Skarns replacing igneous rocks

摘要: Most skarns replace carbonates (limestone-dolostone) and calcareous rocks, therefore the exploration of skarns has been focused on carbonate areas. In recent years several large skarns have been found and they replace intrusive or extrusive igneous rock. This opens new districts for skarn exploration. In this contribution we introduce examples of skarns replacing igneous rocks, summarize their features, identify ways to distinguish such skarns from skarns replacing carbonates, and discuss the exploration implications. Skarns replacing igneous rocks may have three types of occurrences: Type-1 occurs in purely igneous rocks without any traces of carbonates around, Type-2 skarns dominantly replaces volcanic rocks at and close to its contact with carbonates, rather than the carbonates, and Type-3 replaces intrusions as endoskarns in carbonate area side-by-side with skarns replacing carbonates (exoskarn). Type-1 examples include the Tongon Au skarns in Cote d’Ivoire, containing 3.8 Moz Au@ 2.5 g/t Au and replacing basaltic-andesitic crystal tuffs (Lawrence et al., 2017); Meishan Fe skarn, China, with 334 Mt Fe ores@ 39% Fe (NGAC, 1979), replacing a basalt porphyry intrusion and andesitic volcanic wallrocks; the Nihe Fe skarn, China, containing 184 Mt Fe ores with 21% magnetic Fe (NGAC, 2010), replacing diorite porphyry intrusion and the trachyandesite wallrocks, and the Magnum Bonum, Jumna and Jack-in-a-box Sn skarns replacing metabasalt, plus the Tommy Burns Sn deposit (11,520 t Sn) at metabasalt-chert boundary, NE Queensland, Australia. Type-2 includes the Lake, the Merry Widow, and the Iron Hill Fe skarns where …