Composition of skarn grossular from Al-Madhiq area, SW Saudi Arabia

A skarn, located about 20 km south of Khamis Mushayat city, contains a brownish red grossular associated with skarn minerals including epidote, zoisite, hedenbergite, aluminian sphene, albite, quartz, and amphibole. The skarn forms dike-like bodies, veins, and disseminations in the amphibolite as an exoskarn zone at the upper contact of a granitoid intrusive. Higher modal concentrations of grossular define the zoning and occur towards the contact of granitoid intrusion, and the mineral assemblage resembles that of the proximal type skarns. Idiomorphic grossular is developed in the garnet-poorer zone and the xenomorphic-poikiloblastic grossular occurs in the garnet-richer zones. Both types exhibit similar XRD patterns. Microprobe compositions of both types span over the garnet end-member percentage ranges as follows: grossular, 58.27-83.54; andradite, 6.5-21.6; uvarovite, 0-0.15; pyrope, 0-0.47; almandine, 4.33-18.75; spessartine, 0.4-8.58. Zoning in one 0.45 mm garnet crystal was determined that shows lower andradite, almandine, and spessartine molecules and higher grossular molecule towards the core part. Grossular compositions match those of grossuular associated with the economic skarn deposits of W-Sn at other world localities. The grossular composition and other attributes indicate it evolved in a reducing environment, and its likely association would be with a tungsten tactite deposit in preference to Sn. The grossular resulted from initial anhydrous, prograde metasomatism, that later developed retrograde epidote and amphibole.