Orthomagmatic 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 8C 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 (dOSMOW 1⁄4 9:4–4:8‰; dDSMOW 1⁄4 245 to 2 49‰) of quartz and the sulfur isotopic compositions ðdSCDT 1⁄4 20:3 to 0:9‰Þ 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 pipecontrolled. 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. q 2004 Elsevier Ltd. All rights reserved.