Orogenic Gold Mineralization in Luk Ulo Complex,Karangsambung,Central Java,Indonesia

AHMAD TERMIZI, MUHAMMAD HARITZ IFTIQAR, Ir. Dr. I Wayan Warmada;Dr. Nugroho Imam Setiawan

2018 | Tesis | S2 Teknik Geologi

Kompleks Luk Ulo, Karangsambung, Jawa Tengah adalah salah satu singkapan akresi di Indonesia. Kompleks ini merupakan produk subduksi konvergen zaman Kapur, yang menyebabkan tersingkapnya pelbagai jenis batuan metamorf di wilayah kompleks Luk Ulo. Bersama dengan tersingkapnya batuan metamorf tersebut adalah pengayaan emas terkait proses orogeni yang dipanggil tipe endapan emas orogenik. Kehadiran endapan emas tipe yang sama didukung oleh penelitian sebelumnya yang dilakukan oleh beberapa peneliti terdahulu terhadap endapan emas placer di wilayah Luk Ulo yang menemukan jejak bahwa sumber emasnya terkait dengan jenis emas orogenik. Sementara itu, asal usul emas placer di Sungai Loning, Kompleks Luk Ulo masih belum diketahui. Untuk menjawab fenomena ini, penelitian ini dijalankan dengan menggunakan beberapa metode analisis seperti petrografi, mikroskop bijih, Difraksi Sinar-X (XRD), Scanning Electron Microscope - Spektroskopi Dispersi Energi (SEM-EDS), Spektrometri Serapan Atom (AAS) dan mikrotermometri. Dari hasil penelitian, distribusi batuan metamorf di daerah Loning terdiri dari garnet-muskovit sekis (fasies sekis hijau), amfibolit dan juga zeolite-metabreksi. Bersama dengan lapisan batuan garnet-muskovit sekis, adalah jaringan intrusi urat kuarsa-karbonat. Ada dua jenis urat hidrotermal yang dikenal pasti di daerah penelitian yang diklasifikasi sebagai tipe 1 dan tipe 2. Keduanya ditandai dengan hubungan silang dan komposisi mineral penyusun. Tipe 1 yang di interpretasi sebagai urat generasi awal sangat dikontrol oleh struktur foliasi dan deformasi, biasanya dapat ditemukan tersegmentasi, sigmoidal, terputus-putus dan sejajar dengan foliasi batuan induk. Komposisi mineral tipe 1 terdiri dari kuarsa + karbonat + albit +klorit± epidot. Sementara, urat tipe 2 memiliki hubungan silang dengan batuan metamorf induk. Perubahan urat tipe 2 terdiri dari kuarsa + karbonat + albit + klorit + mineral sulfida± grafit. Mineral sulfida yang ditemukan mengandung banyak pirit dengan jumlah kecil kalkopirit, emas, sfalerit dan galena. Pengambilan sampel geokimia menggunakan sampel batuan dan konsentrat mineral dari sedimen sungai juga dilakukan. Dari hasil spektrometri serapan atom (AAS), anomali geokimia mendukung kebarangkalian terbentuknya mineralisasi emas di daerah sungai Loning. Berdasarkan analisis, diketahui bahwa konsentrasi masing-masing elemen berkisar antara 2 sampai 10 ppm Au, 2-20 ppm Ag, 8-105 ppm Cu, 12-1000 ppm Zn, dan 0,9-130 ppm Pb. Tes terhadap inklusi fluida dari tiga sampel KSV2, KSV4 dan KSV5 bercirikan suhu 192.95-388.96 °C dengan salinitas antara 7.90-14.62 wt% NaCl equiv. Suhu homogenisasi rata-rata dari urat kuarsa-karbonat menunjukkan suhu 324.89 °C dengan salinitas rata-rata 10.67 wt% NaCl equiv. Dari plot homogenisasi melawan salinitas dapat diintrepetasi bahawa mineralisasi di Sungai Loning merupakan endapan emas jenis orogenik.

Luk Ulo Complex, Karangsambung, Central Java was one of an accreted terrain in Indonesia formed during Cretaceous orogeny tectonic event leads to expose a various type of metamorphic rock across Luk Ulo Complex region. Together with the event was the enrichment of gold related to mountain-building process profoundly refers to orogenic gold clan. This is supported by previous study was carried out by several researchers on placer gold occurrence within Luk Ulo region suggest the gold origin point toward orogenic gold type. Meanwhile, new founding of placer gold in Loning River, Luk Ulo Complex remains obscured. In order to answer this deficiency, a research was done by utilized several analytical method such as petrography, ore microscopy, X-Ray Diffraction(XRD),Scanning Electron Microscope - Energy Dispersive Spectroscopy (SEM-EDS),Atomic Absorption Spectrometry (AAS) and Microthermometry. From the result, the distribution of metamorphic host rock in Loning River area was composed of garnet-muscovite schist (green schist facies), amphibolite, and also zeolite-metabreccia. Together with the exposed of garnet-muscovite schist bedrock, was network of localized massive quartz-carbonate vein intrusion. There are two type of hydrothermal vein recognized in the Loning River area which namely type 1 and type 2 veins. Both characterized by the cross-cutting relationship and mineral composition. Type 1 which believed the early generations of quartz veins were strongly controlled by the foliation and deformation structure and normally to be found as segmented, sigmoidal, discontinuous and parallel to the foliation of the host rock. The mineral composition of type 1 vein was mainly consisting of quartz + carbonate + albite+chlorite ± epidote. Whilst, type 2 vein having cross cutting relationship with the metamorphic host rock. The composition of type 2 vein was consisting of quartz + carbonate + albite + chlorite + sulfide ± graphite. The sulfide mineral found inside type 2 veins, contain abundant of pyrite with minor amount of chalcopyrite, native gold, sphalerite and galena. The systematic geochemical sampling conducted using rock and heavy mineral-panned concentrate of geochemical samples. From the atomic absorption spectrometry (AAS) result, geological anomalies pointing toward the occurrence of gold mineralization in Loning River area was overwhelming. Based on the analysis, it is known that the concentration of element in each samples ranging between 2-10 ppm of Au, 2-20 ppm of Ag, 8-105 ppm Cu, 12-1000 ppm Zn, and 0.9-130 ppm Pb. As for the fluid inclusion signature, three samples KSV2(barren), mineralized vein (KSV4 and KSV6) characterized between 192.95 -388.96 °C homogenization temperature with salinity range between 7.90 -14.62 wt% NaCl. As for average homogenization temperature of fluid inclusion in Loning River hydrothermal vein fall under 324.89 °C with average salinity of 10.67 wt% NaCl. From the homogenization and salinitydata later it was known the mineralization on Loning River track back into orogenic kind of gold deposit.

Kata Kunci : Loning River, orogenic gold, green-schist facies, quartz-carbonate vein