A Geochemical Study of a Fossil Oceanic Hydrothermal Discharge Zone in the Oman Ophiolite (Zuha Sulphide Prospect): Evidence for a Polyphased Hydrothermal History

  • Regba M
  • Agrinier P
  • Pflumio C
 et al. 
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Abstract

A geochemical study (of major and trace elements (including REE), as well as hydrogen, oxygen, neodymium and strontium isotopes) of the Zuha sulphide prospect in the Salahi block (northern part of the Sumail ophiolite, Oman) was focused on a pair of boreholes which were drilled at the foot of the main gossan, on the border of a 400 ppm Cu geochemical anomaly. The geochemical data establish two main alteration styles (type I and type II) which developed in the V1 lavas around the Zuha prospect. Both types exhibit contrasting secondary mineralogy. Type I rocks display a classical prehnite-pumpellyite facies mineral assemblage (quartz, albite, chlorite, epidote, sphene), whereas type II rocks bear a typical stockwork alteration paragenesis (quartz, chlorite, rectorite, sphene, sulphides). The diversity of chemical and isotopic compositions of the type I and type II alterations reflects distinct temperatures, water/rock ratios and fluid compositions, and suggests that these two types of alteration were formed in different parts of the same hydrothermal system. In this system, the type I mineral assemblage results from an interaction between V1 lavas and Na/Mg rich sea-water derived fluids in recharged zones, and the type II assemblage from an interaction between V1 lavas and upwelling hydrothermal fluids in a discharge zone, at higher temperature. The δD compositions of these two types of alteration assemblages support the contention that the fluids responsible for these recrystallizations were sea-water derived. The oxygen isotope compositions indicate fluid rock interaction temperatures around 200°C for type I rocks and above 250°C for type II rocks, in agreement with mineralogical assemblage estimates. Comparison of type I and type II rock compositions suggests that the fluids at the origin of the Zuha stockwerks were globally enriched in Fe, Si, Cu, and shows they were characterized by LREE and Eu enrichments. Thus, these fluids show strong analogies with modern, active hot springs. Both type I and type II rocks however display 87Sr/86Sr ratios (87Sr/86Sr 0.705) which are higher than modern oceanic hydrothermal compositions. A polyphase hydrothermal history is advanced in order to explain these peculiar compositions. It is proposed that the hydrothermal system at the origin of the Zuha prospect developed in previously altered oceanic crust whose Sr isotopic composition was further shifted. These two alteration stages were successively activated by the first axial magmatic event (V1 type magmatism) and the second post accretion magmatic event (V2 type magmatism). The secondary mineralogical assemblages observed in the V1 lavas (prehnite, pumpellyite facies) and in the diabases from the dike complex (greenschist facies) were recrystallized during this second alteration stage. Preliminary data obtained on epidote-rich dikes (episodites) and their host diabases in this Salahi block dike complex are also presented. The close similarities found between the δ180 and 87Sr/86Sr compositions of some of these episodites and the type II stockwork rocks support the genetic relationship between these two rock facies. As advanced by several authors, these data support the contention that the epidosite zones represent fossil conduits of the hydrothermal, metal-bearing uprising solutions.

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Authors

  • Michel Regba

  • Pierre Agrinier

  • Catherine Pflumio

  • Michel Loubet

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