Hydrothermal alteration, fluid flow and volume change in shear zones: the layered mafic–ultramafic Kettara intrusion (Jebilet Massif, Variscan belt, Morocco)

Abstract

During emplacement and cooling, the layered mafic-ultramafic Kettara intrusion (Jebilet, Morocco) underwent coeval effects of deformation and pervasive fluid infiltration at the scale of the intrusion. In the zones not affected by deformation, primary minerals (olivine, plagioclase, clinopyroxene) were partially or totally altered into Ca-amphibole, Mg-chlorite and CaAl-silicates. In the zones of active deformation (centimetre-scale shear zones), focused fluid flow transformed the metacumulates (peridotites and leucogabbros) into ultramylonites where insoluble primary minerals (ilmenite, spinel and apatite) persist in a Ca-amphibole-rich matrix. Mass-balance calculations indicate that shearing was accompanied by up to 200% volume gain; the ultramylonites being enriched in Si, Ca, Mg, and Fe, and depleted in Na and K. The gains in Ca and Mg and losses in Na and K are consistent with fluid flow in the direction of increasing temperature.When the intrusion had cooled to temperatures prevailing in the country rock (lower greenschist facies), deformation was still active along the shear zones. Intense intragranular fracturing in the shear zone walls and subsequent fluid infiltration allowed shear zones to thicken to metre-scale shear zones with time. The inner parts of the shear zones were transformed into chlorite-rich ultramylonites. In the shear zone walls, muscovite crystallized at the expense of Ca-Al silicates, while calcite and quartz were deposited in Ôen echelonÕ veins. Mass-balance calculations indicate that formation of the chlorite-rich shear zones was accompanied by up to 60% volume loss near the centre of the shear zones; the ultramylonites being enriched in Fe and depleted in Si, Ca, Mg, Na and K while the shear zones walls are enriched in K and depleted in Ca and Si. The alteration observed in, and adjacent to the chlorite shear zones is consistent with an upward migrating regional fluid which flows laterally into the shear zone walls. Isotopic (Sr, O) signatures inferred for the fluid indicate it was deeply equilibrated with host lithologies.

Hydrothermal Alteration in the Main Sulfide Zone at Unki Mine, Shurugwi Subchamber of the Great Dyke, Zimbabwe: Evidence from Petrography and Silicates Mineral Chemistry

“…Corresponding lithologic units are shown in (b). Variation of calculated pressures based on the Equations (38) and (19) of Putirka (2008) [66] across the MSZ. Pressures that were calculated using Equation (39) are slightly higher than those calculated using Equation (38).…”

Geology and wall rock alteration at the Hercynian Draa Sfar Zn–Pb–Cu massive sulphide deposit, Morocco

“…Beauchamp et al (1991) interpreted the argillites of Central Jebilet to have been deposited in an anoxic, "shallower" water environment. The end of the Carboniferous was also characterized by three types of magmatic activity (Bordonaro, 1983, Aarab and Beauchamp, 1987and Bernard et al, 1988: (1) bimodal, subvolcanic, rift-related tholeiitic gabbro and minor rhyodacite and rhyodacite flows; (2) syntectonic calc-alkaline granite intrusions and (3) post-tectonic dykes and sills of microdiorite (Huvelin, 1977, Bernard et al, 1988and Essaifi et al, 2004.…”

“…The D 1 deformation was associated with low-grade regional metamorphism, typified by a quartz-muscovite-chlorite assemblage. D 1 shortening was followed by an N-S D 2 shortening (S 2 crenulation cleavage) and late brittle deformation (Huvelin, 1977, Bordonaro, 1983, Bernard et al, 1988and Essaifi et al, 2004.…”

Draa Sfar, Morocco: A Visean (331 Ma) pyrrhotite-rich, polymetallic volcanogenic massive sulphide deposit in a Hercynian sediment-dominant terrane

“…The argillites of the CJD are interpreted by Beauchamp et al (1991) to have been deposited in an anoxic, shallower water environment. The end of the Carboniferous is characterized by the development of the Mesetian basin where intense volcanic and volcano-sedimentary activity ([Bordonaro, 1983], [Aarab and Beauchamp, 1987] and [Bernard et al, 1988]) is characterized by three types of magmatism: (1) bimodal, subvolcanic rift-related tholeiitic gabbro and minor rhyolite and rhyodacite flows; (2) syntectonic calc-alkaline granite intrusions and (3) post-tectonic, microdiorite dykes and sills ( [Huvelin, 1977], [Bernard et al, 1988] and [Essaifi et al, 2004]). …”