The geology, mineralization, and alteration of Ilkwang Mine, Republic of Korea; a Cu-W-bearing tourmaline breccia pipe

Abstract

Ilkwang mine is situated approximately 25 km northeast of Pusan, Republic of Korea. The breccia pipe is contained within a small quartz monzonite stock which intrudes a sequence of gently dipping Upper Cretaceous sediments and pyroclastics. The steeply plunging pipe which measures 80 m by 50 m is almost elliptical in shape and defined by sets of slickensided surfaces. The shape of the brecciated fragments varies from tabular in the upper levels to angular at depth. The blocks have been cemented initially by tourmaline and quartz, and finally by tungstates and sulfides.Alteration zones have developed both within the brecciated fragments and around the pipe within the quartz monzonite. The majority of the stock lies within the propylitic zone, which has been divided into two subzones: an outer tourmaline subzone characterized by the development of tourmaline rosettes, and an inner garnet subzone which lies within 10 m of the pipe margins. The garnets within this subzone are essentially spessartine-almandine garnets and are either disseminated throughout the rock or contained within tourmaline-filled fractures. Within the pipe the majority of the fragments lie within the sericitic zone except for the larger ones in the deeper parts of the mine which have cores of the garnet subzone.The minerals found within the breccia pipe are dominantly tourmaline, quartz, pyrrhotite, chalcopyrite, wolframite, scheelite, arsenopyrite, and marcasite, with minor quantities of native copper and bismuth, galena, sphalerite, gudmundite, meneghinite, boulangerite, bournonite, and tetrahedrite. Ore zones occur throughout the pipe in the upper levels but are restricted to the peripheries in the lower levels. A primary geochemical halo has developed within the quartz monzonite to a distance of approximately 70 m from the pipe margins.The age of intrusion of the Ilkwang Stock, from K-Ar data on hornblende, is 81 + or - 3 m.y., whereas sericite from the brecciated fragments within the pipe gave an age of 69 + or - 3 m.y.The shape of the pipe is considered to have been controlled by cone-shaped fractures resulting from shock; subsequent solution of the shattered fragments in the deeper parts of the cone by rising hydrothermal fluids resulted in the collapse of the central block of the pipe. Open spaces formed by this collapse, which were largest over and around this central block, were later filled by the gangue and ore minerals.