Permo-Triassic Granite Metallogeny of the New England Orogen

Phillip L. Blevin and Bruce W. Chappell, National Key Centre for Geochemical Evolution and Metallogeny of Continents, Geology Department

Australian National University, Canberra ACT 0200

Introduction

The New England Orogen can be broken into two principal metallogenic domains. The northern NEO (nNEO) is principally a Cu-Mo-Au province while the southern NEO (sNEO) is principally a Sn-Mo-W-polymetallic province. Importantly, these metallogenic associations are replicated through time within each province. Thus Cu-Mo-Au mineralisation occurs within the nNEO associated with magmatism during the Devonian, Permo-Carboniferous, Triassic and Cretaceous, while in the sNEO Sn mineralisation is associated with magmatism during the Carboniferous, Permo-Triassic and Triassic.

Magmatic framework of New England

Permian to Triassic I-type magmatism dominates the New England Batholith of the sNEO. Mineralisation is mainly associated with the high-K Moonbi Supersuite and with two highly fractionated leucomonzogranites (the Gilgai and Mole plutons. Granites of the Moonbi Supersuite are K-rich and have distinctive trace element compositions, most notably high Sr, Ba, Pb, Th and Cs and relatively low Y. Both I- and S-type granites within the sNEO are isotopically primitive despite their generally felsic nature, indicating that they were derived from only slightly older source rocks. Granites of the Clarence River Supersuite have distinctly lower K2O than the Uralla and Moonbi Supersuites. This may also be applicable to the Gundle, Carrai, Daisy Plain and Round Mountain plutons. A geographic separation of the I-types in the sNEO can therefore be made into a western high-K association and a low-K I-type association to the east of the Hillgrove Supersuite.

The nNEO was the site of extensive plutonism in the late Carboniferous to early Permian and the early Triassic, extending down along the central NEO into the sNEO. These granites are typically low- to medium-K diorites, tonalites and granodiorites, with chemical and isotopic signatures indicative of continental margin affinity. The limit of the northerly Permo-Carboniferous province is unclear but probably does not necessarily coincide with the northerly limit of the NEO as currently determined from near-surface geology.

Igneous metallogenic relationships

The NEO was divided into several intrusive metallogenic provinces by Blevin et al. (1) spanning the Devonian to the Cretaceous (Figure 1). The two provinces of Permo-Triassic age are the Moonbi-Bathurst Intrusive Metallogenic Province in the sNEO, comprising oxidised high-K granites, and the Central NEO Intrusive Metallogenic Province, comprising Permo-Triassic intrusions extending from the north coast of New South Wales to the Rockhampton area in Queensland. Further to the north magmatism is dominantly either Permo-Carboniferous (Urannah Batholith) or Cretaceous. Devonian igneous rocks are developed in the Rockhampton region extending to the north along the coast.

Moonbi-Bathurst Intrusive Metallogenic Province: In the sNEO most mineralisation is related to the Moonbi Supersuite, the Mole Granite and the Gilgai Granite. The high-K association extends from Tamworth to Warwick. It is probably more extensive to the north. The Carboniferous granites of the north eastern Lachlan Fold Belt are most probably related. Indeed, similar Mo-W skarns, minor Cu and Au mineralisation are associated with these granites and with the granites in the Moonbi-Attunga region of the sNEO. To the north within the sNEO, granites of the Moonbi Supersuite become more felsic and fractionated and are associated with Sn, Mo and W (Blevin and Chappell, this volume). It also becomes more shallowly exposed. Gold is an accessory in many deposits. The majority of Sn mined was alluvial, shed from low grade disseminated and sheeted vein or stockwork deposits. Sn mineralised granites were polymetallic, often associated with a wide range of metals (Sn, W, Mo, Bi, As, Ag, Pb etc). Complex metal zonation patterns is also present around the plutons. The region is characterised by very large numbers of small deposits, only a few of which approach the tonnage and grade of viable hard rock mines.

Central NEO Intrusive Metallogenic Province: Very little published data is available on the chemistry of intrusive rocks associated with mineralisation in the nNEO. Cu dominant systems such as Coalstoun and Moonmera are associated with granodiorite porphyries, andesitic tuffisites and related dykes inferred to be later stage differentiates of the Bouldercombe Complex. Regionally, the Bouldercombe Complex is also associated with porphyry Cu-Mo style mineralisation at Sandy Creek-Gordon, Struck Oil and (?) Moongan. The Coalstoun deposit is associated with shallow porphyritic tonalitic to quartz dioritic intrusives. Intrusive rocks associated with Cu deposits in the nNEO are oxidised and intermediate to felsic, and in broad compositional terms are not dissimilar to intrusive rocks associated with porphyry style Cu deposits in other continental marginal settings.

Mo dominant systems include Mo-Cu pipes in the Mount Perry area which are associated with medium-K series granites unlike similar pipe-like deposits associated with more fractionated high-K granites in north Queensland and in the sNEO. The Mount Perry pipes are however Mo-Cu rather than being associated with elevated W-Bi as elsewhere. Further to the south east porphyry Mo mineralisation occurs at Anduramba. While the rhyolitic portions of the deposit have textural characteristics similar to Climax style porphyry Mo deposits, the Anduramba intrusions differ in that they are significantly less fractionated.

Discussion and conclusions

Several unusual features are apparent in the metallogeny of the NEO. The NEO has distinct zones of contrasting metallogenic that correlate, in the first pass, with broad scale variations in granite type. The associations are not time specific. That is, there are no "metal specific" metallogenic epochs in the NEO, rather metallogenic epochs mirror the major magmatic episodes that have occurred in the orogen.

The NEO is relatively easily accommodated into a continental margin model, particularly when compared with the Lachlan Fold Belt. The role of plate tectonics as a dominant control on ore element ratios in igneous related mineralisation has long held currency in the economic geology and tectonic literature. Porphyry Cu deposits occur overwhelmingly along linear calcalkaline subduction-related volcanoplutonic arcs. Plutonic rocks associated with porphyry copper deposits within island arc settings are dominantly hornblende and hornblende-biotite diorites and quartz diorites while those in continental marginal settings are more typically granodiorites and quartz monzonite. Both are magnetite bearing. Sr isotope data indicates that intrusives located in continental margin type situations are more isotopically evolved than those in island arc settings. Existing evidence strongly indicates that there is no one "source" for potential porphyry Cu producing magmas.

Considerable metallogenic diversity can be present in any single igneous supersuite. An example is the Moonbi Supersuite which is associated with Cu-Mo-Au at the less evolved end, through Mo to Sn at the most evolved (fractionated) end. The relationships indicate that mineralisation is a product of magmatic and hydrothermal processes and is not a product of anomalous metal contents inherited by magmas.

Relationships within the NEO are also consistent with broad redox-fractionation controls on ore metal-igneous associations. In the nNEO many uneconomic porphyry style Cu-Mo-Au systems occur, all with relatively low grades. From what little is known from the related igneous rocks, there is no a priori reason why these rocks should be considered poor candidates for producing substantial mineralisation.

References

1 Blevin, P. L. Chappell, B. W. and Allen, C. M. 1996. intrusive metallogenic provinces in eastern Australia based on granite source and composition. Transactions of the Royal Society of Edinburgh: Earth Sciences. (In press)

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