The airborne magnetics and rock float indicate that this anomaly is associated with a high-level intrusive, possibly a Sintang intrusive which has created brecciation and hydrothermal fluid release within carbonate rocks, thus creating skarns, breccias and stockwork mineralization. This is interpreted to be a similar age and mineralization type as that seen at Busang, Kelian and Mt. Muro. The initial airborne survey was flown at a height of 100 meters with 500-meter spacing, Walters noted. "Now we have gone back and re-flown a portion of northern Block 1 at 250-meter spacing so we'll know just that much more about this exceptional magnetic anomaly (From Yamana literature)
The mineralization type seen at Busang? So who saw this?
I disagree with one component of the Strathcona report. The Busang rock I saw was NOT that strongly altered, or carbonatized which is what you look for in a gold deposit. Also it was EXTREMELY tight and unsheared. Localized rubble areas are NOT plumbing systems often I have found unless part of a major shear which could plumb depths. A major exception to this seemingly was the Kerr addison Flow ore. This was actually a micro shear that was never properly analysed, and it had euhedral pyrites which threw a lot of people off. It was a continuation of well developed veining and ladder work in the carbonates.
Strathcona report:
Geology
Published Reports
Busang is situated at the Kalimantan Suture, a major tectonic boundary in central Kalimantan, which separates continental basement rocks to the northwest from shelf sediments to the southeast, and forms the margin of an Early to Mid-Tertiary magmatic arc (Corbett and Leach, 1997). The suture hosts the Kelian gold deposit located 100 kilometres SW of Busang, and Mt. Muro and other gold occurrences (Figure 2).
Felderhof et al., 1995, describe the Busang area as being underlain by folded clastic metasediments and limestone of Lower Tertiary age, which are in fault contact with older metamorphosed sedimentary, volcanic and ultramafic intrusive rocks. Volcanic activity during Mid-Miocene to Early-Pliocene led to the development of a 10 x 15 kilometre maar diatreme and intrusion of the Busang dacitic dome, within a 20 x 20 kilometre graben structure. Maar sediments consisting of carbonaceous shale with lignite lenses and of siltstone and sandstone form xenolithic blocks up to 50 x 200 metres in size at surface, and have been observed in breccias to 260 metres depth in drill holes. The dome lithologies are divided into medium to coarse grained massive crystalline quartz-hornblende dacite in the centre, porphyritic rhyodacite at the periphery and locally flow-banded porphyritic hornblende-feldspar andesite at the outer flanks of the dome. Basalt-andesite-dacite-rhyolite plugs and dikes form the youngest members of the dome complex.
Three zones of gold mineralization, the Central Zone, Southeast Zone I and Southeast Zone II are described by Felderhof et al., 1995. The Central Zone has an east-west strike and a length of approximately one kilometre and is situated at the northwest end of the Southeast zones which strike northwest-southeast, and are claimed to have lengths of six and seven kilometres respectively. Carbonate-argillic, sericitic and phyllic alteration is described to occur at progressively deeper levels in the intrusives, and mineralization including pyrite, marcasite, arsenopyrite, pyrrhotite, sphalerite, galena, chalcopyrite, orpiment, realgar, stibnite, cinnabar, bornite, tennantite is associated with hydrofracturing, disseminations, interstitial in-filling, fluidization, hydrobrecciation and stockwork veining. Gold is described to occur dominantly in "free, native form" and as "submicroscopic particles in pyrite, arsenopyrite and as electrum occluded in sphalerite" (Felderhof et al., 1995).
Srathcona's Observations:
The dominant rock type encountered in all holes is a feldspar-porphyritic intrusive, which, based on its macroscopic mineralogical composition - plagioclase and hornblende but no apparent quartz phenocrysts - was classified as andesite porphyry. In contrast, Bre-X geologists call this rock a dacite, which is characterized by a higher silica content and should have quartz phenocrysts. Whether this rock is an andesite or dacite is somewhat academic and probably not of great significance for the gold potential of Busang, with the exception that a siliceous rock is more brittle and more prone to fracturing allowing better penetration of fluids. A siliceous intrusive - dacite or rhyolite - was intersected in hole 75S, from 239 metres to the end of the hole at 274.7 metres. The texture of the andesite porphyry is rather uniform and survives all but the strongest alteration or brecciation. The least altered andesite porphyry is green grey, massive, homogeneous, carries 15-20% plagioclase phenocrysts, 1-4 mm in size, some 5% hornblende and minor magnetite in a fine-grained matrix. The rock is variably propylitic altered, with plagioclase being saussuritized (calcite-sericite-epidote-albite) and mafic minerals (hornblende, biotite) probably chloritized.
