Mineralogi dan Geokimia Endapan Skarn Logam Dasar (Pb-Zn)-Perak Ruwai Selatan dan Sekitarnya, Kabupaten Lamandau, Provinsi Kalimantan Tengah, Indonesia

RIMA WARDHANI, Ir. Anastasia Dewi Titisari, M.T., Ph.D., IPU

2020 | Tesis | MAGISTER TEKNIK GEOLOGI

Mineralogi alterasi hidrotermal di daerah penelitian dibagi menjadi 2 subtipe yaitu tipe alterasi prograde dikarakterisasi oleh kehadiran oleh mineral kalk-silikat anhydrous seperti garnet, klinopiroksen dan wollastonit,sedangkan skarn retrograde dicirikan oleh mineral hydrous seperti tremolit, epidot, klorit, serisit, kalsit dan zeolit. Mineral bijih dikarakterisasi oleh kehadiran galena (PbS), sfalerit (ZnS), kalkopirit (CuFeS2), pirit (FeS2),pirhotit (FeS2), markasit (FeS2), arsenopirit (FeAsS) dan magnetit (Fe3O4). Mineralisasi bijih di daerah penelitian umumnya terbentuk pada tahap skarn retrograde awal. Studi karakterisasi geokimia batuan samping difokuskan pada batulanau dan batugamping. Akibat proses alterasi hidrotermal, geokimia batulanau terskarnisasi menunjukan penambahan massa batuan sebesar 66,67±41,67%. Penambahan massa batuan tersebut seiring dengan bertambahnya beberapa unsur seperti SiO2, Fe2O3, CaO, MnO dan MgO, juga unsur logam seperti Cu, Pb, Zn dan Ag. Pada batugamping terskarnisasi juga terjadi penambahan oksida utama seperti data di atas, kecuali CaO berkurang dikarenakan kalsit tergantikan oleh kuarsa dan mineral kalk-silikat. Unsur logam Cu, Pb, Zn dan Ag mengalami penambahan cukup signifikan dibandingkan dengan batuan precursor, hal ini mencerminkan batuan terskarnisasi tersebut tumpangtindih dengan mineralisasi bijih yang hadir seperti kalkopirit, sfalerit dan galena. Geokimia bijih menunjukan korelasi positif (R2= 0,715) antara total kadar Pb+Zn (wt.%) dengan kadar Ag (ppm), yang mencerminkan semakin melimpah galena dan sfalerit, maka kandungan perak dalam bijih semakin tinggi. Karakteristik kimia mineral sfalerit memiliki kadar Fe tinggi yaitu antara 2,16 dan 11,98 wt.% dengan rata-rata 8,71 wt.% (N=6). Sfalerit juga menunjukan kadar rerata Ga (galium, N=5) sebesar 0,23 wt.% dan kadar Ge (germanium, N=3) sebesar 0,635 wt.%. Kadar Ga dan Ge dalam sfalerit diaplikasikan dalam geotermometer sfalerit dan menghasilkan temperatur mineralisasi bijih sekitar 155°C. Galena mengandung perak yang cukup signifikan yaitu sekitar 0,13 wt.% atau 1.300 ppm Ag. Data kimia galena dianalisis dengan EPMA (Electron Probe Micro Analyser) menunjukan kadar Ag mencapai 0,45% (4.500 ppm Ag), sehingga dapat disebut argentiferous galena (cf. Idrus dkk., 2011). Model genetik pembentukan mineralisasi bijih logam dasar (Pb-Zn)-perak di daerah penelitian dibagi menjadi 6 (enam) tahap pembentukan yaitu, (1) pengendapan dan pembentukan batuan sedimen berupa batugamping dan batulanau sebagai batuan induk (host-rock) mineralisasi, serta pembentukan satuan tuf, (2) pembentukan sesar naik (trust fault) sebagai channelway fluida hidrotermal, (3) intrusi diorit dan metamorfirme isokimia, (4) pembentukan skarn prograde, (5) pembentukan skarn retrograde, dan (6) intrusi andesit sebagai post-mineralization intrusion.

The hydrothermal alteration mineralogy in the study area is divided into 2 subtypes, namely the type of prograde alteration characterized by the presence of minerals. anhydrous calc-silicates such as garnet, clinopyroxene and wollastonite, whereas retrograde scars are characterized by hydrous minerals such as tremolite, epidote, chlorite, sericite, calcite and zeolite. Ore minerals are characterized by the presence of galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS2), pyrite (FeS2), pyrrhotite (FeS2), headquartersite (FeS2), arsenopyrite (FeAsS) and magnetite (Fe3O4). The ore mineralization in the study area is generally formed during the early retrograde scarn stage. The geochemical characterization study of side rock is focused on siltstone and limestone. Due to hydrothermal alteration processes, geochemistry of siltstone.The geochemical hydrothermal characteristic showed the addition of rock mass by 66.67± 41.67%. The addition of the rock mass goes hand in hand with the addition of several elements such as SiO2, Fe2O3, CaO, MnO and MgO, as well as metal elements such as Cu, Pb, Zn and Ag. On limestone In addition, there is also the addition of the main oxide as the data above, except that CaO is reduced because calcite is replaced by quartz and calc-silicate minerals. The metal elements Cu, Pb, Zn and Ag have increased significantly compared to precursor rocks, this reflects the chalcopyrite, sphalerite and galena. The geochemistry of the ore shows a positive correlation (R2= 0.715) between the total Pb + Zn (wt.%) And Ag (ppm) levels, which abundant in galena and sphalerite, the silver content in the ore is getting higher high. The chemical characteristics of sphalerite minerals have high Fe levels between 2.16 and 11.98 wt.% With an average of 8.71 wt.% (N = 6). Sphalerite showed that the mean levels of Ga (gallium, N = 5) were 0.23 wt.% and Ge levels (germanium, N = 3) were 0.635 wt.%. Ga and Ge levels in sphalerite were applied in a sphalerite geothermometer and produced the temperature mineralization of ore around 155 -200° C. Galena contains a significant amount of silver, which is about 0.13 wt.% Or 1,300 ppm Ag. Galena chemical data analyzed by EPMA (Electron Probe Micro Analyzer) showed that Ag levels reached 0.45% (4,500 ppm Ag), so that called argentiferous galena (cf. Idrus et al., 2011). The genetic model for the formation of basic metal ore (Pb-Zn) -prim mineralization in the study area is divided into 6 (six) stages of formation, namely, (1) deposition and formation of sedimentary rocks in the form of limestone and siltstone as a host rock for mineralization, as well as the formation of unit of tuff, (2) formation of a trust fault as a hydrothermal fluid channelway, (3) intrusion of diorite and isochemical metamorphosis, (4) formation of prograde scars, (5) formation of retrograde screens, (6) andesite intrusion as post-mineralization intrusion.

Kata Kunci : Mineralisasi, Skran, Ruwai