Identifying deep hydrothermal fluids that leach metals from the oceanic crust and generate seafloor VMS deposits

Richter, Lisa; Diamond, Larryn William (19 August 2019). Identifying deep hydrothermal fluids that leach metals from the oceanic crust and generate seafloor VMS deposits. Proceedings of the 15th SGA Biennial Meeting, Glasgow, Scotland, 1, pp. 76-79. The Society for Geology Applied to Mineral Deposits (SGA)

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Metasomatic alteration of the deep oceanic crust by heated seawater produces a variety of hydrothermal–metamorphic mineral assemblages including spilites and epidosites. Epidosites have been proposed as markers of deep upflow in hydrothermal convection cells and as source rocks for the metals in seafloor VMS deposits. Whereas spilitization is attributed to fluid of seawater salinity, the salinity of epidotizing fluids is debated. In addition, the metal contents of the fluids have been constrained by experiments and observations of vent fluids, but no direct analyses are available.
We have analyzed primary fluid inclusions in spilites and epidosites in the Semail ophiolite (Oman). The results confirm that both fluids have salinity close to that of seawater (2.6–4.1 wt.% dissolved solids). The fluid inclusion data reveal that the alteration occurred over a range of P-T conditions: 170–430 °C for spilitization and 235– 400 °C for epidotization, all at hydrostatic pressures of 27–50 MPa. Laser-ablation-ICP-MS analyses show that the spilitizing fluid is enriched in Fe, Cu and Zn, whereas the epidotizing fluid is metal- depleted. Therefore, we view the spilitizing fluid rather than the epidotizing fluid as the major carrier of metals to black-smoker sulfide deposits on the seafloor.

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