Formation and mineral chemistry of a calcic skarn from Al-Madhiq, SW Saudi Arabia

A newly identified skarn occurrence is described from the Neoproterozoic rocks of the SW Arabian shield. It is exposed to the SE, E and NE of the Al-Madhiq town. The skarn attributes correspond to those typical of the calcic skarns that host W-deposits. It is characterized as an exoskarn of the proximal type, related to a granitoid contact close to an impure quartzite bed within the regional metamorphic rocks of mixed sedimentary and volcanic derivation. The skarn is localized along a shear zone parallel to the regional faults and other major shear zones. Samples from the studied area contain characteristic skarn minerals that include both the prograde (brownish red grossular, ferrosalite, aluminian titanite-grothite, albite-oligoclase, scapolite), and retrograde (epidote, quartz, hornblende, calcite) assemblages. The pyroxenes are ferrosalites, Mn-bearing, and more like those from “oxidized” skarns; although garnets indicate it to be a “reduced” type skarn. Epidote mimicks that from typical skarns, as it bears a pistacite content of 15.9–20.7%. Grossular composition reflects a largely reduced genetic environment; as it is in solid solution with 6.5–21.6% andradite, 0–0.15% uvarovite, 0–0.47% pyrope, 4.33–18.75% almandine, and 0.4–8.58% spessartine molecules. Titanite composition varies from aluminian titanite to grothite, that may be analogous to the newly described Al-rich titanite from the low-pressure calc-silicate rocks.