Orthornagmatic origin for the Ilkwang Cu-W breccia-pipe deposit, Southeastern Kyongsang Basin, South Korea

Four distinct types of fluid inclusion are associated with brecciation, alteration and mineralization in the Ilkwang Cu-W breccia-pipe deposit in the southeastern Kyongsang Basin, South Korea. The earliest fluid inclusions in igneous quartz have salinities of 35-49 wt% NaCl equiv. and homogenize by halite dissolution. This fluid exsolved directly from the melt. A CO2-bearing fluid (2-4 wt% NaCl equiv.) was trapped during the initial stages of brecciation and pressure decrease and later brecciation events and continued decreasing pressure in the magma chamber generated low-salinity, supercritical fluids that boiled to produce brine (40-48 wt% NaCl equiv.) and coexisting vapor (2-13 wt% NaCl equiv.) at 343-493 degreesC and 120-400 bars. The latest fluid (5-24 wt% NaCl equiv.) was trapped during the waning stages of hydrothermal activity. The oxygen and hydrogen isotopic compositions of the fluid (delta(18)O(SMOW) = 9.4-4.8parts per thousand, deltaD(SMOW) = -45 to - 49parts per thousand) of quartz and the sulfur isotopic compositions (delta(34)S(CDT) = -0.3 to 0.9parts per thousand) of sulfide minerals cementing the breccia fragments, are consistent with a magmatic origin for the hydrothermal fluids. Fluid inclusion data and stable isotopic compositions suggest that Ilkwang Cu-W mineralization and alteration are products of orthomagmatic hydrothermal processes that were strongly pipe-controlled. The various types of fluid inclusions in the Ilkwang deposit reflect the evolution from a high pressure to a low pressure magmatic system. The evolution to a low pressure stage and concomitant aqueous fluid immiscibility in the Ilkwang deposit are major factors in the formation of economic mineralization. (C) 2004 Elsevier Ltd. All rights reserved.