Graham Begg

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.

A unified model for gold mineralisation in accretionary orogens and implications for regional-scale exploration targeting methods

Accretionary orogens are the sites of long-lived convergent margin tectonics, both compressional and extensional. They are also the hosts to the majority of the world’s important gold deposits. A very diverse range of deposit types occurs within accretionary orogens, commonly in close proximity in space and time to each other. These include porphyry and associated high-sulphidation Au–Cu–Ag deposits, classic low-sulphidation Au–Ag deposits, low-sulphidation Au deposits centred on alkalic intrusive complexes, Carlin-type Au deposits, Au-rich volcanic-hosted massive sulphide deposits, orogenic Au deposits, intrusion-related Au deposits and iron oxide Cu–Au deposits. Empirical patterns of spatial distribution of these deposits suggest there must be fundamental generic controls on gold metallogeny. Various lines of evidence lead to the proposal that the underlying key generic factor controlling accretionary orogen gold metallogeny is regional-scale, long-term, pre- and syn-subduction heterogeneous fertilisation of the lithospheric mantle that becomes a source of mineralisation-associated arc magma or hydrothermal fluid components. This process provides a gold-enriched reservoir that can be accessed later in a diverse range of tectonomagmatic settings. Based on this concept, a unified model is proposed in which the formation of a major gold deposit of any type requires the conjunction in time and space of three essential factors: a fertile upper-mantle source region, a favourable transient remobilisation event, and favourable lithospheric-scale plumbing structure. This framework provides the basis for a practical regional-scale targeting methodology that is applicable to data-poor regions.

Plume-subduction interaction forms large auriferous provinces

Gold enrichment at the crustal or mantle source has been proposed as a key ingredient in the production of giant gold deposits and districts. However, the lithospheric-scale processes controlling gold endowment in a given metallogenic province remain unclear. Here we provide the first direct evidence of native gold in the mantle beneath the Deseado Massif in Patagonia that links an enriched mantle source to the occurrence of a large auriferous province in the overlying crust. A precursor stage of mantle refertilisation by plume-derived melts generated a gold-rich mantle source during the Early Jurassic. The interplay of this enriched mantle domain and subduction-related fluids released during the Middle-Late Jurassic resulted in optimal conditions to produce the ore-forming magmas that generated the gold deposits. Our study highlights that refertilisation of the subcontinental lithospheric mantle is a key factor in forming large metallogenic provinces in the Earth’s crust, thus providing an alternative view to current crust-related enrichment models.The lithospheric controls on giant gold deposits remain unclear. Here, the authors show evidence for native gold in the mantle from the Deseado Massif in Patagonia demonstrating that refertilisation of the lithospheric mantle is key in forming metallogenic provinces.

Geoscience Data Integration: Insights into Mapping Lithospheric Architecture

In order to develop a 4D understanding of the architecture of the entire lithosphere, it is necessary to embrace integration of multi-disciplinary, multi-scale data in a GIS environment. An holistic understanding has evolved whereby geologic, geochemical and geophysical signals are consistent with a subcontinental lithospheric mantle (SCLM) dominated by a mosaic of domains of Archean ancestry, variably overprinted by subsequent tectonothermal events. Pristine Archean SCLM is mostly highly depleted (high Mg#), low density, high velocity and highly resistive, and preserves intact Archean crust. There is a first order relationship between changes to these signals and the degree of tectonothermal overprint (by melts, fluids). Continental crust is comprised largely of reconstituted Archean components, variably diluted by juvenile addition, symptomatic of the various overprinting events. These events impart crustal fabrics and patterns dictated by SCLM architecture, influenced by the free surface and crust-mantle decoupling.

Global- to Deposit-Scale Controls on Orthomagmatic Ni-Cu(-PGE) and PGE Reef Ore Formation

Large orthomagmatic Ni-Cu(-PGE) and platinum-group-element (PGE) deposits dominantly form during the assembly and peak of supercontinents, when mantle plumes bombard the Subcontinental Lithospheric Mantle (SCLM) roots. The thinner edges of thick resilient SCLM at (paleo)craton margins internal to continents focus mantle melting and transfer of melts into the crust via active translithospheric faults. An interconnected intrusive network provides the plumbing system to upper-crustal sites of deposit formation. Most PGE reef deposits form slightly inboard of margins at major subhorizontal crustal interfaces. Os isotope data are consistent with melt interaction with the SCLM, which may increase PGE contents of the melts. The degree of mantle melting, regional geodynamics, and interaction with lithospheric architecture determine the crustal setting, melt composition (High MgO versus Low MgO end-members), mechanisms of sulfide saturation, and deposit style. Tectonic stress switches can initiate dynamic injection of key ore-forming sulfide melts from deeper in the magmatic conduit system. Ore-forming concepts are translated into global- to deposit-scale exploration parameters.

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.