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

摘要

This study is based mainly on detailed mapping of the Mt Lyell mine lease, particularly theudLyell Comstock, North Lyell and Iron Blow areas, with the aim of clarifying the relationshipudbetween the volcanic schists and the Owen Group conglomerate sequence, determining theudgeneral nature of the alteration zone and the setting of the various orebodies within the zone,udand reconstructing the geological history of the area.udThe upper part of the Mt Lyell system is preserved at Lyell Comstock, where the alterationudzone cross-cuts the upper andesitic unit of the Central Volcanic Complex and culminates in theudbasal unit of the overlying Tyndall Group. An exhalative zone is present in the Lower Tyndall,udconsisting of small lenses of massive lead-zinc ore associated with breccias containing clasts ofudchert and sulphide, and showing strong sericite-pyrite alteration at the base. Also present areudlenses of limestone, many containing abundant fossil fragments of late Middle Cambrian age.udAlteration and mineralisation die out rapidly up section, and the volcaniclastic rocks of theudMiddle and Upper Tyndall Group clearly post-date the alteration, providing unequivocaludevidence of a Cambrian age for the system.udWithin the upper part of the alteration zone at Comstock are numerous bodies of pale chertyudsilica ('silica heads'), wrapped around by sericite-pyrite schist. The chert bodies culminate inudthe 300 m wide mass of the Comstock chert body which caps the system. This huge mass ofudchert, up to 200 m thick and extending at least 600 m down dip, has discordant lateraludmargins, and, like the other chert bodies, is largely of replacement origin. Numerous veins andudmasses of bright red hematite- jasper- barite material cut the chert body, indicating a majorudperiod of oxidation. The presence of clasts of this distinctive red vein material in the MiddleudOwen Conglomerate along the Great Lyell Fault scarp, together with abundant chert detritus,udprovides clear evidence that the chert-bearing part of the alteration zone had been uplifted,udveined with hematite, and exposed to erosion by Middle Owen time in the Late Cambrian. TheudComstock ore lenses of pyrite-chalcopyrite with minor bornite are located in the footwalludposition of the Comstock chert body.udAt North Lye!!, a large displaced mass of schists, with many chert bodies, extends some 500 mudinto the margin of the Owen basin, and appears to have collapsed eastwards from the scarp ofudthe Great Lyell Fault. Other masses of less altered schist, partly connected to the main schistudbelt to the west, and to the North Lyell mass, lie within the `Tharsis Trough' along the basinudmargin between North Lyell and the Iron Blow mine, obscuring the surface expression of theudGreat Lyell Fault. Several remnants of the sole of these collapsed masses are exposed restinguddirectly on upturned beds of conglomerate and sandstone along the Tharsis Ridge- RazorbackudRidge 'shoulder' structure. A large mass of pale chert, identical to the Comstock chert, lies atudthe schist-Owen contact at the eastern margin of the schist mass at North Lyell, and appears toudrepresent part of the cap-like chert zone at the top of the alteration system. It has beenudmisinterpreted, however, as silicified Owen beds, leading to a widely-held misconception ofudpost-Owen (i.e. Devonian) silicification and associated bornite mineralisation, since theudbornite-rich North Lyell orebody lies along the footwall of the chert. This study clarifies thatudcritical relationship.udBodies of semi-massive hematite, closely associated with hematite-chert-rich breccias, areuddeveloped along the schist-Owen contact from North Lyell to the Iron Blow. These bodies,udand the associated breccias, interdigitate with the Owen sediments, indicating that they wereudformed during deposition of the Middle and Upper Owen beds. The hematite alteration udextends below the bodies into the underlying schist, indicating that the schist mass wasudexposed at the surface, and that it was oxidised and eroded as it collapsed or rolled into place.udThe abrupt change from the coarse fluvial Lower Owen to the red, hematite-rich, shallowudmarine-deltaic facies of the Middle Owen, with its abundant volcanic-derived chert andudhematite clasts, appears to coincide with the exposure and erosion of the schist mass. Thisudmass contained abundant pyrite and other sulphides, and its rapid exposure at surface (possiblyudwhile still hot) appears to have resulted in intense oxidation to produce large quantities ofudhematite and barite, much of which was deposited as clasts in the Owen sediments. Similarly,udmuch of the cherty alteration material now appears as a widespread and abundant elasticudcomponent in the Middle and Upper Owen beds.udA 100 m-wide zone of upturning and folding of Upper Owen sandstone beds occurs along theudschist-Upper Owen contact at the eastern margin of the schist masses. The HaulageudUnconfonnity is developed where the younger Pioneer Sandstone, of probable MiddleudOrdovician age, truncates these folded beds as it transgresses across them to rest, in a fewudplaces, on the schist mass. The folds lack cleavage and were apparently formed when the bedsudwere only semi-consolidated. The folded zone can be attributed to a further advance of theudschist mass against the Owen contact some time in the early Ordovician.udRecognition of the North Lyell schist mass as a section from the chert-rich upper part of theudalteration zone, closer to the geographic centre of the overall system than Comstock (which isudat the northern margin), allows some reconstruction of the major elements of the Mt Lyelludsystem. Within the overall alteration zone was a core zone, 200-500 m wide, of pyrite-richudsericite-chlorite schists, with an upper section of 500 m or so dominated by cherty silica bodiesudup to 50 m across. This culminated near the top in a cap-like zone of chert masses up to 200 mudthick, with bornite-rich orebodies located just beneath the cap over the central part of theudsystem. The presence of pyrophyllite and other indicator minerals in this upper zone indicatesudthat 'high-sulphidation' conditions may have applied during bornite deposition.udSmall lead-zinc massive sulphide bodies were formed in the exhalative zone towards the lateraludmargins of the system, as at Comstock, but in the central parts of the system it appears thatudmassive pyrite-chalcopyrite bodies were more typical, as exemplified by the Iron Blowudsulphide body. This body also lies against the Upper Owen contact in a schist collapse zone,udand has a gossan-like Cambrian hematite mass developed on it, indicating its exposure atudsurface during Owen time. Slightly deeper in the system, within the zone of silica heads, areuddeposits dominated by disseminated pyrite-chalcopyrite (e.g. Crown Lyell 3) or with a smalludamount of bornite (e.g. Western Tharsis, Comstock), and deeper still, below the chert-richudzone, are the large, low-grade, disseminated orebodies such as Prince Lyell.udThere is clear evidence at Mt Lyell that a large part of the alteration system was uplifted,udexposed, and eroded during Owen Group deposition, including much of the chert-rich (andudore-rich) upper part. The cover of Tyndall Group rocks must have been stripped off duringuddeposition of the thick Lower Owen sequence, and the alteration zone was exposed, oxidised,udand eroded, and large sections of it collapsed into the Owen basin during Middle and UpperudOwen time. A possible explanation for the cupwelling' of the schist mass could be that thereudwas significant volume increase associated with the large-scale hydrothermal alteration,udparticularly the hydration of feldspars to sericite and of ferromagnesian minerals to chlorite, asudnoted by Edwards (1939). Further chemical and mass balance studies are required to test this,udhowever.