MINERAL SYSTEMS AND THE CRUST-UPPER MANTLE OF SE AUSTRALIA: IGNEOUS METALLOGENY OF S.E. AUSTRALIA.

Phillip L. Blevin, GEMOC ANU

Many schemes exist for the metallogenic classification of igneous-related mineral deposits. These schemes have traditionally emphasised such factors as time ("metallogenic epochs"), tectonic setting ("metallotectonic provinces"), deposit type or style, and commodity associations (e.g. "Cu provinces" versus "Sn provinces"). Such schemes however are ultimately unsatisfactory from a genetic perspective because they do not take into account the role of magma source, composition and process in determining ore element ratios in related deposits. Because of these relationships, it is the grouping of ore-related granites by suites and supersuites that is preferred as the primary division for metallogenic classification purposes. This methodology, which is independent of genetic models relating granite compositions to source types, seeks to group together suites of similar character as a way of defining regions on the surface of the presently exposed crust where similar intrusive-deposit relationships may also exist. These regions were termed "Intrusive Metallogenic Provinces" by Blevin et al (1996).

An important consequence of this model is that magmatic suites that have fractionated over a large compositional range may be associated with a range of types and styles of mineralisation, as will intrusive metallogenic provinces defined on this basis. These provinces may be time transgressive, and may not correlate with tectonostratigraphic boundaries as defined by near-surface geology.

Some Intrusive Metallogenic Provinces In SE Australia

Lachlan Fold Belt

Mafic to felsic igneous rocks of Ordovician age are exposed in N-S trending belts in the central zone of the LFB. The mineralised complexes are not exclusively (or even dominantly) "shoshonitic" as previous thought. Related mineralisation comprises significant Cu-Au±Mo porphyry and minor skarn types. Alaskan-type ultramafic complexes are associated with Pt mineralisation.

The Silurian-Devonian and Carboniferous granites comprise 20% of the total exposed area of the LFB. Significant mineralisation associated with these granites is restricted to Sn in the western/central portion of the LFB in New South Wales, and Sn-W mineralisation in both eastern and western Tasmania. Numerous minor deposits of Sn, W, Mo, Au, W and Cu also occur elsewhere. Some gold deposits are clearly genetically related to Silurian magmatism, while most penecontemporaneous mesothermal vein-type Au deposits are ambiguous in their genetic relationships. Some specific supersuites (e.g. Boggy Plain) are associated with a range of deposits, while specific deposit types (e.g. Sn in the Wagga Tin Belt, Mo in the Bega Batholith), are associated with a number of granite suites of broadly similar character within discrete regions previously recognised as "granite basement terranes" by Chappell et al. (1988). Gold and base metal mineralisation is also associated with contemporaneous Silurian basins, however the (often presumed) syngenetic nature of many of these deposits remains in doubt (e.g. see Glen, 1995). The Carboniferous granites of the north east LFB are associated with Mo and Au mineralisation, consistent with their generally oxidised nature, and similar to their compositional equivalents (the Moonbi Supersuite) in the Southern New England Orogen (SNEO).

New England Orogen

Mid to Late Devonian magmatism in the NEO is widely distributed in the form of volcaniclastic sediments and related flows. The chemical composition of related intrusive rocks, such as the Mount Morgan Tonalite Complex, are oceanic in character and similar to the tonalitic intrusives of New Britain. Permian to Triassic I-type magmatism in the SNEO is related to Sn, W, Mo and minor Cu mineralisation. Mineralisation is mainly associated with the high-K Moonbi Supersuite and other highly fractionated leucoadamellites. The central and northern portions of the NEO (CNEO and NNEO) were the sites of extensive plutonism in the Late Carboniferous to early Permian, and the early Triassic. These granites comprise typically low- to medium-K diorites, tonalites and granodiorites, with chemical and isotopic signatures indicative of continental margin and/or subduction affinity. While earlier magmatic-plutonic episodes in the CNEO and NNEO were probably subduction related, early Cretaceous magmatism was related to rifting and opening of the Tasman Sea (Ewart et al. 1992). All these magmatic stages are associated with numerous subeconomic porphyry style Cu-Mo-Au systems of the continental margin type.

