Mount Dimer Goldfield, Yilgarn Shire, Western Australia, Australiai

Regional Level Types
Mount Dimer Goldfield	Ore Field
Yilgarn Shire	Shire
Western Australia	State
Australia	Country

This small goldfield is a modern entity, named by the mining company Mt Dimer. The hill itself is some distance to the east. The site is remote, and requires heading along a rough track north of Koolyanobbing. Just before the Aurora and Helena Ranges, a track east needs to be taken approximately 15 kilometres. The nearest civilisation is Koolyanobbing some 50 kilometres south-west as the crow flys. The area is little visited, but has some of the most beautiful country in the goldfields, with almost lush salmon gum woodland backed by the ranges.

Just before the mines, will be a gate. This can be by passed. The track apparently continues east to the Carina iron ore mine, although the writer has not travelled it.

Located in the southern portion of the variously named Marda or Diemals-Marda or Marda-Diemals Greenstone Belt. Exploration by Western Mining Corporation in the late 1980's identified a lateritic with underlying quartz vein gold resource here. The tenements were sold to Glengold Holdings Pty Ltd in 1993 who identified three quartz vein lode systems beneath the laterite. In 1994 the tenements were sold to Tectonic Resources NL who developed the L1 and L2 open pits and underground mines beneath the pits, as well as discovering several satellite deposits called Golden Slipper, Karli West and Frodo. Mining was conducted between 1990 to 1997, producing 639 790 tonnes at 6.5 g/t gold with 133 000oz gold and 183 000 oz silver produced. The majority (83%) of this was from the L1 and L2 mines.

The deposits stretch east-west across 2 kilometres. At its eastern end is the Mt Dimer lodes in close proximity to each other (and from east to west) are L4, L1, L2 and L3 in that order. Heading west is Anomaly 2, Lightning, Golden Slipper, Frodo, and Karli West (the furthest west).

The Marda Greenstone Belt is a succession of ultramafic/mafic volcanic rocks and sediments, unconformably overlain by clastic sediments, felsic volcanic rocks, and possibly a comagmatic granitoid. The deposits are located just south of a structurally complex contact with the greenstone belt, located in shears in a granitic sequence with abundant mafic xenoliths with a broad east trend.

Running roughly east-west truncating the Mt Dimer lodes to the south is the South Bounding Fault and north by a steep dipping ultramafic unit.

The deposit contains a complex Au bearing regolith of Archaean rocks. A unit called 'Billy', 1-2metres thick, is composed of colluvial poorly sorted rounded clasts of vein quartz in a quartz-sand matrix, locally siliceous and ferruginous, laterized in places. There is also a lateritic pisolitic to nodular ferruginous duricrust over mottled leached rock and saprolite. Palaeochannels up to 50 metres thick filled with coarse grained polymetallic congolmerate, interbedded with clay rich sand, and minor clean sand layers. Lastly recent lag developed on residual areas and loamy soil depositional areas with carbonate horizons on or near the surface.

Gold mineralization is found in a number of elongate shoots, 5 to 50 metres strike, 0.5 to 6 metres wide, plunging 45 degrees north. L1 is the largest at a 300 metre strike and to 200 metres deep.

In fresh rock, the L1, L2 and L3 Au is found in laminated veins containing quartz-pyrite-sphalerite-galena-chalcopyrite-silver-gold assemblage. The gold is found entirely within the veins and some is electrum. The gold grades show a strong correlation with the sphalerite. The wall rock shows intense but narrow alteration, and is composed of sericite-silica-pyrite in granite country rock, and biotite-calcite-pyrite in mafic units.

Gold mineralisation at L1, L2 and L3 is also found in the lateritic regolith to 3 metres deep, averaging 1.5 g/t Au. Gold grades increase at the basal contact with the underlying silcrete-calcrete horizon. Supergene enriched quartz veins with Au penetrates the silcrete. A zone of supergene enrichment at 20-30 metres depth has seen remobilization of Au along structures and into the oxidised wall rock. Weathering has occurred to 50 metres with quartz lodes containing sulphides and Fe, Cu and Mn oxides.

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