Melt metasomatism and enrichment in metals in the uppermost Earth’s mantle

Cu-rich sulfide deposits of economic importance in ophiolites such as Troodos in Cyprus or Semail in Oman often occur along the crust-mantle transition zones (e.g. Begemann et al., 2010). Although secondary sulfides formed during serpentiniztion now prevail, the relicts of primary magmatic sulfides indicate the igneous nature of enrichment in sulfides at the oceanic Moho level. Crust-mantle transition zones in situ in the oceans are suggested to be enriched in sulfides and many chalcophile (e.g. Cu, Zn, Pb, Se, Te) metals via melt-mantle reaction (Ciazela et al., 2017; 2018). The enrichment in sulfides seems to be ubiquitous along the crust-mantle transition zone (Ciazela et al., 2018) and might be expected even at the continental Moho. This is possible as sulfides precipitate during melt-mantle reaction independently on pressure. The process seems to work at low pressures of the oceanic crust-mantle transition zone (0.1–0.2 GPa) (Marciniak et al., this session; Ciazela et al., 2018), medium pressures of the continental crust-mantle transition zone (~1.0 GPa) (Pieterek et al., this session), and in high pressures related to various melt-metasomatized mantle xenoliths (up to 2.5 GPa) (Mazurek et al., this session; Patkó et al., 2021). Metal refertilization due to variable melt-peridotite reactions at the crust-mantle transition zone and along melt channels in the upper mantle may affect the local, regional, and even global metal mass balance of the oceanic and continental lithosphere. The distribution of mantle sulfides is heterogeneous. The zones of enrichment in metals occur mostly at the crust-mantle transition or in melt-modified mantle rocks along melt channels in the upper mantle. These zones are important for subsequent ore formation in secondary processes. In the oceans, especially along slow-spreading ridges, shallow magmatic sulfide horizons are penetrated by hydrothermal fluids operating along faults to form massive sulfides on the seafloor. On land, the re-mobilization of the mantle sulfides horizons by sulfide-undersaturated melts or by buoyant CO₂ bubbles can contribute to the formation of porphyry and related epithermal mineral deposits.

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Ciazela J., Dick H. J. B., Koepke J., Pieterek B., Muszynski A., Botcharnikov R. and Kuhn T. (2017) Thin crust and exposed mantle control sulfide differentiation in slow-spreading ridge magmas. Geology 45, 935–938.

Ciazela J., Koepke J., Dick H. J. B., Botcharnikov R., Muszynski A., Lazarov M., Schuth S., Pieterek B. and Kuhn T. (2018) Sulfide enrichment at an oceanic crust-mantle transition zone: Kane Megamullion (23°N, MAR). Geochim. Cosmochim. Acta 230, 155–189.

Patkó L., Ciazela J., Aradi L. E., Liptai N., Pieterek B., Berkesi M., Lazarov M., Kovács I. J., Holtz F. and Szabó C. (2021) Iron isotope and trace metal variations during mantle metasomatism: In situ study on sulfide minerals from peridotite xenoliths from Nógrád-Gömör Volcanic Field (Northern Pannonian Basin). Lithos 396–397, 106238.