Hydrothermal Alteration of Oman Ophiolite Extrusives in Ghuzayn Area

Abstract

Convective circulation of seawater through oceanic crust at mid-oceanic ridges (MOR), on ridge flanks and at back arc basins has wide effects on heat transport, chemical and isotopic compositions of oceanic crust and seawater, mineralization in the crust and on the physical properties of the oceanic basement. The mineralization which occurred in the oceanic crust was resulted by metals leached from large volumes of altered crust at various depths from the surface of hydrothermal circulation (Hannington et al., 1995). Therefore, the study on the ophiolites which are accessible fragments of oceanic crust exposed on land and studied as analogues for tectonic, magmatic and hydrothermal processes occurring at modern MORs and back arc basins that are much more difficult to sample and examine directly, has been used to model chemical and isotopic fluxes between seawater and the ocean crust during hydrothermal alteration and the effect on the compositions of seawater and the crust (e.g., Gregory and Taylor, 1981;Bickle and Teagle, 1992;Alt, 1994).Mineralization and massive sulfide deposits in ophiolites are also cited as analogues of black smokers forming on modern oceanic crust (Hannington et al., 1995). At the same time, the alteration patterns documented for modern oceanic crust and ophiolites record the short and long time-integrated effects of fluid-rock interaction related to the oceanic hydrothermal systems.Geological, geophysical and drilling surveys were applied to investigate the mineralizations in the explo- RESOURCE GEOLOGY, vol. 56, no. 2, 167-182, 2006 167 Abstract: The hydrothermal alteration in Ghuzayn Volcanics was associated with mineralization and accumulation of three massive sulfide deposits. The Ghuzayn Volcanics were discriminated into basaltic and andesitic lavas. The crossplots of Zr versus Nb, Y, Hf, La and Lu show that they fall in the same linear fractionation trends with more evolved affinities in the andesitic lavas compared to the less evolved affinities in the basaltic lavas. The immobile trace element contents of the Ghuzayn Volcanics show that they are of oceanic to continental tholeiitic affinity which fall in the fields of back-arc basin basalt, N-MORB and island arc tholeiite all meet and match with the field of the Lau-Tonga back-arc basin lavas. The ore-body No. 2 is underlain by a zone of intense argillization and silicification which are so-called alteration pipe as in some of the Cyprus-type massive sulfide deposits. The alteration in the proximity of the ore-body No. 2 is characterized by a zone of epidotization and slight silicification and sulfide disseminations, and surrounded by another zone of slight silicification and sulfide dissemination in the outermost rim around the ore-body No. 2. The alteration zones in the proximity of the ore-body No. 2 were divided into Zones I and II based on the abundance of the secondary minerals in the altered basaltic lavas. The Zone I is located in the most northern upper part of the ore-body No. 2 and characterized by corrensite, sa...

Characterizing Hydration of the Ocean Crust Using Shortwave Infrared Microimaging Spectroscopy of ICDP Oman Drilling Project Cores

“…Second, to the extent that the average water contents of core from Hole GT1A are influenced by the presence of highly altered fault zones, readers should keep in mind that the frequency and width of such zones have not yet been fully quantified for the Samail ophiolite, and are almost unknown for lower oceanic crust formed at intermediate-to fast-spreading, Pacific crust. Third, while OmanDP investigators have, like prior investigators of the Oman ophiolite (e.g., A'Shaikh et al, 2006;Kawahata et al, 2001;Pflumio, 1991), inferred that most of the low temperature, high H 2 O-content alteration minerals in the core (i.e., zeolites) formed by water-rock reactions at or near the Samail spreading center, we did not do independent work to fully rule out the alternative that some of these minerals may have formed during or post-obduction.…”

“…The Oman ophiolite records hydrothermal alteration and metamorphism, mostly absent overprinting by weathering or obduction (e.g., A'Shaikh et al., 2006; Kawahata et al., 2001; Pflumio, 1991). Overall, we find that the H 2 O content of the ocean crust at 250–260 μm/pixel spatial resolution in the OmanDP core ranges between approximately 0 and 18 wt% H 2 O, with the majority of the ocean crust containing 0.5–3 wt% H 2 O. Hole GT2A (foliated and layered gabbros) is the most hydrated borehole in this study, with both the highest mean and median hydration values.…”