Saturday, November 28, 2015

Geology Of Buru Island Indonesia, Related Metamorphic Gold Mineralization

Buru is the third largest island after Seram and Halmahera within Maluku Islands of Indonesia. The island belongs to Maluku Province and includes the Buru and South Buru Regencies. Buru is shaped as an oval elongated form from west to east. The maximum length is about 130 km from east to west and 90 km from north to south. The highest point on the island (2,700 m) is the peak of Mount Kapalatmada. The relief is mostly mountainous, especially in central and western parts. With the length of about 80 km, Apo is the largest river of Buru. It flows nearly straight to the north-east and empties into the Kayeli Bay. Buru Island constitutes one of the islands in the Banda Islands, Central Maluku, Indonesia. Geologically, it is part of the outer Banda Arc of non-volcanics (Guntoro, 2000).

Buru Island provides a key example of the processes involved in mountain building and continental collision. So far, it is generally accepted that Buru Island is a microcontinent derived from Australian continent that had been detached during the Mesozoic. The emplacement of Buru Island to the present position is still subject to debate. Figure 1 shows that presently Buru Island is tectonically situated at the fore arc of western-eastern trending Sunda-Banda magmatic arc, which is terminated in the east at the Banda Islands (Carlile and Mitchell, 1994).

Regional geological map of Indonesia
Figure 1. Regional geological map of Indonesia. Some major gold-copper mineralizations are indicated.
Major Tertiary magmatic-arcs are also shown (Carlile and Mitchell, 1994). Buru Island is part of -
outer Banda arc (nonvolcanic) situated at the fore arc of the western-eastern trending Tertiary-Quaternary
Sunda-Banda magmatic arc.

The description of geological framework of Buru Island is based on Geological Map of Buru Island sheet, Moluccas (Tjokrosapoetro et al.,  1993).  Stratigraphically, the  lithologies of the Buru Island from the oldest to the youngest are successively occupied by Wahlua Complex (Pzw), Rana Complex (Pzr), Ghegan Formation (Tg), Dalam Formation (Td), Tm (Mefa Formation), Kuma Formation (MTk), Wakatin Formation (Tmw), Hotong Formation (Tmh), Leko Formation (Tpl), and Qa (Alluvium).

The Wahlua Complex (Pzw) mostly comprises moderate grade metamorphic rocks ranging from green schist to lower amphibolites, phyllite, slate, meta-arkosic sandstone, quartzite, and marble. The complex is widely distributed in the eastern part of the Buru Island (Figure 2). The Rana Complex (Pzr) occupies the central part of the island around the Rana Lake. This rock complex is composed of phyllite, slate, meta-arkose, meta-greywacke, and marble. The Ghegan Formation (Tg), Dalam Formation (Td), Kuma Formation (MTk), Wakatin Formation (Tmw), Hotong Formation (Tmh), and Leko Formation (Tpl) are mostly characterized by carbonaceous clastic sediments and widely extended in the western part of the Buru Island. The Mefa Formation (Pm) is typified by basaltic lava and tuff and the presence of pillow structure and diabase intrusion in the easternmost of the island.  Quaternary sediments are  represented by lake deposit in Rana (Qd), reef limestone (Qt), and Quartenary alluvial deposit (Qa). Qa is characterized by fragments, gravel, sand, silt, and mud, which are distributed within the valley of rivers and along the stream. Studied area is situated in the Wahlua metamorphic complex (Figure 2),which is of Upper Carboniferous until Lower Permian in age.

Geological map of Buru Island
Figure 2. Geological map of Buru Island (modified from Tjokrosapoetro et al., 1993).
Gunung Botak and Gogorea are occupied by Pzw (Wahlua metamorphic rock complex).

Ore Deposit Geology of Buru Island

Lithologically, primary gold mineralization in Buru Island is hosted by metamorphic rocks (in mica schist) of Carboniferrous to Permian Wahlua Metamorphic Complex (Pzw). The same case has been recognized in Bombana gold deposit which is also hosted by Carboniferrous-Permian Pompangeo  Metamorphic  Complex  (PMC)  (Idrus  and  Prihatmoko, 2011). PMC is typically characterized by mica schist. Hence, it is important to note that the rock characteristics and the ages of both Wahlua Complex and PMC are exactly the same. Petrographic study exhibits that mica schist in Buru Island is composed  of  muscovite,  chlorite,  and  sericite suggesting a green schist facies (Yardley, 1989). Gold mineralization in Buru occurs in the form of quartz veins/veinlets/reef. In general, there are two types/generations of quartz veins namely (1) Quartz veins which are segmented, sigmoidal, discontinous, and parallel to the foliation of the metamorphic rocks. Thevein distribution and pattern is intimately controlled by foliation orientation in the area. Mineralogically, the quartz vein is lack of sulfides, weak mineralized, crystalline, relatively clear, and maybe poor in gold (Figure 3a); (2) Quartz veins occurs within a ‘mineralized zone’ of about 100 m in width and ~1,000 m in length (Figure 3b). Gold mineralization is strongly overprinted with argillic alteration zone (Figure 3b). Although it is still lack of field data, the mineralization-alteration zone is probably parallel to the mica schist foliation.According to field data and Buru geological map (cf. Tjokrosapoetro et al., 1993), it is interpreted that gold mineralization may be strongly controlled by N-S or NE-SW-trending geological structures (strikeslip faults?) (Figure 2). Artisanal and small scale gold  mining  (ASGM)  activities are  currently concentrated along the structural controlled mineralization zone.

