Magnetite/Ilmenite–series Classification and Magnetic Susceptibility of the Mesozoic-Cenozoic Batholiths in Peru

Abstract: Plutonic rocks of the Coastal Batholith of Peru were evaluated in terms of the granitoid-series classification using the bulk ferric/ferrous ratio from the literature and new measurements of magnetic susceptibility. The batholith is largely composed of magnetite-series plutonic rocks; the magnetite series make up 85% by number of chemical analyses (n=130) and 80% by measurement of magnetic susceptibility (n=210). The ilmenite-series rocks are mostly found in the felsic facies of the batholith. Asymmetrical distribution of magnetic susceptibility is not clear as in the Japanese Islands and Peninsular Range Batholith, but the magnetic susceptibility may decreases continentward (i. e., Peninsular Range type).
The Cordillera Blanca Batholith and stocks are also composed of mainly magnetite series plutonic rocks, but ilmenite-series rocks may be more predominant than in the Coastal Batholith, which is also indicated by the presence of Sn and W mineralizations.     

Cretaceous granitoids and their zircon U–Pb ages across the south‐central part of the Abukuma Highland,Japan

Zircons separated from Cretaceous granitoids are dated from a south‐central transect of the Abukuma metamorphic and granitic terrane. The zircon ages do not follow ‘older’ and ‘younger’ granitoid ages that are used conventionally. In the western part of the study area (Zones I, II and III) where the Takanuki and Gosaisho metamorphic rocks are exposed, the Iritono quartz dioritic stock intruding the greenschist facies rocks in Zone III exhibits the oldest age of 121 Ma in the studied region. Quartz diorite located northward shows 112 Ma, but the other four granitoids intruding into the Takanuki and Gosaisho metamorphic rocks are younger and 103–99 Ma. Two‐mica and biotite granites belong to the youngest age group of 99 Ma. The granitic activities of both the Abukuma and Ryoke belts were initiated by intrusion of quartz dioritic magmas and were ended by two‐mica granite activity. The ages of the eastern two batholiths vary from 110 to 106 Ma (four samples), and show no age common to the Kitakami granitoids farther to the north. Throughout the Japanese Islands arc, Cretaceous granitic activities became younger toward the marginal sea side from the Kitakami Mountains, to the Abukuma Highland, and the Ryoke Belt, then to the Sanin belt of the Inner Zone of Southwest Japan.     

Granitoid Series and Mineralization in the Circum-Pacific Phanerozoic Granitic Belts

Abstract: Throughout the Phanerozoic Era, magnetite-series and ilmenite-series granitoids are heterogeneously distributed on both sides of the Pacific Ocean. Ilmenite-series granitoids are dominant in the western Pacific rim, while magnetite-series granitoids prediminate in the eastern Pacific orogenic belt. Regional distribution patterns of the two series of paired belt are also recognized differently, implying different tectonic and geochemical settings on both sides of the Pacific Ocean. The differences reflect on the predominance of Sn-W deposits in the western Pacific but sulfide-forming mineral commodities in the eastern Pacific rims. To determine redox state of granitoids is the first step in mineral exploration of granitoid affinity.     

Sulfur isotopic characteristics of late Cenozoic ore deposits at the arc junction of Hokkaido, Japan

Abstract Sulfide minerals of late Cenozoic vein-type deposits of southwest Hokkaido and Kuril Islands yielded δ³⁴_CDT S values of 2 to 8 permil, which are typical green-tuff values of magnetite-series igneous terrane. Sulfides of the Kitami district of northeast Hokkaido, on the other hand, are characterized by negative δ³⁴S_CDT values, ranging from 0 to - 7 permil. This unique value among ore deposits in the late Cenozoic back-arc terranes in the Japanese Islands is considered to have resulted from extraction of ³²S enriched sulfur from the basement rocks, because of well-developed N-S fracturing in the basement, which is characteristic of the axial belt and Kitami district of Hokkaido.     

Sulfur isotopic composition of the magnetite-series and ilmenite-series granitoids in Japan

Sulfur isotopic composition has been measured on 30 granitoids and 11 gabbroids from the Cretaceous and Tertiary granitic terranes of Japan. The two series of granitoids, the magnetite-series and ilmenite-series, defined by Ishihara (1977), show two specific isotope trends. The magnetite-series granioids all have positive (su34)S (CDT) values from +1 to +9, while the ilmenite-series rocks are dominated by negative values between –11 and –1. The trend in the ilmenite-series is consistent with the thesis that the magma has been influenced by light biogenic sulfur from the continental crust. The inferred large scale magma-crust interaction in the ilmenite-series granitoids indicates that the emplacement of this series of magma has been governed by a stoping mechanism.In contrast, the magnetite-series granitoids have little if any evidence for significant magma-crust interaction, indicating that the intrusion of this series of magma may have been more or less of fissure-filling type. Their trend towards positive (su3 4)S values (average +5) argues for the introduction of some heavy sulfur, probably of seawater origin, into the mantle derived sulfur. This is most likely to occur in an arctrench system by the subduction of an oceanic plate which accompanies the sulfate-bearing pelagic sediments.The isotopic data of gabbroids, mostly between –1 and +3, are close to the commonly assumed value for mantle sulfur. Nevertheless, the gabbroids from the magnetite-series granitic terranes tend to have higher (su34)S value than those from the ilmenite-series belts. It is inferred that the factors controlling the isotope characteristics of the granitoid sulfur have also been operative in these grabbroids at least to some extent.     

