[H-RE12] 資源地球科学　

[HRE12-P03] Mineral textures and assemblages, sulfur isotopes, and the whole rock major and trace element compositions of hydrothermal veins in Cordon, northern Luzon, Philippines

*Hertz Balmater¹、Ryohei Takahashi¹、Hinako Sato¹、Andrea Agangi¹、Pearlyn Manalo¹、Jillian Aira Gabo-Ratio³、Graciano Yumul Jr.² (1.Akita University - Graduate School of International Resource Sciences - Economic Geology Lab、2.Cordillera Exploration Company, Incorporated.、3.University of the Philippines Diliman - National Institute of Geological Sciences)

キーワード：alkalic, mineralization, sulfur isotopes, northern Luzon, Philippines

This study presents the mineral textures and assemblages, sulfur isotopes, and the whole rock major and trace element compositions of the hydrothermal veins that were collected from the Cordon deposit, northern Luzon, Philippines. The geology of Cordon is composed mainly of the Oligocene to early Miocene syenite, tinguaite, and monzonite of the Cordon Syenite Complex, which have intruded the Oligocene andesites, pyroclastic rocks, and the shallow marine sandstone and siltstone. The hydrothermal veins are composed of the quartz-sulfide-carbonate veins. They are hosted by the syenite and monzonite porphyries that were later intruded by syenite dikes. The younger andesitic plugs formed near the contact between the syenite and monzonite porphyries. The syenite porphyry have medium-grained, equigranular, euhedral to subhedral phenocrysts of K-feldspar and biotite. They have subordinate clinopyroxene, apatite, and magnetite. The pyrite and chalcopyrite occur as dissemination. The biotite and the clinopyroxene phenocrysts are being replaced by chlorite, epidote and clay minerals. The monzonite porphyry has around medium-grained, tabular plagioclase feldspar phenocrysts. They have minor fine-grained biotite and dark green clinopyroxene. The groundmass consist of K-feldspar, biotite and clinopyroxene. The monzonite porphyry is weakly altered but relatively fresh compared with syenite porphyry. The andesitic plugs are composed of phenocrysts of dark-gray augite, plagioclase, and alkali feldspar. The groundmass consists of granular plagioclase, augite, alkali feldspar, biotite, and apatite. They are relatively less altered compared with syenite and monzonite porphyries. Bulk chemical compositions of the syenite and monzonite porphyries, and the andesite plugs, show negative correlations for TiO2, MnO, MgO, Fe2O3, and CaO vs. SiO2 contents. Linear trends of major element variation of the CSC in Harker diagrams imply simple crystal fractionation of magma. With the exception of K2O and Na2O, all the major elements decrease in abundance, with increasing SiO2. Based on the total alkali and silica (TAS) compositions, the ten Oligocene andesitic rocks that were intruded by the CSC are classified as basaltic andesite, basaltic trachyandesite and trachyandesite. TAS compositions of fifteen intrusive rocks of the CSC (syenite, monzonite, and tinguaite) indicate alkalic basalt or alkalic gabbro, monzogabbro, monzonite, and syenite in composition. Compared with the primitive mantle, the intrusive rocks of the CSC are found to be enriched in large ion lithophile elements (LILE: Cs, Pb, Rb, Ba, Sr, and K) in agreement with their alkalinity. The intrusive rocks likewise exhibit depletion with the high field strength elements (HFSE: Th, Ta, Nb, Zr, Hf, Ti, and Lu) compared to the LILE. The wall rocks were found to be moderately altered, up to around 50-100 meters, from hydrothermal veins. They are composed mainly of quartz, pyrite, sanidine, clinochlore, and phlogopite in the southwest of the main deposit (Adit 3); quartz, pyrite, sanidine, and microcline in the northeast of the main deposit (Adit 5/9); quartz, muscovite, fluorapatite, illite, orthoclase in the south of the main deposit (Adit 6); and sanidine, quartz, pyrite, and orthoclase, in the northwest of the main deposit (Adit 8). Phyllosilicate minerals, mainly of clay, chlorite, and illite are present in the bulk analysis. The hydrothermal veins commonly range from centimeters up to a hundred centimeters in width. They are hosted by the syenite and monzonite porphyries. The veins consist mainly of quartz, pyrite ± chalcopyrite, sphalerite, and galena, with minor covellite and tennatite-tetrahedrite. The bulk chemical compositions of the ten vein samples show positive correlations for Au versus Ag, and for Au versus Cu. It showed 26.7 ppm Au, 123 ppm Ag, and 710 ppm Cu for Dallao, and 5.88 ppm Au, 156 ppm Ag, and 1141 ppm Cu for Anonang. Sulfur isotope analysis of pyrite from thirteen (13) samples from Adit 3 have yielded a δ34S average value of -3.2 ‰, from isotopic values ranging from -4.5 to -2.1 ‰. Twenty-one (21) samples from Adit 5/9 have yielded a δ34S average value of -3.9 ‰, from isotopic values ranging from -5.8 to -2.4 ‰. Eight (8) samples from Adit 6 have yielded a δ34S average value of -3.5 ‰, from isotopic values ranging from -6.4 to -1.4 ‰. Seven (7) samples from Adit 8 have yielded a δ34S average value of -4.2 ‰, from isotopic values ranging from -4.9 to -3.5 ‰. These negative δ34S values, in a narrow range close to 0 ‰, suggest that the sulfur came mostly from igneous sources, that can either be as magmatic fluids or igneous sulfides dissolution.