The Rustlers Roost deposit is located about 90 km southeast of Darwin in the Northern Territory, Australia.
The deposit was discovered in 1948 or 1949 by Jim Escreet. The areas of high gold concentration were termed, from north to south, Sweat Ridge, Dolly Pot, Beef Bucket and Backhoe. It is estimated that 200 to 250 t of ore was subsequently mined by Jim Escreet from these deposits for the production of about 3.7 kg Au (Rabone 1995). Exploration by a number of other companies was conducted from 1978, and in 1993, Valdora Minerals NL (Valdora) announced a resource of 8.1 Mt of ore at 1.2 g/t Au. Additional exploration by Valdora increased this to a JORC-compliant resource of 34 Mt at 1.17 g/t Au (Rabone 1995). Between 1994 and 1998, Valdora produced about 3.515 tonnes of Au from 4.71 Mt @ 1.05 g/t Au from the deposit by heap leaching. The remaining mineral resource, at a cut-off grade of 0.70 g/t, amounts to an indicated resource of 15.14 Mt, grading 1.34 g/t Au, and an inferred resource of 1.46 Mt grading 1.23 g/t Au (Valencia Ventures Inc, press release 14 April 2005).
The deposit occurs within the Pine Creek Orogen, which covers an area of ~47 500 km2 and comprises a thick (>4 km) succession of Palaeoproterozoic 2200 to 1870 Ma supracrustal carbonate and carbonaceous sedimentary and volcanic rocks, unconformably overlying Neoarchaean ~2670 to 2500 Ma granitic and gneissic basement. This succession was subjected to regional metamorphism and deformation of varying grades and intensities in different parts of the orogen in the period ~1867 to 1850 Ma, and syn to post-tectonic granite emplacement at ~1830 to 1800 Ma (Ahmad and Hollis, 2013).
The Rustlers Roost deposits is hosted within the Mount Bonnie Formation of the South Alligator Group, which is in turn part of the Cosmo Supergroup. The South Alligator Group is exposed from east of the Rum Jungle Complex to the South Alligator Valley, and is a succession of iron-rich sedimentary rocks, tuff, carbonate rocks, shale, greywacke and siltstone. It has been divided into three formations, the Koolpin Formation, Gerowie Tuff and Mount Bonnie Formation. The thickness of the South Alligator Group increases from 1000 m in the west, near Rum Jungle, to as much as 5000 m in the South Alligator Valley (Ahmad and Hollis, 2013).
The Koolpin Formation consists of ferruginous siltstone, banded ironstone, pyritic carbonaceous shale, ferruginous breccia and minor dolostone. The overlying Gerowie Tuff comprises green to grey mudstone, siliceous shale, siltstone, dark grey chert and tuff, while thin beds of iron-rich sedimentary rocks and minor quartz nodules similar to those of the Koolpin Formation are also mapped (Ahmad and Hollis, 2013).
The host Mount Bonnie Formation comprises a succession of interbedded slate, mudstone, phyllite, siltstone, feldspathic greywacke and minor tuffaceous chert, vitric crystal tuff, lithic crystal tuff, and rare banded ironstone and dolostone. It represents a transition from the low energy, shallow water environments of the Koolpin Formation and Gerowie Tuff to high energy, turbiditic environment of the overlying Finniss River Group, a thick succession of sedimentary rocks of turbiditic origin. The basal unit of the Finnis River Group is the Burrell Creel Formation, host other significant gold deposits in the region (Ahmad and Hollis, 2013).
Dolerite dykes intrude the host Mount Bonnie Formation succession.
Mineralisation occurs as a stockwork of thin quartz-sulphide stringers and stratabound iron-rich beds. It is predominantly stratabound and occurs within three, spatially separate stratigraphic intervals of fine- to medium-grained, thinly bedded, dolomitic clastic sedimentary rocks. Mineralisation occurs over an area of 1.5 x 0.5 km, and is truncated to the south by a north-trending fault dipping at 75°E. The host sequence is only weakly metamorphosed to sub-greenschist facies, with no visible contact metamorphic minerals. The deposit occurs at the crest of the south-plunging Dolly Pot Anticline, which is asymmetric, with limbs dipping 35°E and 50 to 70°W. Mineralisation is found on both limbs. Several gold-bearing lenses with variable strikes have been outlined within the area, with most gold occurring in dark dolomitic, carbonaceous and pyritic shale. Some gold is also found in flat-lying, discordant quartz veins. The ore mineral assemblage includes 1 to 50 µm gold grains, with pyrite, arsenopyrite, chalcopyrite, marcasite, pyrrhotite and sphalerite. The oxidised ore extends to a depth of about 80 m.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2015)
