emplaced across the Georgetown region, particularly in the south (Figure 5). They include the Copperfield Batholith (including the Oak River Granodiorite), Dumbano, Dido, Glenmore, Ingham, Robin Hood, Tate and White Springs batholiths. The granitoids are also geochemically subdivided into the White Springs, Dido and Mount Webster supersuites (Bain and Draper, 1997; Jell, 2013). Batholiths are mostly elongated to the north-east. The granitoids have significant compositional variations, both between and within supersuites, ranging from gabbros (rare) and diorites to granites with >75% SiO₂. They are mostly unfractionated, with primary oxidation state varying from oxidised to moderately reduced.

U-Pb zircon geochronology for plutons of the Pama Igneous Association in the Georgetown region (summarised in Table 2.1) and other parts of the Etheridge Province (discussed in Bain and Draper, 1997 and Jell, 2013) indicates the dominant magmatic emplacement age in the middle to late Silurian, between 418 Ma and 435 Ma. This is consistent with U-Pb zircon geochronology from numerous samples of Cainozoic stream sediments in the Etheridge Province (Murgulov et al., 2007) and Devonian sedimentary rocks from the adjacent Mossman Orogen (Jell, 2013; Kositcin and Bultitude, 2015; Kositcin et al., 2015), indicating the common presence of a major detrital zircon population with the same range of measured ages.

Historic Rb-Sr whole rock and mineral geochronology obtained from several plutons of the Pama Igneous Association in the region recorded ages in the range of 385 – 425 Ma (Richards et al., 1966; Black, 1973). Older measured ages overlap age estimates obtained from U-Pb zircon geochronology, while the younger Devonian ages have been subsequently interpreted as spurious (Bain and Draper, 1997; Jell, 2013). K-Ar geochronology (summarised in Table 4.1) recorded apparent ages between 370 Ma and 415 Ma. To date, Devonian magmatic ages have not been recorded by robust U-Pb zircon geochronology from any larger plutons of the Pama Igneous Association in the Georgetown region. However, magmatic crystallisation age of 377.2 ± 2.0 Ma (Table 2.1 – from Cross et al., in prep.), estimated for a population of 26 zircon grains from a quartz-feldspar porphyry dyke from the West 24 prospect (part of the Lineament camp) confirmed that at least minor intrusive magmatism occurred in the region in the Late Devonian. Similar magmatic ages, although rare in north-east Queensland, have also been recorded for a granite intersected by a drill hole 225 km south-west of Georgetown (382.1 ± 2.9 Ma; Carson et al., 2011) and plutons of the Mt Formantine Supersuite in the Hodgkinson Province, ~300 km to the north-west (378.8 ± 2.7 Ma and 376.0 ± 3.0 Ma; Kositcin et al., 2015a, b). Widespread Early to Middle Devonian plutonic magmatism (between 390 and 410 Ma) of the Pama Igneous Association has been documented in the adjacent regions to the north (the Cape York Peninsula Batholith) and south-west (the Reedy Springs and Lolworth batholiths in the Charters Towers Province).

Recorded Devonian Rb-Sr and K-Ar ages from intrusive rocks of the Pama Igneous Association in the Georgetown region thus probably reflect re-setting of the Rb-Sr and K-Ar isotopic systems by the same Devonian regional thermal event which produced large-scale plutonic magmatism in other parts of north-east Queensland.

Table 4.1: Radiometric Ages on Palaeozoic rock units (upper section) and mineralisation related intrusions (lower section).

4.3 LATE DEVONIAN TO EARLY CARBONIFEROUS SEDIMENTARY ROCKS

Mostly undeformed clastic sedimentary rocks of the Gilberton Formation (Withnall et al., 1980) occur near Gilberton and in several isolated outcrops in the Georgetown region (Figure 5). They unconformably overlie low-grade metamorphic rocks of the Etheridge Group and are locally overlain by the early Permian Agate Creek Volcanic Group. The formation is dominated by poorly sorted fluvial sandstones and polymictic conglomerates, with minor mudstones and sandstones. Its preserved stratigraphic thickness is estimated to commonly range from 100 m to 500 m. The age of the formation is biostratigraphically constrained to the lLate Devonian (Frasnian) at the base to early Carboniferous (Visean) at the top (Withnall et al., 1980; Bain and Draper, 1997). The formation is commonly tilted and locally cut by high-angle faults, some of which are intruded by rhyolitic dykes.

The existing radiometric ages for the geological units in the Georgetown region have been augmented by recent ages from various sources on intrusions that are associated with the mineral deposits (Table 4.1). An additional 17 samples of mineralised intrusions have been submitted U-Pb zircon dating and the results are awaited. The dating reinforces the established broad epoch subdivision and highlights the likely justifiable separation between early Permian, late Carboniferous and early Carboniferous mineralising events. The most interesting feature is the recognition that dikes related to mineralisation in the Gilberton District are part of the early Permian Agate Creek suite rather than early Carboniferous Kidston-Lochaber suite as had been anticipated.

Although there are no new zircon U-Pb ages on the Silurian intrusive suites the available ages do reinforce the Silurian as the emplacement age and the early Devonian as a regional tectonic-hydrothermal overprint that has reset biotite and hornblende K-Ar and Rb-Sr ages. This is a feature of the Pama Province granitoids throughout north Queensland and is a key support for the idea that the extensive Early Devonian gold mineralisation is not directly related to the host granitoids either in Georgetown or Charters Towers.