The LFB and NEO compared.

The fold belts of south eastern Australia contain a range of granite compositions that may be grouped into three broad categories in terms of source: those produced directly from the mantle with or without contemporaneous subduction (e.g. Ordovician LFB magmatism); those derived dominantly from the fusion of older crust (e.g. Silurian-Devonian LFB magmatism); and those resulting from the reworking of juvenile crustal materials recently added to the crust, where fusion was associated temporally and spatially with active subduction or rifting process (e.g. magmatism in the NEO). There is a general correlation in the nature and style of mineral deposits and commodity associations with igneous rocks representing these categories, from Cu-Au porphyry styles with subduction-related and/or arc-like magmatism, through to lithophile dominated mineralisation associated with granites derived dominantly from older crustal materials. This is in part due to the degree of compositional evolution of the igneous source materials. Exceptions do occur however, for example, the presence of significant lithophile mineralisation (Sn, Mo, W) in the SNEO associated with granites derived from juvenile Palaeozoic source materials.

Contrasts in the magmatic and tectonic evolution of the NEO and LFB are profound. Notably, the NEO is much more easily accommodated into subduction and continental margin models than is the LFB. The igneous metallogeny of the LFB evolves with time, from Cu-Au through to lithophile dominated mineralisation during the Palaeozoic. In the NEO, lithophile (Sn, Mo, W) dominates in the SNEO while Cu-Mo-Au dominates in the CNEO and NNEO. There is a recurrence of these metallogenic patterns throughout time in both portions of the NEO.

Emplacement and Preservation Considerations

While the metallogenic scheme outlined above provides a useful conceptual framework for relating ore deposits back to igneous rocks and their sources, it cannot predict whether igneous suites may have generated mineralisation, and whether that mineralisation may have been preserved. A challenge for the future then is to give such schemes enhanced predictive capacity by assigning ore generative, and preservation potentials to granite suites and any related mineralisation. This will involve assessing the level of emplacement of the magma and the subsequent level of (tectonic) erosion that it has undergone.

The timing and efficiency of volatile saturation and extraction relative to the progress of crystallisation within granite plutons is very important (Candela & Holland, 1986). Most intrusive-related mineralisation is generated at shallow levels within the crust, particularly at epithermal to hypabyssal depths (1-5 km). Porphyry-type systems become less prominent below 4-6 km, with mineralisation styles comprising mainly skarns at greater depths (to 8-10 km). "Mesothermal" gold-quartz veins are problematical in their genetic relationships to contemporaneous magmatism, and typically form at depths of 4-15 km.

An unusual feature of the LFB is that the character of deposits through time do not correlate with their assumed preservation potentials. Ordovician volcanism in the LFB is shallow in nature, becoming subareal in the Late Ordovician. The associated porphyry style mineralisation is typical of subvolcanic porphyry style alteration and mineralisation elsewhere in the nature of its veining and infill, and relationship to shallow volcanism. Preservation of only mildly deformed Ordovician volcaniclastics requires that subsequent compression, uplift and erosion of the LFB, at least in central NSW, cannot have occurred evenly across the belt. The apparent time-space juxtaposition in the Silurian-Devonian of both volcanism and plutonism, and the apparent absence of subsequent crustal thickening and/or mountain building was accommodated into a "granite tectonics" model involving vertical crustal reworking driven by granite ascent in concert with basin formation (Blevin et al., 1997). However, problems remain with the dynamics of such models, and particularly evidence for diapirism.

Differential uplift of the order of only a few kilometres may be sufficient to alter the metallogenic character of a region. The S-type granites of the Wagga and Kosciusko Granite Basement Terranes are compositionally very similar at their mafic ends, however mineralisation is almost entirely restricted to small, high level stocks that are just being unroofed within the "Wagga Tin Belt". Preservation of these intrusives may be an artefact of differential uplift and exposure, with granites of the Kosciusko region perhaps representing more deeply eroded equivalents of those further inboard. In the Bega Batholith, low grade Mo mineralisation is associated with several granite suites. Determination of crystallisation depths in the weakly zoned Bemboka Pluton suggest that volatile exsolution occurred in these magmas at pressures too great to allow the efficient segregation of fluids and metals into large, high grade mineral deposits (Candela & Blevin 1995).

