Abstract The strata-bound Century zinc-lead-silver deposit is located approximately 250 km north-northwest of Mount Isa in northwestern Queensland, Australia. High-grade mineralization at Century occurs mostly in black shales of the Middle Proterozoic Lawn Hill Formation, dominantly as fine-grained sphalerite and galena lamellae with siderite and minor pyrite. An intricate network of associated, but paragenetically later, strata-bound microfractures overprints these lamellae, mineralized with progressively more discordant sphalerite, galena, and siderite. The black shale units are separated by less mineralized, siderite-rich, silt-stone horizons. The deposit is unmetamorphosed, only weakly deformed, and displays excellent lateral stratigraphic and grade continuity apart from small-scale fault dislocations. Century lies within a 20-km-diam cluster of relatively small quartz-siderite-galena-sphalerite veins. These lodes are for the most part confined to a distinctive stratigraphic interval in the lower to middle Lawn Hill Formation, several hundred meters beneath the mineralized strata at Century. The lodes formed relatively late in the deformational and mineralization history of the district but have a strong kinship to the earlier, deep-level diagenetic mineralization at Century. A major strata-bound district-scale siderite-ferroan dolomite-ankerite-illite alteration system partly overlaps the major vein cluster and is roughly centered on the Century deposit. The deposit, the regional lodes, and the strata-bound alteration are genetically linked and were products of a single district-scale fluid system. The Century ores were emplaced in the relatively early stages of the system and correlate with the first-stage basin inversion event (D 1 ). The regional lodes are the products of continuing but less voluminous fluid migration during a regional D 2 fold event. The regional alteration has textural aspects suggestive of both early- and late-stage burial diagenesis and per-haps developed intermittently throughout the entire life of the system in subtly changing lithostructural environments that were not conducive to sulfide mineral deposition. The discovery and subsequent development of an economic resource at Century is the result of explo-ration persistence and conceptual flexibility in an area with a greater than 100-yr history of exploration and mining. The Century orebody was discovered by drilling a zinc, lead, and silver soil geochemical anomaly. Surface expression of mineralization was visually subtle. Only a small part of the mineralization had an out-crop expression because of the synclinal structure of the orebody and later postmineral Cambrian lime-stone cover. These outcrops lacked a classic iron-rich gossan profile because of the low iron sulfide content of the ore. The soil anomaly was not drill tested for 3 yr because of its unpromising visual appearance and lack of a conductive geophysical response. Key concerns that remain unresolved at Century include the absolute timing of different phases of the system, detailed controls on sulfide-rich versus sulfide-deficient alteration assemblages, the geometry of major fluid conduits, and the definition of boundary conditions for ore fluid compositions. A better under-standing of these characteristics will provide critical insights for the generation of new targets in alternative stratigraphic and/or structural positions.

ABSTRACT Airborne radiometric (gamma-ray) data provide estimates of the concentrations of potassium (K), thorium (Th), and uranium (U) in soil, regolith, and bedrock. Radiometric data constitute an important source of geochemical information, commonly used in mineral exploration and for geological mapping of Earth and other planets. Airborne radiometric data have rarely been applied to the exploration and analyses of impact structures, in contrast with other conventional geophysical tools (e.g., gravimetry, magnetism, and seismic reflection/refraction). This work represents the first systematic survey of the K, Th, and U radiometric signatures of Australian impact structures, based on the continent-wide airborne radiometric coverage of Australia. We first formulated several hypotheses regarding the possible causes of formation of circular radiometric patterns associated with impact structures. Then, the radiometric signatures of 17 exposed impact structures in Australia were documented. Our observations confirmed the supposition that impact structures are commonly associated with circular radiometric patterns. We then selected the five structures with the most prominent circular radiometric patterns (Gosses Bluff, Lawn Hill, Acraman, Spider, and Shoemaker), and we discuss the possible origin of these anomalies. Based on these five case studies, we argue that such patterns result from either crustal deformation induced by the impact event and/or from postimpact superficial processes controlled by the crater topography. This work also suggests that airborne radiometric data may be useful, in combination with other geophysical tools, in the search for new possible impact structures.