Geology and origin of the Century Zinc deposit

The Century deposit represents an important new genetic variation of the style of sediment-hosted Zn-Pb-Ag mineralisation that is so well represented in the Mount Isa Inlier and McArthur Basin of north-eastern Australia. The deposit is hosted by shale and siltstone of the Mesoproterozoic age Lawn Hill Formation. Mineralisation comprises fine-grained sphalerite with minor galena and pyrite. Most sulphides (80-90%) occur as delicate replacive lamellae in black shale units, separated by siderite-rich silts tone marker horizons. The remainder are present as stratabound progressively coarser grained and more discordant fracture filling forms. The position of the highest grade mineralisation migrates upward within the mineralised sequence from southeast to northwest. Despite systematic lateral variation in grades from 3-5% zinc to greater than 25% zinc in individual units, this grade variation occurs without concomitant changes in thickness of the host shale, suggesting that the mineralisation is dominantly of replacement origin. The host sedimentary rocks show no lateral chemical or textural changes suggestive of exhalative facies.\ud \ud Two dominant textural varieties of stratabound sphalerite are recognised: "porous", which has a high pyrobitumen content and "non-porous", which has a relatively low pyrobitumen content. These varieties appear to be co-genetic and almost co-abundant. They are respectively interpreted to represent the products of oil- and gas- mediated thermochemical sulphate reduction (TSR) and sulphide deposition from a metal-and sulphate-rich fluid. A paleo source-reservoir type hydrocarbon- reservoir model is invoked to explain the onset of TSR, the cross-stratigraphic migration of the mineralisation and the zoning of the "porous" and "non-porous" sphalerite types.\ud \ud Sulphur isotope values of mineralisation appear to evolve with time to progressively heavier values. Paragenetically early stratabound sulphides have δ³⁴S values of between 5 and 10 ‰CDT, while the later transgressive and fracture filling styles evolve to values of between 10 and 20 ‰CDT. This isotopic evolution appears to follow through into more widespread syn-deformational vein-style lodes in the 100-200 km² area surrounding the deposit. In the regional lodes, early sphalerite has a sulphur composition of 20-25 ‰CDT while later sphalerite generations have progressively heavier values, reaching a maximum of 25-30 ‰CDT in the final stages of vein mineralisation. This progressive enrichment in ³⁴S suggests that the total mineralising fluid system was a large closed-system reservoir in the latter stages of the deformation and mineralisation event.\ud \ud Migration of the mineralising fluids at regional scale was triggered by the early stages of basin inversion and regional deformation, continuing through into the development of gentle north-south trending folds. In the early stages of the fluid flow system, gravity-driven recharge from the developing Mt Isa Orogen to the south and east of the Lawn Hill district possibly helped maintain the regional hydrodynamic regime. As regional deformation proceeded, faults progressively developed or were reactivated, modifying and redistributing fluid flow. The closure of the district-scale fluid system is attributed to this progressive tectonically driven fragmentation of the regional scale fluid system. These changes were manifested initially by the development of a stratabound network of hairline fractures throughout the Century orebody, which were mineralised with progressively more discordant sphalerite, galena and siderite. As tectonism continued, vein-style lode deposits were emplaced into faults. The process culminated in the reactivation of major regional structures such as the Termite Range Fault and preservation of the orebody in an extensional fault duplex parasitic to the Termite Range Fault