Sang Joon Pak

Geochemistry and petrogenesis of mafic-ultramafic rocks from the Central Indian Ridge, latitude 8°–17° S: denudation of mantle harzburgites and gabbroic rocks and compositional variation of basalts

This study investigates the formation of lower oceanic crust and geochemical variations of basalts along the Central Indian Ridge (CIR, lat. 7°45′–17°10′ S). Harzburgites, various gabbroic cumulates, medium- to fine-grained oxide gabbros, diabases, and pillow basalts were recovered by dredging from segment ends such as ridge-transform intersections (RTIs), non-transform discontinuities (NTDs), and transform offset areas. The occurrence of both harzburgites and gabbroic rocks with minor basalts at all segments ends, and leucogabbro intrusive into harzburgite at the 12°45′ S NTD indicates that oceanic crust at segment ends exposes mantle-derived harzburgites and gabbroic intrusions with a thin basaltic cover due to sparse magmatic activity. Basalts collected along the entire ridge show wide compositional variations between N (normal)- and E (enriched)-mid-ocean ridge basalt (MORB). T (transitional)-MORBs with enriched affinities are more prominent than N-MORBs. There is no tendency of enrichment towards specific directions. (La/Sm)N variations in MORB along the CIR (8°–21°S) fluctuates at a regional scale with local high positive anomalies reflecting compositional heterogeneity of the sub-CIR mantle domain.

Genetic environment of the Yuryang Te-bearing Au deposit: Batholith-type orogenic Au mineralization in Korea

The Yuryang Au deposit, comprising a Te-bearing Au-Ag vein mineralization, is hosted in Precambrian gneiss and closely related to pegmatite. The mineralized veins display massive quartz textures, with weak alteration adjacent to the veins. Ore mineralization took place in two stages; early Fe-sulfide mineralization and late Fe-sulfide and Au-Te-Bi mineralization. The pressure corrected temperatures (= 500?) indicate that the deposit was formed at a distinctively high temperature from fluids with moderate to low salinity (< 12 wt. % eq. NaCl), CH4 (1 ∼ 22 mol %) and moderate CO2. The sphalerite geobarometry yield an estimated pressure about 3.5 ∼ 2.1 kb. The measured and calculated isotopic compositions of fluids [δ18Ofluid =10.2 to 12.2‰; δDfluid=−52 to −77‰] may indicate that the gold deposition originated from S-type magmatic waters. The physicochemical conditions observed in the Yuryang Au deposit indicate that the deposit is a batholith-type Au deposits with Te mineralization at a considerable depth, and consistent with those of other Jurassic Aurich deposits in the Cheonan metallogenic province.

Mineralogical and Fluid Inclusion Study on Seafloor Hydrothermal Vents at TA25 Subsea Caldera in Tongan Waters

The extensive hydrothermal deposits have been found, for the first time, on the western TA25 seamount caldera in the Tonga arc. The seafloor hydrothermal vents are active and immature, emitting the transparent fluids of which temperatures range from to (average

Rock-magnetic Properties of Chimneys from TA25 Seamount in the Tofua Arc, Southwest Pacific

To identify rock-magnetic properties of volcanogenic hydrothermal sulfide deposits, chimneys were obtained from the Tofua Arc in Southwest Pacific, using a remotely operated vehicle (ROV) and Grab with AV cameras (GTVs). Three different types of chimneys used in this study are a high-temperature chimney with venting fluid-temperature of about 200 o C (ROV01), a low-temperature chimney of about 80 o C (GTV01), and an inactive chimney (ROV02). Magnetic properties of ROV01 are dominated by pyrrhotite, except for the outermost that experienced severe oxidation. Concentration and grain-size of ROV01 pyrrhotite are relatively low and fine. For GTV01, both magnetic concentration and grain-size increase from interior to margin. Pyrrhotite, dominant in the core, becomes mixed with hematite in the rim of the chimney due to secondary oxidation. High concentration and large grain-size of magnetic minerals characterize the ROV02. Dominant magnetic phases are pyrrhotite, hematite and goethite. In particular, the outermost rim shows a presence of magnetite produced by magnetotactic bacterial activity. Such distinctive contrast in magnetic concentration, grain-size and mineralogy among three different types of chimney enables the rock-magnetic study to characterize an evolution of hydrothermal deposits.