Geology and characteristics of low sulphidation epithermal vein in Senepo area, East Java

Abstract

Senepo area is one of several epithermal mineralization prospects in Southern Mountains of Java Island. This study is aimed to investigate the geological framework and hydrothermal alteration of the prospect as well as to characterize the low sulphidation (LS) epithermal vein including vein textures, ore mineral, ore chemistry and hydrothermal fluid that are responsible for the formation of the quartz vein. Detailed field mapping was completed to understand the geological framework and hydrothermal alteration zones. Petrographic, ore microscopic, XRD and AAS analyses were applied to characterize the quartz vein. The study area is occupied by pebbly sandstone, andesite breccia, andesite units and alluvial deposit. Ore mineralization is mainly hosted by andesite of Oligo-Miocene of Watupatok Formation. The host-rock has been suffered by overprinted hydrothermal alteration and weathering. The epithermal veins are found as N-S-trending quartz veins with a thickness of 1-2 meters. These veins show massive texture, crustiform, cockade, druzy, comb, and disseminated sulphides, which is included in Crystalline Quartz Zonation (X). Optical microscopy and XRD analysis indicate that these veins contain several ore minerals such as chalcopyrite, sphalerite, galena, pyrite, hematite, covellite and malachite, associated with gangue mineral including quartz, felspar, chlorite and clay minerals. AAS analysis displays variable chemical composition of precious and base metals such as Au (17-37 ppb), Ag (8.1-32.6 ppm), Cu (122.1-81,450 ppm), Pb (35.4-883.6 ppm) and Zn (18.3-2,628 ppm). Gold and silver are relatively low compared to the base metals. Fluid inclusion micro thermometric analysis indicates that ore mineralization originated at temperatures of 187-263 o C and salinity of 0.18-0.53 % wt. NaCl eq. This salinity corresponds with a minimum hydrostatic pressure of about 10-55.1 bars. Based on those characteristic data, mineralized quartz veins in the study area are classified into a base metal horizon of LS epithermal system. , Ag (8,6 ppm), Cu (122, Pb (35,6 ppm)

Analysis of Grain Morphology, Mineral Composition, and Ore Grade on Gold Placer Deposits in Bantimurung, Pangkep Regency, South Sulawesi, Indonesia

“…Meanwhile, covellite, magnetite, sphalerite, and garnet were found only slightly at the sampling station. This mineral composition indicates that the source of the placer deposits at the study site is epithermal low sulfidation or epithermal high sulfidation (Aluç et al, 2020;Burkett et al, 2015;Enríquez et al, 2018;Idrus and Handayani, 2017;Pratomo et al, 2021;Purwanto et al, 2018;Syafrizal et al, 2017;Villaplaza et al, 2017;Wahyudi, 2011). Although Augrain visually on microscope present only on 1 station it could be cause the location is quite near of mineralization so the Au-grain is still in very fine grain size which it's hard to identify through microscope beside that the Au-grain might be still trapped in carrier mineral is the other reason Au-grain doesn't appear on ore petrography but appear on chemistry analysist.…”

“…Meanwhile, covellite, magnetite, sphalerite, and garnet were found only slightly at the sampling station. This mineral composition indicates that the source of the placer deposits at the study site is epithermal low sulfidation or epithermal high sulfidation (Aluç et al, 2020;Burkett et al, 2015;Enríquez et al, 2018;Idrus and Handayani, 2017;Pratomo et al, 2021;Purwanto et al, 2018;Syafrizal et al, 2017;Villaplaza et al, 2017;Wahyudi, 2011).…”

“…The results showed that quartz and plagioclase minerals were appear at each station, biotite absence on station 1 and pyroxenes absence on station 2 (Table 3). The abundant of quartz indicate the present of vein, with plagioclase and pyroxene it might be indication of mineralization of epithermal (Burkett et al, 2015;Idrus and Handayani, 2017;Purwanto et al, 2018;Wahyudi, 2011). Biotite present indicate the low sulfidation mineralization (Enríquez et al, 2018;Syafrizal et al, 2017;Villaplaza et al, 2017;Irfan et al, 2017)).…”