Mobilisation of deep crustal sulfide melts as a first order control on upper lithospheric metallogeny

Abstract

Magmatic arcs are terrestrial environments where lithospheric cycling and recycling of metals and volatiles is enhanced. However, the first-order mechanism permitting the episodic fluxing of these elements from the mantle through to the outer Earth’s spheres has been elusive. To address this knowledge gap, we focus on the textural and minero-chemical characteristics of metal-rich magmatic sulfides hosted in amphibole-olivine-pyroxene cumulates in the lowermost crust. We show that in cumulates that were subject to increasing temperature due to prolonged mafic magmatism, which only occurs episodically during the complex evolution of any magmatic arc, Cu-Au-rich sulfide can exist as liquid while Ni-Fe rich sulfide occurs as a solid phase. This scenario occurs within a ‘Goldilocks’ temperature zone at ~1100–1200 °C, typical of the base of the crust in arcs, which permits episodic fractionation and mobilisation of Cu-Au-rich sulfide liquid into permeable melt networks that may ascend through the lithosphere providing metals for porphyry and epithermal ore deposits.

Distinct magma evolution processes control the formation of porphyry Cu–Au deposits in thin and thick arcs

“…This could be the (subordinate?) effect of additional processes that were not considered in the modeling, like gold recycling from Aurich sulfides in the lower crust (Shafiei et al, 2008;Richards, 2009;Holwell et al, 2022) or Au precipitation efficiencies higher than the range here simulated (Table S1). Figs.…”

Tracing Volatiles, Halogens, and Chalcophile Metals during Melt Evolution at the Tolbachik Monogenetic Field, Kamchatka

“…We therefore conclude that while several models have explored the importance of sulphide melt resorption and chalcophile metal release back into an accompanying silicate melt (e.g. Kerr & Leitch, 2005;Reekie et al, 2019;Wieser et al, 2020, Chen et al, 2021Holwell et al, 2022), we currently do not have the necessary data to decipher any contributions from this process to the compositions of these unusual Tolbachik samples. Thus, we suggest that the simpler model for sulphide-undersaturated evolution across the entirety of the compositional range of the new monogenetic cone melts analysed in this study is the most likely explanation for the trends discussed here.…”

“…9) Further targeted Se and Ag analysis of the primitive high-Mg and high-S melt inclusions from both sulphide-bearing and sulphide-free monogenetic cone eruptions would be key in unravelling any contributions from sulphide melt resorption and chalcophile metal release in the Tolbachik plumbing system (e.g. Holwell et al, 2022).…”

Imaging the Whole‐Lithosphere Architecture of a Mineral System—Geophysical Signatures of the Sources and Pathways of Ore‐Forming Fluids

“…If the sulfides form an interconnected network, for example, by wetting the grain boundaries of mantle olivine (Gaetani & Grove, 1999), they can substantially contribute to reducing the bulk electrical resistivity. Recent work has demonstrated that Cu–Au‐rich sulfide melt likely exists in cumulates near the base of the crust in regions with long‐lived magma generation, and investigated mechanisms for the mobilization and ascent of Cu–Au rich fluids upon reheating (Holwell et al., 2022). Metal preconcentration in fluids and/or magmas is thought to be a critical step in the formation of some mineral deposits and is closely related to the segregation of sulfide phases, which have been shown to persist at the base of the crust (Richards, 2011; Xu, Yang, et al., 2021; Zheng et al., 2019).…”

“…Another explanation for the events is one where ore‐forming fluids are generated from the lithospheric mantle that has been fertilized and metasomatized by fluids and melts derived from a much earlier geodynamic event, such as subduction (e.g., Groves, Santosh, & Zhang, 2020; Groves, Zhang, & Santosh, 2020). In such a scenario, former intensive and long‐lived arc magmatism may have left abundant sulfide‐bearing and metal‐rich cumulates near the base of the crust and a subsequent thermal anomaly, such as a mantle upwelling, triggered remelting, which provided abundant metals and S for post‐subduction fluid generation and mineral deposit formation (Richards, 2011; Hou et al., 2015; Xu, Hou, et al., 2021; see also Holwell et al., 2022) with a temporal gap suggesting independence of the fluid and mineral formation from the metamorphic and volcanic host rocks. This is consistent with studies that suggest the initial enrichment of metals in fluids and/or magmas is an important step in the formation of mineral deposits (Xu, Yang, et al., 2021; Zheng et al., 2019; see also Heinrich & Connolly, 2022).…”