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.In contrast, the subsequent Mount Isa Group sediments mark a change in sedimentation to a carbonate-rich black shale environment. old, and can be sub-divided into a sequence of dolomitic siltstones with minor dolomites, overlain by tuffaceous (that is, volcanic ash-bearing) dolomitic siltstones and shales.The Mount Isa Group sediments themselves are estimated to be about 1650 million A. Within these tuffaceous upper Mount Isa Group sediments is the Urquhart Shale, the rock unit that contains all the known economic mineralisation at Mount Isa.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.
At Mount Isa subsequent post-depositional deformation and metamorphism has recrystallised, folded and partly re-distributed the metal sulphides, but bedding features such as particle size grading are still evident.
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.