Karakteristik Geologi, Zonasi Alterasi, dan Mineralisasi Bijih pada Pit Gojo, Kabupaten Lamandau, Provinsi Kalimantan Tengah

Azzaini Ghoni, Dr. Eng. Ir. Lucas Donny Setijadji, S.T., M.Sc., IPU.; Dr.rer.nat. Ir. I Wayan Warmada, IPM.

2026 | Skripsi | TEKNIK GEOLOGI

Daerah Pit Gojo, Kabupaten Lamandau, Provinsi Kalimantan Tengah, merupakan salah satu wilayah mineralisasi hidrotermal yang berkembang pada lingkungan magmatik Busur Schwaner dan dikenal sebagai endapan skarn Fe–Zn–Pb–Ag. Seiring menurunnya potensi ekonomis mineralisasi skarn utama, eksplorasi lanjutan menunjukkan adanya indikasi anomali Au yang diduga berkaitan dengan sistem hidrotermal tahap lanjut. Penelitian ini bertujuan untuk mengetahui karakteristik geologi, zonasi alterasi, mineralisasi bijih, faktor pengontrol geologi, serta proses pembentukan sistem mineralisasi di daerah penelitian. Metode penelitian dilakukan melalui pemetaan geologi permukaan, analisis petrografi, mineragrafi, Analytical Spectral Device (ASD), X-Ray Fluorescence (XRF), Fire Assay, serta interpretasi data core logging. Hasil penelitian menunjukkan bahwa stratigrafi daerah penelitian tersusun atas satuan batugamping, batulanau, dan tuf lapili yang kemudian diterobos oleh intrusi monzonit kuarsa, diorit, andesit porfiri, dan dolerit. Struktur geologi utama berkembang dengan orientasi NE–SW yang kemudian dipotong oleh struktur NW–SE dan E–W sebagai fase deformasi lebih muda. Alterasi hidrotermal berkembang secara luas dan terbagi menjadi dua periode utama, yaitu alterasi periode skarn dan alterasi periode epitermal. Alterasi periode skarn dicirikan oleh zona garnet+klinopiroksen+kuarsa±magnetit, klorit+epidot+kuarsa, serta kaolinit+dikit±kuarsa±gibsit, sedangkan alterasi periode epitermal dicirikan oleh asosiasi kaolinit+serisit. Mineralisasi bijih yang berkembang terdiri atas magnetit, pirit, kalkopirit, sfalerit, galena, kovelit, malakit, hematit, goetit, dan jarosit dengan tekstur penggantian masif (massive replacement), sebaran (disseminated), dan pengisian rongga (cavity filling). Pembentukan alterasi dan mineralisasi dikontrol oleh interaksi antara intrusi magmatik, batuan karbonat, dan sistem struktur geologi yang berfungsi sebagai jalur migrasi fluida hidrotermal. Evolusi mineralisasi diawali oleh pembentukan sistem skarn akibat metasomatisme kontak antara fluida magmatik dengan batuan karbonat yang menghasilkan mineral kalk-silikat dan mineralisasi sulfida logam dasar. Tahap selanjutnya ditandai oleh alterasi retrograde berupa klorit, epidot, dan kuarsa, kemudian diikuti oleh perkembangan sistem epitermal yang menyebabkan overprinting alterasi dan potensi mineralisasi Au. Penelitian ini menunjukkan bahwa Pit Gojo memiliki sistem mineralisasi magmatik-hidrotermal kompleks yang berkembang secara multistage dan dikontrol kuat oleh aktivitas magmatisme serta struktur geologi.

The Pit Gojo area, located in Lamandau Regency, Central Kalimantan Province, represents one of the hydrothermal mineralization systems developed within the magmatic environment of the Schwaner Arc and is recognized as an Fe–Zn–Pb–Ag skarn deposit. Along with the declining economic potential of the main skarn mineralization, further exploration activities revealed indications of Au anomalies that are interpreted to be related to a later-stage hydrothermal system. This study aims to determine the geological characteristics, alteration zoning, ore mineralization, geological controlling factors, and the mineralization-forming processes in the study area. The research methods included surface geological mapping, petrographic and mineragraphic analyses, Analytical Spectral Device (ASD) analysis, X-Ray Fluorescence (XRF), Fire Assay, and interpretation of core logging data. The results indicate that the stratigraphy of the study area consists of limestone, siltstone, and lapilli tuff units, which were subsequently intruded by quartz monzonite, diorite, andesite porphyry, and dolerite. The main geological structures trend NE–SW and are crosscut by younger NW–SE and E–W structural systems. Hydrothermal alteration is widely developed and can be divided into two major periods, namely the skarn-stage alteration and the epithermal-stage alteration. The skarn-stage alteration is characterized by garnet+clinopyroxene+quartz±magnetite, chlorite+epidote+quartz, and kaolinite+dickite±quartz±gibbsite zones, whereas the epithermal-stage alteration is characterized by kaolinite+sericite assemblages. Ore mineralization consists of magnetite, pyrite, chalcopyrite, sphalerite, galena, covellite, malachite, hematite, goethite, and jarosite, occurring as massive replacement, disseminated, and cavity-filling textures. The formation of alteration and mineralization was controlled by the interaction between magmatic intrusions, carbonate host rocks, and geological structures that acted as pathways for hydrothermal fluid migration. The mineralization evolution began with the development of a skarn system caused by contact metasomatism between magmatic fluids and carbonate rocks, resulting in calc-silicate minerals and base-metal sulfide mineralization. The subsequent stage was marked by retrograde alteration represented by chlorite, epidote, and quartz, followed by the development of an epithermal system that overprinted argillic alteration and potentially introduced Au mineralization. This study demonstrates that Pit Gojo hosts a complex multistage magmatic–hydrothermal mineralization system strongly controlled by magmatic activity and geological structures.

Kata Kunci : Pit Gojo, skarn, epitermal, alterasi hidrotermal, mineralisasi bijih, Kalimantan Tengah