Abstract The integrated interpretation of aeromagnetic data is a key exploration tool to define the concealed, potentially prospective geology that we plan to explore. It helps define the district-scale morphology of structural networks and predict which structures may be associated with the formation of mineral deposits. Aeromagnetic data is particularly useful in guiding geologic mapping, exploration targeting, and strategy because the data available is usually broad, geologic processes and features are normally well imaged in the data, and it is relatively cheap to acquire and process. A foundation to the interpretation of the geophysical data is that the interpreter should be a geoscientist familiar with the geology of the area in question, maximizing the integration of geologic knowledge of the area into the interpretation product. The interpreter must think geologically when building the interpretation, drawing on the parallels between aeromagnetic interpretation and geologic mapping/air photo interpretation. Geologic mapping observations have direct parallels in aeromagnetic interpretation (e.g., lithology, structure, alteration). The interpretation process is outlined using the Lake Lefroy region, Western Australia, including form line construction, identification of magnetic rock units, domain definition, data set integration, definition of structural elements, lithological definition, interpretation of the structural framework, and evaluation of the interpretation. Case studies are then provided at a range of scales from the Pine Creek inlier in northern Australia, the Superior province in eastern Canada, and the Zambian Copperbelt Northwest province to illustrate the connection between the interpretations and exploration targeting. The final integrated interpretation is a supplement to outcrop maps, not a competitor. The purpose is to generate a structural and lithological framework that combines the geophysical data of different types with the mapped geology, which can be interrogated by mineralization models over a much wider area than can be achieved if structural elements and lithology are restricted to areas of outcrop.

Abstract The development of more predictive models for the distribution patterns of large ore deposits and districts is critical for future discovery success in mineral exploration. Some studies have suggested that the distribution of orogenic Au and porphyry Cu deposits appears ordered with a periodic spacing in some mineral provinces, but it remains unclear if spatial periodicity is a common feature of diverse mineral systems. We present evidence for spatial periodicity of large mineral deposit clusters along 20 major structural corridors from nine world-class mineral provinces with five ore deposit types (orogenic Au, porphyry Cu, sediment-hosted Cu and Zn-Pb-Ag, and diamondiferous kimberlites). For orogenic Au deposit clusters, spatial periodicity commonly occurs around 30 to 40 km (range 19–50 km) in the Eastern Goldfields (Australia), Abitibi (Canada), and Sierra Foothills (United States) provinces. Periodicity of moderate- to giant-sized sediment-hosted Cu deposit clusters occurs around 27 km in the Central African Copperbelt (Zambia, Democratic Republic of Congo). Large porphyry Cu deposit clusters show periodicity around 65 to 122 km in the American Cordillera (United States, Mexico, Chile). Large shale-hosted Zn-Pb-Ag deposit clusters have a periodicity around 116 km in the Carpentaria province (Australia). Finally, kimberlite clusters have a spatial periodicity around 121 to 237 km in southern and central Africa. We also observed a dual periodicity along some structural corridors, with smaller deposits located at half the spacing of giant deposits. Whereas the mineral provinces studied were selected on the basis that they seemed to show spatial periodicity, many other provinces worldwide do not appear to display spatial periodicity of ore deposits. We link our results to the phenomenon of self-organization, which explains emergence of large-scale spatial (and temporal) order in complex systems as an effective mechanism to dissipate large energy gradients. As the best examples of spatial periodicity of ore deposits identified to date are associated with some of the world’s best endowed mineral provinces, it is possible that overall province endowment is linked to both the degree of self-organization and the magnitude of regional energy gradients. Further research is required to identify relevant underlying geologic causes for spatial periodicity. Nevertheless, we provide two case studies suggesting that the intersection of preexisting basement fault sets with at least semiregular spacing may be a common control on spatial periodicity of mineral deposits. Where spatial periodicity of mineral deposits is observed to occur, it can improve the predictive capacity of exploration models and ore discovery rates.

Abstract The Central African Copperbelt is the world’s premier sediment-hosted Cu province. It is contained in the Katangan basin, an intracratonic rift that records onset of growth at ~840 Ma and inversion at ~535 Ma. In the Copperbelt region, the basin has a crudely symmetrical form, with a central depocenter maximum containing ~11 km of strata positioned on the northern side of the border of the Democratic Republic of Congo and Zambia, and marginal condensed sequences <2 km in thickness. This fundamental extensional geometry was preserved through orogenesis, although complex configurations related to halokinesis are prevalent in central and northern parts of the basin, whereas to the south, relatively high-grade metamorphism occurred as a result of basement-involved thrusting and burial. The largest Cu ± Co ores, both stratiform and vein-controlled, are known from the periphery of the basin and transition to U-Ni-Co and Pb-Zn-Cu ores toward the depocenter maximum. Most ore types are positioned within a ~500-m halo to former near-basin-wide salt sheets or associated halokinetic structures, the exception being that located in extreme basin marginal positions, where primary salt was not deposited. Stratiform Cu ± Co ores occur at intrasalt (Congolese-type), subsalt (Zambian-type), and salt-marginal (Kamoa-type) positions. Bulk crush-leach fluid inclusion data from the first two of these deposit types reveal a principal association with residual evaporitic brines. A likely signature of the ore fluids, the brines were generated during deposition of the basin-wide salt-sheets and occupied voluminous sub and intrasalt aquifers from ~800 Ma. Associated intense Mg ± K metasomatism was restricted to these levels, indicating that capping and enclosing salt remained impermeable for prolonged periods of the basin’s history, isolating the deep-seated aquifers from the upper part of the basin fill. From ~765 to 740 Ma, the salt sheets in the Congolese part of the basin were halokinetically modified. Salt was withdrawn laterally to feed diapirs, ultimately leading to localized welding or breaching of the former hydrological seal. At these points, deeper-level residual brines were drawn into the intrasalt stratigraphy to interact with reducing elements and form the stratiform ores. It is probable that salt welding occurred diachronously across the northern and central parts of the basin, depending upon the interplay of original salt thickness, rates and volumes of sediment supply during accumulation of salt overburden, and tectonism. The variable timing of this fundamental change in hydrologic architecture is poorly constrained to the period of halokinetic onset to the earliest stages of orogenesis; however, the geometry of the ores and associated alteration patterns demands that mineralization preceded the characteristically complex fragmentation of the host strata. Thus, while an early orogenic timing is permissible, mineralization during the later stages of extensional basin development was more likely. In situ reducing elements that host Zambian-type stratiform Cu ± Co ores were in continuous hydrological communication with subsalt aquifers, such that ore formation could have commenced from the ~800 Ma brine introduction event. The nonhalokinetic character of the salt in this region allowed the intact seal to have maintained suprahydrostatic pore pressures, facilitating fluid circulation until late stages of basin growth and possibly early stage orogenesis. Leachate data from ores positioned in the depocenter maximum and southern parts of the basin that underwent relatively high grade metamorphism record mixing of residual and halite dissolution-related brines. Salt dissolution was likely triggered by emergence of diapirs or thermally and/or mechanically induced increased permeability of halite. While it is certain that halite dissolution occurred during and after orogenesis, conditions favorable for salt dissolution may have existed locally during extension in the depocenter maximum. The permeability of salt increased to a point where it became the principal aquifer. The salt’s properties as an aquiclude lost, originally deep-seated residual brine mixed with new phases of evaporite dissolution-related brine to produce ores at middle levels of the basin fill. During the final stages of ore formation, recorded by postorogenic Pb-Zn-Cu mineralization in the depocenter maximum, the salinity of fluids was dominantly derived from the dissolution of remnant bodies of salt.