Picture of Captains Flat

Picture of the Lake George Mine Entrance at Captains Flat

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Captains Flat Tour - Geology

"Captains Flat is important to Australian Geologists and mining engineers because it is the classical archetypal syngenetic stratiform sulphide ore deposit and one of the first such large commercial deposits worked on the Australian Mainland."

John Erskine - Mining Engineer
The Captains Flat Mining Record
4 October 1980

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"The type of deposit to which I am referring is a lead, zinc, copper sulphide deposit with subsiding amounts of silver and gold. Geologists have only recently recognised the true nature of such deposits and worked out how they must have formed as the result of slow movement of continental plates across the earth's surface (Australia, for example, is drifting north at the rate of about a mile every thousand years). Where one continental plate, collides with another, one of them gets pushed down and the other rides over the edge of it; and volcanoes of acid lavas (acid volcanics) pour out at the earth's surface when the deeply buried down thrust plate melts.

Below the surface flows of acid volcanics there is often a great mass of molten granite, and the lead, zinc, copper, sulphur, gold and silver separate out from the main molten granite mass (boil out from the top of it) and sometimes make their way to the earth's surface, through cracks and fissures (faults); and if such material boils out below sea level, and solidifies, and gets covered with sea floor sediments, it becomes what we term a syngenetic stratiform sulphide deposit. The first such syngenetic, stratiform sulphide deposit to be recognised as such in Australia was in fact that at the great Rosebery lead, zinc, copper mine in Tasmania, but the first one recognised on the Australian mainland is the one at Captain's Flat.

The valuable elements (the lead, zinc, copper, sulphur, silver and gold) came welling up from deep within the earth (being some of the relatively easily volatilised material in the molten granite) in Silurian times (about 420 million years ago). It boiled off the top of the granite mass at the last gasp of the volcanic activity that had earlier pushed up the acid volcanics so that finally the lead, zinc. copper, gold and silver were either deposited in the acid volcanics or boiled out into the air and were dissipated, or boiled out below sea level and precipitated in the cold sea water to form the Captain's Flat type deposit, later to be covered over with sediments washed down onto the sea floor.

The roots of the volcanic plugs from this type of volcanic action are also targets for todays mineral exploration, because they often contain payable gold, and the numerous cracks and fissures for miles around the plugs are often filled with veins of white quartz containing occasional silver and gold. The lead, zinc, and copper metals came welling up through the cracks in the form of chloride volatiles (lead chloride for example) in the last gasp of the volcanic activity, because normal silicate minerals cannot contain them in their mineral lattice. The metals are then precipitated on the sea floor as sulphides (see the diagram). The lead and zinc (and a little copper) at Captain's Flat were finally deposited on the floor of the ancient Silurian sea, and covered over with silt, to be later rediscovered as the Captain's Flat mine long after the sea had gone and the land had risen up as a mountain range. Some of the gold and silver went in with the lead and zinc and copper sulphides, but some stayed lower down in the quartz reefs and in the acid volcanic material in the root of the volcanic plug itself."


This information is taken in part from an article written by John Erskine (Mining Engineer) and published on 4 October 1980 in The Captain's Flat Mining Record


Content 2006 Gregory S. Davies
Content: 2006 Captains Flat Community Association
Site: www.captainsflat.org