Insung Lee

Review of the stable isotope geochemistry of Mesozoic igneous rocks and Cu–Au deposits along the middle–lower Yangtze Metallogenic Belt, China

Ore deposition took place in the Yangtze Valley episodically during the Jurassic and Cretaceous periods, generating approximately 200 polymetallic Cu–Fe–Au, Mo, Zn, Pb, and Ag deposits. We analysed the stable isotopes of sulphur, oxygen, and hydrogen from the Cu–Au deposits and correlated our new data with published stable isotope for associated Yanshanian (Mesozoic) igneous rocks. The latter bears a close relationship to Cu–Au mineralization in the area. Cu–Au deposits in the middle–lower Yangtze Valley can be divided into three types: skarn, porphyry, and volcanic. The S–O–H isotopic values allow constraints to be placed on the conditions of origin of these famous Cu–Au ores and their related igneous rocks. Sulphur from the sulphide ores mostly was derived from a magmatic source; however, a few deposits reflect a sedimentary source of sulphur. Oxygen isotope values in quartz from the Shaxi porphyry Cu–Au deposit and from the Tongling skarn Cu–Au deposits range from 2.6‰ to 12.5‰ and from −1.3‰ to 24.5‰, respectively; these values represent larger variations compared with those from other Cu–Au deposits in this metallogenic belt. Hydrogen versus oxygen isotope plots of the Cu–Au ore-forming fluids demonstrate that the fluids came from different sources: the most important involved the mixing of magmatic and meteoric water; the second most important was strictly magmatic water; and the third most important may have been a mixture of formation water or meteoric water that had reacted with carbonate wall rocks.

Carbonate-hosted talc deposits in the contact aureole of an igneous intrusion (Hwanggangri mineralized zone, South Korea): geochemistry, phase relationships, and stable isotope studies

Many talc deposits occur in the Hwanggangri Mineralized Zone (HMZ) in dolomitic marbles of the Cambro-Ordovician Samtaesan Formation within 1 km of the contact with the Cretaceous Muamsa Granite. Talc commonly forms fine-grained, fibrous aggregates, or pseudomorphs after tremolite; abundant tremolite is included as impurities in the talc ore. Talc generally was derived from tremolite in calc-silicate rock within the dolomitic marble. Calc-silicate rock, consisting mainly of tremolite and diopside, was generated from silicic metasomatism during the prograde stage, which promoted decarbonation reactions until dolomite was exhausted locally. Hydrothermal alteration of calc-silicate rock to talc is marked by the addition of Mg and Si, and the leaching of Ca; Cr, Co, and Ni were relatively immobile during the retrograde stage. Contact metamorphism related to the granite intrusion generated the successive appearance of tremolite, diopside, and forsterite, or wollastonite-bearing assemblages in the marble, depending on the bulk rock composition. The XCO2 content of the metamorphic fluids rose initially above XCO2=0.6, and decreased steadily toward a water-rich composition with increasing temperature above 600 °C in the calcitic marble, while buffered reaction of the dolomitic marble occurred at higher XCO2 conditions above 600 °C. Talc mineralization developed under metastable conditions with infiltration of large amounts of igneous fluids along a fault-shattered zone during the retrograde stage and is characterized by the loss of Ca2+ with the addition of Mg2+. Oxygen and carbon isotopic variations of carbonate and calc-silicate minerals are in agreement with theoretical relationships determined for decarbonation products of contact metamorphism. Talc formation temperatures obtained from oxygen isotope fractionation, T–XCO2 relationships, and activity diagrams range from 380 to 400 °C.

Geochemical study of Cretaceous magmatic rocks in Chuzhou region, low Yangtze River metallogenic belt: implications for petrogenesis and Cu–Au mineralization

ABSTRACT The low Yangtze River metallogenic belt (LYRMB) is one of the most important poly-metal deposit belts in China. The Chuxian, Machang and Shangyaopu intrusions in the LYRMB are intermediate rock series, mainly composed of monzonite and quartz monzonite. In this study, bulk rock major and trace elements, zircon U–Pb dating and Hf isotope were analysed. Five ages have been obtained as (1) Chuxian, 121.8 ± 1.9 and 124.0 ± 1.4 Ma, respectively, (2) Machang intrusion, 123.1 ± 2.0 Ma and (3) Shangyaopu, 126.6 ± 1.8 and 123.4 ± 1.9 Ma, indicating that the regional igneous activity was in Early Cretaceous, being consistent with the massive Yanshanian magmatic events in eastern China. These three intrusions are identified as a high-Mg adakite, most of them showing geochemical features of high Si, high Na and low Sr, which can be interpreted as partial melting of subducted oceanic crust. High Mg# characteristics indicate the magmas reacted with the mantle. The negative zircon εHf values of these adakites suggest that the magmas have assimilation of old crustal material, e.g. Archaean continental crust, the basement of the south Tancheng–Lujiang (Tan–Lu) fault. Biotite Ti temperature result (about 700°C) shows that intrusive magma has a relatively low temperature. Petrogenesis and regional Cu–Au mineralization mechanism may be explained by Pacific plate subduction during about 125–180 million years subducted to southwest towards the LYRMB. Magmas formed by partial melting of subducted oceanic crust have systemically high Cu–Au contents, which are conducive to corresponding mineralization. Graphical Abstract

