Hidehiko Shimazaki

Oxygen isotope study of the Kamioka Zn-Pb skarn deposits, Central Japan

The Kamioka Zn-Pb deposits consist of clinopyroxene-rich skarns replacing limestone lenses in Hida gneissose rocks. Paleozoic metabasite, early Jurassic Funatsu granitic rocks and late Cretaceous porphyritic dikes and stocks, are the known igneous rocks in the mining area. Oxygen isotopic compositions of eight separates each of clinopyroxene and quartz from the deposits (Table 1), and some minerals and rocks from the major rock units around the deposits (Table 2), have been analyzed. The estimated oxygen isotopic ratios of the fluids responsible for the formation of the deposits, range as low as -4‰ to +3‰ (SMOW). These values are considerably lower than expected from magmatic waters, and demonstrate that none of the igneous rocks in the area could be a direct source of the skarn- and ore-forming fluids. The data instead show that the deposits are formed by a huge convective circulation of solutions of meteoric water origin, promoted by a hidden batholithic intrusion at the time of the late Cretaceous.

Geology and Metallogeny of the Shizhuyuan Skarn-Greisen Deposit, Hunan Province, China

The Shizhuyuan deposit is the largest among the economically important polymetallic tungsten deposits in China. The deposit occurs within the thermal aureole of Yanshanian felsic intrusions that were emplaced into Devonian carbonates and marls. The mineralization can be divided into three phases that are genetically associated with three episodes of granitic emplacement-pseudoporphyritic biotite granite, equigranular biotite granite, and granite porphyry. During the emplacement of pseudoporphyritic biotite granite, thermal metamorphism and subsequent skarnization developed around the stock. The pure limestone was transformed to marble, whereas marls and argillite interlayers were changed to a series of metamorphic rocks such as grossular-diopside hornfels, wollastonite hornfels, diopside hornfels, wollastonite-vesuvianite hornfels, muscovite-K-feldspar-anorthite hornfels, and prehnitevermiculite hornfels. Because of the subsequent strong skarn development, most hornfelses later were transformed into skarn...

Oxygen and carbon isotope zonations of wall rocks around the Kamioka PbZn skarn deposits, central Japan: application to prospecting

Mapping of the oxygen and carbon isotopic composition of hydrothermally altered wall rocks was conducted during blind ore prospecting for PbZn skarn deposits in the Kamioka mining district, central Japan. The wall rocks consist of heterogeneous rock units. Oxygen and carbon isotope ratios were determined for 35 limestones and 33 silicate rocks from the area around the Mozumi deposit (3 km × 3 km) in the Kamioka mining district. The results (δ18Ospsmow of − 1.1 to + 17.3% and δ13Csppdb of − 5.0 to +4.8% for limestones, and δ18Ospsmow of −0.8 to + 12.5% for silicate rocks) show isotope zonations of the wall rocks, with lighter isotopic compositions toward the center of mineralization. The isotope zonations likely formed by interaction of thermal waters with the wall rocks during skarn mineralization. The isotopically light zone indicates a higher paleotemperature or higher water-to-rock ratios, and occurs in the footwall of the 7Gohi fault. This structure is spatially related to the distribution of orebodies, indicating that the fault was the main conduit of the ore-forming fluids. The oxygen and carbon isotopic compositions of limestones vary regularly across limestone bodies hosted by the silicate wall rocks, suggesting that the thermal waters were pervasive throughout the wall rocks at the time of mineralization. An isotopically light zone was also found in the southeastern corner of the study area, where significant mineralization had yet to be identified. This suggested an extension of the extinct hydrothermal system to this area, and the possibility of hidden orebodies underneath. Recent drilling in this area has intercepted a zone 45 m thick with a grade of 13.4% Zn, 0.03% Pb and 8 g per metric ton at about 380 m depth.

Geology and Geochemical Aspects of Ore Formation at the Prestea Mesothermal Vein Gold Deposit in the Birimian System of Ghana

The Prestea lode gold deposit occurs in a graphitic shear zone in the Birimian system of Ghana. The Birimian is an Early Proterozoic greenstone assemblage (≈ 2100 Ma) with large gold deposits, consisting of a lower unit predominantly of metasediments and an upper unit of metavolcanics with interbedded sediments. The metamorphic stage is of greenschist facies grade. The gold generally occurs as free gold or closely associated with sulfides, particularly arsenopyrite and sulfosalts. It is usually coarse grained and occurs along grain boundaries, as inclusions and in fractures with the ore minerals. The gold is nearly pure, with analyzed grains containing 95-96 wt% Au, 4-5 wt% Ag, and less than 0.1 wt% Sb, Bi, Zn, or Cu. Associated metallic phases consist of pyrite, arsenopyrite, chalcopyrite, pyrrhotite, sphalerite, galena, and sulfosalts such as tetrahedrite, boulangerite, bournonite, and jamesonite. The fluid associated with the gold mineralization was H2O-CO2-NaCl in composition, with a mean salinity = 3...

