northwest The geology and Geochemistry of some Indicators in Victoria’s Golden Triangle
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australian electronic gold prospecting forum.com  |  Geology and occurrence of gold deposits  |  Gold deposits  |  Gold in Victoria  |  Topic: The geology and Geochemistry of some Indicators in Victoria’s Golden Triangle 0 Members and 1 Guest are viewing this topic. « previous next »
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Doug
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« on: Thursday July 22 2010 11:29:09 AEST AM »

The geology and Geochemistry of some Indicators in Victoria’s Golden Triangle
Part1

A notable feature of this area is the remarkable alignment of rich nuggety gold occurrences including the world’s largest nugget the “Welcome Stranger” in a north trending zone between Ballarat and Wedderburn and the  constant association of this line with  some rich “indicator”  Gold  reefs. There is still controversy as to an adequate explanation as to the origin of these indicators and the rich, nuggety gold associated with them.

Geology of the Golden Triangle
The geology has been well described in the literature. The sedimentary turbidite beds are believed to be Ordovician in age although fossil evidence is lacking in the Western part of the region. The sedimentary beds are a thick succession of silty sandstones, clays shales, mudstones and siltstones.

The Indicators and associated rocks
Gold occurs in quartz reefs and is frequently associated with sulphide minerals most commonly pyrite and arsenopyrite with smaller amounts of lead, copper and zinc sulphides. Most of the significant gold mineralization in the Western part of this region is in 25km wide north trending zone that goes from Wedderburn in the North to  Ballarat in the South. Rich gold was almost always found to be associated with carbonaceous (containing graphite) silt and mudstones (shale’s) with a characteristic spotted appearance due to  phyllites ( cleaved metamorphic coarse grained rock, metamorphosed at low temperatures ) which are often green in color  and with a “greasy’ feel and often contain large cubes of  iron sulphide. These indicator beds are often no more than 100m wide.

These pyritic shale’s and spotted green or purple slates ( Maryborough ) were regarded by the old miners as highly prospective and when they found them they would search for the “indicators”. When found these often consisted of shale beds containing finely bedded iron sulphides and (but not always) one or more layers of ferruginous (iron containing) laminated quartz often referred to as “bookleaf” quartz. The thickness of these indicators is rarely thicker than 5cm and some like the “pencil mark indicator” are very thin. Interestingly this indicator has been traced over a strike length of over 5Km.

The most marked development of the indicators occurs in the  Wedderburn, Tarnagulla and Duunolly area where they are more prominent and tend to be wider. In his  1911 report Whitelaw noted that laminated quartz which he regarded as  metasomatic ( a metamorphic change which involves the introduction of material from an external source ) ,was an essential component of the indicators at  Wedderburn and that he knew of no instance where it did not accompany of rich patches of gold. He said in 1911,” two-thirds of the gold in the  Ballarat  east goldfield came from large formations and spurs apart from indicators and  one-third from the points of enrichment along the lines of the indicator …… at Wedderburn……nine tenths of the reef gold has come from direct contact of the reefs with the  indicators
Part2
In many instances, particularly in the Ballarat  Goldfields the indicators did not carry payable gold and assays of recently  collected specimens support this observation. However the of specimens available for assay have been small. In 1911 the geologist Whitelaw obtained some larger specimens from the Wedderburn goldfield and found them all the quartz free specimens to contain gold on assay, with gold values up to 36gm/ton. Some indicators from the Queens Gully area at  Wedderburn have been reported to have gold  values as high as 20oz/ton .but these specimens may not have been free of  quartz.

Frequent references are made in the literature to the “pitted” or “spotted” or “nodular” appearance of the host, turbidite sedimentary rocks of the gold mineralization. Petrographic examination of some host rocks from near Dunolly shows some of them to be phylittes( a cleaved metamorphic rock having affinities with both slates and  mica schists, rocks are coarse grained and less perfectly cleaved than slates, but are finer grained and have better cleavage than  mica  schists{another metamorphic rock}.they are formed from low temperature regional metamorphism) which contain small nodules of  iron carbonates and a little iron pyrite(iron sulphide) which may constitute up to 15% of the host rock. These iron carbonate nodules average about .5mm in diameter.

It is believed that most of these “pits” and “nodules” observed in many of the host rocks are due to these iron carbonates or replacement of them by other minerals like pyrite. I have some indicator slates from St Arnaud which have contained some large cubic crystals of pyrite. In 1953 the Geologist Baragwanath recorded the presence of a number of different minerals in the in the wider portions of the bedded, laminated quartz reefs at Ballarat West goldfield. These minerals included calcite, dolomite, ankerite, pyrite , arsenopyrite, other minor sulphides  and graphite in the black shale’s, some of which themselves were indicators( eg pencil mark and black seam indicators).
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