The Relationship among the Orebody, Volcanic Rocks, and a Rhyolite Dike at the Epithermal Kupol Deposit (Western Chukotka)

The relationship among the orebody, volcanic rocks, and a rhyolite dike in the large epithermal Kupol deposit (Western Chukotka) is considered. Almost throughout its entire length (more than 3000 m), the orebody of the deposit is intersected by rhyolite dikes. In the studied profiles in the mine excavations at the horizons 425, 455, 470, 515 m, a rhyolite dike clearly intersects and bisects the orebody, which is a 20-m-thick dominantly quartz vein with some adularia. The rhyolite has a porphyritic structure. The groundmass is devitrified with the formation of an aphanite quartz-feldspar aggregate with a mosaic microstructure. Some patches are fluidized. The rhyolites contain from 72.96 to 74.97 wt % SiO 2 , and the composition corresponds to biotite granites of the low-alkaline series. The following criteria have been identified to indicate the post-mineralization age of the dike: a clear intersection of a thick ore vein by the dike; vitrification of the dyke material at the contact with the vein; rhyolite cementing the crushed quartz vein material; the emergence of some small tongues from the dike that penetrate the quartz vein; xenoliths of quartz vein material in the dike body; the orebody tongues do not penetrate the dike; increased concentration of vein and ore minerals on the contact surface of the dike or in cracks of the endocontact zone. The formation of obsidian (vitrophyre) in rhyolite at contact with the orebody indicates that the dike was emplaced in a cold system. Preservation of the productive vein thickness, its morphological features, and the Au and Ag contents at the cabs of the rhyolite dike; identical textures and repeated rhythms in the areas of rhythmic-banded layering in the vein on both sides of the dike shows that in the Kupol deposit, post-mineralization dikes do not replace or impoverish the productive vein system, which is probably due to their dilatation nature and the infilling of separation cracks.