Jon Hronsky

The Lithospheric architecture of Africa : seismic tomography, mantle petrology and tectonic evolution

We present a new analysis of the lithospheric architecture of Africa, and its evolution from ca. 3.6 Ga to the present. Upper-lithosphere domains , generated or reworked in different time periods, have been delineated by integrating regional tectonics and geochronology with geophysical data (magnetic, gravity, and seismic). The origins and evolution of lower-lithosphere domains are interpreted from a high-resolution global shear-wave tomographic model, using thermal/compositional modeling and xenolith/xenocryst data from volcanic rocks. These data are integrated to map the distribution of ancient highly depleted subcontinental lithospheric mantle (SCLM), zones of younger or strongly modified SCLM and zones of active mantle upwelling, and to relate these to the evolution of the upper lithosphere domains. The lithospheric architecture of Africa consists of several Archean cratons and smaller cratonic fragments, stitched together and flanked by younger fold belts; the continental assembly as we see it has only existed since lower Paleozoic time. The larger cratons are underlain by geochemically depleted, rigid, and mechanically robust SCLM; these cratonic roots have steep sides, extending in some cases to ≥300-km depth. Beneath smaller cratons (e.g., Kaapvaal) extensive refertilization has reduced the lateral and vertical extent of strongly depleted SCLM. Some cratonic roots extend ≥300 km into the Atlantic Ocean, suggesting that the upper lithosphere may detach during continental breakup, leaving fragments of SCLM scattered in the ocean basin. The cratonic margins, and some intracratonic domain boundaries, have played a major role in the tectonics of Africa. They have repeatedly focused ascending magmas, leading to refertilization and weakening of the SCLM. These boundaries have localized successive cycles of extension, rifting, and renewed accretion; the ongoing development of the East Africa Rift and its branches is only the latest stage in this process. The less depleted SCLM that underlies some accretionary belts may have been generated in Archean time, and repeatedly refertilized by the passage of magmas during younger tectonic events. Our analysis indicates that originally Archean SCLM is far more extensive beneath Africa than previously recognized, and implies that post-Archean SCLM rarely survives the collision/accretion process. Where continental crust and SCLM have remained connected, there is a strong linkage between the tectonic evolution of the crust and the composition and modification of its underlying SCLM.

Science of targeting: definition, strategies, targeting and performance measurement

Mineral exploration comprises three sequential steps: development of a business strategy, creation and application of a targeting model, and follow-up with direct detection in defined high-priority domains. The main geoscientific challenge is the conceptual targeting phase which can lower geological risk and ensure cost-effective direct-detection exploration. A fundamental tenet of conceptual targeting is that ore deposits are part of much more extensive systems, and hence that targeting must be carried out at global through province to district scales. The heterogeneous distribution of ore deposits and their power-law size frequency distribution in individual provinces leads to alternative ‘Elephant Country’ and ‘First Mover’ strategies, both of which employ conceptual targeting, but at different scales. The first stage of targeting science involves development of robust, multi-scale targeting models for ore-deposit types, particularly larger examples. The targeting models can then be applied to identify specific targets by interrogating databases compiled as layers of spatially referenced key themes or parameters. At larger scales in immature terrains, a Hierarchical approach is commonly used to progressively reduce terrains and identify targets, whereas a Venn-diagram approach, the basis of most GIS-based prospectivity analyses, is more commonly used in mature terrains where spatial databases are of higher, more homogenous quality. Target ranking is best achieved using a multiplicative probability approach in which it is required that all essential processes in a mineral system must have operated to form a significant ore deposit. In practice, one or more critical spatially referenced parameters are used as proxies for the essential processes to develop a target score, which is a semi-quantitative estimate of probability of the presence of a large ore deposit. Such target ranking can be used in both proactive ground acquisition and reactive submittal-based project acquisition. Once targets have been defined and explored, it is important that there is critical feedback on the robustness of the targeting exercise such that new information is used to build superior databases and/or targeting models for future area-selection programs.

