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

Discussion

The modelling demonstrates the ease with which Pt can be removed from potential source rocks within the Semail ophiolite by a variety of oxidized natural waters. Rocks richest in Pt however, aren’t necessarily the best source rocks as elevated Pt is usually accompanied by higher masses of reductant minerals. The reducing capacity as well as platinum concentration of the source rock also has a crucial effect on the Pt composition of successive batches of outgoing fluid. Most rocks studied here would produce a spike of Pt-enriched water, while the chromitite would generate a fluid with lower but constant Pt content. Porosity has been identified as a key variable, controlling the amount of fluid throughput required to effect Pt removal. Redox buffering of the fluid is more effective at lower porosity requiring greater fluid throughput in order to extract Pt. In reality, porosity probably equates to fracture density and morphology.

An important question is whether the processes described above could give rise to an economic deposit of Pt. Grades similar to those in nature have been simulated by interacting Pt-rich fluid from various ophiolite lithologies with “fresh” serpentinite. Such models simulate movement of oxidized fluid through a section of the serpentinised basal ophiolite (e.g. along a fault) and its subsequent interaction with unaltered serpentinite, perhaps as the result of a second faulting event and consequent fluid flow. Platinum grade in the altered rock is a function of the Pt content of the incoming fluid and the bulk composition of the depositional site. The former is shown to be dependent on time (fluid flow history), flow path (i.e. rock types traversed, mainly influencing oxidation state and pH), porosity (fracture density?) and mass of fluid. Understanding the hydrodynamic regime in the area is therefore critical to the prediction of ore deposits.

Conclusion

Two types of Tertiary hydrothermal alteration have been described in serpentinite of the Semail ophiolite, Oman. Alteration involved massive addition of carbonate or silicification along faults that developed during extension and after obduction of the Semail ophiolite, a period of regional high heat flow reflected in intrusion of basanite and basaltic magma of continental affinity. Both alteration types are somewhat enriched in Pt. Likely hydrothermal fluids include meteoric, seawater and bittern, due to periodic emergence of the Tertiary landmass from an environment of mangrove swamp and saline coastal flats.

Geochemical modelling has provided some insights into hydrothermal processes. Important constraints on the Pt content of hydrothermal waters within the Semail ophiolite are fluid flux (integrated fluid:rock), porosity (probably density of fracturing), oxidation state, pH and the availability of discrete Pt minerals. The ionic strength and cation ratios of the fluid (i.e. derivation) appear to be of much lesser importance. Knowledge of the hydrodynamic regime and better understanding of fluid chemistry at the time of alteration would permit prediction of ore-grade Pt occurrences.

References

Al Harthy, M. S., Coleman, R. G., Hughes-Clark, M. W., Hanna, S. S., 1990, Tertiary basaltic intrusions in the Cedntral Oman Mountains: in Peters, T., Nicolas, A., Coleman, R. J., (eds) “Ophiolite Genesis and the Evolution of the Oceanic Lithosphere”, pp. 675-682.

Bethke, C. M., 1996, Geochemical Reaction Modelling: Oxford Uni. Press, 397 pp.

Buisson, G. & LeBlanc M., 1985, Gold in Carbonatized Ultramafic Rocks from Ophiolite Complexes: Econ. Geol., V80, pp. 2028-2029.

Buisson G. & LeBlanc M., 1987, Gold in Mantle Peridotites from Upper Proterozoic Ophiolites in Arabia, Mali & Morocco: Econ. Geol., V82, pp. 2091-2097.

Cotton J., Bechenec, F., Caroff, M. and Marcoux, J., 2001, Magmatic Evolution of the Tethyan Permo-Triassic Oman Margin: Abstracts International Conference on the Geology of Oman, p.60.

El Beialy, S. Y., 1998, Stratigraphic and Palaeoenvironmental Significance of Eocene Palynomorphs from the Rusayl Shale Formation, Al Khawd, Northern Oman: Rev. Palaeobotany & Palynology, v.102, pp. 249-258.