Evaluation of the volatilization and infiltration effects on the stable isotopic and mineralogical variations in the carbonate rocks adjacent to the Cretaceous Muamsa Granite, South Korea

The Muamsa Granite in the Hwanggangri Mineralized Zone, South Korea, intruded the Cambro-Ordovician Samtaesan Formation, which dominantly consists of calcitic marble and dolomitic marble with minor intercalations of pelitic rock and amphibolite. Intrusion of the granite caused an isotopic depletion of 13C and 18O in the carbonate rocks. Calcitic carbonates show systematic depletion from unrecrystallized limestone through calcitic marble to skarn. It was initiated with volatilization accompanying calcsilicate-forming reactions, and then, with increased permeability, H2O-rich siliceous fluids of igneous origin actively infiltrated the system promoting fluid-rock interaction. For the skarn fluid infiltration yielded fluid–rock ratios as high as 4.2 in open system, and caused increased 18O depletion rates compared to the volatilization stage. More than 70% of the isotopic depletion in the calcitic marble is attributed to the infiltration process. With such strong infiltration, mineral assemblages such as phlogopite in the absence of K-feldspar, phlogopite+diopside, and wollastonite are commonly observed in the marble. The dolomitic marble was comparatively less depleted in heavy isotopes compared to the calcitic marble and formed under a wide range of XCO2 and low fluid–rock ratios (<1.0). Active decarbonation in dolomitic marble was restricted by silica-poor dolomite protolith, which caused limited fluid infiltration and resulted in high XCO2 mineral assemblages represented by dolomite+calcite+diopside+forsterite and more 13C-depleted nature compared to dolomite+calcite+tremolite+forsterite assemblage.

Measurement of ocular torsion using iterative Lucas-Kanade optical flow method

This paper presents a new method for measuring ocular torsion using the optical flow. Iris image is cropped and transformed into rectangular image to make an orientation invariant image. Feature points were selected at iris region from a reference and a target image, and then shift of each feature was calculated using iterative Lucas-Kanade method. The feature points were selected according to the strength of corner on the iris image. The accuracy of the algorithm was tested using printed eye image and compared with traditional cross-correlation method. Measurement error was less than 0.15deg

Textural and geochemical characteristics of Fe−Mn crusts from four seamounts near the Marshall Islands, western Pacific

Textural and geochemical properties of ferromanganese crust (Fe−Mn crust) samples from four adjacent seamounts near the Marshall Islands were investigated to delineate the paleoceanographic condition on their growth history. The Fe−Mn crust samples of this study show four distinct layers (layers 1 to 4 from top to bottom). The uppermost layer 1 is massive and black, and is enriched in hydrogenetic elements such as Mn, Co, Ni, and Mo. The next layer 2 is porous and filled with sediment. Detrital (Al, Rb, and Ti) and biogenic (Cu, Zn, and Ba) elements are enriched in layer 2. The layers 3 and 4 are phosphatized layers which are impregnated with carbonate fluorapatite (CFA), and therefore their primary mineralogy and geochemistry were not preserved. The property of layer 2 suggested that this layer had grown under the condition of high biogenic and detrital flux. Such a condition may be met in the Inter-tropical Convergence Zone (ITCZ) where the northeast and southeast trade winds meet. Considering the present location and paleotracking of seamounts, layer 2 appears to have formed when these seamounts were beneath the ITCZ. On the other hand, layer 1 may have started to grow after the seamounts moved out of the ITCZ with the northwestward movement of the Pacific plate. Our study indicates that the Fe−Mn crusts can be used to trace the paleolocation of ITCZ when precise age determination and information on the plate movement are provided.

