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EAST ELANG 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 (Hamilton 1979). The geology of Sumbawa is characterized by an island-arc type volcano sedimentary succession of Late Oligocene to Quaternary age.

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). Overlying dacitic rocks are locally mapped in the region, which mainly occupy higher topography. The youngest rocks in the region are products of Recent and Quaternary volcanism and are generally located in the north of the islands and have not been a focus of exploration because of the limited exposure of the hydrothermal systems by erosion.

A series of eroded overlapping volcanic centres with associated fringing sediments make up the Tertiary arc, and are represented by a thick mass of andesitic pyroclastics, flows, intermediate intrusives, shallow marine sediments and limestones. The sequence is exposed in the southern two-thirds of western Sumbawa with elevations up to 1,500 metres. Tertiary rocks are also evident in the north of Sumbawa, where they are thought to form the basement rocks to more recent volcanism.

Intrusive rocks are commonly exposed in an east-west belt in southern Sumbawa. Older intrusives are commonly diorites and microdiorites and occur as dykes and stocks intruding the volcanics and sediments. Later tonalite intrusions are spatially and temporally related to the main porphyry Cu-Au mineralization, whereas older dioritic intrusions and stocks are related to earlier similar mineralization types but commonly lower grades (0.2 to 0.4 % Cu). Diatreme dome breccia complexes have been mapped peripheral to dacite porphyry intrusives. Dacitic pyroclastics unconformably overly the andesitic sequence. A thick blanket of volcanoclastic rocks (debris flow sequence) is exposed along the southern coast of Sumbawa.

Strong conjugate systems of NW and NE faults are the dominant structural features 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. NE trending lineaments are evident from airphotograph analyses 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).

Elang East Property Geology

The majority of the East Elang property area is underlain by an andesitic volcanic unit comprising tuff, lapilli tuff, breccia and lava, with localized intercalations of calcareous sediments and limestone. Dacitic volcanics, which are restricted to high topography, are inferred to unconformably overly the andesitic volcanic unit. Maryono (2005) infers, based on mapped porphyry-type stockwork mineralization at the Newmont Elang-Dodo gold-copper porphyry discovery, that the dacitic volcanic unit predates the main Cu-Au porphyry mineralization event.

Mineralization at Elong-Dodo comprises a complex of polyphased felsic to intermediate intrusives that generated multiple mineralized porphyry centres (Eland, Gergang, Kuning and Sepekat) along a NNE trending structural corridor. Rock geochemistry of surface and drill hole samples demonstrate a NE-SW elongate copper zone more than 1.5 km long by 0.8 km wide.

The andesitic to dacitic volcanics include crystal tuffs, tuff breccias, with intercalated volcaniclastics and andesite aphanitic to porphyritic textured flows and dykes. Limited exposures of andesite porphyry intrusive have been noted. Propylitic alteration is ubiquitous throughout the area, with argillic assemblages limited to fault/shear zones. Minor quantities of vuggy crystalline quartz and jasperite have been noted as a component of the stream bedload.

Polyphasal intrusive events of diorite to dacite composition are observed as small dykes up to large stock bodies. Intrusive rocks observed include porphyritic andesite, hornblende diorite, quartz diorite, feldspar porphyry, tonalite and intrusive diatreme breccia in the appropriate chronological order (Clode, 2002).

A thick veneer of Quaternary epiclastic breccias blanket the southern part of the property. The topography is extremely steep. In the northeast corner of the property Quaternary alluvium deposits are prolific along fault valleys and catchment basins.

Structural studies carried out by Newmont of mineralized districts suggest that there are three main structural grains and these NW, N-S and NE trends are related to copper and gold mineralization in the region (Clode, 2002). "...The NNE to NE trends (extensional or synthetic faults) form the mineralized corridors hosting porphyry Cu-Au mineralization, whilst later through-going NW trending faults are interpreted as late stage or post-porphyry. Late stage epithermal veins and post mineral dykes are developed along the NW and N-S trending faults..."

Remote sensing studies together with modeling of airborne magnetic data indicate that the controlling NNE trending mineralized structural corridor present on the Newmont property extends onto Southern Arc's property. Limited surface evaluation work and aerial sorties by the company support this finding.

Historical Geochemistry

Earliest reports of precious mineral occurrences in the district began in 1910 when a soldier noted quartz-pyrite veining near Dodo Hill (Brouwer, 1943) on what is the Elang-Dodo property currently held by by Newmont. Van Rheden (1914) visited the area in 1912 and noted base metal mineralization.

NNT, a subsidiary of the Newmont Mining Corporation, undertook regional drainage sampling of the area as part of a larger regional survey during 1986 to 1988. From this work a number of anomalous areas were defined including the Air Panas Anomaly.

Air Panas Anomaly

Air Panas is located immediately to the east of the Brang Dodo River, which drains Newmont's Elang porphyry deposit. Access to the prospect is via a series of footpaths from the road heads at Lawin and Kemilas villages.

This anomaly does not appear to be an extension of the Elang gold-copper porphyry discovery. Initial drainage sampling reported BLEG anomalies of 24.2 ppb Au/13.5 ppb Ag and 6.07 ppb Au/13.5 ppb Ag. No anomalies were reported from -80* fraction or pan concentrates. Upstream of reported anomalies there were occurrences of banded quartz-sulfide veining hosted by argillically altered andesite breccia. Rock chip samples from outcrops returned 0.83 g/t Au/34.0 g/t Ag/0.93 ppm Hg and 3.6 g/t Au/310 g/t Ag/0.75 ppm Hg.

A large alteration system was mapped in the river bed of the Air Panas drainage. Numerous narrow quartz veins were mapped in siliceous and argillized zones. Veins are hosted in a bedded sequence of predominantly polymictic pebble and boulder conglomerates, thin sandstone and siltstone units with minor andesitic lavas and airfall pyroclastics. Argillic alteration is confined to the selvedges of vein and stockwork systems, whereas propylitic alteration is ubiquitous.

Mineralization is controlled by three structural sets. The most characteristic is the east-west set with abundant sooty pyrite, poddy colloform chalcedonic quartz containing cinnabar. A 300 to 330 degree set is the possible conjugate set; often sheeted vein sets or stockworks are noted in the apex of the two vein sets. Less common are N-S structures with similar mineralogy to the other two sets.

Hot spring activity is observed several kilometres to the south, where crystalline calcite is being deposited with algal material.

 

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