In the NNEO, emplacement levels of the magmas versus their current level of exposure may be important in understanding the nature of mineralisation within the region, particularly from a comparative metallogenic standpoint. For example, the Urannah Batholith is similar to the Sierra Nevada Batholith in terms of its gross compositional and petrographic character (Allen & Chappell, 1993). The two regions differ tectonically however, in that the Sierra Nevada have undergone significant uplift and erosion. Associated mineralisation in the Sierra Nevada is mostly skarn type (W, Mo, Cu) formed at intermediate crustal depths (Barton, 1996). Inboard of this region, similar age Cu mineralisation is dominantly porphyry style, and is associated with intrusive complexes exposed at considerably shallower erosional depths (0-4 km). Mineralisation in the NNEO includes a number of apparently metal-starved, porphyry-like Cu-Mo systems. Allen and Chappell (1993) have demonstrated from textural and Al-in-hornblende geobarometry data a lack of major uplift and erosion in the NNEO, in contrast to the Sierra Nevada.

Future Studies

In addition to developing better methodologies in assessing parameters such a volatile history and emplacement levels of magmas, and the preservation potential of related mineral deposits, more attention will need to be focussed on understanding the role of tectonics and crust/mantle processes in defining, or generating mineral provinces. This is being pursued petrologically by GEMOC through 4D lithospheric mapping utilising chemical and isotopic databases on granites, basalts and xenoliths. Such studies will help refine the petrological, temporal and spatial evolution of the crust/mantle system in south eastern Australia over time.

References

Allen C. M. & Chappell B. W. 1993. Contrasting Carboniferous-Permian and Cretaceous plutonism in the Urannah Batholith, northern New England Fold Belt. In: Flood P. G. & Aitchison J. C. eds New England Orogen, Eastern Australia, pp. 573-579. Armidale, University of New England.

Barton M. D. 1996. Granitic magmatism and metallogeny of southwestern North America. Transactions of the Royal Society of Edinburgh: Earth Sciences 87, 261-280.

Blevin P. L. & Chappell B. W. 1992. The role of magma sources, oxidation states and fractionation in determining the granite metallogeny of eastern Australia. Transactions of the Royal Society of Edinburgh: Earth Sciences 83, 305-316.

Blevin P. L., Chappell B. W. & 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 87, 281-290.

Blevin P. L., Ellis D. J. & Warren R. G. 1997. Metallogenic implications of granite tectonics: The Lachlan Fold Belt as a case study. Geological Society of Australia, Abstracts No. 44, pp. 12.

Candela P. A. & Blevin P. L. 1995. Do some miarolitic granites preserve evidence of magmatic volatile phase permeability? Economic Geology 90, 2310-2316.

Chappell B. W., White A. J. R. & Hine R. 1988. Granite provinces and basement terranes in the Lachlan Fold Belt, southeastern Australia. Australian Journal Of Earth Sciences 35, 505-521.

Ewart A., Schon R. W. & Chappell B. W. 1992. The Cretaceous volcanic-plutonic province of the central Queensland (Australia) coast - a rift related 'calc-alkaline' province. Transactions of the Royal Society of Edinburgh: Earth Sciences 83, 327-345.

Glen R. A. 1995. Thrusts and thrust-associated mineralisation in the Lachlan Orogen. Economic Geology 90, 1402-1429.

Horton D. J. 1978. Porphyry-type copper-molybdenum mineralisation belts in eastern Queensland. Economic Geology 73, 904 - 921.

Shaw S. E. & Flood R. H. 1993. Carboniferous magmatic activity in the Lachlan and New England Fold Belts. In: Flood P. G. & Aitchison J. C. eds. New England Orogen, Eastern Australia, pp. 113-121. Armidale, University of New England.

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