The Keumhak Cu deposit is located within the Cretaceous Gyeongsang Basin in the southeastern part of the Korean peninsula. Quartz and calcite vein mineralization in Early Cretaceous sedimentary rocks occurred in three stages: Stage I, quartz+sulfides+ hematite+sulfosalts mineralization; Stage II, quartz+sulfides stage; and Stage III, barren calcite. The main ore mineralization, stage I, can be classified into three substages (pyritic, base-metal, and sulfosalt substages) according to ore mineral assemblages and paragenesis. K-Ar radiometric dating of alteration sericite (77.6±1.6 Ma) indicates that the Cu mineralization of the Keumhak deposit was Late Cretaceous and probably associated with Late Cretaceous granitic rocks that occur near the Keumhak mine.
The Stage I mineralization occurred between >360° and <200°C from fluids with salinities between 5.6 and 1.4 equiv. wt.% NaCl. Most of the Cu mineralization occurred at higher tempera-tures (320° to 280°C). Fluid inclusion data indicate that the main Cu-Pb-Zn ore mineralization resulted from a complex history of CO₂ effervescence coupled with boiling, cooling and dilution of ore fluids. The Cu mineralization was mainly a result of CO₂ effervescence coupled with boiling, whereas the Pb-Zn and the late sulfosalt mineralization resulted mainly from cooling and dilution by an influx of cooler meteoric waters coupled with retrograde boiling. Evidence of fluid unmixing indicates pressures of about 500 bars to 800 bars, and later retrograde boiling evidence indicates that the pressure fluctuated by 200 to 300 bars during hydraulic fracturing.
Sulfur isotope compositions of sulfide minerals in the stage I decrease systematically with paragenetic time with the resultant decrease of calculated δ³⁴S values of H₂S of 8.4 to 4.7‰. These values, together with the observed change to hematite+sulfides (and/or sulfosalts) assemblages, suggest progressively increasing oxidizing conditions, with a corresponding increase of the sulfate/H₂S ratio of hydrothermal fluids. Gradual increase in the oxidized/reduced sulfur ratio of ore stage fluids led to later mineralization in the stage I through breakdown of metal complexes. Measured and calculated hydrogen and oxygen isotope values of the middle to late ore-forming fluids of the stage I suggest meteoric water dominance, approaching unexchanged meteoric water values, except the ¹⁸O-shift.

View full abstract

The Hanawa pluton reported by Tanaka and Ochiai (1988) is devided into main bodies and small fine-grained quartz diorite-tonalite bodies. K-Ar ages using hornblende by Shibata and Utsumi (1983) are 117 Ma for fine-grained quartz diorite-tonalite bodies and 99 Ma for main bodies which intrude the former.
The main bodies are devided into granodiorite, foliated granodiorite-tonalite and medium to fine-grained tonalite-granodiorite facies which are probably formed by succesive intrusion of magma differentiated at deeper level. Furthermore the granodiorite is subdivided into mylonitelike, large sphene bearing, porphyritic and fine-grained sub-facies, which are seem to be differentiated at shallower level.
Each facies contains zircons showing characteristic morphology; granodiorite facies (110 dominant type), foliated granodiorite-tonalite facies (100 dominant type), medium to fine-grained tonalite-granodiorite facies (110/100 intermediate type), fine-grained quartz diorite-tonalite facies (E/F dispersed type).
The morphology of zircons in each rock facies is different in response to petrochemical characters. Prism index of zircon from main bodies decreases systematically with increasing SiO₂ contents of the rocks and with decreasing FeO, CaO, and MgO contents. Therefore the prism index of zircon is related to differentiation of granite magma. While prism index of zircon in fine-grained quartz diorite-tonalite of older stage is completely different from the above trend of the main bodies.
On the other hand U, Th and HREE contents in the zircons of the Hanawa pluton show a wide range from several ten to several hundreds ppm. Elongation index of zircon is clearly related to concentration of these elements substituting Zr.
In conclusion, morphological variations of zircons in granitic rocks reflect evolution of magma. Therefore the study of morphology of zircon is useful for understanding of petrogenesis of granite magma.

View full abstract

Seven whole-rock K-Ar ages ranging from 2.5 to 0.9 Ma were obtained for the Ueno basaltic rocks, central Honshu, Japan. These and existing data collectively show that, in the study area, monogenetic volcanoes were formed before ?? 1 Ma whereas a composite volcano (On-take) was formed after ?? 1 Ma. It maybe suggested that the crustal stress state was changed at about 1 Ma in central Honshu.

View full abstract