Isochron radiometric dating wikipedia
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.Statistical analysis of the cyclicity and lateral zonation of the ore sulphides and host sediments in the silver-lead-zinc orebodies enables calculation of a deposition rate that could have produced the whole Mount Isa deposit in less than 20 days!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.
Other forms appeared to be double-walled cells, commonly infilled with pyrite, which Love and Zimmerman, however, has demonstrated conclusively that the host Urquhart Shale contains abundant organic remains of micro-organisms.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.resulting in adjacent deposition of black carbonaceous metal sulphide muds.Since its discovery in 1923 the major mineralisation at Mount Isa has been the subject of many studies and much controversy. The Mount Isa deposit lies in the western basin of an intensely deformed, multiply intruded and variably metamorphosed belt of Lower and Middle Proterozoic ‘age’ sediments. Following a period of erosion of the basement, deposition of a variety of sediments (conglomerates, sandstones, siltstones, shales, tuffs, cherts and dolomites) and basaltic lavas began.The basement to these sediments consists of acid volcanics complexly intruded by different phases of granite, and basic dykes, all of Lower Proterozoic (1800–2000 million A. Sedimentation appears to have been in two ‘waves’, the first ‘wave’ of sediments being dominantly quartz-rich sandstones, with some thick basalts, conglomerates, siltstones and other sandstones.
Controversy regarding the formation of the Mount Isa copper orebodies in the ‘silica dolomite’ has been a little more difficult to settle, but in spite of some dissenters It is now considered that conditions necessary for deposition of all sulphides began with the development, through penecontemporaneous faulting, of a restricted basin of sedimentation.