Abstract
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•Unusual mineral zones developed when pegmatite bodies cross-cut through ultramafic rocks.•Anthophyllite, actinolite, vermiculite, hercynite, corundum and tourmaline zones are formed.•A traverse of lateral variation of various zones geochemically and mineralogically investigated.•Small-scale pegmatite veins and refractory ultramafic host preclude gem corundum generation.•Mineral assemblage, bulk and mineral chemistries support metasomatic origin by diffusional model.
Unusual series of hybrid metasomatic mineral zones are well developed when the granitic pegmatite veins cross-cut through ultramafic rocks at the Hafafit complex, South Eastern Desert of Egypt. Two groups of mineralogical zones are developed; the first group related to the serpentinized ultramafic rocks (peridotite-related zones) comprises anthophyllite-, actinolite- and vermiculite-bearing zones, whereas the second group related to the pegmatite veins (pegmatite-related zones) comprises hercynite- and corundum-bearing zones. Tourmaline-rich zone only occurs when the pegmatite veins come in contact between ultramafic rocks and biotite-rich granitic gneiss. The mineral assemblages, bulk rock geochemistry and in situ mineral chemistry suggest a metasomatic origin for the hybrid zones. The mineral assemblages: corundum + margarite ± Ca-plagioclase, hercynite + corundum + garnet ± chlorite, and anthophyllite + actinolite + enstatite, in addition to the constrained garnet-biotite/phlogopite geothermometer, and the higher tourmaline XMg, strongly indicate amphibolite facies metamorphism for the ultramafic-pegmatite hybrid zones. The constrained temperature ranges between 500 and 700 °C and a pressure around 7 kbars, with pronounced increases of metamorphism from the outermost zone in the serpentinized ultramafic ongoing to the pegmatite mineral zones. Juxtaposition of two contrasting rock types (highly reactive hot SiO2-, alkali-rich melt and cool silica-deficient ultramafic rocks) side by side would results in an intense contact bimetasomatic elemental exchange. The SiO2, H2O and alkalies would be offered by pegmatite melt, while MgO, FeO and CaO were supplied by serpentinized ultramafic rocks. The small volume of pegmatite melt relative to the ultramafic rocks would be easily desilicated by SiO2 diffusion into the ultramafic rocks to form the monomineralic SiO2-rich minerals in the peridotite-related zones. Consequently, Al2O3 would be concentrated in the residual melt to form corundum and other Al-rich mineral phases in the pegmatite-related zones. The small size of pegmatite veins and refractory nature of the ultramafic rocks could be the reasons precluding the formation of colored gem corundum in the Hafafit complex. Restricted occurrence of tourmaline-rich zone at the ultramafic-granitic gneiss contact is attributed to the diffusion of B-rich fluids generated from the leaching of biotite-rich granitic gneiss.