Abstract of Arunta Inlier Synthesis

Record 2001/12

Compiled by Anthony Budd

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• Executive Summary - Geology
• Executive Summary - Metallogenic Potential

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Please note you can download the complete Arunta Inlier data as one large PDF file, or download the same data in three smaller parts.

• Download the complete Arnhem Block / Mc Arthur Basin Synthesis report in pdf format [PDF_3.4MB].
• Arunta Inlier - Part 1 [PDF_4.5MB]
• Arunta Inlier - Part 2 [PDF_4.4MB]
• Arunta Inlier - Part 3 [PDF_4.9MB]

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Executive Summary - Geology

The Arunta Inlier has a very limited history of mining and exploration, and although several mines have been operated, total production is low. The deposits mined include gold, copper, lead and zinc, tin-tungsten-tantalum, fluorite and mica. Exploration has been limited, partly due to poor outcrop, isolation, and a perception that the Inlier is too highly metamorphosed to host significant deposits.

The regional geology of the Arunta Inlier was reviewed in terms of its tectonic setting in two papers: Stewart, Shaw and Black (1984); and Shaw, Stewart and Black (1984). In these works, the Arunta Block was seen to be made up of a partly fault bounded Central Tectonic Province of high-grade metamorphic rocks and a few granites, flanked by the Northern and Southern provinces, which contain low to high-grade metamorphic rocks and numerous granite intrusions. The stratigraphy thought to comprise three major divisions. Division 1 was inferred to be the oldest, and was made up of mafic and felsic granulites that in part represented a bimodal metavolcanic assemblage interlayered with minor metasediments. This was most common in the central province, but also occurred in the northern province. Division 2 rocks, mainly immature metasediments of turbiditic origin, covered extensive areas of the northern province. Division 3 rocks were platform-style sediments comprising quartzite-shalecarbonate successions that locally unconformably overlay the two other divisions. Later work by Collins and Shaw (1995) suggested that no major structural discontinuity exists between the northern and central tectonic provinces, but that the Redbank Thrust Zone is, at least in part, a province boundary. Also, these authors state that rocks originally assigned to divisions are better grouped into a larger number of lithological assemblages until better stratigraphic and isotopic correlations can be made. These authors proposed a tectonic history nomenclature consisting of several chronologically constrained tectonic events, orogenies and uplift phases.

Granites were intruded mostly syntectonically during the history of the Arunta Inlier. The earliest known granites are dated at ~1880 Ma, and the youngest at ~1140 Ma. The largest granite intrusive event is dated at about 1770 Ma. The two most mineralised granite events are dated at 1713 Ma and 1570 Ma. Generally, the granites in the Arunta Inlier were emplaced at deep crustal levels, and show little sign of extensive alteration. Most are Sr-depleted, Y-undepleted, indicating a plagioclase-residual source, and an ensialic rifting environment during emplacement. Most are I-type, with few S-types.

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Executive Summary - Metallogenic Potential

This compilation has assessed the potential of each granite suite based on the criteria set out in the Project Proposal. Suites which have been identified as having high potential for granite-related mineralisation are as follows:

The Atnarpa Suite at 1880 Ma has unknown potential, due to the difficulty in determining the extent of fractionation in this suite. It shares some similarities with Phanerozoic magma systems associated with porphyry-style deposits, although it is more felsic. There are numerous Au occurrences nearby and within this suite, and although some of these deposits have been related to the Palaeozoic Alice Springs Orogeny (Warren et al. 1974), some of the gold may be sourced from this suite. On this basis, the suite may have a high potential for further gold occurrences.

The Napperby Suite at 1770 Ma is one of the most extensive suites in the Inlier. It shows fractionation and evidence of the activity of a fluid phase, however, it crystallised over a very narrow silica range, and there are very few known mineral occurrences near this suite. This suite therefore has only moderate potential for copper, gold and tungsten-molybdenum.

The Jinka Suite at 1713 Ma is felsic, fractionated, enriched in heat-producing elements, shows evidence of a fluid phase, is an I-type, and is associated with known mineralisation. It is also a high-fluorine suite, associated with known fluorite and scheelite deposits, and has high potential for further tungsten, tantalum and molybdenum deposits.

The Alarinjela Suite, thought to be intruded at 1713 Ma, is fractionated, red to pink, oxidised, shows evidence of a fluid phase, intrudes suitable host rocks, and is possibly associated with mineralisation. It is assigned high potential for copper, lead and zinc, and moderate potential for gold.

The Barrow Creek Suite at 1713 Ma, is one of only a few S-type granites in the Inlier. It is fractionated, has abundant pegmatite, is host to many small occurrences of tintantalum-tungsten and molybdenum, and its oxidation state increases with fractionation. It is considered to have a high potential for further similar mineralisation.

The Ali Curung Suite at 1713 Ma is metaluminous, oxidised, fractionated, has a mapped contact aureole, and is near several known mineral deposits. However, it is volumetrically small, has a short fractionation range, and is not strongly fractionated or enriched in heat-producing elements. It is considered to have moderate potential for copper, gold, lead and zinc mineralisation.

The Mount Webb Suite at 1615 Ma shows evidence of fractionation, brecciation, latestage fluids and hydrothermal alteration. It has probably been emplaced at shallow levels and thus epithermal or vein style mineralisation are possibilities. It has high potential for copper and gold mineralisation. Little exploration has been conducted in the area, therefore this suite may be considered as having ‘greenfields’ potential.

The Ennugan Mountains Suite at 1600 Ma is enriched, abundant in fluorine, and has several tin, tantalum and uranium occurrences associated with it. It has moderate potential for further similar mineralisation.

The Southwark Suite at 1570 Ma is fractionated, has a broad compositional range, has pegmatite, aplite, microgranite and greisen phases, and is associated with copper, molybdenum and silver deposits. It is similar to the Cullen Batholith in the Pine Creek Inlier, and is considered to have moderate to high potential for mineralisation of many types.

The Teapot Suite at 1140 Ma is the youngest Proterozoic granite in the Arunta Inlier. It is fractionated, enriched, oxidised, and ranges from tonalite to granite in composition. It has no known mineralisation nearby, but is considered to have moderate potential for copper and gold.