Graham-Ruzicka, Julie Louise (2014) Gold-bismuth occurrences in the Kennedy igneous province, North Queensland: constraints on tectonic, magmatic and hydrothermal processes in intrusion-related gold deposits. PhD thesis, James Cook University.

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Abstract

The Kennedy Igneous Province (KIP) in north Queensland, Australia is host to a gold mineralising event (8.5Moz) associated with Carboniferous–Permian aged magmatism. Gold production was mainly from the shallowly emplaced (<5km) hydrothermal breccia pipe hosted gold deposits like Kidston, Mt Leyshon and Mt Wright. The Kidston and Mt Leyshon deposits share characteristics with intrusion-related gold systems of the Tintina Gold Province, Yukon and Alaska. This suggests north Queensland is prospective for intrusion-related gold deposits or intrusion-related gold systems.

The thesis focused on whether two gold occurrences, the Empire Stockworks and Douglas Creek gold mineralisation, share similarities with other intrusion-related gold systems. Empire Stockworks gold mineralisation is temporally and spatially associated with rhyolite dykes located on the eastern margin of the Empire dacite hydrothermal breccia pipe. A number of Empire Stockworks characteristics are similar to other shallow intrusion-related gold systems. The geochemistry of the Empire Stockworks rhyolite dykes suggest they are weakly to strongly oxidised (Fe₂O₃\FeO, 0.5-8), sub-alkaline (<10wt. % Na₂O+K₂O and~60 to 85wt. % SiO₂), peraluminous, moderately to highly fractionated (Rb/Sr >1, -ve Eu compared to REECN) intrusions derived from a crustal source (depleted in Ba, Nb, La, Ce, Sr, P and Ti compared to primitive mantle). The veins are associated with potassic alteration and contain <2% sulphide. The mineralising fluid was CO₂ poor with variable salinities from ~5 to 45 equivalent wt. % NaCl. The best estimate of conditions of mineralisation is ~260-437ºC at 1.5kbar. This estimate is based on a primary L-V inclusion entrapment temperatures calculated using an entrapment pressure (1.5kbar) from a primary ms-L-V inclusions, in a Stage 2 quartz K-feldspar vein, that homogenised by halite disappearance. Further work is required to confirm the entrapment conditions.

Mineralisation contains gold associated with Bi-chalcogenides, although the bulk geochemistry of the vein samples suggests the correlation is generally low. Based on the timing and, the close spatial relationship between mineralised quartz veins and rhyolite dykes the mineralisation was genetically related to the rhyolite dykes. The dates were not conclusive but suggest an age of ~316Ma (Zircon, ID-TIMS) for a mineralised rhyolite dyke from Empire Stockworks. This age is comparable with the age of the Red Dome deposit, near Chillagoe. The rhyolite dyke post-dates the emplacement of the breccia pipe and provides the upper age limit of the formation of Empire Breccia Pipe complex.

The Empire rhyolite dykes are similar to other north Queensland breccia hosted gold deposits that comprise mainly oxidised intrusions (weakly to strongly). This distinguishes them from the reduced intrusions associated with intrusion-related gold deposits. Compared to the larger breccia pipe hosted gold deposits, Empire Stockworks has a number of differences in geological setting. Empire Stockworks had a lower volume of rhyolite intrusions that lack magmatic-hydrothermal transition textures (unidirectional solidification textures). At Empire Stockworks, there is no evidence of a crowded porphyry dyke phase, mineralised tuffisites, miarolitic cavities or porosity filled gold mineralisation, as observed in the larger Kidston and Mt Leyshon deposits. Unlike the larger breccia pipe hosted gold deposits the mineralised quartz veins, at Empire Stockworks were emplaced at the pipe contact and post-date the sealing of the Empire Breccia Pipe.

Empire Stockworks and the breccia pipe hosted gold deposits are distinguished from Moporphyry deposits. This is because the main commodity is gold, the tectonic setting of the Kennedy Igneous Province is back-arc with little or no juvenile mantle input, and the intrusions are high-K calc-alkaline. The breccia pipe hosted gold deposits are associated with fractionated metaluminous to peraluminous rhyolites, rather than quartz-monzonite to granodiorite plutons, despite the overlap in emplacement depth of the intrusions (1- 6km). In this respect Empire Stockworks and the breccia hosted gold deposits of north Queensland are a distinct group of deposits that have some affinities with both Mo- Porphyry deposits and intrusion-related gold systems.

