STRUCTURAL CONTROL RELATED WITH MEDIUM-TO-VERY HIGH Au GRADE AT PIT B EAST AND B WEST, TUJUH BUKIT MINE, EAST JAVA

Abstract

Tujuh Bukit generally consists of Late Oligocene – Middle Miocene volcanic and volcaniclastic Batuampar Formation. After tectonomagmatism took place at Pliocene, this stratigraphic unit altered and became a host rock for Pliocene economic mineralization. The research area located in open pit mining at Pit B East and B West. Both pits have an extensive area of ± 700 x 500 m², projected on ± 9045100 – 9045600 mN and ± 174400 – 175100 mE UTM WGS 1984 zone 50S coordinate system. This research discusses structural control as the main role of alteration characteristics and Au high sulfidation epithermal mineralization system forming in open pit mining at Pit B East and B West, Tujuh Bukit mine. Main developed structural are Pliocene strike-slip system, relatively NW-SE and N-S orientation, with NNW-SSE principal stress following pure shear model. There is also relatively NW-SE normal fault and ENE-WSW thrust fault. All of these faults in the regional scale are NNW-SSE compressive tectonic compensation within one tectonic period. The strike-slip system resulting in normal fault and ridge zones within releasing stepover, releasing bend, and restraining bend structural geometry. Strike-slip faults in Pit B East and B West are the smallest strike-slip system within Tujuh Bukit regional scale. Developed structural geology representing recent tectonic of Java. Based on the field observation, core drilling, ASD instrument scanning, and petrographic analysis, the alteration unit divided into five, there are quartz, quartz-alunite, quartz-kaolinite, kaolinite-montmorillonite-chlorite, and kaolinite-montmorillonite alteration unit. Based on field and mineragraphic observation, there found ore minerals such as pyrite, chalcopyrite, covellite, bornite, tetrahedrite, azurite, malachite, hematite, and goethite. All these minerals generally found associated with silicification alteration, such as quartz, quartz-alunite, and quartz-kaolinite alteration unit. Development of medium to very high-grade mineralization within silicification alteration unit generally took place in normal fault zones caused by the strike-slip system on the releasing bend (can be found on the bending line of BE 2 and BE 3 Fault) and releasing stepover geometry (on the area between BE 1-BE 2 Fault, the area between BW 2-BW 3 Fault, and the area between BW 5-BW 6-BW 7 Fault), and BW 4 normal fault which parallel to the principal stress on a regional scale.