The Union Reefs gold deposit is located some ~15 km north of Pine Creek township and 185 km SE of Darwin in the Northern Territory of Australia.
The Union Reefs Goldfield was discovered in 1873. It produced some 1.76 t Au from 0.58 Mt of ore during the period from 1880 to 1910. The old workings consisted of some 1600 pits, open cuts and shafts concentrated in an area about 5 km long and 450 m wide, in two sub-parallel northwest-trending zones about 200 m apart. The western zone is known as the Union Reefs and the eastern is the Lady Alice Line (Hissfield 1936b, Shields et al 1967, Newton et al 1998). The Lady Alice Line hosts the Millers, Ping Ques, Lady Alice and Lady Alice North workings. The Union Reefs Line hosts the Union South, Crosscourse, Union Central, Prospecting Claim and Union North workings.
After programs of exploration and proving between 1984 and 1984, open-pit mining by Acacia Resources Ltd (later AngloGold Ltd) commenced in January 1995 and continued to 2003, with the production of 29.11 tonnes of Au from 20.225 Mt of ore averaging 1.47 g/t Au.
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 goldfield is hosted within turbidites of the Palaeoproterozoic Finnis River Group of the Pine Creek Geosyncline, and lies within the NNW trending, 300 m wide Pine Creek shear zone. The Finniss River Group represents a thick succession of sedimentary rocks of turbiditic origin. In the Central Trough. These sedimentary rocks are generally finer grained and are dominated by shale and sandstone/shale in contrast to the Batchelor Shelf, where conglomerate and sandstone are relatively more significant. The Finnis River Group in the Central Domain is subdivided into the Burrell Creek and Tollis formations. Several units of felsic volcanic rocks occur, interbedded with the Burrell Creek Formation in the westernmost part of the Central Domain on the Batchelor Shelf. The Burrell Creek Formation conformably overlies the Mount Bonnie Formation and is conformably overlain by the Tollis Formation. It consists of interbedded phyllite, slate, mudstone, feldspathic greywacke and minor pebble conglomerate. Laminated to thinly bedded, well cleaved grey or brown phyllite, slate, mudstone and quartz-mica schist are the dominant rock types (from Ahmad and Hollis, 2013).
The Finnis River Group sequence is intruded by the 1835 to 1800 Ma Cullen Supersuite granite that has imposed a broad contact metamorphic aureole.
The Union Reefs lodes lie within the Pine Creek Shear Zone and are hosted by greywacke, slate and minor conglomerate of the Burrell Creek Formation. The rocks are strongly deformed into a series of isoclinal folds overturned to the northeast and in many places, are dislocated by beddingparallel faults or shears (Turner 1990). The isoclinal folds form an anticlinorium with an average plunge of 25°S (Ahmad and Hollis, 2013).
Mineralisation extends over a length of 3.5 km, 400 m width and to a vertical depth of 300 m. It is associated with quartz-sulphide veins that are present as i). stockworks, ii). sheeted veins and iii). 1 mm to 2 m thick lode-style quartz reefs (Hellsten et al., 1994). The thick quartz reefs are usually situated in sheared, dominantly pelitic wall rocks. Stockwork veining is commonly restricted to greywacke, whilst sheeted veins are commonly present in the thinly interbedded intervals. These three vein styles may occur separately, overlap or merge. The lodes generally outcrop as en echelon, pinching and swelling, NW-plunging lenses that are parallel to near-vertical shears trending 10°, 330° and 355°. Steeply plunging saddle reefs are up to 3 m thick comprising massive barren quartz in the centre, wirh mineralised margins (Ahmad and Hollis, 2013).
The primary minerals comprise pyrite, arsenopyrite, galena, sphalerite, marcasite and pyrrhotite. Gold is present as rare small grains and as submicroscopic inclusions in sulphides. Some coarse visible gold is also observed, although this is mainly confined to selvages of larger quartz veins, and is also present as single grains and as clusters of small grains up to 5 mm in diameter in small cracks within sulphide veins. A crude mineral zonation is present, whereby sphalerite and galena tends to be marginal to gold ore that is rich in pyrite and arsenopyrite. Gangue minerals comprise quartz, chlorite, calcite, dolomite and albite. In places, relatively small rich shoots with grades of up to 60 g/t Au and 25 g/t Ag over a true width of 2 m have been intersected at 100 m depth (Shields et al., 1967; Turner 1990; Ahmad and Hollis, 2013).
The pervasive wall rock alteration is best developed in psammites and comprises in decreasing order of abundance quartz-muscovite-chlorite-arsenopyrite-pyrite±dolomite. Statistically gold is also best developed in psammite. The veins are composed of quartz, dolomite, chlorite, pyrite, arsenopyrite, sphalerite, galena, marcasite and minor bismutite.
At the end of 1997 the resource totalled 19.06 Mt @ 1.98 g/t Au. Total production to 2009 was 20.225 Mt @1.47 g/t Au, with a remaining resource estimated at 0.25 Mt at 2.7 g/t Au (Gillman et al., 2009).