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

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Mewet Vein : Jelai Gold Project
quartz. Overprinting acidic/silica enriched fluids partially replaced the carbonate with amorphous
silica, leading to massive chalcedonic silica with patches of bladed calcite pseudomorphs, lattice
infilled textures and crustiform-colloform banded quartz veining.
Photograph 1. Sample of
Mewet Vein material showing
the distinct banded nature of the
higher grade vein material.
Massive carbonate cavity fill in JM018 has been partially
replaced by chalcedonic silica with the zone proximal to the
colloform-crustiform banded quartz + adularia vein being
completely replaced to massive chalcedonic silica. The
elevated pH conditions from the draw down of acid
sulphate rich water lead to the deposition of adularia
disseminations and as banding in the colloform-crustiform
banded quartz. No zone of advanced argillic alteration has
been noted in the Mewet Vein system or surrounds,
indicating that the upper section (mixing zone) has been
eroded down to the relative level of the boiling zone.
Continued boiling of the epithermal system lead to several
telescoping phases of overprinting events, representing
varying degrees of pH and concentrations of silica + CO 2 .
The system represents several phases of sealing and rebrecciation
with several phases of clasts of colloformcrustiform
quartz clasts observed in the chalcedonic
quartz. Colloform banded quartz clasts formed as a
product of an acidic fluid are observed within a matrix of
carbonate. Drusy quartz is in places overprinted with
rhombic calcite with the calcite rarely displaying a late
drusy quartz coating. Clasts of earlier chalcedonic quartz
are often observed within the massive appearing carbonate
veining and the quartz veining is also often represented by
several phases of sealing and cracking. The collapse of the
hydrothermal system led to the draw-down of meteoric
waters leading to the deposition of glassy crystalline quartz
+ calcite. Rarely amethystine quartz veining is noted as a late overprinting event associated with
the late draw down event.
High grade gold values were restricted to the finely laminated chalcedonic quartz-adulariasulphide
veins. Variable high gold values were encountered in brecciated chalcedonic quartz
vein with the grade being dependant on the frequency of chalcedonic-colloform banded quartz
vein clasts within the breccia.
Significant gold mineralisation is hosted in typical high grade, low sulphidation veins. Maximum
gold values were obtained from finely laminated colloform-crustiform banded quartz-adularia
veins with zones of multiphase brecciation of the colloform banded quartz and healing by
chalcedonic quartz returning the highest values. Brecciation of the colloform banded quartz and
subsequent additional deposition of chalcedonic quartz + adularia lead an upgrade of the gold
grade. The mineralisation was tightly structurally controlled with there being no gold
mineralisation disseminated in the footwall to the vein. Low grade gold values were obtained
within the breccia column on the hanging wall to the quartz vein. Silver grades are sporadic with
the most elevated value occurring with the high grade gold in the finely laminated, sulphidic
quartz veins.
Unaltered andesitic basalt dykes were intersected; these were un-mineralised and appear to
predate mineralisation. Weak to moderate propylitic alteration occurs within the andesitic dykes,
predominately as fracture coatings.
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