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The Angkola Volcanic Formation (Tmav) is widespread and divided into four facies (Figure 3.3).

3.5.1.1 Facies Tmav 1 – Coherent basalt

The coherent basaltic facies is observed in the Aek Pahu river, at the Pelangi deposit to the southeast and at the Parbotikan prospect to the west. Generally these units could be mapped along the strike for tens of meters but their overall extent was difficult to determine due to vegetation and erosion. Several flows were mapped within the volcanic breccia (Tmab) sequence at Parbotikan. Exposures at the Parbotikan prospect and in the Aek Pahu river show that flow thickness can be as great as 5–10 m. The coherent basaltic lava facies is aphanitic, black to dark greenish, unaltered to weakly altered with a weak magnetic response (Figure 3.17 and 3.18). It consists of fine laths of plagioclase with lesser amount of olivine, pyroxene and relict magnetite (?) set in a glass groundmass (Figure 3.18). Phenocryst abundances range from 2 to 5%. Sub- to euhedral tabular plagioclase exhibits multiple twinning and oscillatory zoning that ranges from 1 to 3 mm. The fine grained groundmass consists of mafic minerals and feldspar. Weak fracture-controlled alteration consisting of chlorite, clays and calcite is observed.

Figure 3.17. (A) Aphanitic, black unaltered basaltic lava (APSD001_55.2). (B). Contact between coherent porphyritic trachytic-andesite and chloritic-altered basaltic andesite (APSD001_35.3).

Figure 3.18. (A) Aphanitic, fine grained plagioclase laths and olivine in glass groundmass (APSD001_55.2). (B) Fine laths of plagioclase showing flow texture in basaltic lava. (APSD001_55.2).

Numerous basaltic dykes and minor sills occur in the basaltic lavas at Purnama. They are grey to dark green, massive and typically aphyric. Locally, fine pyroxene and/or olivine phenocrysts are visible. Wall rock contacts are either sharp or irregular and marked by zones of basalt-cemented, wall-rock-clast breccia as seen at Pelangi. Dykes range in thickness from 10 cm to 5 m, but are generally 1 to 3 m thick, and they are predominantly hosted by basaltic flow facies. The flows range from 3 to more than 40 m in thickness. At Purnama, dykes occur in drill cores APSD001, APSD003 and APSD005. A few dykes have also intruded the Sibarus formation at Pelangi. Due to the limited amount of drill core, the dykes can not be correlated.

The coherent basalt facies is commonly associated with coherent basaltic andesite, coherent andesite, volcaniclastic and/or volcanic breccia facies. Textures associations indicate that the coherent basalt facies represents the coherent parts of basalt dikes and flows. The majority of this facies is extrusive flows.

A B A B Trachytic andesite Chlotie altered basalt

3.5.1.2 Facies Tmav2 - Coherent trachytic-basalt

This facies is part of the mafic rocks with trachytic texture. This facies consists of mainly augite, labradorite- bytownite and titanomagnetite+ilmenite. A subparallel

arrangement of microcrystalline lath-shaped feldspars in the groundmass of the holocrystaline or hypocrystalline rock is common (Figure 3.19).

The coherent trachytic-basalt facies is commonly associated with the coherent basaltic facies. It has been identified only in drillcores APSD001, APSD003 and APSD005.

Figure 3.19. (A) Fine laths of aligned plagioclase showing flow texture in trachytic basaltic (APSD003-86.9). (B) Fine- to medium olivine phenocrysts in groundmass (APSD089_9).

3.5.1.3 Facies Tmav3 - Coherent andesitic basalt

The coherent andesitic basaltic facies contains fine grained phenocrysts of dull white feldspar in a dark brown to grey, bleached, weakly to moderately altered, aphanitic matrix (Figure 3.20). Phenocryst abundances range from, 2 to 15% (average of 5%) and are predominantly plagioclase with smaller amount of pyroxene, amphibole and magnetite (Figure 3.21). Sub- to euhedral tabular plagioclase phenocrysts, that can exhibit flow alignment, range in size from 1 to 5 mm. The fine-grained groundmass consists of micro- granular feldspar and glass. This facies is variably altered to chlorite, calcite and illite.

This facies is commonly associated with other coherent basalt, tuff, non-stratified monomict andesite breccia facies and non-stratified sediment matrix andesite monomict breccia facies. Textures and facies association indicate that the coherent andesitic basalt facies represents extrusive lava flows.

The coherent andesitic basalt facies is observed throughout the Purnama and Pelangi deposits. It is dominant facies in the Purnama footwall, but is rare in the Baskara. Lithologic contacts between coherent flows are not always clearly defined. Single units range in thickness from 50 cm in Purnama to 20 m at Pelangi footwall. Varying degrees of

alteration along contacts and variations in phenocryst size between layers are the discriminator of unit boundaries.

Figure 3.20. (A) Coherent andesitic basalt altered to chlorite (APSD003_15). (B) Weakly to unaltered coherent andesitic basalt with abundant hornblende and plagioclase phenocrysts (APSD005_24).

