Plume-subduction interaction forms large auriferous provinces

Lithospheric-scale processes involved in the precursor stage of formation of the Deseado Massif auriferous province. Stage A: plume activity during Early Jurassic related to the initial stages of Gondwana break-up induces metasomatic Au enrichment in the overlying SCLM and coeval partial melting. The inset shows the transfer of Au to the enriched domains and partial melting processes responsible for the early magmatic stages of the CA-SLIP. Stage B: onset of the subduction zone at the western margin of Gondwana provides fluids capable of scavenging Au from formerly enriched domains and generates calc-alkaline magmatism represented by the middle-late magmatic stages of the CA-SLIP that hosts the Au deposits. The inset shows the process of partial melting of enriched domains and Au transport to crustal levels; some portions of enriched lithosphere remain unmodified

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 traditional notion of Au endowment in a given metallogenic province is that Au accumulates by highly efficient magmatic-hydrothermal enrichment processes operating in a chemically ‘average’ crust. However, more recent views point to anomalously enriched source regions and/or melts that are critical for the formation of Au provinces at a lithospheric scale. Within this perspective, Au-rich melts/fluids might originate from a mid or lower crust reservoir and later migrate through favourable structural zones to shallower crustal levels where the Au deposits form. Alternatively, the subcontinental lithospheric mantle (SCLM) may also play a role as a source of metal-rich magmas.

This model involves deep-seated Au-rich magmas that may infiltrate the edges of buoyant and rigid domains in the SCLM producing transient Au storage zones. Upon melting, the ascending magma scavenges the Au as it migrates towards the uppermost overlying crust. Discontinuities between buoyant and rigid domains in the SCLM provide the channelways for the uprising of Au-rich fluids or melts from the convecting underlying mantle, and when connected to the overlying crust by trans-lithospheric faults, a large Au deposit or well-endowed auriferous province can be formed. Thus, the generation of Au deposits in the crust may result from the conjunction in time and space of three essential factors: an upper mantle or lower crustal source region particularly enriched in Au, a transient remobilisation event and favourable lithospheric-scale plumbing structures.

The giant Ladolam Au deposit in Papua New Guinea gives a good single-deposit case example of this mechanism since deep trans-lithospheric faults connect the crustal Au deposit directly with the mantle source, and similar Os isotopic compositions are exhibited by Au ores and metal-enriched peridotite of the underlying mantle. Despite these evidences, the genetic relation between a pre-enriched mantle source and the occurrence of gold provinces in the upper crust remains controversial since limited evidence is available at a broader regional scale.

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Reference:
Plume-subduction interaction forms large auriferous provinces. Santiago Tassara, José M. González-Jiménez, Martin Reich, Manuel E. Schilling, Diego Morata, Graham Begg, Edward Saunders, William L. Griffin, Suzanne Y. O’Reilly, Michel Grégoire, Fernando Barra & Alexandre Corgne. DOI:10.1038/s41467-017-00821-z

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