Sabalong Project Geology
The island of Sumbawa is located in the central part of the tectonically active, east-west trending Sunda-Banda magmatic arc that marks the convergence of three major tectonic plates. The geology of Sumbawa is characterized by an island-arc type volcano sedimentary succession of Late Oligocene to Quaternary. The southern parts of Sumbawa Island are overlain by Late Oligocene to Middle Miocene, low-K calc-alkaline to weakly alkaline andesitic volcanic and interbedded volcaniclastic rocks, associated low-K intermediate intrusions and shallow marine sedimentary rocks and limestones (Garwin, 2002) on Tertiary volcano-plutonic complexes.
Overlying dacitic rocks are locally mapped in the region, which mainly occupy higher topography. The northern part of the island consists of the youngest rocks in the region, which are products of Recent and Quaternary volcanism and are generally located in the north of the islands.
Extensive hydrothermal alteration of intermediate pyroclastics and intrusive rocks, hosting high-sulfidation epithermal quartz veins has been previously identified at Sabalong. Rock chip assays made during previous exploration activities have reported a maximum tenor of 0.77 g/t Au and 260 g/t Ag. There appears to be potential for carbonate-replacement and base metal mineralization further to the east.
An area of 3 km by 2 km of phyllic alteration assemblages, hosting zones of residual silica and enargite-bearing quartz veins typical of high-sulfidation epithermal systems, has also been previously identified.
Tertiary rocks are also evident in the north of Sumbawa, where they are thought to form the basement rocks to more recent volcanism.
Strong conjugate systems of northwest (NW) and northeast (NE) faults are the dominant structural feature at both district and regional scales. On the basis of structural features in the Batu Hijau district and other localities, NW trending structures are more readily observed than NE fabrics. Northeast trending lineaments are evident from airphoto analysis and satellite-airborne image interpretation, but observed only as minor and discontinuous structures in the field. The structures are interpreted as results of north-south directed subduction related compression and subsequent relaxation along the volcano-plutonic arc, where plate convergence is nearly orthogonal to the arc (Garwin, 2002).
