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 sedimen t-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 gal ena and pyrite. Most sulfides (80-90%) occur as delicate replacive lamellae in black shale units, separated by siderite-rich siltstone horizons. The r emainder are present as progressively coarser-grained and more discordant f racture-filling forms, strata-bound within the ore sequence. The mineraliza tion envelope transgresses stratigraphy, with the position of the highest-g rade material migrating upward within the mineralized sequence fi-om southe ast 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 shale s, 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 alm ost co-abundant. Porous sphalerite is interpreted as the product of oil-med iated thermochemical sulfate reduction (TSR) and sulfide deposition. Nonpor ous sphalerite is the result of gas mediated TSR. A paleosource reservoir-t ype hydrocarbon trap is proposed as a model to explain the grade distributi on trends and zoning of the mineralization. Paragenetically early sulfides have delta(34)S between 5 and 10 per mil, wh ile later fracture-filling and replacement styles have values up to 20 per mil. This trend appears to follow through into more widespread synde-format ional vein-style lodes in a 100- to 200-km(2) area surrounding the deposit. In the regional lodes, early sphalerite has delta(34)S values from 20 to 2 5 per mil while later sphalerite generations have progressively higher valu es, reaching a maximum of 25 to 30 per mil in the final stages of vein mine ralization. This progression suggests that the total mineralizing fluid sys tem was a large closed-system reservoir, with progressive enrichment in hea vy sulfur over the life of the deformation and mineralization event. Migration of the mineralizing fluids was triggered by the early stages of b asin inversion and regional deformation, continuing through into the develo pment of gentle north-south-trending folds. As deformation proceeded; the r egional-scale fluid system was progressively redistributed into late fractu res and faults. This is recorded by the initial development of a dense netw ork of hairline fractures throughout the Century orebody. These fractures w ere 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 r eactivation of major regional structures such as the Termite Range fault. Genetic links between shale-hosted Pb-Zn mineralization and classical Missi ssippi Valley-type deposits have often been alluded to, but comparisons hav e been obscured by specific geological attributes assigned to the classic " sedex" model for mineralization; for example, rifting and high heat flow Th e Century deposit appears to represent a shale-hosted mineralization style formed during tectonically driven migration of basin fluids into an overpre ssured/undercompacted shale sequence. This scenario is much more reminiscen t of some models of Mississippi Valley styles of mineralization than those traditionally associated with sedimentary exhalative processes. Century dem onstrates an intimate relationship, becoming increasingly identified, in ex ploration, between petroleum reservoirs and traps and sites of base metal a ccumulations formed during deep burial. The recognition of this style of mi neralization greatly expands the framework in which to explore for shale-ho sted zinc and lead in the Proterozoic Mount Isa inlier and McArthur basin.