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

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Mewet Vein : Jelai Gold Project
High gold values were obtained from finely banded, colloform-crustiform, quartz-adularia veins.
The highest gold values occur in zones of multiphase brecciation of the colloform banded quartz
which are healed by chalcedonic quartz and adularia. No gold mineralisation occurs in the
footwall to the vein but low grade gold values occur within the breccia on the hanging wall to the
quartz vein. Silver grades are sporadic with the most elevated values occurring with the high
grade gold in finely banded, sulphidic, quartz veins (Photograph 1).
Drilling to date has only been carried out over a 325 metre strike length of the Mewet Vein. Most
of the drill holes targeted at the vein intersected mineralised sections of vein and vein breccia
material with variable mineralisation reported from the analysis of the core samples.
7.2.3
Alteration
Pervasive propylitic alteration represents the predominant alteration throughout the andesitic
volcanic sequence. The fine grained mafics always show a certain degree of chloritic alteration.
Haematite is often present in the fine grained matrix of volcanic flow breccias, fracture coatings
and in vesicles as coatings to chlorite and calcite. Up to 1% fine disseminated pyrite occurs with
the chloritic alteration. Sporadic epidote occurs in zones of intense propylitic alteration and
preferentially replaces feldspar phenocrysts and also occurs rarely as fracture coatings. In
zones of intense propylitic alteration epidote occurs in vesicles with calcite and chlorite. Epidote
alteration was not observed in the vicinity of the Mewet vein.
Sericite alteration occurs as an halo to the hydraulic brecciation and to the epithermal
mineralisation. The intensity of sericite is a function of the proximity to the mineralisation.
Sericite is in all cases observed overprinting proplytic alteration. Sericite alteration is not
specifically restricted to any one single phase of quartz veining/silicification. The intensity of the
sericite – silica alteration is also a function of the intensity of brecciation. The strongly brecciated
hanging wall acted as a more permeable medium for the alteration fluids than did the relatively
unfractured footwall; thus sericite alteration is moderate to strong in the hanging wall while it
quickly dissipates to strong chloritic alteration in the footwall.
An advanced argillic zone is not present at Mewet. This is due to the system being eroded down
to the top of the precious metal zone, at the boiling zone. Adularia is found as fine
disseminations within the colloform-crustiform banded quartz veins in this zone.
7.3
Interpreted Genesis of the Epithermal Veins
Multiple phases of brecciation and vein formation are evident at the Mewet Vein. Similar
systems in other parts of Indonesia, and indeed the world, follow similar patterns of
development and, although no specific studies have been undertaken to collaborate the
observations, the following section describes the development of a typical low-sulphidation
quartz vein, within the context of the Mewet Vein.
North-south structures, originating from the major Jelai Fault System to the south, provided a
conduit for ascending near-neutral chloride fluids which led to rapid silicification. The silica
solution, with the silica originating from the leaching of the wall rocks at high temperatures,
remained saturated and was deposited slowly, resulting in the formation of
mesothermal/crystalline quartz observed in the early phase of silicification in the Mewet Vein.
Sealing of the system with silica resulted in an over-pressuring effect, which was relieved
through explosive brecciation of the system. Brecciation of the earlier phase of quartz veining
resulted with sub-rounding of some clasts indicating that there has been some transportation
from depth. No clasts of the underlying sedimentary basement were encountered in the drilling.
Mixing of CO 2, with meteoric water, lead to the deposition of rhombic and bladed carbonate as
breccia matrix flood and cavity infill. Subsequent mixing of the supersaturated acidic fluids with
meteoric water resulted in the rapid deposition of fine grained microcrystalline to chalcedonic
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