Fournier, M., Lepvrier, C., Jolivet, L., 2001, Post-Obduction Extension in Northern Oman: Abstracts International Conference on the Geology of Oman, p.33.

Frimpong, A, Fryer, B.J, Longerich, H.P, Chen, Z. and Jackson, S.E., 1995, Recovery of precious metals using nickel sulphide fire assay collection: Problems at nanogram per gm concentrations: Analyst. V120, pp. 1675 – 1680.

Goodall, J.G.S., & Racey, A., 2001a, Palynology of the Early-Middle Eocene Rusayl Formation, Northern Oman: Abstracts International Conference on the Geology of Oman, p.37.

Goodall, J.G.S., Al Sayigh, A.R.S. & Racey, A., 2001b, Sequence Stratigraphy and Petroleum potential of the Early-Middle Eocene Rusayl Formation, Northern Oman: Abstracts of the International Conference on the Geology of Oman, p. 38.

Grant J.A., 1986, The Isocon Diagram – A Simple Solution to Gresens’ Equation for Metasomatic Alteration: Econ. Geol., V81, pp. 1976-1982.

Hanna, S. S., 1992, The Geology, Tectonics and Structures of the Oman Mountains from Pre-Permian to Recent: Abstracts of the 29th International Geological Congress, pp. 431.

Hanna, S. S. & Rodgers, D., 2001, Post-Orogenic Extension in Northern Oman: The Fanjah Range-Front Fault: Abstracts of the International Conference on the Geology of Oman, p. 40.

Hanna, S. S. & Rodgers, D., 1996, The Range-Front Fault of Northern Oman And Its Effect On The Hydrocarbon Potential Of The Gulf of Oman Basin: GeoArabia V1/1, p. 145.

Haynes, L., 2001, Mineralization Styles in Northern Oman Unrelated to the Ophiolite: Field Guide to Excursion A08, International Conference on the Geology of Oman, MCI, Oman, 56 pp.

Hopkinson, L., 2001, A Novel Mineralisation Style Associated with Evaporite Diapirism Through Ophiolite Assemblages: Muscat, Sultanate of Oman: Abstracts 25th AGM Mineral Deposits Studies Group, p. 36.

Keen M.C., Racey, A., 1991, Lower Eocene Ostracods from the Rusayl Formation of Oman: J. Micropal., V10/2, pp.227-233.

Lachize, M., Lorand, J.P., Juteau, T., 1991, Cu-Ni-PGE Magmatic Sulfide Ores and Their Host Layered Gabbros in the Haymiliyah Fossil Magma Chamber (Haylayn Block, Semail Ophiolite, Oman): in Peters, T., Nicolas, A., and Coleman R. G., (eds), Ophiolite Genesis and Evolution of Oceanic Lithosphere: Kluwer, pp. 209-229.

LeBlanc M., Ceuleneer G., Al Azri H., Jedwab J., 1991, Concentration Hydrothermale de Palladium et Platine dans les Peridotites Mantellaires du Complex Ophiolitique: C. R. Acad. Sci. Paris, pp. 1007-1012.

Lorand, J. P., & Juteau, T., 2000, The Haymilyah Sulphide Ores (Haylayn Massif, Oman Ophiolite): In Situ Segregation of PGE-poor Magmatic Sulphides in a Fossil Oceanic Magma Chamber: Marine Geophys. Res., V21, pp. 327-349.

Moreno T., Gibbons, W., Prichard, H. M., Lunar, R., 2001, Platiniferous Chromite and the Tectonic Setting of Ultramafic Rocks in Cabo Oretegal, NW Spain: J. Geol. Soc. Lond., V158, pp. 601-614.

Nehlig, P. & Juteau, T., 1988, Flow Porosities, permeabilities and preliminary data on fluid inclusions and fossil thermal gradients in the crustal sequence of the Sumail Ophiolite: Tectonophysics, V151, pp. 199-221.

Page N. J., Pallister J.S., Brown M.A., Smewing, J.D., Haffty, J., 1982, Palladium, platinum, rhodium, ruthenium and iridium in chromite-rich rocks form the Semail ophiolite, Oman: Can. Mineral., V20, pp. 537-548.

