Unpublished report for Borneo Gold Corporation, Toronto, Canada, pp. 135. (Epithermal Au prospect).
In the Tanah Laut area of southern Kalimantan, gold-in-soil and trenching-RC drilling anomalies in weathered basement rocks are being explored by Borneo Gold Corporation and its Indonesian arm PT Aurum Framindos. Regional basement rocks outcropping in the nearby Meratus Range comprise a complex sequence of Jurassic ophiolites and schists, and Cretaceous to Tertiary volcanic, intrusive and sedimentary rocks. Lithological contacts and schistosity generally strike SW-NE. Diamond drilling at Tanah Laut in 1996 explored the weathered zone and basement in the Kandiwalan, Rasnu, Munggukalang, Saranghalang, Ketapang and Galam areas, to depths of some 50 to 100 m. Basement rocks are commonly schistose and altered, making them difficult to identify and correlate. This problem is intensified by strong surfical weathering effects in the uppermost 20-40 m of the holes, which have degraded the rocks to soft, whitish material. The current study has focused on the identification of primary basement lithologies in the drilled areas, the nature of basement hydrothermal alteration, the effects of surficial weathering on rock compositions, and the overall tectonic setting of the volcanic and intrusive rocks. The approach used to address these problems is mainly geochemical, involving the testing and application of immobile element methods based on some 100 samples from 20 holes, but is closely tied to petrographic examination and detailed core logging. Initial sampling of outcrops in the Besar region (not as yet drilled) was also carried out.
Immobile element methods were used to establish compositional groupings in the drilled areas. It was found that the Zr/TiO2 ratio was the most effective discriminant, with Zr/Al2O3 and Zr/Y ratios also providing generally consistent separations of rock types. These three ratios can also be used effectively to identify the precursors of hydrothermally altered basement rocks. In addition, assessment of the behavior of these ratios across surficially weathered and degraded intersections indicates that they can also be used to identify the precursors of such material. The methods discussed in this report can be applied to exploration in other areas of lithological complexity and deeply weathered profiles (e.g. identifying rocks in trenching programs).
A wide diversity of rocks is present within the drilled areas. The main chemical compositions are andesite and basaltic andesite, with lesser basalt and dacite. The basaltic to dacitic rocks form a fairly coherent suite with typical island-arc chemistry. They display a distinct range of affinities (based on Zr/Y ratios), in general from tholeiitic basalts to transitional andesites to calc-alkaline dacites, which is consistent with the presence of magmas derived from both primitive and more evolved island-arc sources. These basaltic to dacitic rocks encompass a wide range of igneous textures ranging from fine-grained and aphyric, through porphritic, to equigranular and coarse-grained, which suggests that they were emplaced as flows, subvolcanic intrusives, and deeper intrusives. Other igneous rocks which are present in some drillholes include serpentinized ultramafics, pyroxenite, picritic and high Mg-Cr basalts, depleted mafic rocks of boninitic affinity, vesicular and oxidized high-Zr basalts, and an alkaline mafic dyke. In addition, a series of colour banded, schistose rocks are present in the Rasnu-Munggukalang-Saranghalang area which probably represent quartz-rich layered sediments. Intrusive rocks in the Besar area include several distinct compositional groups, including monzodiorite and monzonite (with near-alkaline affinity), as well as granitic and granodioritic rocks. Identical monzodiorite is also present near the drilled Galam andesite sequence, which is strongly hydrothermally altered.
Basement rocks which are altered commonly display K addition and Na-Ca loss, with variable amounts of silica, carbonate and base metal addition; these rocks contain sericite and K-feldspar and locally chlorite. In some holes, epidote-albite-carbonate-quartz-chlorite veins are present, suggesting higher temperature alteration. In general, alteration effects appear to be restricted to strongly veined areas of metre-scale cross-strike extent; these locally contain Au enrichments in the order of 1 to 3 g/t over 1 to 2 m. It is, however, generally difficult to decipher spatial trends in alteration and mineralization due to the effects of multiple generations of dykes, local faulting, and superimposed weathering. Vein sytems are developed in many different rock types, but seem to be concentrated within tens of metres of contacts between strongly contrasting lithologies (e.g. in metasedimentary rocks near ultramafic bodies). The metasedimentary rocks host systems of quartz veins (mm-dm-width) which are commonly boudinaged subparallel to regional foliation.
The diversity of lithological assemblages in the Tanah Laut area suggests that it represents a tectonically imbricated series of rocks, which is probably the result of accretion of both arc and fore-arc sequences, and which includes sheets of mantle-derived ultramafic and related cumulate rocks, plus some rocks of boninitic affinity. The monzodiorite-monzonite intrusives in the Besar to Galam area have an alkaline-arc chemistry which contrasts with the volcanic and subvolcanic rocks in the Pelaihari to Galam area. It is therefore possible that these intrusions were injected during the final stages of tectonic imbrication, producing potassic alteration and Au-base-metal-carbonate-quartz epithermal veins which traversed a diversity of imbricated panels. Areas of potential exploration interest include the margins of the monzonitic intrusions (porphyry Cu-Au), mesothermal Au-base metal vein systems developed near these margins, and more distal vein systems located mainly near structural contacts within the imbricated sequence.
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