The geology of the Mount Lyell mines area, Tasmania : a re-interpretation based on studies at Lyell Comstock, North Lyell and the Iron Blow area

This study is based mainly on detailed mapping of the Mt Lyell mine lease, particularly the Lyell Comstock, North Lyell and Iron Blow areas, with the aim of clarifying the relationship between the volcanic schists and the Owen Group conglomerate sequence, determining the general nature of the alteration zone and the setting of the various orebodies within the zone, and reconstructing the geological history of the area. The upper part of the Mt Lyell system is preserved at Lyell Comstock, where the alteration zone cross-cuts the upper andesitic unit of the Central Volcanic Complex and culminates in the basal unit of the overlying Tyndall Group. An exhalative zone is present in the Lower Tyndall, consisting of small lenses of massive lead-zinc ore associated with breccias containing clasts of chert and sulphide, and showing strong sericite-pyrite alteration at the base. Also present are lenses of limestone, many containing abundant fossil fragments of late Middle Cambrian age. Alteration and mineralisation die out rapidly up section, and the volcaniclastic rocks of the Middle and Upper Tyndall Group clearly post-date the alteration, providing unequivocal evidence of a Cambrian age for the system. Within the upper part of the alteration zone at Comstock are numerous bodies of pale cherty silica ('silica heads'), wrapped around by sericite-pyrite schist. The chert bodies culminate in the 300 m wide mass of the Comstock chert body which caps the system. This huge mass of chert, up to 200 m thick and extending at least 600 m down dip, has discordant lateral margins, and, like the other chert bodies, is largely of replacement origin. Numerous veins and masses of bright red hematite- jasper- barite material cut the chert body, indicating a major period of oxidation. The presence of clasts of this distinctive red vein material in the Middle Owen Conglomerate along the Great Lyell Fault scarp, together with abundant chert detritus, provides clear evidence that the chert-bearing part of the alteration zone had been uplifted, veined with hematite, and exposed to erosion by Middle Owen time in the Late Cambrian. The Comstock ore lenses of pyrite-chalcopyrite with minor bornite are located in the footwall position of the Comstock chert body. At North Lyell, a large displaced mass of schists, with many chert bodies, extends some 500 m into the margin of the Owen basin, and appears to have collapsed eastwards from the scarp of the Great Lyell Fault. Other masses of less altered schist, partly connected to the main schist belt to the west, and to the North Lyell mass, lie within the `Tharsis Trough' along the basin margin between North Lyell and the Iron Blow mine, obscuring the surface expression of the Great Lyell Fault. Several remnants of the sole of these collapsed masses are exposed resting directly on upturned beds of conglomerate and sandstone along the Tharsis Ridge- Razorback Ridge 'shoulder' structure. A large mass of pale chert, identical to the Comstock chert, lies at the schist-Owen contact at the eastern margin of the schist mass at North Lyell, and appears to represent part of the cap-like chert zone at the top of the alteration system. It has been misinterpreted, however, as silicified Owen beds, leading to a widely-held misconception of post-Owen (i.e. Devonian) silicification and associated bornite mineralisation, since the bornite-rich North Lyell orebody lies along the footwall of the chert. This study clarifies that critical relationship. Bodies of semi-massive hematite, closely associated with hematite-chert-rich breccias, are developed along the schist-Owen contact from North Lyell to the Iron Blow. These bodies, and the associated breccias, interdigitate with the Owen sediments, indicating that they were formed during deposition of the Middle and Upper Owen beds. The hematite alteration extends below the bodies into the underlying schist, indicating that the schist mass was exposed at the surface, and that it was oxidised and eroded as it collapsed or rolled into place. The abrupt change from the coarse fluvial Lower Owen to the red, hematite-rich, shallow marine-deltaic facies of the Middle Owen, with its abundant volcanic-derived chert and hematite clasts, appears to coincide with the exposure and erosion of the schist mass. This mass contained abundant pyrite and other sulphides, and its rapid exposure at surface (possibly while still hot) appears to have resulted in intense oxidation to produce large quantities of hematite and barite, much of which was deposited as clasts in the Owen sediments. Similarly, much of the cherty alteration material now appears as a widespread and abundant elastic component in the Middle and Upper Owen beds. A 100 m-wide zone of upturning and folding of Upper Owen sandstone beds occurs along the schist-Upper Owen contact at the eastern margin of the schist masses. The Haulage Unconfonnity is developed where the younger Pioneer Sandstone, of probable Middle Ordovician age, truncates these folded beds as it transgresses across them to rest, in a few places, on the schist mass. The folds lack cleavage and were apparently formed when the beds were only semi-consolidated. The folded zone can be attributed to a further advance of the schist mass against the Owen contact some time in the early Ordovician. Recognition of the North Lyell schist mass as a section from the chert-rich upper part of the alteration zone, closer to the geographic centre of the overall system than Comstock (which is at the northern margin), allows some reconstruction of the major elements of the Mt Lyell system. Within the overall alteration zone was a core zone, 200-500 m wide, of pyrite-rich sericite-chlorite schists, with an upper section of 500 m or so dominated by cherty silica bodies up to 50 m across. This culminated near the top in a cap-like zone of chert masses up to 200 m thick, with bornite-rich orebodies located just beneath the cap over the central part of the system. The presence of pyrophyllite and other indicator minerals in this upper zone indicates that 'high-sulphidation' conditions may have applied during bornite deposition. Small lead-zinc massive sulphide bodies were formed in the exhalative zone towards the lateral margins of the system, as at Comstock, but in the central parts of the system it appears that massive pyrite-chalcopyrite bodies were more typical, as exemplified by the Iron Blow sulphide body. This body also lies against the Upper Owen contact in a schist collapse zone, and has a gossan-like Cambrian hematite mass developed on it, indicating its exposure at surface during Owen time. Slightly deeper in the system, within the zone of silica heads, are deposits dominated by disseminated pyrite-chalcopyrite (e.g. Crown Lyell 3) or with a small amount of bornite (e.g. Western Tharsis, Comstock), and deeper still, below the chert-rich zone, are the large, low-grade, disseminated orebodies such as Prince Lyell. There is clear evidence at Mt Lyell that a large part of the alteration system was uplifted, exposed, and eroded during Owen Group deposition, including much of the chert-rich (and ore-rich) upper part. The cover of Tyndall Group rocks must have been stripped off during deposition of the thick Lower Owen sequence, and the alteration zone was exposed, oxidised, and eroded, and large sections of it collapsed into the Owen basin during Middle and Upper Owen time. A possible explanation for the cupwelling' of the schist mass could be that there was significant volume increase associated with the large-scale hydrothermal alteration, particularly the hydration of feldspars to sericite and of ferromagnesian minerals to chlorite, as noted by Edwards (1939). Further chemical and mass balance studies are required to test this, however.