Racey, A., 1994, Biostratigraphy and Palaeobiogeographic Significance of Tertiary Nummmulitids (foraminifera) from Northern Oman: in Simmons M.D. (ed.) “Micropalaeontology and Hydrocarbon Exploration in the Middle East”, Brit. Micropal. Soc. Pub. Series., pp. 343-369.

Sassani and Shock , 1998, Solubility and Transport of Platinum group Elements in Supercrital Fluids: Summary of Estimates of Thermodynamic Properties of Ru, Rh, Pd, Pt solids, aqueous ions and complexes to 1000¼C and 5 Kb: Geochim. Cosmochim. Acta., V.62, pp. 2643-2671.

Stanger, G., 1984, The Hydrogeology of the Oman Mountains: PhD Thesis, Open University, UK, 355 pp.
Stanger, G., 1985, Silicified serpentinite in the Semail Nappe of Oman: Lithos, V18, pp13-22.

Stanger, G., Neal, C., 1984, Calcium and Magnesium Oxide Precipitation from Alkaline Groundwaters and their Significance in the Process of Serpentinization: Min. Mag., V48/2, pp. 237 – 241.

Villey M., Le Metour, J. and De Gramont, X., 1986, Geological Map of Fanjah, Sheet NF40-3F. Explanatory Notes, BRGM & Oman Ministry of Petroleum & Minerals, 67 pp.

Worthing M.A. and Wilde A.R., 2002, Petrology, Geochronology and Tectonic Setting of Basanites from Eastern Oman: J. Geol. Soc. Lond., 159, pp. 469-483.

Ziegler U., Stoessel F., and Peters, T., 1991, Meta-Carbonatites in the Metamorphic Series Below the Semail Ophiolite in the Dibba Zone, Northern Oman Mountains: in Peters T. (ed), “Ophiolite Genesis and Evolution of the Oceanic Lithosphere”, Ministry of Petroleum and Minerals, Sultanate of Oman, pp. 627 – 645.

