District-scale alteration

The distribution of alteration mineral zones in part of the Martabe district is shown in Figures 6.23 and 6.24. Some of the alunite-rich zones follow northwest-southeast

structures. Siliceous alteration passes outwards into advanced argillic alteration which passes outwards to argillic and propylitic alteration. At Martabe, hydrothermal alteration, especially advanced argillic and siliceous alteration, is generally located in areas recognized as steep and/or distinctive topographic highs. The zones are mostly situated in hilly terrain at the east side of the Purnama fault. Whereas the moderate to flat areas are dominated by widespread propylitic and argillic zones, as notably the abrupt low elevation on the west side (foot wall) of the Purnama Fault (Figures 6.2 and 6.3). The Purnama deposit is associated with the NW trend of the Purnama fault which controls weathering-resistant silicified ridge of the hypogene breccias and porphyritic andesite of Toru Volcanic Formation. As the Baskara and Kejora deposits are situated within the centre of a dome, their topographic high is mainly controlled by siliceous alteration. Minor intense kaolinite alteration and quartz veining zones are recognized at the small ridge in the Parbotikan area associated with a NW trending fault at the west (Figure. 6.23). To the southeast, sub- sequent faulting and ore mineralization associated with quartz veins along these structures occurred during a possible younger, low-sulfidation hydrothermal episode in Pelangi. Vuggy quartz is best developed at Baskara and Gerhana. At Purnama, siliceous alteration forms extensive sub-horizontally zones of massive microcrystalline quartz with minor vuggy quartz texture, associated with dickite, pyrite, enargite/luzonite and minor tennantite-tetrahedrite. The contact between advanced-argillic and argillic/propylitic altered rocks on the west-slope at Purnama follows the structure and lithological sharp contact between breccias and lavas of the Toru Volcanic Formation. Figure 6.24 shows the distribution of hydrothermal alteration zone in the Martabe district.

There are some zones of kaolinite+alunite that occur in weakly altered Angkola Volcanic andesites down a ridge to the west at Parbotikan, indicating that the alteration probably extended down this ridge. Compared to the Purnama area, alteration at Baskara is more alunite-rich, with very little dickite. A propylitic-altered intrusion (hornblende-bearing andesite) was mapped at Purnama, but at Baskara and Kejora this intrusion is partly altered to siliceous and advanced argillic alteration. Overall, in the Martabe district propylitic

sulfidation veins occur. Several small ‘alunitic’ and ‘kaolinitic’ zones are also mapped in the Golf Mike area, approximately 4 km south of the Martabe district (Figures 6.23 and 6.24). Breccia-hosted siliceous and advanced argillic outcops were also observed at about 800 meters to the south-southeast of Pelangi. This area is also adjacent to the north-northwest structure which controlled the formation of the Pelangi deposit.

At Purnama, the vuggy quartz zone has mainly developed from polymict- monomict breccia and porphyritic andesite of the Toru Volcanic Formation; while at Baskara and Kejora, the vuggy altered zone is predominantly hosted in a hornblende- phyric andesite to dacite dome. At Baskara and Kejora, the vuggy quartz zone occurs persistently at the elevations of 425 m and 575 m above sea level. Large vuggy textures are found in Pelangi and Gerhana, showing vugs up to 30 cm in diameter which is due to solution of large breccia clasts. The existence of a vertical feeder, probably associated with a remnant fault, is indicated by Induced Polarization/resistivity anomaly. This distribution suggests that hydrothermal fluid flow was channeled preferentially along faults and lithological contacts (Figure 6.24). Chalcedony veins are ubiquitous at Purnama. In contrast, quartz veins of low-sulfidation epithermal character are also clearly exposed on the ridges at Pelangi deposit 2 km to SE of Purnama. At Purnama, the advanced argillic alteration is identified by quartz+dickite± kaolinite±alunite which are mostly hosted in breccias and andesite volcanic rocks. At Baskara, Kejora and Gerhana, quartz+alunite is the characteristic of the advanced argillic alteration which occurs in the dacitic-andesitic dome and breccias. Kaolinite is observed to replace feldspar in both volcanic and volcaniclastics rocks as well as in breccias fragments. In sedimentary rocks, kaolinite and dickite infill cavities and fractures and coexists with quartz veinlets. At Baskara the

advanced argillic alteration is restricted to a narrow, <20 m wide zone on the margin of the ore deposit. The argillic mineral assemblage passes outward into the propylitic altered rocks at Baskara and Kejora, which are mainly hosted by dacitic-andesitic intrusions. The western limit of mineralization at Purnama is controlled by structure with patchy to intensive argillic alteration (Figure 6.24).

The three cross sections (Figures 6.25 to 6.27) demonstrate the spatial relationship of the alteration zones in Purnama, Baskara and Gerhana, respectively. Figure 6.25 is the east-west Purnama cross-section compiled using all available drill hole data within 25 m of the line of intersection. This cross-section illustrates the dominant sub-horizontal and minor sub-vertical nature of the hypogene alteration and the spatial relationships of the propylitic alteration in the western part of the Purnama deposits. Figure 6.26 is the east- west cross-section of the Gerhana deposit. Figure 6.27 is the oblique northwest-southeast cross-section of Baskara. These cross-sections illustrate the dominant vertical nature of the

hypogene alteration which is structurally controlled. Figure 6.28 is a conceptual long section showing district wide alteration.

Figure 6.23. District alteration map showing the location of the deposits overlain on the digital orthophoto image of the Martabe district. The lines and transparent polygons are the structure (white lines) and alteration zones, respectively. Siliceous zones are shown in red, alunitic zones in orange, kaolinitic zones in yellow, argillic zones in light blue and propylitic zones in green.

Figure 6.24. Alteration zones and main structures of the Martabe district. Structural control on the distribution of the ‘alunitic’ and siliceous alteration zone is illustrated.

Fig. 6.25.East-west cross-section of the Purnama deposit (Section 167200 mN) showing (a) resistivity and (b) the distribution of alteration zones. Sections were prepared from logging of diamond drill-core holes, petrographic investigation, PIMA and XRD data.

Figure 6.26. East-west cross-section of the Gerhana deposit (Section 171200 mN) showing comparison between (a) geology, (b) distribution of alteration zones and (c) resistivity data. Sections were prepared from logging of diamond drill-core holes, petrographic investigations, PIMA and XRD data. Induced polarization geo-modeled section illustrates the thickness of the siliceous alteration and small root zones.

Figure 6.27. Northwest-southeast cross-section of the Baskara deposit (section OB-096) showing comparison between (A) geology, (B) distribution of alteration zones and (C) gold, Cu and Ag grades and resistivity data. Induced polarization geo-modelled section illustrates the thickness of the siliceous alteration and small root zones. Sections were prepared from logging of diamond drill- core holes, petrographic onvestigation, PIMA and XRD data.

Figure 6.28. Schematic north-south (or longitudinal) section through the central part of the Martabe district showing spatial distribution of the alteration zones interpreted from diamond drillcore, petrographic investigation, PIMA and XRD data.