Geochemical studies of banded iron formations (BIFs) in the North China Craton: a review

Majority of the banded iron formations (BIFs) in China are hosted in North China Craton (NCC). The BIFs in NCC are interlayered with metamorphics including amphibolite, and mainly composed of quartz, magnetite, and hematite with minor minerals such as garnet, epidote, chlorite, calcite, biotite, and pyrite. They were formed during from Paleoarchean to the early Paleoproterozoic with a peak at Neoarchean. Wall rock and Fe ore bodies in the BIFs have experienced greenschist to granulite facies metamorphism. Because of the intensive post-ore geologic activities, ores in the BIFs in NCC are characterized as low Fe grade (30.35 wt% of Fe in average) compared to those in the world. Most of the BIFs in China are affiliated into Algoma-type BIF. The Fe ores are composed of SiO2 and Fe2O3T with minor Al2O3, MgO, CaO, Rb, Sr, Zr, Hf, Cr, and Co. Post Archean Average Shale (PAAS) normalized REY (rare earth elements with Y) patterns in the Fe ores show depletion in light REE (rare earth elements; LREE) compared to heavy REE (HREE) with positive La and Eu anomalies, suggesting that the Fe ores in BIFs were precipitated from the seawater with hydrothermal fluids. Trace elements ratios further suggest that the Fe and Si in the BIFs were derived from volcanic-related submarine hydrothermal fluids. Incorporation of continental-derived clastic materials are insignificant in most of the BIFs, while a few exhibit some incorporations. Ce anomalies in the Fe ore indicate an evolution of redox state during the BIF formation. Tectonic models in NCC and the formation of the BIFs were suggested by the geochemical and geological investigations in this study.

Sulfur Isotope Composition of Seafloor Hydrothermal Vents in the Convergent Plate Boundaries of the Western Pacific: A Role of Magma on Generation of Hydrothermal Fluid

Seafloor hydrothermal system occurs along the volcanic mid-ocean ridge, back-arc spreading center, and other submarine volcanic regions. The hydrothermal system is one of the fundamental processes controlling the transfer of energy and matter between crust/mantle and ocean; it forms hydrothermal vents where various deepsea biological communities are inhabited and precipitates metal sulfide deposits. Hydrothermal systems at convergence plate boundaries show diverse geochemical properties due to recycle of subducted material compared to simple systems at mid-ocean ridges. Sulfur isotopes can be used to evaluate such diversity in generation and evolution of hydrothermal system. In this paper, we review the sulfur isotope composition and geochemistry of hydrothermal precipitates sampled from several hydrothermal vents in the divergent plate boundaries in the western Pacific region. Both sulfide and sulfate minerals of the hydrothermal vents in the arc and backarc tectonic settings commonly show low sulfur isotope compositions, which can be attributed to input of magmatic SO2 gas. Diversity in geochemistry of hydrothermal system suggests an active role of magma in the formation of seafloor hydrothermal system.

Measurement of ocular torsion using iterative optical flow

This work represents a new method for measuring ocular torsion using optical flow. Feature points are obtained from reference and current image, and the relative optical flows of each point are calculated. The feature points are selected according to the strength of corner on the signature of iris. This method is robust and effective in calculation.

Preliminary study on noble gas isotopes of mantle-derived peridotite xeno- liths along the Tan-Lu Fault belt, Eastern China

In order to understand the nature of noble gas reservoirs beneath continents, elemental and isotopic compositions of noble gases were analyzed from spinel-lherzolites with anhydrous mineral assemblages (i.e., olivine+clinopyroxene+orthopyroxene +spinel±apatite±garnet); these are considered to be typical of the upper lithospheric mantle. Isotopic compositions of the noble gases were measured for peridotite xenoliths enclosed in Cenozoic alkaline basalts in eastern-central China to provide insight on mantle characteristics. The results show that the noble gas abundances and isotopic compositions in mantle-derived xenoliths from eastern-central China vary widely:4He abundances from 0.0024×10−6 to 25.0×10−6 (cm3stp/g);3He/4He ratios range between 0.33RA and 4.62 RA;40Ar/36Ar ratios from 250.56 to 1233.86;40Ar abundances from 0.97×10−6 to 34.18×10−6(cm3/g). Only three measured Ne isotopes show a degree of homogeneity:20Ne from 0.3 to 0.7 (cm3stp/g);20Ne/22Ne ratios from 10.5 to 11.0; and20Ne abundances are also rather uniform, from 35.8×10−13 to 47.8×10−13 (cm3stp/g), and129Xe/130Xe ratios from 6.50 to 6.70. It can be seen that3He/4He ratios vary largely, and40Ar/36Ar ratios are significantly lower than MORB and Proterozoic crust, but close to that of atmospheric values. On the basis of the noble gas abundances and isotope compositions, it can be concluded that Ne, Kr and Xe in mantle-derived xenoliths from NE China have atmospheric origin. The results also show the heterogeneity of noble gases in peridotite from the upper mantle in eastern China, which accords with the results of previous geochemical studies.