Thermal and compositional structure of the subcontinental lithospheric mantle: Derivation from shear wave seismic tomography

Seismic tomography can provide unique information on the structure of the subcontinental lithospheric mantle (SCLM), but seismic velocity reflects both temperature and composition. We present a methodology for evaluating and isolating the relative contributions of these effects, which produces maps of regional geotherm and broad compositional constraints on the SCLM from the inversion of shear wave (Vs) seismic tomography. This approach uses model geotherms quantized in steps of 2.5 mW/m2 and three mantle compositions corresponding to typical Archean, Proterozoic, and Phanerozoic SCLM. Starting from an assumed composition for a volume of SCLM, lithospheric density at surface pressure and temperature is calculated for each geotherm at each point; the optimum geotherm is taken as the one yielding a density closest to the mean value derived from mantle xenoliths (3.31 g/cm3), since density varies with composition. Results requiring densities or geotherms outside the known natural range of these parameters worldwide require the choice of a different mantle composition. This technique, applied iteratively to a 275 km × 275 km Vs model developed by S. Grand (University of Texas, Austin), results in maps of the geotherm and regional density, which allow interpretation of SCLM composition within broad limits. These results can then be compared with local (paleo)geotherms and data for mantle composition, derived from xenolith suites. Application of this technique to the SCLM beneath Africa, Siberia, and North America shows good correlation with regional geological features, xenolith data, and other geophysical data.

The mineral systems concept: the key to exploration targeting

P-T conditions, involving SO2 gas and liquid water, such as those close to volcanic vents (Fegley & Prinn 1989), or in water-free, high-T systems (Henley et al. 2015). However, release of magmatic volatiles can occur at depth where hot magmatic gas meets crustal rocks +/− pore fluids. We have investigated experimentally the viability of such reactions by reacting SO2 gas with a mixture of calcite and a metal-bearing saline fluid at 1.0– 1.5 kbar pressure and 400–800°C. SO2 gas is supplied via thermal decomposition of sodium bisulphite in an unsealed inner capsule (Figure 1). At all temperatures, the calcite-bearing experiments produced anhydrite and sulphide through reaction of SO2 with calcite (Figure 1). In an experiment where calcite was replaced by quartz, no sulphide was produced, establishing that Ca is a crucial component of this reaction. Our experiments show that production of anhydrite and sulphide through calcite-mediated SO2 disproportionation takes place on timescales of just a few hours. This could occur where hot magmatic SO2 gas encounters magmatic brines in igneous rocks, basinal brines within sedimentary rocks, or calcium-bearing saline groundwaters. We suggest that reaction of hot magmatic SO2 with Ca-bearing crustal rocks containing saline, metal-bearing fluids may be a potent mechanism for scrubbing volcanic gas and precipitating metal sulphides, as previously suggested on the basis of andesite-dacite reaction experiments involving S-bearing magmatic volatiles and brines (Blundy et al. 2015).

CET exSim: mineral exploration experience via simulation

Undercover mineral exploration is a challenging task as it requires understanding of subsurface geology by relying heavily on remotely sensed (i.e. geophysical) data. Cost-effective exploration is essential in order to increase the chance of success using finite budgets. This requires effective decision-making in both the process of selecting the optimum data collection methods and in the process of achieving accuracy during subsequent interpretation. Traditionally, developing the skills, behaviour and practices of exploration decision-making requires many years of experience through working on exploration projects under various geological settings, commodities and levels of available resources. This implies long periods of sub-optimal exploration decision-making, before the necessary experience has been successfully obtained. To address this critical industry issue, our ongoing research focuses on the development of the unique and novel e-learning environment, exSim, which simulates exploration scenarios where users can test their strategies and learn the consequences of their choices. This simulator provides an engaging platform for self-learning and experimentation in exploration decision strategies, providing a means to build experience more effectively. The exSim environment also provides a unique platform on which numerous scenarios and situations (e.g. deposit styles) can be simulated, potentially allowing the user to become virtually familiarised with a broader scope of exploration practices. Harnessing the power of computer simulation, visualisation and an intuitive graphical user interface, the simulator provides a way to assess the user’s exploration decisions and subsequent interpretations. In this paper, we present the prototype functionalities in exSim including: simulation of geophysical surveys, follow-up drill testing and interpretation assistive tools.

The evolution of lithospheric domains: A new framework to enhance mineral exploration targeting

A new approach to global exploration targeting is essential for the discovery of new world-class ore deposits. Understanding the lithosphere-scale context of resource formation and location may provide the next step-change in enhancing exploration success. Knowledge of the nature of trans-lithospheric structure and discontinuities and the delineation of deep lithospheric domains with fundamentally different composition, architecture and evolution is providing a new framework for exploration. This enhances understanding and prediction of the location of ore-deposits derived from a variety of deep lithospheric processes including mantle-derived magmatic and fluid flow (and associated thermal transfer) and deep crustal reworking and partial melting. Relevant resource deposits include Ni-PGE, Cu, Au and diamonds, while these methods also reveal important parameters about basin formation that are potentially important for oil and gas occurrence.