METALLOGENIC STUDY OF THE GEORGETOWN, FORSAYTH AND GILBERTON REGIONS, Part 7

PLATE 23: Sample from the Caledonia workings, Gilberton. Shows brecciated early, white, tightly packed, euhedral buck quartz (Plutonic style) cemented by clear, comb textured quartz, colloform banded and in places moss textured chalcedonic silica and siderite (Epithermal style).
METALLOGENIC STUDY OF THE GEORGETOWN, FORSAYTH AND GILBERTON REGIONS, Part 6
August 19, 2019
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METALLOGENIC STUDY OF THE GEORGETOWN, FORSAYTH AND GILBERTON REGIONS, Part 7

PLATE 29 : Kidston breccia ore showing stubby, medium grained, comb textured quartz crystals lining vughs with later carbonate, pyrite and base metal sulphide filling vugh cores.

PLATE 29 : Kidston breccia ore showing stubby, medium grained, comb textured quartz crystals lining vughs with later carbonate, pyrite and base metal sulphide filling vugh cores.

Report Number 114062, METALLOGENIC STUDY OF THE GEORGETOWN, FORSAYTH AND GILBERTON REGIONS, NORTH QUEENSLAND is now available for download from QDEX. The authors are Dr Gregg Morrison, Harry Mustard, Ashley Cody, Dr Vladimir Lisitin (GSQ), Jose Veracruz and Dr Simon Beams.

A collaboration between Terrasearch Pty Ltd, Klondike Exploration Services and the Geological Survey of Queensland, funded under the Queensland Government Future Resources Program.

TABLE OF CONTENTS

1.0……… SUMMARY.

2.0……… INTRODUCTION.

3.0……… GEORGETOWN MINE ENDOWMENTS and HISTORICAL GOLD PRODUCTION.

4.0……… GEOLOGY.

4.1 PROTEROZOIC METAMORPHIC AND INTRUSIVE ROCKS.

4.2 SILURIAN – DEVONIAN INTRUSIVE ROCKS.

4.3 LATE DEVONIAN TO EARLY CARBONIFEROUS SEDIMENTARY ROCKS.

4.4 PERMO – CARBONIFEROUS INTRUSIVE AND EXTRUSIVE ROCKS.

5.0……… THE METALLOGENIC DATABASE AND GIS.

6.0……… AGES AND EPOCHS OF MINERALISATION.

7.0……… MINERALISATION STYLE AND DEPTH CLASSIFICATION BASED ON QUARTZ TEXTURES.

7.1 QUARTZ TEXTURES OF PLUTONIC DEPOSITS.

7.2 QUARTZ TEXTURES OF INTRUSION RELATED DEPOSITS.

7.3 QUARTZ TEXTURES OF EPITHERMAL DEPOSITS.

7.4 QUARTZ TEXTURE ASSEMBLAGES AND CAMP DEFINITION.

8.0……… MULTI-ELEMENT GEOCHEMISTRY.

8.1 METHODOLOGY OF MULTI-ELEMENT GEOCHEMISTRY INTERPRETATION.

8.2 INTERPRETATION OF GEORGETOWN MULTI-ELEMENT GEOCHEMICAL DATA.

8.3 GEOCHEMICAL ZONATION PATTERNS.

9.0 ……. DEPOSIT MODELS.

9.1 MOUNT HOGAN flat plutonic lode.

9.2 PLUTONIC LODE DEPOSITS.

9.3 GILBERTON DISTRICT.

9.5 KIDSTON INTRUSIVE RELATED MESOZONAL BRECCIA HOSTED GOLD DEPOSIT.

9.6 MOUNT TURNER.

9.7 HUONFELS, INTRUSION RELATED EPIZONAL LODE AG – AU PROSPECT.

9.8 AGATE CREEK EPITHERMAL GOLD DEPOSIT.

10.0……. OVERALL METALLOGENIC CLASSIFICATION.

11.0……. CONCEPTUAL MODEL.

12.0……. GEORGETOWN EXPLORATION POTENTIAL.

13.0……. CONCLUSIONS.

14.0……. REFERENCES.

