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Mount Isa in north-west Queensland, Australia, is one of the world’s largest concordant base metal deposits, with both silver-lead-zinc and copper orebodies in the same Middle Proterozoic shale beds.
Galena, sphalerite and pyrite were said to have been deposited by hydrothermal replacement of selected beds within the shales, despite the theoretical necessity for complex alternation of deformation and introduction of fluids to explain mineral textures.These microfossils are brownish-grey to black in colour as a result of the mild metamorphism of the Mount Isa Group sediments, but are reasonably well-preserved in other respects apart from occasional distortion of cells as a result of internal growth of pyrite or other minerals.Love and Zimmerman found large masses of both double-and single-walled cells resembling colonies.They found these algal stromatolites both immediately below the ore-bearing Urquhart Shale in the Native Bee Siltstone, and above in the Kennedy-Spear Siltstone.They described them and concluded that their shape and internal fabric closely resemble algal structures that occur today in shallow marine environments such as at Hamelin Pool, Shark Bay, Western Australia, and the Persian Gulf, while similar stromatolites are also known on the shores of the Great Salt Lake, Utah, and in the Coorong area, South Australia.She concluded that the greatly abundant microfossils were the remains of blue-green algae (cyanobacteria).
Even more recently Neudert and Russell have reported their discovery of stromatolites, the layered structures formed as a result of the accretion of fine grains of sediment by matted colonies of micro-organisms, principally blue-green algae (cyanobacteria), in sediments closely associated with the Mount Isa orebodies.
Chemical analyses of the isolated organic matter revealed a carbon content of approximately 93% for the Mount Isa ore, while both transmission electron microscope and electron diffraction examination of the crystalline structure suggests that the Mount Isa organic matter is more akin to graphite than anthracite.
Thus Saxby were, in fact, the first to describe the possibility of microfossils in the Urquhart Shale at Mount Isa.
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In the past 25 years careful research has resulted in most geologists favouring a syngenetic origin for the silver-lead-zinc orebodies, that is, the sulphides were deposited contemporaneously with the sediments which now form the enclosing rock.