Yangzhumingite, KMg2.5Si4O10F2, a new mineral in the mica group from Bayan Obo, Inner Mongolia, China

Yangzhumingite, ideally KMg2.5Si4O10F2, occurs as subhedral to euhedral platy crystals up to ca . 0.5 mm across in metamorphosed carbonate rock from Bayan Obo, Inner Mongolia, China. It is associated with dolomite, calcite, tremolite, norbergite and huanghoite-(Ce) with minor phlogopite, barite, bastnasite-(Ce), parisite-(Ce), and fluorite. Yangzhumingite is transparent and colorless with white streak and pearly luster. It is optically biaxial (−), n = 1.532–1.537 (589 nm), and 2 V = 5–10°. The hardness is 3 on Mohs’ scale (measured on a synthetic equivalent). It has a monoclinic unit cell with a = 5.249(4), b = 9.095(5), c = 10.142(5) A, β = 99.96(6)°, V = 476.9(5) A3, Z = 2, space group C 2/ m , and D calc. = 2.807 g/cm3. The five strongest lines in the powder XRD pattern are [ d (A), I / I , hkl ]: (10.03, 95, 001); (3.37, 48, 022), (2.90, 49, 113), (2.59, 67, 131200) and (2.41, 100, 132201). The mean of 15 electron microprobe analyses and SIMS analysis for Li lead to the empirical formula (K0.70Li0.30Na0.01)(Mg2.48Fe0.06) (Si3.96Al0.03)O10[F1.92(OH)0.08] on the basis of 12 anions with (F + OH) = 2. Yangzhumingite is a member of the mica group (Nickel-Strunz grouping 9.EC.10), and it represents the Mg-dominant analogue of montdorite.

Zhangpeishanite, BaFCl, a new mineral in fluorite from Bayan Obo, Inner Mongolia, China

Zhangpeishanite, ideally BaFCl, occurs as inclusions up to 100 μm across in fluorite from Bayan Obo, Inner Mongolia, China. It is associated with barite, hematite and norsethite in fluorite. Zhangpeishanite occurs as transparent, colourless crystals with a white streak and vitreous luster. It is tetragonal, P 4/ nmm , a = 4.3951(8), c = 7.223(2) A, V = 139.52(7) A 3 , Z = 2. The five strongest lines in the powder XRD pattern are [ d (A), I / I 0 , hkl ]: (3.75, 100, 101), (3.11, 94, 110), (2.36, 82, 112), (2.79, 67, 012) and (1.898, 49, 211). The mean of twelve electron-microprobe analyses is (wt.%); Ba 70.90, F 9.88, Cl 18.85, and leads to the empirical formula Ba 0.99 F 0.99 Cl 1.02 on the basis of 3 apfu. Zhangpeishanite is a member of the matlockite group (Struntz–Nickel grouping 3DC.25), the Ba-dominant analogue of matlockite and rorisite. The calculated density of zhangpeishanite is 4.54 g/cm 3 . Zhangpeishanite is non-fluorescent. The hardness is 2½ on Mohs’ scale (measured on a synthetic equivalent). The optical data obtained from the synthetic equivalent are: uniaxial (−), ω = 1.656(2), e = 1.652(2) (589 nm).

Mineralogy and geochemistry of the Au-Ag ore deposits of the South Korean Peninsula

Pyrite and arsenopyrite are the predominant ore minerals in the Korean Au-Ag deposits of this study. The XNipy, XCopy, XNiapy, and XCoapyvalues range between 100 and 3,000 ppm, 200 and 6,000 ppm, 200 and 8,200 ppm, and 100 and 10,200 ppm, respectively. Most XNipy/XCopyvalues fall in the field lower than values varying 0.16–1.30. Arsenopyrite also tends to prefer cobalt rather than nickel showing XNiapy/XCoapyvalues between 0.20 and 1.40. The concentrations of minor elements in ores and gangue minerals vary 1–55 ppm Au and 1–1,120 ppm Ag for the former and 4–57 ppm Ni and 2–45 ppm Co for the latter. The Au/Ag ratio in ore has a good correlation to the Ni/Co ratio of arsenopyrite to gangue. The (Ni/Co)py-(Ni/Co)gangue and (Ni/Co)apy-(Ni/Co)gangue diagrams revealed that the values from the Korean Au-Ag deposits plot in the field lower than 900 °C which is the lowermost temperature determined by previous partitioning experiments.