Figure 3.21. (A) Coherent andesitic basalt with pyroxene and plagioclase phenocrysts set in a fine grained groundmass. The rocks are weakly altered to chlorite and calcite (APSD090_98-2). (B). Euhedral plagioclase and pyroxene in a fine grained groundmass. Plagioclase, pla and pyroxene, px (APSD090_98-2). A B A 1 cm 1 cm pla pla px px B

3.5.1.4 Facies Tmab – Basaltic-andesite volcanic breccia

This volcanic breccia is a coarse, heterolithic, fragmental rock composed primarily of sub-rounded, volcanic clasts but locally contains sandstone and siltstone clasts. It is unstratified, greenish to reddish to grey, often bleached, clast- to matrix-supported and poorly sorted.

The clasts consist of coherent porphyritic, coherent andesite facies (Tmav 3) and trachytic basaltic facies (Tmav2) as polyhedral blocks, ranging in size from 1 to 10 cm, with curvi-planar and angular surface set in fine-grained matrix (Figure 3.22). The primary minerals of the fresh volcanic rock are quartz, andesine, hornblende, pyroxene, biotite, titanomagnetite and magnetite. Basaltic (Tmav2) clasts, consisting of plagioclase, augite and a small proportion of magnetite, show a range in sizes of plagioclase and augite crystals (<0.01-0.5 mm). Phenocryst abundances range from 2 to 10% and are predominantly remnants of plagioclase with lesser amounts of pyroxene and amphibole. Phenocrysts range from 1 to 3 mm in size. The fine grained groundmass consists of micro-granular feldspar (Figure 3.23). The matrix is fine grained, hematitic mudstone that is composed of micro-granular feldspar.

An intense quartz alunite alteration commonly occurrs at the contact between the Tmab and the porphyritic andesite (Tmtv) or diatreme breccias. Some basaltic andesite clast are strongly bleached, oxidized and have illite-altered rims (Figure 3.22). Alteration intensity may vary between clast and matrix. These are generally weakly- to strongly-altered to dickite and kaolinite at Purnama, while clasts in this breccia west of the Purnama fault are generally unaltered to weakly altered. This volcanic breccia is an upper part of the sequence of the Angkola volcanic unit.

This monomict andesite breccia facies occurs above the coherent andesite (Tmav 3) flows at the Purnama, but rarely at Pelangi. Individual beds range from 1 to 5 m in thickness. This facies is gradational into the coherent basaltic andesite (Tmav 1) facies and is commonly associated with other flow, coherent porphyritic andesites (Tmtv), coherent trachytic basalt andesite (Tmav 2) and coherent basalt (Tmav 1).

The clast shapes, monomict composition and association with coherent andesite facies of the same composition suggest that the monomict breccia facies represents the clastic portion of andesite lavas. These clasts likely formed from brittle fragmentation (autobrecciation; Pichles, 1965) of solidified magma, due to tensile stresses exerted by adjacent or underlying ductile flowing magma.

The Tmab volcanic breccias were mapped at southeastern of Pelangi close to other breccias body (Pelangi breccias. The volcaniclastic (Tmab – ‘Tertiary Miocene Angkola breccia’) is observed on the Purnama cliff and at northern part of Pelangi. These coherent volcanic are generally unaltered to weakly altered and massive unit. On the contrary, the volcanic breccia is mostly altered to chlorite, kaolinite and dickite, in particular along the contact with upper unit.

In some intervals in the Purnama drill core, this volcanic breccia shows vuggy and massive quartz alteration. The coherent basaltic lava is likely the lower most facies of the Tmav with grading into trachytic-textured volcanic and andesitic lava composition. A volcanic breccia appears to be an upper part of the sequence. The volcanic breccia is mostly overlain by the andesite Toru volcanic unit (Tmtv). However, little is known about exploration potential of this facies to the west of Purnama Fault due to the lack of

significant alteration and also because of the mostly gentle relief and infrequent exposure.

Figure 3.22. Comparison between weakly altered (bottom) and oxidized-kaolinite altered volcaniclastic (top). The volcanic breccias are coarse, heterolithic fragmental rocks composed primarily of sub-round volcanic clasts, but locally contains sandstone and siltstone fragments. The clasts commonly show hyaloclastic texture.

Figure 3.23. Clasts of trachytic basaltic set in fine grained plagioclase groundmass (A) in plain light. (B) in cross-nichols (APSD003_86.9).

3.5.1.5 Miocene Angkola Volcanic - origin and depositional environment

A K/Ar age of 18.91 ± 0.07 Ma (Turner, 2004) was obtained from basaltic andesite samples in drill hole APSD001_23. Other radiometric ages for the Angkola Volcanic unit are as young as 17 Ma (Barber, 2005). In the Martabe district, basalt lavas overlie the Sibarus formation units, therefore the extrusive basaltic units in the Martabe district are interpreted to have been deposited in a proximal volcanic setting during the early Miocene. Basalt dykes are inferred to be the sub-volcanic feeders to the extrusive units.