Geology and mineralisation of the Mewet Vein, Jelai Gold Project ...

PT GMT Indonesia
Mewet Vein : Jelai Gold Project
palaeogeographic domains. The characteristics of these domains are conditioned by the
relationships between a continent, situated to the west and a fluvial drainage system draining
into an ocean, located to the east (Lefevre et al., 1982). The lithologies observed are flysch type
sequences, with a marine environment found to the north and east. Lefevre et al. (1982)
estimate a combined thickness of 7,000 metres for the various units. For simplicity, the
Mentarang Formation is preferred, with the dacitic tuff/lava component represented by the Geh
sub-Formation.
6.2.2
Late Oligocene-Early Miocene
Several corridors of north-northeast trending granitoid intrusive complexes occur within the
project areas. One belt occurs at Long Laai, the other southwest of Long Bia. The Long Laai
complex has been studied in some detail by the BRGM/DSDM (Le Bel et al., 1985 & 1986) and
found to consist of leuco adamellite, with the development of exo and endo skarn facies, at the
contact of the intrusives with the older sediments.
The complex situated southwest of Long Bia has been termed the Yamuk Intrusive Complex by
Sennitt et al. (1995). It consists of biotite and hornblende granodiorite, granodiorite porphyry,
quartz diorite porphyry, andesite porphyry and a “dry” quartz-feldspar porphyry. On the margins
of the Yamuk Intrusive Complex, the older sediments are highly fractured and deformed, with
hornfels and collapse breccias occasionally developed. Within the intrusive complex, intrusion
breccias, autobreccias and xenolith rich zones are found.
Age dating by the BRGM (Le Bel et al., 1985) obtained a 22.6 Ma age for the Long Laai
leucoadamellite, indicating a Late Oligocene-Early Miocene age. A similar age is preferred for
the Yamuk Intrusive Complex.
6.3
Regional Structure
The regional structure of the project area is dominated by the north-northeast trending Jelai
Fault System, which can be traced for at least 200 kilometres south-southwest from the project
area and defines the main trend of volcanics and intrusives in the area. This fault system is
interpreted to represent an arc-parallel structure, reactivated on numerous occasions and
initially formed during the Cretaceous subduction event below Borneo from the northwest. The
northern extension of the fault system is hidden beneath younger sediments of the Tarakan
Basin.
The Jelai Fault System contains a number of jogs to the south of the project area and the major
rivers in the area, such as the Kayan River, follow the main fault line as a series of kinked
drainages extending to the south from the project area.
Subsidiary faults lie parallel to the main fault system and a number of splays can be see
extending from the fault line on the north-western side, one of which hosts the epithermal vein
system at the Jelai Gold Project. These splays may actually be conjugate fault sets developed
from the main Jelai Fault System.
Other structures noted on a regional basis around the project include west-northwest trending
arc-normal structures, again assumed to have developed during the Cretaceous subduction
event beneath Borneo.
6.4
6.4.1
Description of Lithologies in the Project Area
Andesitic Lava
The predominant host rocks are fine to coarse andesitic lava (Figure 4). The rocks have a
distinctive lava texture and comprise clasts containing crowded fine to coarse grained
0065 JCM/GEO/2009.04 Page 18