Mineralogy of Metacarbonate Rocks and Garnet Depositsat two Selected Areas at Asir Region, Southwestern KSA

Abstract

The mineralogical data materialized in the present work suggest that the previously described skarns at Ad Darb in the literature are actually marble deposits intercalated with schists and phyllites of different compositions. The marble and associated metasediments lie to the west of striking ridges of marbles that are nearly aligned in the NNW-SSE direction. Garnet at Al Madhiq occurrence often occurs in the form of bands conformable with rock foliation (gneissosity and schistosity). It is suggested that the paragenesis “quartz-gar- net-epidote” is developed due to the percolation of some Al-rich solutions along rock foliation of the horn- blende gneiss, i.e. metasomatic garnet. Careful field investigation collaborated with petrographic and SEM studies, suggest the occurrence of another garnetiferous paragenesis associating quartz, mica and feldspar in pegmatites, aplites and quartz veins, i.e. exclusively igneous garnet. Metasomatic garnet in the calc-silicates of Al Madhiq is of grossular composition. It is commonly unzoned but some distinctly to slightly zoned crystals are observed where the core is andradite-rich and the rim is grossular. Metasomatic events responseble for growth of garnet in the calc-silicates led also to formation of epidote post-dating grossular. Hand specimens, microscopic investigation and BSE images prove that this epidote post-dates and replaces gros sular, and even rims it in some instances. Igneous garnet at Al Madhiq (almandine-spessartine) is found only in pegmatites and aplites that are genetically related to alkali granitoids. Sulphides (dominated by pyrite) occur in intemate association with domains rich in grossular and hence these sulphides are more likely hydrothermal indicating reducing conditions for formation of grossular.