EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Metal migration and ore minerals across the crust-mantle transition zone (Oman DP ICDP holes CM1A, CM2B)

Dariusz Marciniak1, Ciążela Jakub1, Jesus Ana2, Pieterek Bartosz3, Koepke Jürgen4, Strauss Harald5, Lazarov Marina4, Horn Ingo4, Słaby Ewa1, Prell Marta6, and Blutstein Konrad7
Dariusz Marciniak et al.
  • 1Institute of Geological Sciences, Polish Academy of Sciences, Poland (
  • 2Instituto Dom Luiz, LA Faculdade de Cienncias Universidade de Lisboa (FCUL), Portugal
  • 3Institute of Geology, Adam Mickiewicz University, Poland
  • 4Intitut für Mineralogie, Leibniz Universität Hannover, Germany
  • 5Institut für Geologie und Paläontologie, University of Münster, Germany
  • 6Institute of Geological Sciences, University of Wrocław, Poland
  • 7Wrocław University of Science and Technology, Faculty of Geoengineering, Mining and Geology, Poland

Holes CM1A and CM2B of the International Continental Scientific Drilling Program (ICDP) Oman Drilling Project (OmanDP,  drilled  through  the Moho  Transition  Zone  (MTZ).  CM1A is composed  of layered gabbro (0–160 meters below surface, mbs), dunite (160–310 mbs), and harzburgites (310–405 mbs), whereas CM2B contains dunite (20–120 mbs) and harzburgites (120–300 mbs). The drillholes provided an unprecedented opportunity to study the behavior of metals in the MTZ, where arriving primitive MORB melts  extensively  react  with  the  mantle.  Here,  melts, typically  enriched  with  sulfur and  chalcophile  elements,  are supposed to enrich the mantle and lower crust with sulfides (Gonzalez-Jimenez et al., 2020 – Ore Geol. Rev.; Ciążela et al., 2018 - GCA).          

            Modal sulfide content increases downwards the gabbro sequence from ~0.004 vol.‰ to ~1.0 vol.‰ but decreases again from 0.8 vol.‰ to 0.01 vol.‰ in the lower part of the MTZ and in the harzburgite of the upper mantle. This is reflected in the S concentration increasing from 341 ± 17 ppm, 2sd (standard deviation = σ) to  832  ±  37  ppm,  2sd,  in  the  gabbro  section  and  decreasing  downwards  from  the middle part of  Moho into harzburgites from 475 ± 21, 2sd ppm to 63 ± 3 ppm, 2σ. The sulfides in olivine gabbro from MTZ are mostly (56–87% of all sulfides) pyrrhotite-pentlandite-chalcopyrite assemblages indicating the magmatic origin. Sulfides in layered gabbro sequence are consisted of similar magmatic assemblages (36-100%) with minor chalcopyrite, bornite, heazlewoodite, chalcocite, millerite, siegenite and sphalerite with secondary origin. In dunite and harzburgite sequences sulfides are exclusively hydrothermal.

Based on EMPA and LA-ICPMS measurements, Zn, Co and Cu seem to reach their maximum concentrations in magmatic sulfides from the MTZ. Although, no significant differences are observed between the Fe isotope signatures in magmatic pyrrhotites from the lower crust (–0.73 to –0.24, 2sd [‰] of δ56Fe) and the MTZ (–0.73 to –0.53, [‰] of δ56Fe), we found different δ56Fe for pyrrhotite (–0.24‰) and chalcopyrite +0.36‰ within the same sulfide grain. The bulk signature of δ56Fe for this grain is –0,12‰ being in accordance with the mass balance calculated δ56Fe 0.025‰ ± 0.025‰ of the mantle (Craddock et al., 2013 – Earth Planet. Sci. Lett).

            The  enrichment in sulfides and selected metals (Zn, Co, Cu) towards the  MTZ  might  result  from  melt-mantle  reaction  as  we  proposed previously for the slow-spread oceanic lithosphere based on the Kane Megamullion Ocean Core Complex (Ciążela et al., 2018 - GCA).  In the CM1A/2B ultramafic rocks: dunites and harzburgites, most sulfides are, however, secondary, formed by the same secondary fluids which caused the pervasive serpentinization. To verify whether these sulfides replaced the primary magmatic sulfides or were brought from late-stage seawater-derived fluids, we plan to measure sulfur in whole-rocks and in situ and more iron isotopes in sulfides in situ. Preliminary δ56Fe signature isotope data give us evidence for magmatic origin of the sulfides from upper part of the MTZ section.

How to cite: Marciniak, D., Jakub, C., Ana, J., Bartosz, P., Jürgen, K., Harald, S., Marina, L., Ingo, H., Ewa, S., Marta, P., and Konrad, B.: Metal migration and ore minerals across the crust-mantle transition zone (Oman DP ICDP holes CM1A, CM2B), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-517,, 2022.