Geneses of Two Types of Mafic Rocks to Carry Placer‐Magnetite Ores in the Sanin Granitic Belt,SW Japan

Two types of mafic rocks from the central Sanin district, and their mafic minerals, were studied chemically and microscopically. They are classified into pyroxene‐containing gabbroid and hornblende–biotite quartz diorite. The gabbroid had higher color index but lower magnetite content; while the quartz diorite had lower color index, but higher magnetite content. The magnetite contents are also related to the amounts of hydrous mafic silicates. The gabbroic magma having pyroxene–amphibole assemblage, originated in the upper mantle, was considered essentially anhydrous, but became partly hydrous on the way to the site of solidification in the continental crust, and crystallized some magnetites with hypersthene and amphibole. The quartz dioritic magma was formed by partial melting of possibly subducting ocean‐floor basalts, once exposed to the sea‐floor then altered; thus the magmas became hydrous and oxidized originally, and precipitated abundant magnetite and hydrous mafic silicates from the early crystallization stage onward. Their weathered parts provided the most placer magnetite ores in the history.     

Mo-related Adakitic Granitoids from Non-island-arc Setting: Jecheon Pluton of South Korea

Abstract. The Jecheon granitoids, having an elongated shape of NE-SW 27 km and NW-SE 13 km (190 km²), are composed mostly of magnetite-series hornblende-biotite granodiorite and biotite granite, which intrude into the Neoproterozoic metamor-phic and Paleozoic sedimentary rocks of the Ogcheon Belt. The granitoids have Triassic-Jurassic age of 202.7 ±1.9 Ma with very high ⁸⁷Sr/⁸⁶Sr initial ratio of 0.7140. The granodiorite has 63–69 % SiO₂, 15.1–17.3 % Al₂O₃, <1.6 % MgO, 6–15 ppm Y and Sr/Y ratios of 24–76, and is depleted in HREE. Biotite granite together, the Jecheon pluton has adakitic characteristics, which are unique in a continental tectonic setting. The granitoids may have been generated by partial melting of an older adakitic granitoid of I-type basement, or by separation of early crystallized garnet and hornblende from an anatectic melt.     

REE Mineralization of Weathered Crust and Clay Sediment on Granitic Rocks in the Sanyo Belt,SW Japan and the Southern Jiangxi Province,China

Rare earth element (REE) geochemistry and mineralogy have been studied in the weathered crusts derived from the Early Yanshanian (Jurassic) biotite granites of Dabu and Dingnan, as well as in the Indosinian (Permian) muscovite–biotite granite of Aigao in southern Jiangxi province, China, and the weathered crusts and clay sediments on biotite granites in the Sanyo belt, SW Japan, that is, Okayama, Tanakami, and Naegi areas. In all of the weathered crusts, biotite and plagioclase commonly tend to decrease toward the upper part of the profile, whereas kaolinite and residual quartz and K‐feldspar increase. The weathered crusts of the Dingnan granites and some Naegi granites, which are characterized by the enrichment in light REE (LREE) in C horizons, have higher total REE (ΣREE) content than the parent REE‐enriched granites. Weathering of LREE‐bearing apatite and fluorocarbonates in the Dingnan granites and allanite and apatite in some Naegi granites may account for the leaching of LREE at the B horizons. The leached LREE must result in subsequent enrichment of LREE in the C horizons. The enrichment is probably associated with mainly adsorption onto kaolinite and partly formation of possible secondary LREE‐bearing minerals. In Japan it was found that REE mineralization occurs not in the weathered granitic crusts but in reworked clay sediments, especially kaolinite‐rich layers, derived mainly from the weathering materials of REE‐enriched granitic rocks. The clay sediments are more enriched in LREE, which likely adsorbed onto kaolinite. Concentration of heavy REE within almost all the weathered crusts and clay sediments, however, may reflect mainly residual REE‐bearing minerals such as zircon, which originated in the parent granitic rocks. The findings of the present study support the three processes for fractionation of the REE during weathering: (i) selective leaching of rocks containing both stable and unstable REE‐bearing minerals; (ii) adsorption onto clay minerals; and (iii) presence of possible secondary LREE‐bearing minerals.