الفهرس 
AbstractOnly 14 pages are availabe for public view
 |  | from | 210 |  |  |
| | | from | 210 | | |
Abstract
4.1. Abu Dahr peridotites and associated Ni-Cu-PGE mineralization
1- Ultramafic rocks of Abu Dahr (AD) unit represent thrust sheet of about 102 km2 which was tectonically overlying the Arayis ophiolitic mélange in the southern Eastern Desert, Egypt. Harzburgites (with minor dunite bodies) are the dominant lithologies. Podiform chromitites occur locally within the mantle harzburgite, as well as ilmenite deposit at Wadi Abu Nekheil. Occasional orthopyroxenitic and mafic dykes crosscut AD peridotites.
2- Although the E-W trending Wadi Abu Nekheil Fault Zone (WANFZ) separates AD peridotites into two parts, the northern and southern AD peridotite sections, the two sections are similar to each other in terms of petrographical, mineralogical and geochemical features. The tectonic fabric of the harzburgites indicates that they primarily the residue of melting at an ocean ridge. The harzburgites are essentially the residue left after extraction of tholeiitic MORB-magma.
3- The two parts of AD peridotites are very similar with respect to the compositional data presented for olivines, chromian spinels, orthopyroxenes, clinopyroxenes, and amphiboles. The Fo content of olivine in dunite ranges from Fo91 to Fo93 and in harzburgite from Fo91 to Fo92. NiO content in olivine shows no compositional difference and ranges from 0.21 to 0.57 wt. %. Clinopyroxenes have very low TiO2 (up to 0.03) and Al2O3 (0.65 - 1.22) contents. There are remarkable differences compared with those from abyssal peridotites and clinopyroxenes from subcontinental lithospheric mantle (SCLM), which have much higher TiO2 content. The Ti contents exhibited by the amphiboles confirm metamorphism under greenschist-lower amphibolite facies condition.
Core compositions of accessory chromite from AD peridotites retain high Cr# (60 – 74) and Mg# (35 – 58) values, consistent with type III peridotites of arc-related ophiolite of Dick and Bullen (1984). Plots of Fe3+# versus Mg# and Cr# versus Mg# confirm that AD peridotites represent alpine-type comparable to Troodos ophiolite. Accessory chromites are zoned, mineralogically and chemically, and rimmed by “ferrichromite”. Chromian spinel from chromitites has higher Cr# (83 to 88) and Mg# (66 – 79), and lower TiO2 content between 0.03 to 0.12 wt. %.
4- AD harzburgites plot within the olivine – spinel mantle array (OSMA) on the plot of Mg# (olivine) versus Cr# (spinel) particularly in the oceanic supra-subduction zone (SSZ) peridotite field (Pearse et al., 2000). This indicates that the harzburgites originated as residue from (20 - 25%) melting at a ridge, and were subsequently modified to dunites by interaction with arc magmas. The dunites, however, were plot within the OSMA, indicating that they represent the products of extreme interaction between mantle and melt and not cumulates. The high Cr# and Fo values of these dunites are consistent with a supra-subduction zone origin (Dick and Bullen, 1984; Bonati and Michael, 1989). Plot of Cr# versus TiO2 for the chromian spinels confirm that AD peridotites represent residual mantle that interacted with boninitic melt, the degree of interaction increasing from the harzburgites to dunites. This plot demonstrates also that the residual mantle end member was depleted. The depleted character of AD peridotites is confirmed also by the extreme low concentrations of Al2O3 and CaO of bulk rock compositions.
5- The composition of the chromian spinel in AD chromitites suggests that the parent magma was boninitic similar to those hosting chromitites in the mantle section of ophiolites. On the Cr# versus Mg# plot, AD chromitites are plot within the boninite field. In comparison with chromitites from various geological settings, AD chromitites resemble the high-Cr, low-Ti podiform chromitites hosted in the mantle section of ophiolites. Moreover, the identified platinum-group minerals (PGM) comprise laurite (Ru, Os, Ir)S2 and minor erlichmanite (Os, Ru, Ir, Rh)S2 associated with native gold (Au) and Ni-sulfides. Laurite compositions plot well within the field of laurite inclusions in mantle-hosted chromitites from various ophiolite complexes.