Figure 6 : Metallogenic camps plotted on the broad scale geology and structure.

Broad-scale structural controls are evident in the distribution of the camps relative to the regional geology and structure (Figure 6). The first is the apparent restriction of the early Devonian camps to an approximately 20km wide corridor adjacent to the western margin of the Newcastle Range Volcanic Group and a coincidence with the exposures of the Meso-Proterozoic granitoids. The Newcastle Range Volcanic Group does not obscure this mineralisation in the east as the NNW-trending boundary clearly extends in the same direction in the basement south of the Newcastle Range and east of Percyvale and Mt Hogan.

Within this corridor the Early Devonian lodes follow east-trending structures like the Drummer Hill and Big Wonder Faults at Georgetown and the ESE-trending Big Reef structure at Forsayth. The Big Reef structure can be traced as a series of faults and lodes for 40km from Greenhills in the NW to Kidston in the SE. The deposits along this structural corridor range in age from Permian to Early Devonian and represent many mineralisation classes and styles. Maybe such structural corridors can be considered long-lived, crustal-scale channel-ways for magma and fluids. A similar ESE-trending corridor extending from Agate Creek to north of Christmas Hill localises the Percyvale dike swarm, but cuts across the more E-trending Early Devonian lodes in the Percyvale area.

The NE-trending Gilberton Fault is drawn as the southern boundary of the Gilberton District, but does not clearly localise mineralisation. Extensions of the Gilberton Fault and a series of parallel faults seem to bound the Silurian intrusions east of the Newcastle Range and probably control the orientation of the Lochaber Bagstowe Complex across the Gilberton Fault from Kidston, but don’t influence the Kidston mineralisation (Morrison, 2007).

The N-trending Delaney Fault cuts through the Early Devonian mineralised corridors, may in part limit their distribution, does have some sub-parallel veins in the Georgetown camp and is cut by several Early Permian plugs with mineralisation (Figure 5). It is not as significant a mineralised structure as Big Reef and Big Wonder and may represent a hinge fault formed by evacuation of the magma-chamber that fed the Newcastle Range Volcanics in the Carboniferous.

4.4 PERMO – CARBONIFEROUS INTRUSIVE AND EXTRUSIVE ROCKS

Permo-Carboniferous age intrusive and extrusive rocks are widespread throughout the Georgetown, Forsayth and Gilberton regions and some of the major mineral deposits have direct links to this phase of magmatism e.g. Kidston, Agate Creek. The bulk of the Permo-Carboniferous volcanic rocks are hosted in the Newcastle Range Volcanic Group which forms a northerly trending zone of preserved cauldron collapse structures 100kms long and 20kms wide (Figure 6). The Newcastle Range Volcanic Group is dominated by rhyolitic ignimbrite, lava and tuff with rocks of intermediate to basaltic compositon making up only a minor component (Champion & Bultitude, 2013). The oval shaped Lochaber and Bagstowe intrusive complexes lie 30kms south of the Newcastle Range and are composed of biotite to hornblende-biotite granite, microgranite and rhyolite and represent more deeply eroded equivalents of the Newcastle Range Volcanic Group. Rhyolite dyke swarms are common throughout the region and typically possess a northerly or northwesterly trend, parallel to the main regional structures (Figure 6).

Dates of the Permo-Carboniferous intrusives range from ~345 to 280Ma however magmatism appears to have been intermittent with discrete Carboniferous (~345 – 335Ma) and Permian (~290-280Ma) episodes with very few dates between 320 and 290Ma (Withnall et al, 1997).

Two isolated volcanic centres composed mainly of rhyolitic to dacitic igimbrite lie 30 kilometres northwest and southwest of Georgetown respectively. The volcanics belong to the Carboniferous Cumberland Range Volcanic Group. Mineralisation in the Huonfels Camp is spatially and probably temporally related to a swarm of rhyolite dykes emanating from the northern volcanic centre (See Section 9.7). Numerous, scattered, porphyritic microgranite and microgranodiorite intrusives of late Carboniferous to early Permian age, crop out within and between the two volcanics centres and also at the nearby Mount Turner i.e. Mount Sircom and Mount Darcy Microgranodiorites and the Prestwood Microgranite (Table 6.1) (See Section 9.6). Disseminated and breccia style porphyry copper – gold mineralisation at the Phyllis Mae, Log Creek and Mount Turner Camps are related to these relatively young intrusives (Figure 6).

The Agate Creek volcanic centre is located on the Robertson River Fault, 30kms north of Gilberton and consists of a northwest elongate oval shaped block of Permian age, intrusive, volcanic and volcaniclastic rocks, 12kms long and 6kms wide. The volcanic sequence is up to 1000m thick and is composed of crystal-lithic rhyolite ignimbrite and basaltic andesite with abundant agate filled amugdules, capped by arenite and rudite derived from the erosion of the underlying volcanic units (See Section 9.8). Mineralisation at the Sherwood gold deposit is partly hosted in rhyolite dykes and sills of the Agate Creek Volcanic Group that have been dated as early Permian (Tables 4.1 & 6.1).