Quartz veins in Buru Island
Figure 3. Photographs of quartz veins. (a) The first quartz vein type with typical sigmoidal
structure, and (b) Second quartz vein in Gunung Botak associated with mineralization argillic
alteration zone (~100 m width and ~1,000 m length). Artisanal and small scale mining (ASGM)
activities are concentrated along the mineralization zone (second quartz vein type).

Ore Textures of Buru Island

Field and handspecimen observations indicate that gold-bearing quartz veins are characterized by vuggy, banded texture particularly colloform following host rock foliation, and sulphide banding (Figures 4a and b) and brecciated texture (Figure 4c). Bladed-liketexture is also observed, but it is rare (Figure 4d). Those textures more likely developed in classic LS epithermal vein deposits. However, a few anomalies from shallow gold systems in the Yilgarn Block of Western Australia are notable. Comb, cockade, crustiform, and colloform textures at the Racetrack deposit, Australia, deposited from CO2 poor fluids in lower greenschist facies rocks are also recognized (Gebre-Mariam et al., 1993). Similar textures at the Wiluna gold deposits in subgreenschist facies rocks, as well As 18 O quartz measurements as light as 6 - 7 per ml, provide some of the strongest evidence of meteoric water involvement in some of the ‘mesothermal’ hydrothermal  systems (Hagemann et  al., 1992, 1994). Although it is uncommon, pseudomorph bladed  carbonate  texture  could be present in orogenic  quartz  veins/reefs if  the  hydrothermal fluids forming the ore deposit have the right phase separation  condition  (personal  communication, Richard J. Goldfarb, 2011).

Gold-bearing quartz veins in Buru Island
Figure 4. Photographs of gold-bearing quartz veins. (a) Handspecimen of the second
quartz vein type with banding (colloform texture quartz vein following foliation), graphite,
and sulphide banding, (b) The microphotograph of graphite banding (dark) and sulphide
banding (light) identified as arsenopyrite with white-grey colour and strong anisotropy,
(c) Outcrop of brecciated quartz vein and silicified mica schist in Gunung Botak, and
(d) Handspecimen of highly oxidized/limonitic quartz vein with bladed-like texture. indicating
a boiling condition.

Alteration and Ore Mineralogy of Buru Island

Hydrothermal alteration style is identified according to the field observation and petrographic analysis. As outlined above, the gold mineralization zone is intimately associated with argillicaltered mica schist delineating an obvious high Au grade zone of about 100 m width and 1,000 m length. Clay mineral types characterizing argillic alteration zone are unknown. The petrographic analysis shows host rock is also propyllitically altered typified by the presence of chlorite, calcite, and sericite.  Carbonation  alteration  style represented by graphite banding (Figure 4a) and carbon flakes (Figure 5a,b) is a typical alteration type occurring in metamorphic-related hydrothermal ore deposits. The field observation and ore microscopic analysis indicate that the ore mineralization is characterized by pyrite, native gold (Figure 6a &  b), pyrrhotite, and  arsenopyrite (Figure 6c). However, cinnabar, stibnite, chalcopyrite, galena, and sphalerite are rare or maybe absent. In general, sulphide minerals are rare (<3%). This is consistent with mineralogical features of other metamorphic rock-hosted gold mineralizations worldwide (cf. Groves et al., 1998, 2003).

Carbonation altered rock in Buru Island
Figure 5. The carbonation-altered rock/quartz vein microphotographs: (a) Quartz (light)
and graphite banding (brown) in parallel nicols, (b) Quartz (light) and graphite (dark) in

Ore mineralization in Buru Island
Figure 6. Microphotographs of ore mineralization. (a) Microphotograph of native gold grain in
quartz vein, (b) native gold (nugget gold) panned from thumb-size quartz vein, and (c) closeup
of arsenopyrite banding in Figure 4a & 4b with a whitegrey colour and strong anisotropy.

References : Indonesian Journal on Geoscience by Arifudin Idrus, dkk.

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