Geology and origin of shale-hosted Zn-Pb-Ag mineralization at the Century Deposit, Northwest Queensland, Australia

The Century deposit represents an important new member of the major sediment-hosted Zn-Pb-Ag deposits of the Mount Isa inlier and McArthur basin. The deposit is hosted by shales and siltstones of the Mesoproterozoic Lawn Hill Formation. Mineralization comprises fine-grained sphalerite with minor galena and pyrite. Most sulfides (80-90%) occur as delicate replacive lamellae in black shale units, separated by siderite-rich silt-stone horizons. The remainder are present as progressively coarser-grained and more discordant fracture-filling forms, strata-bound within the ore sequence. The mineralization envelope transgresses stratigraphy, with the position of the highest-grade material migrating upward within the mineralized sequence from southeast to northwest. Despite systematic lateral variation in grades from 3 to 5 percent Zn to greater than 25 percent Zn in individual units, this grade variation occurs without concomitant changes in thickness of the host shales, suggesting that the mineralization is dominantly of replacement origin. The host sediments show no lateral chemical or textural changes suggestive of exhalative facies within the preserved portions of the deposit.Two principal textural varieties of strata-bound sphalerite are recognized: "porous," which has a high pyrobitumen content, and "nonporous," which has a relatively low pyrobitumen content. These appear to be cogenetic and almost co-abundant. Porous sphalerite is interpreted as the product of oil-mediated thermochemical sulfate reduction (TSR) and sulfide deposition. Nonporous sphalerite is the result of gas mediated TSR. A paleosource reservoir-type hydrocarbon trap is proposed as a model to explain the grade distribution trends and zoning of the mineralization.Paragenetically early sulfides have delta (super 34) S between 5 and 10 per mil, while later fracture-filling and replacement styles have values up to 20 per mil. This trend appears to follow through into more widespread syndeformational vein-style lodes in a 100- to 200-km (super 2) area surrounding the deposit. In the regional lodes, early sphalerite has delta (super 34) S values from 20 to 25 per mil while later sphalerite generations have progressively higher values, reaching a maximum of 25 to 30 per mil in the final stages of vein mineralization. This progression suggests that the total mineralizing fluid system was a large closed-system reservoir, with progressive enrichment in heavy sulfur over the life of the deformation and mineralization event.Migration of the mineralizing fluids was triggered by the early stages of basin inversion and regional deformation, continuing through into the development of gentle north-south-trending folds. As deformation proceeded, the regional-scale fluid system was progressively redistributed into late fractures and faults. This is recorded by the initial development of a dense network of hairline fractures throughout the Century orebody. These fractures were mineralized with progressively more discordant sphalerite, galena, and siderite. As tectonism continued, vein-style lode deposits were emplaced at a more regional scale in fault structures. The process culminated in the reactivation of major regional structures such as the Termite Range fault.Genetic links between shale-hosted Pb-Zn mineralization and classical Mississippi Valley-type deposits have often been alluded to, but comparisons have been obscured by specific geological attributes assigned to the classic "sedex" model for mineralization; for example, rifting and high heat flow. The Century deposit appears to represent a shale-hosted mineralization style formed during tectonically driven migration of basin fluids into an overpressured/undercompacted shale sequence. This scenario is much more reminiscent of some models of Mississippi Valley styles of mineralization than those traditionally associated with sedimentary exhalative processes. Century demonstrates an intimate relationship, becoming increasingly identified, in exploration, between petroleum reservoirs and traps and sites of base metal accumulations formed during deep burial. The recognition of this style of mineralization greatly expands the framework in which to explore for shale-hosted zinc and lead in the Proterozoic Mount Isa inlier and McArthur basin.