Gold deposits of the Northern Territory

The following updated notes are derived from the abstract of Ahmad M, Wygralak, AS and Ferenczi PA, 1999, reprinted 2001. Gold deposits of the Northern Territory. Northern Territory Geological Survey, Report 11. Contact the Information Centre (telephone +61 8 8999 6443 or email geoscience.info@nt.gov.au) for copies of this report, available free-of-charge.

Gold deposits of the Northern Territory have a structurally controlled mesothermal setting and on the basis of host rock and mineral association can be divided in to seven types: (1) gold-quartz veins, (2) gold-ironstone bodies, (3) gold in iron-rich sediments, (4) polymetallic deposits, (5) gold-PGE (platinum group element) deposits, (6) uranium-gold deposits and (7) placer deposits. Over half the occurrences are gold-quartz veins typical of the Pine Creek Orogen. The gold-ironstone association characterises the Warramunga Province of the Tennant Region and constitutes about a third of the deposits. Gold deposits hosted by iron-rich sediments are present in the Pine Creek Orogen and the Tanami Region. Although forming only five percent of occurrences, these have contributed significantly to the gold inventory. In the other hard-rock types gold is present as a secondary commodity and though there are only a few examples (Coronation Hill, Jabiluka, Koongarra, Mount Bonnie, Iron Blow) these could form an important source of future gold. Future uranium exploration in Arnhem Land may add significantly to the present gold inventory. Small placer deposits are present in the Pine Creek Orogen and the Arunta Province.

Native gold is the main ore mineral and is commonly present as micron-sized grains; coarse gold nuggets are rare. Gold is commonly associated with pyrite, arsenopyrite and pyrrhotite, and in places with minor base-metal sulfides. Copper and bismuth minerals are common in ironstone-hosted deposits and lead-zinc sulfides dominate the polymetallic deposits. Common gangue minerals in the quartz vein-hosted deposits are quartz, chlorite, sericite and carbonates. Ironstone-hosted gold deposits have magnetite, chlorite, muscovite, sericite and quartz. Gold deposits in iron-rich sediments contain chlorite, sericite, cummingtonite, actinolite, chloritoid, garnet and carbonates.

Most deposits show a preference for competency-contrast situations in dilatant or low-pressure zones, such as anticlinal crests, recurrent shear zones and necking zones. Gold mineralisation is invariably late, occurring after orogenic events.

Common factors for most gold deposits include: (1) nearly all are in low-grade (sub-greenschist to lower greenschist facies) regionally metamorphosed sediments (commonly greywacke-siltstone-shale); (2) anticlinal hinges and shear zones are generally the most favourable loci; (3) subsequent to regional metamorphism and deformation the metasediments were intruded by granites with I-type characteristics and most gold deposits are within the contact metamorphic aureole; (4) fluid inclusion data suggest the involvement of moderate to high salinity Na-Ca-Mg-Cl-N2-CH4-CO2 fluids ranging in temperature from 200 to 3500C; and (5) stable isotope data suggest a magmatic/metamorphic origin of these fluids.

The above features can be used to demarcate regions of high gold prospectivity in various tectonic units of the Northern Territory. The most prospective regions are: (1) the central part of the Pine Creek Orogen, specifically a northwest-trending belt between Darwin and Katherine; (2) an east-trending belt comprising the Warramunga Formation in the central Tennant Inlier; and (3) the Dead Bullock Formation in the Tanami Region and their stratigraphic equivalents in the Arunta Region. Other areas which may have potential are the Murphy Inlier (Murphy Metamorphics), the Arnhem Inlier (Grindall Formation) and the basement to the Victoria-Birrindudu Basin (Inverway Metamorphics).

The regions most suitable to host U-Au (eg Jabiluka), and Au-PGE (eg Coronation Hill) deposits include the Murphy Inlier, the Tanami Region and extensions of the Pine Creek Orogen in western Arnhem Land.