6- In view of the contrasting interpretations, which were previously proposed for the tectonic setting of AD peridotites, the results of this study lead to a conlusive answer. AD peridotites show close affinity with supra-subduction zone (SSZ) ophiolite complexes, rather than with Precambrian layered intrusions. This ophiolitic nature has been demonstrated from their mineral and whole rock geochemistry. AD peridotites represent oceanic supra-subduction zone (SSZ) peridotites of back-arc basin.
7- Base-metal sulfides (BMS) and base-metal alloys (BMA) are occasionally found as disseminations in AD peridotites. The following minerals were identified and confirmed by electron-microprobe analyses (in decreasing order of abundance): Ni-Fe alloy, cobalt pentlandite, pentlandite, heazlewoodite, millerite, goldevskite, and chalcopyrite.
The sulfide mineralogy reflects the magmatic and post-magmatic evolution of AD complex. While pentlandite and heazlewoodite were formed from the magma at the late stage of crystallization, Ni-rich sulfides and alloys were formed during serpentinization. Ni librated from olivine during serpentinization participated in the formation of the Ni-rich sulfides and alloys. This suggestion is supported by the silicate, sulfides and alloys chemistry and textures.
8- Pyroxenite dykes, mainly consisting of orthopyroxenites (often bearing Cr-rich spinel), are frequently transected harzburgites and dunites. Their chromian spinels have Cr# (67 - 82), and plot in the trend of boninites. Orthopyroxenes in the orthopyroxenite dykes are enstatites, with Mg# in the range of 84.4 – 88.0. Their En content ranges from En83.2 to En86.8. Amphiboles are calcic amphibole, mainly tremolite. They have Mg# in the range of 90.9 – 92.5. The orthopyroxenite dykes probably crystallized from Mg-rich and Al-poor melts and display boninitic affinity. They represent traces of boninitic magma crystallized in dykes intruded into a host peridotite under mantle conditions.
9- Um Eleiga gabbro intrusion is a small body of sulfide-bearing unmetamorphosed mafic rocks that has intruded AD peridotites from the northern and eastern side. It consists mainly of gobbroic rocks of fresh and altered varieties. The fresh gabbroic rocks are characterized by layering and cumulate textures. The layered gabbros comprise gabbronorite, and hornblende gabbronorite, hornblende gabbro, and pyroxenite. Compositional data presented for plagioclase, orthopyroxene, clinopyroxene, amphibole, and epidote. Plagioclase in the fresh gabbronorites is labradorite ranging in composition from An55.9 to An65.2. Orthopyroxenes are defined as hypersthene (En66.9-69.3, Fs29.1-30.2, Wo1.5-2.9) with Mg# ranges from 69.25 to 70.87 in the fresh gabbronorites; and hypersthene (En65.4-68.4, Fs30.3-33.2, Wo1.3-1.8) with Mg# ranges from 66.8 to 69.7 in the altered gabbros. Clinopyroxenes are defined as diopside (En42.0-43.4, Fs11.2-12.9, Wo43.8-45.7) with Mg# ranges from 76.9 to 80.2 in the fresh gabbronorites; and augite (En42.8-45.3, Fs12.2-9.7, Wo45.0-45.1) with Mg# ranges from 78.0 to 83.2 in the altered gabbros. Amphiboles in fresh gabbronorites are classified as magnesio-hornblende and those in altered gabbro are classified as tschermakite-hornblende, actinolite, actinolite hornblende, and magnesio-hornblende. Apatite was found in Um Eleiga gabbros and there is a positive correlation of the PGE rich sulfides with apatite.
10- The disseminated base metal sulfides in Um Eleiga gabbro are Cu dominant, comprising pyrrhotite, chalcopyrite, bornite, and pyrite. Of the major minerals of Um Eleiga gabbro, pentlandite and mackinawite are the most PGE carrier. Accessory minerals accompanying these major sulfides include magnetite and ilmenite. The platinum-group elements are present as “invisible PGE” in pentlandite and mackinawite. The only phase that accommodates appreciable PGE is pentlandite.
“Invisible PGE” in pentlandite and mackinawite of Um Eleiga gabbro suggest that PGE were concentrated during cooling of a parental sulfide liquid. Diffusion and migration of PGE must have continued to temperatures below that of pentlandite and mackinawite crystallization (~ 600º C).