Ore shoot targeting in the Gosowong Vein Zone, Halmahera, Indonesia
Olberg, Daniel J (2001) Ore shoot targeting in the Gosowong Vein Zone, Halmahera, Indonesia. Research Master thesis, University of Tasmania.
Gosowong is located in the Maluku province of eastern Indonesia, on the north arm of the
island of Halmahera. It is a classic example of a volcanic-hosted, low-sulfidation, epithermal quartz vein deposit. Due to the relatively short mine life, there is a very limited time frame for
increasing ore reserves before mining ceases. Therefore a great emphasis has been placed on
exploring the strike extent of the structure that hosts the Gosowong deposit. This mineralized
structure is known as the Gosowong Vein Zone (GVZ) and has been traced along strike for 2 km,
though the Gosowong deposit encompasses only a 400 m section of the total strike length. The
primary aim of this study is to identifY additional high-grade ore-shoots along the GVZ. To this
end, a multi-faceted approach has been implemented incorporating structure, stratigraphy, vein
textures, alteration zoning, fluid inclusions, and metal zonation, with the ultimate aim being to construct a system model that will allow predictive targeting of high-grade ore-shoots along the
GVZ. Most data are presented on a longitudinal section of the GVZ.
High-grade mineralization at the Gosowong deposit occurs within two gently southplunging
ore shoots: the Quartz-Adularia zone (QA) and the Quartz-Chlorite zone (QC). The
interplay between structure and stratigraphy is thought to be one of the main controls on the
emplacement and distribution of high-grade mineralization at Gosowong. A distinct mappable
volcanic stratigraphy has been recognized within generally intermediate to mafic coherent
volcanic and volcaniclastic rocks of Miocene age. The preferential host rocks to faulting and
subsequent quartz veining are the Gosowong Volcaniclastics, a package of resedimented
volcaniclastic rocks with interbedded ignimbrite and andesitic lava. This unit dips moderately to
the south, striking roughly perpendicular to the strike of the GVZ. The intersection between the volcaniclastic stratigraphy and the GVZ fault is thought to be the key factor in the deposition of high-grade mineralization.
A study of quartz vein textures along the GVZ has shown that high-grade mineralization
is generally developed in discrete shoots within lower grade or barren mineralization. The vein
texture most commonly associated with high-grade mineralization is poly-compositional
crustiform/colloform/cockade banding. The presence of bladed calcite pseudomorphs at various
levels in the system is a positive indication of boiling, though they do not always carry significant
Au· grades. Banded chalcedony and phreatic breccia deep in the system perhaps indicates further
potential at depth.
Alteration zoning ,was mapped out with the use of a PIMA mineral analyzer. The
alteration in the GVZ is typical of low-sulfidation, epithermal deposits: Illite-group minerals are dominant in the ore horizons while propylitic assemblages are usually associated with weakly
mineralized veining. lllite-group minerals and chlorite display a distinct zoning along the fluid
flow pathway, from illite-chlorite ~ illite ~ illite-smectite ~ smectite-illite with decreasing
depth. Alteration zoning mimics stratigraphy, as indicated by gently south plunging paleoisotherms.
Mineralizing fluids are postulated to have ascended the Gosowong fault and then
spread out laterally along the permeable volcaniclastic horizon.
Fluid inclusion analyses indicate that mineralizing fluids have a typical epithermal
signa~e: dilute (generally <1.0 eq. wt. % NaCl) and low temperature (generally 175-265°C).
Coexisting vapor-rich and liquid-rich primary fluid inclusions indicate that boiling processes have
taken place in the GVZ. Trapping temperatures in the QA zone suggest that quartz deposition
took place 100-350 m below the paleo-water table. The variation in trapping temperatures
between the QA (21 0°C) and the QC (236°C) may indicate multiple mineralizing events. Paleoisotherms
mimic the stratigrap.h y, plunging gently to the south, indicating a component of
horizontal fluid flow through the permeable volcaniclastic units.
The GVZ appears to display most of the typical vertical metal zoning common in lowsulfidation
epithermal systems: base metals dominant deep in the system, precious metals
dominant at shallow levels. Base metal values are generally low, averaging 125 ppm Cu, 53 ppm Pb, and 83 ppm Zn. Lead is the base metal most closely associated with Au mineralization.
The distribution of high Au and Ag values indicates a gentle southerly plunge to the precious
metal-rich horizon. Increasing Cu/Zn, Zn/Pb, and precious-metal/base-metal ratios may indicate
vectors to ore-grade mineralization.
It appears that the southerly plunge of the strata, ore-shoots, paleo-isotherms, alteration
zoning, and metal zoning may be in part due to the post-mineral tilting of the GVZ. It is believed
that pre-mineralization deformation has rotated the strata approximately 25-30° to the south,
while post-mineralization deformation has added an additional 10-15° to the overall rotation of
the strata. Thus, deeper levels of the system are exposed closer to the surface on the north end of
A Gosowong specific "prospectivity matrix" has been constructed based on the sum total
of the relative prospectivlties of each of the components analyzed in this study. This matrix
indicates that the most prospective area of the GVZ (outside of the Gosowong deposit area) is the
area deep and to the south of the deposit. Additional, slightly less prospective areas have been
delineated and a total of 5 drill holes have been targeted on these zones of interest.
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