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
Discussion The modelling demonstrates the ease with which Pt can be removed from potential source rocks within the Semail ophiolite by a variety of oxidized natural waters. Rocks richest in Pt however, aren’t necessarily the best source rocks as elevated Pt is usually accompanied by higher masses of reductant minerals. The reducing capacity as well as platinum concentration of the source rock also has a crucial effect on the Pt composition of successive batches of outgoing fluid. Most rocks studied here would produce a spike of Pt-enriched water, while the chromitite would generate a fluid with lower but constant Pt content. Porosity has been identified as a key variable, controlling the amount of fluid throughput required to effect Pt removal. Redox buffering of the fluid is more effective at lower porosity requiring greater fluid throughput in order to extract Pt. In reality, porosity probably equates to fracture density and morphology. An important question is whether the processes described above could give rise to an economic deposit of Pt. Grades similar to those in nature have been simulated by interacting Pt-rich fluid from various ophiolite lithologies with “fresh” serpentinite. Such models simulate movement of oxidized fluid through a section of the serpentinised basal ophiolite (e.g. along a fault) and its subsequent interaction with unaltered serpentinite, perhaps as the result of a second faulting event and consequent fluid flow. Platinum grade in the altered rock is a function of the Pt content of the incoming fluid and the bulk composition of the depositional site. The former is shown to be dependent on time (fluid flow history), flow path (i.e. rock types traversed, mainly influencing oxidation state and pH), porosity (fracture density?) and mass of fluid. Understanding the hydrodynamic regime in the area is therefore critical to the prediction of ore deposits. Conclusion Two types of Tertiary hydrothermal alteration have been described in serpentinite of the Semail ophiolite, Oman. Alteration involved massive addition of carbonate or silicification along faults that developed during extension and after obduction of the Semail ophiolite, a period of regional high heat flow reflected in intrusion of basanite and basaltic magma of continental affinity. Both alteration types are somewhat enriched in Pt. Likely hydrothermal fluids include meteoric, seawater and bittern, due to periodic emergence of the Tertiary landmass from an environment of mangrove swamp and saline coastal flats. Geochemical modelling has provided some insights into hydrothermal processes. Important constraints on the Pt content of hydrothermal waters within the Semail ophiolite are fluid flux (integrated fluid:rock), porosity (probably density of fracturing), oxidation state, pH and the availability of discrete Pt minerals. The ionic strength and cation ratios of the fluid (i.e. derivation) appear to be of much lesser importance. Knowledge of the hydrodynamic regime and better understanding of fluid chemistry at the time of alteration would permit prediction of ore-grade Pt occurrences. References Al Harthy, M. S., Coleman, R. G., Hughes-Clark, M. W., Hanna, S. S., 1990, Tertiary basaltic intrusions in the Cedntral Oman Mountains: in Peters, T., Nicolas, A., Coleman, R. J., (eds) “Ophiolite Genesis and the Evolution of the Oceanic Lithosphere”, pp. 675-682. Bethke, C. M., 1996, Geochemical Reaction Modelling: Oxford Uni. Press, 397 pp. Buisson, G. & LeBlanc M., 1985, Gold in Carbonatized Ultramafic Rocks from Ophiolite Complexes: Econ. Geol., V80, pp. 2028-2029. Buisson G. & LeBlanc M., 1987, Gold in Mantle Peridotites from Upper Proterozoic Ophiolites in Arabia, Mali & Morocco: Econ. Geol., V82, pp. 2091-2097. Cotton J., Bechenec, F., Caroff, M. and Marcoux, J., 2001, Magmatic Evolution of the Tethyan Permo-Triassic Oman Margin: Abstracts International Conference on the Geology of Oman, p.60. El Beialy, S. Y., 1998, Stratigraphic and Palaeoenvironmental Significance of Eocene Palynomorphs from the Rusayl Shale Formation, Al Khawd, Northern Oman: Rev. Palaeobotany & Palynology, v.102, pp. 249-258. Fournier, M., Lepvrier, C., Jolivet, L., 2001, Post-Obduction Extension in Northern Oman: Abstracts International Conference on the Geology of Oman, p.33. Frimpong, A, Fryer, B.J, Longerich, H.P, Chen, Z. and Jackson, S.E., 1995, Recovery of precious metals using nickel sulphide fire assay collection: Problems at nanogram per gm concentrations: Analyst. V120, pp. 