The geology and geochemistry of the Gunung Pani gold prospect, North Sulawesi, Indonesia

Abstract

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 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.