Journal Contents - Preliminary study of Cenozoic hydrothermal alteration and platinum deposition in the Oman Ophiolite.

A. Wilde^1,3, L. Simpson², S. Hanna¹

¹Dept Earth Sciences, Sultan Qaboos University, Al Khodh, Oman
²Post Graduate Institute for Sedimentology, University of Reading, Reading, UK
³Now at CRC for Predictive Mineral Discovery, School of Geosciences, Monash University, Melbourne, Clayton, Australia

We present the preliminary results of an investigation into Tertiary hydrothermal alteration of ultramafic rocks from the Semail ophiolite, Oman. We infer a Tertiary regional thermal event manifested in basaltic and basanitic dyke intrusion, extensional faulting and fluid flow. New data include field and mineralogical observations, geochemical analyses and geochemical models of alteration processes. Two types of hydrothermal alteration are documented. Carbonate-altered serpentinite is mineralogically and chemically similar to travertine that is abundant in northern Oman. The second alteration type is silica-iron oxide also known locally as “listwaenite”. Both rocktypes contain anomalous but sub-economic levels of Ni and Pt, but are depleted in Cu.

Geochemical modelling has shown that ionic strength and cation contents of potential hydrothermal fluids are of less significance than oxidation state and to a lesser extent pH in mobilizing Pt from potential source rocks. Ppm levels of Pt are predicted in oxidized waters that have interacted with serpentinite, gabbro, pyritic gabbro and chromitite containing Pt metal and PtS. The rocks with highest Pt abundance (pyritic gabbro and chromitite) are not necessarily the best source rocks given that they have higher concentrations of reductant minerals. Modelling suggests that Pt release may occur in a rapid “spike” as Pt-undersaturated oxidized fluid overcomes the ability of the rock to buffer oxygen fugacity to low levels.

Prediction of economic Pt deposits requires knowledge of many variables including fluid composition, total fluid flux, flow path (i.e. rock types traversed, mainly influencing oxidation state and pH), porosity (fracture density?) and time. Understanding the hydrodynamic regime in the area is therefore critical to the prediction of ore deposits.

Wilde, A., Simpson, L. and Hanna, S. 2002. Preliminary study of Cenozoic hydrothermal alteration and platinum deposition in the Oman Ophiolite. In: Jessell, M. J. 2002. General Contributions: 2002. Journal of the Virtual Explorer, 6, 7-13.
Preliminary study of Cenozoic hydrothermal alteration and platinum deposition in the Oman Ophiolite
A. Wilde^1,3, L. Simpson², S. Hanna¹ ¹Dept Earth Sciences, Sultan Qaboos University, Al Khodh, Oman ²Post Graduate Institute for Sedimentology, University of Reading, Reading, UK ³Now at CRC for Predictive Mineral Discovery, School of Geosciences, Monash University, Melbourne, Clayton, Australia Abstract We present the preliminary results of an investigation into Tertiary hydrothermal alteration of ultramafic rocks from the Semail ophiolite, Oman. We infer a Tertiary regional thermal event manifested in basaltic and basanitic dyke intrusion, extensional faulting and fluid flow. New data include field and mineralogical observations, geochemical analyses and geochemical models of alteration processes. Two types of hydrothermal alteration are documented. Carbonate-altered serpentinite is mineralogically and chemically similar to travertine that is abundant in northern Oman. The second alteration type is silica-iron oxide also known locally as “listwaenite”. Both rocktypes contain anomalous but sub-economic levels of Ni and Pt, but are depleted in Cu. Geochemical modelling has shown that ionic strength and cation contents of potential hydrothermal fluids are of less significance than oxidation state and to a lesser extent pH in mobilizing Pt from potential source rocks. Ppm levels of Pt are predicted in oxidized waters that have interacted with serpentinite, gabbro, pyritic gabbro and chromitite containing Pt metal and PtS. The rocks with highest Pt abundance (pyritic gabbro and chromitite) are not necessarily the best source rocks given that they have higher concentrations of reductant minerals. Modelling suggests that Pt release may occur in a rapid “spike” as Pt-undersaturated oxidized fluid overcomes the ability of the rock to buffer oxygen fugacity to low levels. Prediction of economic Pt deposits requires knowledge of many variables including fluid composition, total fluid flux, flow path (i.e. rock types traversed, mainly influencing oxidation state and pH), porosity (fracture density?) and time. Understanding the hydrodynamic regime in the area is therefore critical to the prediction of ore deposits.
Index: Introduction Previous Work Geological Framework Four main Tectonostratigraphic Units Tertiary Extension & Inversion Field & Mineralogical Observations Geochemical Analysis Methods Results & Interpretation Geochemical Modelling Software & Computational Methods Choice of Hydrothermal Fluid Composition Possible Sources of Pt and Model Rock Compositions Results Discussion Conclusions References