The volcanic suite of the Ghayth area forms the lowermost lavas of the Oman ophiolite extrusives within Oman ophiolite. Ghayth area is located within Fizh block, one of the northern blocks of the Oman ophiolite. The lithological succession in the Ghayth area consists of Units 1a and 1b (Geotimes Unit, Lasail Unit) overlain by Unit 2 (Clinopyroxene phyric Unit). Texturally, Unit 1a lavas are dominantly aphyric, with few plagioclase and clinopyroxene clots and microphenocrysts. In contrast, Unit 1b lavas are sparsely to moderately phyric with clinopyroxene, plagioclase (altered to albite) and olivine pseudomorphs (replaced by clay minerals) in sub ophitic texture in intersertal intergranular groundmass consisting of albite laths and devitrified mesostasis (altered to clay minerals). Unit 2 lavas are glomerophyritic with clinopyroxene and olivine (pseudomorphed by calcite) glomerocrysts surrounded by plagioclase microlites and devitrified mesostasis (partly altered to calcite). The minor elements data from clinopyroxenes indicate those Units 1a, 1b and 2 showing tholeiitic basalt signatures but with island arc affinities for Unit 2. However, the triangular discrimination diagram of (Y La Nb) for bulk analyses indicated that Units 1a and 1b are falling where the three fields of N MORB(Normal Mid Ocean Ridge Basalt), back arc basin basalt and island arc tholeiite all meet. The clinopyroxenes and bulk chemical compositions of Unit 1a lavas are more evolved and are characterized by enrichment of incompatible HFSEs, REEs (High Field Strength Elements, Rare Earth Elements) and depletion of Cr and Ni relative to the less evolved Unit 1b lavas (which are richer in Cr and Ni and low in HFSEs and REEs abundances). Correlations between Mg# (Magnesium number) and selected major, minor and trace element concentrations indicate their single mantle sources with different magmatic fractionation events. The spider diagram patterns of selected bulk rock trace elements show that Units 1a and 1b have a pattern of mixed affinities between N MORB and back arc basin. This is coupled with the Nb/Ta and Zr/Hf depletions relative to N MORB and the mildly depleted to flat LREE patterns, indicating that their mantle sources are possibly contaminated with back arc basin signatures. Therefore, these lavas might have erupted in the back arc spreading center.

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...