A strong and pervasive carbonate-sericite(or illite)-clay-pyrite/marcasite alteration has affected most of the porphyry intersected, and may reach from incipient bleaching to strong replacement of the hostrock by the above minerals. Hydrofractures - closely spaced dark fractures in light grey altered porphyry with or without pyrite/marcasite - and milled matrix fluidized breccias (Corbett and Leach, 1997) are characteristic alteration features at Busang (see photos on next two pages). The descriptive term 'rubble breccia' was used for milled matrix fluidized breccias in the Strathcona drill logs. The multi-stage alteration is characterized by a relative abundance of pyrite, which occurs fine grained, disseminated in the matrix and coarser grained on young fractures with carbonate-clay and rare sphalerite and galena. Marcasite spreading from hairline fractures, partly with dendritic textures is typical for zones of hydrofracturing. Bright red, euhedral realgar was rarely seen on fractures e.g. in hole 75S at 165.65 and 216.70 metres.
The observed rock types and alteration assemblage confirm the classification of Busang as a "porphyry-related, low sulfidation carbonate-base metal-(gold) system" (Corbett and Leach, 1997). The two major alteration styles - propylitic and carbonate-sericite-clay-pyrite - are easy to recognize in drill core and have been used by the Bre-X geologists during core logging to separate un-mineralized ("dacite") from mineralized("mineralized zone") sections. This classification is correct, as long as "mineralization" refers to pyrite, marcasite, and base metals, and logging by Bre-X and Strathcona (or any other geologists familiar with the rocks), would, and has produced very similar results, as far as identification of these units is concerned. At Busang, "mineralized zone" core was bagged and shipped via Samarinda to Indo Assay Lab in Balikpapan, and generally returned good gold values. Propylitic "dacite" core was considered waste, and was shipped to Indo Assay Laboratories for assaying at a later date, with assay preparation done at Busang.
While we are in general agreement with the geological descriptions, we do not agree with some of the alteration types used by Bre-X, particularly with the amount of silicification described in logs and shown on sections. The carbonate-altered core is without exception soft and silicification has not been observed in the Strathcona drill core. Quartz described by PetraScience Consultants (A.Thompson, 1996) in the matrix of altered rock is part of the original silica content of the andesitic or dacitic rock, but is - in our opinion - not a result of silicification. Quartz veins are very rare, and do occur as hairline stringers in dacitic/rhyolitic intrusive in hole 75S, or as dark fragments of brecciated older veins. The softness of the rock and the lack of quartz veins is apparent from drilling, both by the fast advance of the drill string and by the low wear rate for drill bits, which may last 800 metres. The absence of quartz veins and of silicification is characteristic for carbonate-base metal systems, and does not necessarily indicate an absence of gold.
Arsenopyrite is frequently mentioned in the Bre-X logs, but has not been identified in the Strathcona core, which agrees with findings by PetraScience that arsenopyrite is absent in samples from the Southeast Zone, but does occur in the Central Zone. A sample of core from the Central Zone identified by T. Leach, contained an approximate half-centimetre vein of arsenopyrite. Arsenic assays of the Strathcona core are generally low, some higher values may be due to realgar seen in the core, and possibly to very fine grained arsenopyrite or arsenic in pyrite and marcasite. Based on carbonate chemistry, frequency of arsenopyrite, marcasite and sericite/illite, PetraScience concludes that the Southeast Zone alteration developed during higher temperatures, and is indicative of a close relationship to a porphyry environment. The Central Zone and the Southeast Zone I South also differ in their surface geochemistry. The former has a marked gold soil geochemical anomaly, while the latter has virtually no gold in samples collected in trenches and outcrops along the numerous drill roads.
Conclusion
Busang has characteristics favourable for gold deposition. These include its location on a major tectonic boundary - the Kalimantan Suture - which also hosts the Kelian maar diatreme gold deposit, classified as a carbonate-base metal gold deposit by Corbett and Leach (1997), and other gold occurrences, including Mt. Muro. The proposed maar diatreme setting and the type and large size of the alteration system at Busang are further positive indicators for gold mineralization. Available information indicates, that these conditions may have led to deposition of some gold in the Central Zone, while drilling of the Strathcona holes in the Southeast Zone proves that this zone, despite its strong alteration, contains traces of gold only.
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