Brown's Prospect, Rum Jungle, Coomalie Shire, Northern Territory, Australiai

Regional Level Types
Brown's Prospect	Prospect (Flooded)
Rum Jungle	Locality (Australia)
Coomalie Shire	Shire
Northern Territory	Territory
Australia	Country

The following is from Melville (2013):

Browns is a stratabound base metal deposit containing Pb-Cu-Co-Ni-Zn located in the Rum Jungle Mineral Field approximately 100 km south of Darwin in the Northern Territory, Australia. Browns Deposit, historically known as Browns Prospect, has been a source of mineral specimens for the commercial market since 1969.

The main open pit excavated in 2009 is approximately 700 m long and 300 m wide and is elongated in a northeast-southwest direction – paralleling the strike of the mineralised stratigraphy. A small remnant section of ground located in the northeastern part of the pit was left untouched by the mining process. The surface of this remnant represents the pre-mining land surface and approximates the position of the original sealed road, which led into the old Rum Jungle uranium workings. Of interest is the presence of several metres of compacted ‘fill’ presumably from the 1977 dig overlying the collecting area.

The south facing scarp of this feature is where the recent specimen producing zone is located. The mineralisation (malachite-pyromorphite +/- cerussite) occurs within a vuggy, faulted and sheared interval, consisting of intensely oxidised material intermixed with quartz veining. This structurally disrupted zone appears to dip steeply to the south, paralleling the contact of Whites Formation carbonaceous shale with the underlying Coomalie Dolostone.

Collecting commenced in May 2010, then on several occasions from late September to the end of November. No further collecting was possible until late 2012 due to flooding of the open pit during an exceptionally wet February in 2011.

Specimens from the 2010 collecting episode were uniquely different from the classical Browns material of the 1970s. Plates of variably green malachite, partially to completely covered with drusy to millimetric prismatic pyromorphite crystals were characteristic. Cerussite was notable by its (almost) complete absence. A striking feature of the specimens collected over this period was the variability of colouration of both the malachite and pyromorphite. The malachite varied from bright apple green, to a deep emerald green to and a turquoise-like bluish-green. Pyromorphite occurred as bright yellow, yellow-green, lime and grass green crystals of varying crystal habit. Much of the malachite was typically botryoidal, although flat plates were also recovered. Crystallised “primary” malachite was collected on one occasion from an isolated pocket zone.

A few cerussite specimens were collected towards the top of the bench face. These resembled the 1970s specimens, consisting of simple prisms, cm-sized reticulated aggregates and rarely trilling twins, all implanted on malachite. A rare occurrence consisted of twinned aggregates of cerussite crystals completely coated and partially to fully replaced by yellow microcrystalline pyromorphite. In most cases the original cerussite had been dissolved out leaving casts composed of pyromorphite. Detailed examination of these casts would yield tiny sub-millimetric yellow to orange tabular wulfenite crystals.

Another very short collecting episode took place towards the end of 2012. The water level in the main part of the pit was reduced several metres by pumping (for mine operational purposes) and this action exposed the 2010 collecting bench. As a result, a small lot of malachite-pyromorphite specimens showing a slightly different morphology to those from 2010 were collected by the mine geologist.

Several metres away from this malachite-pyromorphite zone, a vuggy fracture in decomposed dolomite was exposed by the backhoe. This fracture yielded many fine specimens of yellow-green to lime green crystallised pyromorphite with no associated malachite. The crystals, up to 1 cm in length, invest a weathered dolomitic matrix and are characterised by a very pronounced bright luster and bright colour.

Since December 2012 there has been no further collecting. There is no doubt that additional specimen material remains but to access it would involve a significant lowering of the present water level in the pit followed by more earthworks. Most of the specimen material collected since 2010 has been dispersed into the collector market. Given the limited size of the production area, the volume of quality specimens collected has been far less than that from the 1970s digs. The uniqueness of these new specimens however adds another dimension to Browns and reinforces its position as a ‘world class mineral locality’.

Other Regions, Features and Areas containing this locality

References