REPORT TEXT

APPENDIX 1 : GEORGETOWN METALLOGENIC CAMP SUMMARIES

APPENDIX 2 : GEORGETOWN METALLOGENIC Database

APPENDIX 3 : GEORGETOWN METALLOGENIC CAMP STRUCTURAL PLAN

APPENDIX 4: MAPINFO WORKSPACE PACKAGE – Georgetown Metallogenic Simplified Geology (zip)

10.0   OVERALL METALLOGENIC CLASSIFICATION

All the epithermal deposits have rhyolites related to or of the same age as mineralisation so the boundary between epithermal per se and intrusion related epizonal is not clear (Table 10.1). This separation is currently based on presence of chalcedony and boiling textures in Epithermal versus fine comb quartz only in Intrusion-related epizonal deposits, but both types have similar chemistry notably Te significant and Ag>Au most common. The implication is that the epithermal deposits are part of the Intrusion-Related Epizonal system and predominantly of Permian age (Figures 27 & 28).

The Intrusion-Related deposits have early Permian and early Carboniferous groups with two possible Late Carboniferous examples (Mt Turner, Log Creek). All the deposits are polymetallic with prominent Te-Bi and As-Sb and are mesozonal to epizonal. The early Carboniferous deposits in the Kidston region are Au-rich mesozonal hydrothermal breccias whereas the deposits west of Georgetown are early Permian-late Carboniferous Au>Ag lode deposits (Electric light, Cumberland Mine, Beverley, Double Z) or Ag>Au epizonal-mesozonal lodes, stockworks and breccia deposits (Ironhurst, Phyllis May, Mt Turner, Bald Mountain) (Figures 27 – 29).

The Plutonic deposits are those in which there is no demonstrated spatial or temporal link to intrusions. These deposits are Early Devonian shear-hosted lodes that may have steep or shallow dipping orientation and be extensive on kilometre scale along the structure. There are three districts – Georgetown, Forsayth & Gilberton each with a distinct Au>Ag, Bi-Te, As-Sb, base metal signature and zoning on kilometre scale in both quartz textures and metal geochemistry (Figure 27 & 30).

Table 10.1 : Summary classification for each metallogenic camp defined in the Georgetown region.
Table 10.1 : Summary classification for each metallogenic camp defined in the Georgetown region.
Figure 27: Proterozoic geology with all metallogenic camps
Figure 27: Proterozoic geology with all metallogenic camps
Figure 28: Permian geology with Permian metallogenic camps
Figure 28: Permian geology with Permian metallogenic camps
Figure 30: Silurian geology with Early Devonian camps
Figure 30: Silurian geology with Early Devonian camps

11.0   CONCEPTUAL MODEL

The observed features, structural controls, related intrusions, quartz textures and metal geochemistry have been used to classify the deposits and to build models of the hydrothermal systems responsible for the mineralisation as a conceptual guide for exploration.

In the Georgetown region there are two main conceptual models: porphyry-epithermal that applies to the Carboniferous and Permian Intrusion-related deposits and the Plutonic model that applies to the Early Devonian lode gold deposits (Figure 31).

The epithermal-porphyry system is dominated by magmatic fluid that emanates from porphyry plugs and dikes at mesozonal to epizonal-level and interacts with ground-water at shallow levels. There are two variants of the epithermal deposits in the Georgetown region those with boiling textures-chalcedony, crustiform & colloform banding and bladed textures and those with fine crystalline textures only that have been referred to as epizonal IRGS. The field distinction is quite valid but in both variants there are rhyolitic dikes related to the mineralisation and anomalous Bi & Te that suggests that all the deposits have a magmatic link and that the boiling zone variant is just a shallower level variant of the epizonal IRGS.

The plutonic deposits are those formed at mesozonal to hypozonal level above stocks that emanate from an inferred batholith during deformation (Figure 31). There is no direct physical evidence of intrusion involvement, but the textures of the quartz suggest the hypo-mesozonal level of emplacement and the multi-element geochemistry with prominent Te-Bi suggest magmatic fluid is involved in the mineralisation. The best mechanism for this is to form the deposits under moderate confining pressure due to deformation loading, but to uplift the area above the batholith with buoyant forces during batholith emplacement. In this case there is a combination of fluid derived by contact metamorphic dewatering in the aureole of the pluton and local magmatic fluid emanating from the stocks.

Figure 31: Conceptual model of fluid regimes for the plutonic and porphyry-epithermal deposits compared with the metamorphic fluid model generally envisaged for orogenic deposits.
Figure 31: Conceptual model of fluid regimes for the plutonic and porphyry-epithermal deposits compared with the metamorphic fluid model generally envisaged for orogenic deposits.

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.

14.0   REFERENCES

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