In order to constrain its petrogenesis and mineralization, a detailed study of Miocene quartz diorite porphyry from the Ciemas area of West Java, Indonesia, was undertaken, including geochronology and major and trace element analyses. Ciemas quartz diorite porphyry contains medium–coarse-grained quartz and hornblende phenocrysts in a matrix of primarily cryptocrystalline quartz and feldspar. The rocks belong to the calc-alkaline-high K calc-alkaline series. The quartz diorite porphyry and Miocene dacite and andesite wall rocks have similar compositions, both being enriched in large-ion lithophile elements (LILEs) K, Rb, Th, and U, depleted in high-field strength elements (HFSEs) Nb, Ta, Zr, Hf, and P; with light rare earth element (LREE)-enriched patterns and negative Eu anomalies, which are typical for arc magmatic rocks. LA-ICP-MS U-Pb zircon ages of andesite, amphibolic tuff breccia, and quartz diorite porphyry are 17.5 ± 0.3, 16.9 ± 0.3, and 17.1 ± 0.4 Ma, respectively. The quartz diorite porphyry formed by mid-Miocene arc magmatism. This intrusion and surrounding volcanics developed together as the Indian–Australian Plate subducted northward beneath Java, releasing fluids that triggered partial melting of the overlying mantle wedge and felsic melts that evolved further in the crust. Petrogenesis and ages of Ciemas magmatic rocks are important to gaining an understanding of Cenozoic Sunda Arc magmatism, and also provide insights into related epithermal gold deposits.
The Ciemas gold deposit is located in West Java of Indonesia, which is a Cenozoic magmatism belt resulting from the Indo-Australian plate subducting under the Eurasian plate. Two different volcanic rock belts and associated epithermal deposits are distributed in West Java: the younger late Miocene–Pliocene magmatic belt generated the Pliocene–Pleistocene epithermal deposits, while the older late Eocene–early Miocene magmatic belt generated the Miocene epithermal deposits. To constrain the physico-chemical conditions and the origin of the ore fluid in Ciemas, a detailed study of ore petrography, fluid inclusions, laser Raman spectroscopy, oxygen-hydrogen isotopes for quartz was conducted. The results show that hydrothermal pyrite and quartz are widespread, hydrothermal alteration is well developed, and that leaching structures such as vuggy rocks and extension structures such as comb quartz are common. Fluid inclusions in quartz are mainly liquid-rich two phase inclusions, with fluid compositions in the NaCl–H2O fluid system, and contain no or little CO2. Their homogenization temperatures cluster around 240°C–320°C, the salinities lie in the range of 14–17 wt.% NaCl equiv, and the calculated fluid densities are 0.65–1.00 g/cm3. The values of δ18OH2O-VSMOW for quartz range from +5.5‰ to +7.7‰, the δ5DVSMOVV of fluid inclusions in quartz ranges from −70‰ to −115‰. All of these data indicate that mixing of magmatic fluid with meteoric water resulted in the formation of the Ciemas deposit. A comparison among gold deposits of West Java suggests that Miocene epithermal ore deposits in the southernmost part of West Java were more affected by magmatic fluids and exhibit a higher degree of sulfidation than those of Pliocene–Pleistocene.
The Xuanwei Formation is widely distributed in western Guizhou Province, NW China, the lower section of which is primarily composed of gray-white kaolinitic claystone interbedded with thin layers of grayish black carbonaceous mudstone that are extremely enriched with rare earth elements. In order to determine the distribution patterns and existing status of ore-forming elements in these rocks, careful field investigations were performed along a selected geological profile and rock samples were collected and studied in terms of mineralogical and geochemical characteristics. The results show that: 1) REEs are primarily enriched in the grayish white kaolinitic clay sediments and grayish black carbonaceous mudstone. Mineralogical analyses revealed kaolinite as the major mineral in rocks along with smaller amounts of smectite, illite, boehmite, hornblende, pyrophyllite, calcite, dolomite and/or iron-bearing minerals, as well as a certain proportion of feldspar, quartz crystal debris and non-crystal debris. 2) ∑ REE contents are 89.0 to 9965 ppm with an average of 1312 ppm. The thickness of the host rock with ∑ REE higher than 1300 ppm is more than 4 m, which is referred to as the "REE-enriched layer". 3) The REE contents of bulk rocks exhibit a negative correlation with kaolinite, positive correlations with boehmite, hornblende and iron-bearing minerals, and weak positive correlations with smectite, illite and pyrophyllite, indicating that the REE might exist in an ion adsorption state in the space between the layers of clay minerals. 4) Compared with the underlying Emeishan Basalts, the REE patterns of samples are quite similar but are enriched in both LREE and HREE. The degree of enrichment of HREE is relatively high. Based on these results, a model is suggested where the REE-enriched layers originated from the Emeishan Basalts and were controlled by the transportation and deposition of detritus from a paleo-weathering crust. The hard clay rocks have a significant resource potential, as the contents of REE, Ga, Nb and Zr are considerably higher than those in the weathering crust type of REE deposit.
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