Abstract
The 135 Ma Paraná-Etendeka Large Igneous Province (PELIP) is one of the largest areas of continental flood basalt (CFB) volcanism in the world and is widely agreed to be a product of intracontinental melts related to thermal anomalies from the Tristan mantle plume. The province rifted during the break-up of Gondwana, as the plume transitioned into an oceanic geodynamic environment. This study reports analyses of plume-derived basalts from the Brazilian side of the PELIP (the Serra Geral Group) to investigate major, trace and platinum-group element (PGE) abundances in an evolving plume-rift metallogenic setting, with the aim of contextualising metallogenic controls alongside existing magmatic interpretations of the region. The chalcophile geochemistry of these basalts defines three distinct metallogenic groupings that fit with three modern multi-element magma classifications for Serra Geral lavas. In this scheme, Type 4 lavas have a distinctive PGE-poor signature, Type 1 (Central-Northern) lavas are enriched in Pd, Au and Cu, and Type 1 (Southern) lavas are enriched in Ru and Rh. Our trace element melt modelling indicates that the compositional variations result from changes in the melting regime between the garnet and spinel stability fields, in response to the thinning and ‘unlidding’ of the rifting continent above. This process imposes progressively shallower melting depths and higher degrees of partial melting. Accordingly, Type 4 magmas formed from small degree melts, reducing the likelihood of sulphide exhaustion/chalcophile acquisition at source. Type 1 (Central-Northern) magmas incorporated components of the sub-continental lithospheric mantle (SCLM) in higher-degree partial melts; the SCLM was heterogeneously enriched via metasomatism prior to plume melting, and this produced enrichment in volatile metals (Pd, Cu, and Au) in these magmas. In contrast, the Ru-Rh enrichment in Type 1 (Southern) lavas is attributed to increased spinel-group mineral and sulphide incorporation from the mantle into higher degree partial melts close to the continental rift zone. Our models confirm the importance of contributions from SCLM melts in precious metal mineral systems within CFB provinces, and reinforce the role of heterogeneous metasomatic enrichment underneath cratons in boosting intracontinental prospectivity with respect to ore deposits.
Highlights
An observed spatial association between platinum-group element (PGE)-rich ore deposits and Archaean lithosphere may indicate that the ‘inheritance’ of precious metals plays an important role in dictating aspects of the geochemistry of plume-derived magmas and mineralisation (e.g. Downes, 2001; Hawkesworth and Schersten, 2007; Zhang et al, 2008; Maier and Groves, 2011; Maier et al, 2012; Griffin et al, 2013; Barnes et al, 2015)
We investigate lavas associated with the Tristan plume and the Parana-Etendeka Large Igneous Province (PELIP) in the South Atlantic in order to test how the PGE, Au and base metal geochemistry of the continental flood basalt (CFB) province changed through time and space
The bulk major and trace element, PGE and Au data and quantitative modelling presented in this chapter support a progressively thinning lithosphere in response to thermal uplift from the upwelling Tristan plume and congruent crustal extension
Summary
Many of the world’s important magmatic sulphide deposits are found within continental flood basalt (CFB) provinces Thompson and Gibson, 1991; Ellam, 1992; Maier and Groves, 2011), the initial geochemical fingerprint of CFB magmas can have important implications for metallogenic fertility and understanding how metals are mobilised from the mantle Localities with prominent PGE orthomagmatic sulphide deposits share a similar geodynamic setting, with plumederived (i.e. intraplate) magmas interacting with thick continental cratons and their margins. An observed spatial association between PGE-rich ore deposits and Archaean lithosphere may indicate that the ‘inheritance’ of precious metals plays an important role in dictating aspects of the geochemistry of plume-derived magmas and mineralisation An observed spatial association between PGE-rich ore deposits and Archaean lithosphere may indicate that the ‘inheritance’ of precious metals plays an important role in dictating aspects of the geochemistry of plume-derived magmas and mineralisation (e.g. Downes, 2001; Hawkesworth and Schersten, 2007; Zhang et al, 2008; Maier and Groves, 2011; Maier et al, 2012; Griffin et al, 2013; Barnes et al, 2015)
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