1675 – 1680. Goodall, J.G.S., & Racey, A., 2001a, Palynology of the Early-Middle Eocene Rusayl Formation, Northern Oman: Abstracts International Conference on the Geology of Oman, p.37. Goodall, J.G.S., Al Sayigh, A.R.S. & Racey, A., 2001b, Sequence Stratigraphy and Petroleum potential of the Early-Middle Eocene Rusayl Formation, Northern Oman: Abstracts of the International Conference on the Geology of Oman, p. 38. Grant J.A., 1986, The Isocon Diagram – A Simple Solution to Gresens’ Equation for Metasomatic Alteration: Econ. Geol., V81, pp. 1976-1982. Hanna, S. S., 1992, The Geology, Tectonics and Structures of the Oman Mountains from Pre-Permian to Recent: Abstracts of the 29th International Geological Congress, pp. 431. Hanna, S. S. & Rodgers, D., 2001, Post-Orogenic Extension in Northern Oman: The Fanjah Range-Front Fault: Abstracts of the International Conference on the Geology of Oman, p. 40. Hanna, S. S. & Rodgers, D., 1996, The Range-Front Fault of Northern Oman And Its Effect On The Hydrocarbon Potential Of The Gulf of Oman Basin: GeoArabia V1/1, p. 145. Haynes, L., 2001, Mineralization Styles in Northern Oman Unrelated to the Ophiolite: Field Guide to Excursion A08, International Conference on the Geology of Oman, MCI, Oman, 56 pp. Hopkinson, L., 2001, A Novel Mineralisation Style Associated with Evaporite Diapirism Through Ophiolite Assemblages: Muscat, Sultanate of Oman: Abstracts 25th AGM Mineral Deposits Studies Group, p. 36. Keen M.C., Racey, A., 1991, Lower Eocene Ostracods from the Rusayl Formation of Oman: J. Micropal., V10/2, pp.227-233. Lachize, M., Lorand, J.P., Juteau, T., 1991, Cu-Ni-PGE Magmatic Sulfide Ores and Their Host Layered Gabbros in the Haymiliyah Fossil Magma Chamber (Haylayn Block, Semail Ophiolite, Oman): in Peters, T., Nicolas, A., and Coleman R. G., (eds), Ophiolite Genesis and Evolution of Oceanic Lithosphere: Kluwer, pp. 209-229. LeBlanc M., Ceuleneer G., Al Azri H., Jedwab J., 1991, Concentration Hydrothermale de Palladium et Platine dans les Peridotites Mantellaires du Complex Ophiolitique: C. R. Acad. Sci. Paris, pp. 1007-1012. Lorand, J. P., & Juteau, T., 2000, The Haymilyah Sulphide Ores (Haylayn Massif, Oman Ophiolite): In Situ Segregation of PGE-poor Magmatic Sulphides in a Fossil Oceanic Magma Chamber: Marine Geophys. Res., V21, pp. 327-349. Moreno T., Gibbons, W., Prichard, H. M., Lunar, R., 2001, Platiniferous Chromite and the Tectonic Setting of Ultramafic Rocks in Cabo Oretegal, NW Spain: J. Geol. Soc. Lond., V158, pp. 601-614. Nehlig, P. & Juteau, T., 1988, Flow Porosities, permeabilities and preliminary data on fluid inclusions and fossil thermal gradients in the crustal sequence of the Sumail Ophiolite: Tectonophysics, V151, pp. 199-221. Page N. J., Pallister J.S., Brown M.A., Smewing, J.D., Haffty, J., 1982, Palladium, platinum, rhodium, ruthenium and iridium in chromite-rich rocks form the Semail ophiolite, Oman: Can. Mineral., V20, pp. 537-548. Racey, A., 1994, Biostratigraphy and Palaeobiogeographic Significance of Tertiary Nummmulitids (foraminifera) from Northern Oman: in Simmons M.D. (ed.) “Micropalaeontology and Hydrocarbon Exploration in the Middle East”, Brit. Micropal. Soc. Pub. Series., pp. 343-369. Sassani and Shock , 1998, Solubility and Transport of Platinum group Elements in Supercrital Fluids: Summary of Estimates of Thermodynamic Properties of Ru, Rh, Pd, Pt solids, aqueous ions and complexes to 1000¼C and 5 Kb: Geochim. Cosmochim. Acta., V.62, pp. 2643-2671. Stanger, G., 1984, The Hydrogeology of the Oman Mountains: PhD Thesis, Open University, UK, 355 pp. Stanger, G., 1985, Silicified serpentinite in the Semail Nappe of Oman: Lithos, V18, pp13-22. Stanger, G., Neal, C., 1984, Calcium and Magnesium Oxide Precipitation from Alkaline Groundwaters and their Significance in the Process of Serpentinization: Min. Mag., V48/2, pp. 237 – 241. Villey M., Le Metour, J. and De Gramont, X., 1986, Geological Map of Fanjah, Sheet NF40-3F. Explanatory Notes, BRGM & Oman Ministry of Petroleum & Minerals, 67 pp. Worthing M.A. and Wilde A.R., 2002, Petrology, Geochronology and Tectonic Setting of Basanites from Eastern Oman: J. Geol. Soc. Lond., 159, pp. 469-483. Ziegler U., Stoessel F., and Peters, T., 1991, Meta-Carbonatites in the Metamorphic Series Below the Semail Ophiolite in the Dibba Zone, Northern Oman Mountains: in Peters T. (ed), “Ophiolite Genesis and Evolution of the Oceanic Lithosphere”, Ministry of Petroleum and Minerals, Sultanate of Oman, pp. 627 – 645.
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