The Douglas Creek gold-bismuth mineralisation, in the south eastern Hodgkinson Province, comprises gold-bismuth veins temporally and spatially association with Emerald Creek Microgranite at ~314-320Ma. Minor mineralisation is associated with pegmatite and aplite dykes and quartz-aplite veins that post-date W-skarns hosted in the metamorphic aureole of the Emerald Creek Microgranite. Geochemistry of the microgranite suggests it is a sub-alkaline (<8wt. % Na₂O + K₂O, >60wt. % Si₂O), metaluminous to peraluminous, fractionated (Rb/Sr ~1) and reduced (0.1-1 Fe₂O₃/FeO) intrusion derived from a crustal source (depleted in Ba, Nb, La, Ce, P, Zr, Eu and Ti compared to primitive mantle). The mineralising fluid temperature ranged from 100 to 300ºC with salinities from ~1 to 30 equivalent wt. % NaCl and no CO₂. The occurrence is a poorly endowed intrusion-related gold system.

New age dating suggests the Douglas South Granite and the Tinaroo Granite are younger than the Emerald Creek Microgranite but older than other Late Carboniferous intrusions in the Hodgkinson Province. A consistent age of 306.6±1.6Ma (muscovite, Ar-Ar) was obtained for the Douglas South Granite and suggests the intrusion is older than rest of the Whypalla Supersuite (282±9Ma, Rb-Sr, Garrad and Bultitude, 1999) and was emplaced at about the same time as the Emerald Creek Microgranite. The Tinaroo Granite is ~ 278Ma based on a consistent relationship between a laser ablation dates of 277.8±1.2Ma and a muscovite Ar-Ar date of 277.7±1.5Ma.

There are no large gold deposits associated with the Emerald Creek Microgranite because it is a reduced intrusion with low agpaitic number and lacks volatiles such as CO₂. Sheeted vein deposits similar to Fort Knox or Dublin Gulch in Alaska and the Yukon possibly never formed, as suggested by the vein style and distribution and lack of evidence for formation of transitional hydrothermal textures in the upper carapace of the Emerald Creek Microgranite. Compared to other metamorphic aureole hosted intrusion related gold deposits (e.g. Dublin Gulch, Clear Creek) the veins are irregular in style and are variable in their orientation suggesting that they lacked a homogeneous stress field perhaps owing to variation in the Emerald Creek Microgranite contact. The broad distribution of the veins also suggests the fluid flow was not focused into one structure and a low fluid flux is suggested by the restricted distribution, low abundance and small size of the veins.

The deepest eroded portion of the Kennedy Igneous Province is in the Hodgkinson Province where intrusions were emplaced at >7km. There is low potential for deeply emplaced intrusion-related gold deposit styles of mineralisation associated with intrusions in Hodgkinson Province because they are mainly reduced S-type intrusions with low agpaitic numbers and little or no known genetically related gold mineralisation. There are no gold-bismuth occurrences associated with other I-type intrusive suites in the Hodgkinson Province.

The use of the Au-Bi-Te association in intrusion-related gold deposits for exploration was investigated at the regional to prospect scale at Pogo gold deposit, Alaska. It was found that Au-Bi-Te correlations and ratios could be used to rank regional prospects. However, the Au-Bi-Te ratio is not useful as a vector towards higher grade mineralisation within an ore zone. This is because the Au-Bi-Te ratio varies due to more than one hydrothermal process such as redox conditions, sulphidation, and cooling and late stage re-mobilisation of gold along faults. For example, the Au-Bi ratio in the Liese 1 Vein, at Pogo, is highly variable both laterally and vertically. Therefore, the Au-Bi ratio of vein could not be used to differentiate a less prospective from a prospective vein set during a prospect drilling program or to determine which direction to drill within a large mineralised vein.

The main large scale controls on formation of intrusion-related gold systems include: 1) a distal back-arc tectonic setting comprising a source region with the right range of initial oxidation states or as a result of assimilation; 2) the transfer of volatiles and metal to the crust during exposure to the asthenosphere or upper lithospheric mantle; 3) the formation of a large magmatic belt followed by production of isolated magma chambers with efficient magmatic processes to accumulate the gold.

Item ID:	43773
Item Type:	Thesis (PhD)
Keywords:	Au-Bi; breccia pipes; Douglas Creek; Empire Stockworks; gold deposits; gold mineralisation; gold systems; gold-bismuth; intrusions; Kennedy Igneous Province; KIP; North Queensland; NQ; rhyolite dikes; rhyolite dykes
Additional Information:	Appendices 2-7 (data) are not available through this repository.
Date Deposited:	15 Jun 2016 05:39
FoR Codes:	04 EARTH SCIENCES > 0402 Geochemistry > 040201 Exploration Geochemistry @ 100%
SEO Codes:	84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840199 Mineral Exploration not elsewhere classified @ 100%
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