Kavalieris, Imants
Description
The Gunung Pani gold prospect, located near
the coastal village of Marisa in North Sulawesi(longitude 122°E and latitude 0°33'N) is a low grade,large tonnage gold resource, hosted in intrusive
porphyritic rhyodacites and pyroclastics of the Pani Volcanic Complex.
The Pani Complex of probable Pliocene Pleistocene age is a sub-circular volcanic structure approximately3.5 km in diameter composed of rhyodacitic breccia,lava, agglomerate and lapilli tuff, extensively intruded by comagmatic...[Show more] porphyritic rhyodacites. The volcanic assemblage is interpreted in terms of the structure
of volcanic domes.
The geological setting is poorly known, but
important regional components include: (1) a suspected pre-Tertiary basement terrain composed of amphibolite and low K-granitoids; (2) Eocene basalts belonging to
the Tinombo Fm, which regionally dominate the geology of the Marisa hinterland; (3) fine-grained foliated hornblende-biotite microgranodiorite, genetically related to the Pani Volcanics, intrusive into the
basement; and (4) the Pani Volcanics, mainly represented by circular structures, such as the Pani Volcanic Complex.
The Pani Volcanics are divided into two units
based upon phenocryst mineralogy, (1) Tpi, composed of quartz-biotite-sanidine porphyritic rhyodacites, and (2) Tpii, of similar mineralogy to Tpi but characterised
by smaller phenocrysts, fewer quartz phenocrysts and the presence of hornblende in addition to biotite. The Tpii lithologies are unaltered and represent the last
phase of volcanicity in the Pani Volcanic Complex, and may correlate in part to volcanics elsewhere mapped as Pinogu Volcanics. Hornblende microgranodiorites
including ring-dykes in the region are correlated to Tpii.
Geochemically the Pani Volcanics can be best
compared to rocks from continental margin tectonic settings, and they can be classified as high K rhyolites (Ewart, 1979). The mineralogy and geochemistry is
also consistent with I-type magmas (Chappell and White,19 7 4) . The probable young age, distribution and I-type characteristics of the Pani Volcanics suggests they may
be related to subduction in the North Sulawesi Trench, but their continental affinity indicates that a simple relationship is unlikely.
Gold mineralisation is associated only with
the Tpi lithologies and occurs in silicified porphyritic rhyodacite lavas and intrusives, the largest of which may represent a rhyodacite dome. Alteration of these
rocks is weak and is typified by deuteric silicification, Na-alkali metasomatism, Pe-rich chlorite, anatase, sericite, pyrite, manganoan siderite, and quartz-adularia
encrustation of vuggy fractures. In the main area of mineralisation (Pani ridge) , quartz veins are rare or absent, but higher grade mineralisation (> 2 ppm Au) may
be related to abundant vuggy fractures and thin brecciated zones and fractures containing sulphides in silicified rhyodacites and wall rocks.
Gold is present as electrum (20% Ag) and is
associated with pyrite characterised by galena inclusions, as well as sphalerite and chalcopyrite.
Supergene enrichment of gold is negligible.
Low grade silver mineralisation is restricted
to Gunung Baganite and occurs as acanthite in quartz veins hosted in pervasively silicified rhyodacitic lapilli tuff. The silver mineralisation postdates
gold mineralisation and is related to relatively acid hypogene fluids which altered the earlier sulphides to
hematite, barite and possibly jarosite and goethite.
These fluids are related to the waning stages of the hydrothermal syste~ produced by the intrusion of the Baganite rhyodacite dome. Internally the Baganite rhyodacite dome is characterised by low grade
disseminated gold mineralisation in the order of 0.8 ppm.
Chlorite alteration in the Baganite rhyodacite dome records the changing hydrothermal alteration following emplacement and cooling, in exhibiting a wide
range in chemical composition. The chemical variation may be due to hydrothermal alteration of early crystallised chlorite by progressively more acid fluids, non-equilibria crystallisation of chlorite, or chlorite crystallisation in equilibria with the changing fluids.
The first possibility is preferred.
The field and alteration studies suggest
the low grade gold mineralisation is closely related to the cooling and degassing of porphyritic rhyodacite bodies which may exhibit characteristics of both lavas
and intrusives, and the nature and distribution of alteration is primarily a consequence of their emplacement, rather than to a later hydrothermal event.
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