Abstract of Granites-Tanami Block Synthesis

Record 2001/12

Compiled by Lesley Wyborn

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• Executive summary - geology
• Executive summary - metallogenic potential
• Future work

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Executive summary - geology

Gold was first discovered in The Granites-Tanami Block in 1900 and was mined intermittently between 1904 and 1961. More recently it has been the focus of significant exploration efforts with several major new mines operating since 1986. The area as a whole is poorly exposed and is mostly covered by thick regolith. The regional geology of The Granites-Tanami Block was described by Blake et al. (1979) and very little has been published on the regional geology since then. More detailed geological descriptions are available in papers on the gold deposits in the region ( e.g., Ireland and Mayer 1984; Mayer 1990; Ireland 1995) and new structural interpretations are available in Ding (1996, 1997) and Ding and Giles (1996).

Traditionally, The Granites-Tanami Block was believed to consist of two major subdivisions separated by a major unconformity (Blake et al. 1979). The lower subdivision, the Tanami Complex, was deformed and metamorphosed prior to the eruption of volcanics of the Mount Winnecke Formation and deposition of the overlying Supplejack Sandstone. The lowermost Tanami Complex rocks were believed to rest unconformably on Archaean basement (Page and Sun 1994; Page et al. 1995). The Tanami Complex comprises the Killi Killi Beds, Mount Charles Beds, Nanny Goat Creek Beds, Nongra Beds and the Helena Creek Beds. These units consist of greywacke, siltstone, arenite, chloritic and sericitic shale, as well as carbonaceous shale, banded iron formation, and mafic and felsic volcanics. Most of the mineralisation is hosted by the Mount Charles Beds, which contain some of the more reactive rock types of the Tanami Complex including banded iron formation and carbonaceous shale.

Recently Ding (1997) in an abstract has provided a reinterpretation of The Granites-Tanami Block in which he identifies five major stratigraphic packages ranging in age from at least 2450 Ma (Tanami Group) to 1815 Ma. Each package is separated by a major unconformity, and the basal group lies unconformably on basement that is ~2500 Ma old (Ding 1997).

Ding (1997) reports a single crystal zircon age of 2450 Ma on a granite sill which intrudes the Tanami Group. The earliest known Proterozoic 'granites' dated by conventional means occur at ~1880 Ma. These are partial melts of late Archaean gneisses which occur in the Browns Range Dome. Some felsic volcanics have been identified within the Tanami Complex, but only one has been dated at around ~1800 Ma. The dated rock is presumed not to come from the Tanami Complex, but from volcanics belonging to the Granites Supersuite which are dated from about 1825 Ma to 1795 Ma. The members of this suite were intruded mostly after the deformation that affected rocks of the Tanami Complex.

Most felsic igneous rocks in The Granites-Tanami Block are Sr-depleted and Yundepleted, indicating a plagioclase-residual source.

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Executive summary - metallogenic potential

The only suite with recognised metallogenic potential is the Granites Supersuite which consists predominantly of non-magnetic, reduced, fractionated, metaluminous rocks. The highly reduced nature of the early phases of this suite is anomalous, as goldbearing granites are usually assumed to be oxidised and magnetic. It is suggested that the reduced nature of this suite results from infusion of H2 from carbonaceous country rocks into the magma early in the magmatic history. With increasing evolution, the Supersuite then became more oxidised either because the H2 stopped passing into the magma chamber from the country rock, or because H2 diffused into the atmosphere (Czamanske and Wones 1973; Wyborn 1983).

The gold deposits in The Granites-Tanami area are hosted by predominantly iron-rich, graphite-rich or sericite/chlorite-rich rocks. All the deposits appear to have precipitated from reduced fluids and have pyrite-quartz-sericite alteration associated with the mineralising event. As the majority of granites within the area are reduced fractionating I-types it seems more than likely that fluids derived from these granites are a component of the mineralising fluids, a suggestion that is supported by fluid inclusion work (e.g., Tunks 1995, Valenta and Wall 1996). There is a possibility that Sn may be found around the Lewis Granite and there is also potential for W mineralisation. It is also possible that there may be some mineralisation related to the late magmatic phases of the granite, although these will probably have hosts of different composition from those in The Granites-Tanami Block that are associated with reduced fluids.

The better known deposits are located in an area where the granite intrusions are interpreted to be relatively deep (Blake et al. 1979; Wall 1989). In the north, the Winnecke Granophyre has intruded to a much shallower depth and has altered and greisenised its own comagmatic volcanics in the Mount Winnecke Formation. If any mineralisation exists in this area it is more likely to be of an epithermal or porphyry style, and hosted within or close to the volcanics or the granophyre. The limiting factor in this model may be that the magmas in this northern area are also the most fractionated and may have already lost metals such as Au or Cu.

It is accepted that the connection to a granite source for the mineralisation could be regarded as tenuous, as all known mineralisation is distant fromthe granites. Ding (1997) has argued that the Granites Gold Deposit is an example of a stratabound preorogenic deposit (pre-1980 Ma) whilst Wall (1989) and Valenta and Wall (1996) argue for a granite-related model. The point at issue with this study is that The Granites Supersuite shows clear evidence of fractionation and is a type of granite that is similar to those found in both the Pine Creek and Telfer areas. Hence, the granites of this Supersuite must be considered as viable components in any model trying to explain at least some of the controls on the distribution of mineralisation in The Granites-Tanami Block.

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Future work

The felsic igneous rocks of The Granites-Tanami Block are poorly defined both in terms of their chemistry and their ages. The single crystal ages reported by Ding (1987) need to be confirmed by SHRIMP analyses to determine just how representative these individual zircon ages are, not only of the samples dated, but also of the intrusions that each individual sample is taken from.

A systematic granite sampling program also needs to be carried out, firstly to better define the metallogenic characteristics of this suite, and secondly and more importantly, to try to classify the numerous unnamed granite outcrops scattered throughout the area and to try to define whether they are part of the Granites Supersuite or belong to the 1880 Ma or the Archaean suites.