Dr. Dan Wood: Discovery of the Kencana... (2-12-2016)


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Dr. Dan Wood (SEG, U. Queensland)

Discovery of the Kencana Au-Ag ore body at Gosowong, Indonesia

Vendredi 2 décembre 2016 à 15h30 / Friday, December 2, 2016, 3:30 pm

Redpath Museum Auditorium, Université McGill

Résumé / Abstract:

Successive discovery of the bonanza-grade Au-Ag deposits, that comprise the Gosowong, Toguraci andKencana ore bodies on Halmahera Island in Indonesia, owes much to serendipity, good science andnecessity. The ore bodies are examples of low-sulphidation, epithermal Au-Ag mineralisation and werediscovered over 11 years, from 1992; with exploration still continuing. Serendipity contributed to discovery of the first (Gosowong) and third (Kencana) of the ore bodies; theaverage reported Au grades of which were 24 g/t for Gosowong, 27 g/t for Toguraci and 41 g/t forKencana. The Gosowong and Toguraci ore bodies were identified with surface sampling, even thoughoutcrop was mostly absent, and were mined by open pit. The much larger and deeper, Kencana orebody had no identifiable near-surface expression and is being recovered using an underground miningmethod.  Collectively,  the  Gosowong, Toguraci and Kencana ore  bodies contain an in situ mineralresource of more than 6.0 M oz Au, with an average grade in excess of 20 g/t.The   Gosowong   ore   body   was   serendipitously   discovered   while   exploring   for   epithermal  Au   andporphyry-style  Au-Cu   mineralisation   in   Miocene-age   andesitic   volcanic   rocks,   which host severalsmall,   low-grade   porphyry   deposits,   and   also   host   younger   (about   2.8   Ma),   epithermal   Au-Agmineralisation. Discovery of the ore body is generally attributed to sampling a very short stream whichclosely parallels the strike of the epithermal vein. The high adularia content of the major ore shoot isconsidered the reason for the absence of Gosowong vein outcrop.Discovery of the Toguraci cluster of ore shoots in several adjacent epithermal veins, during a necessity-driven search for additional ore to sustain the Gosowong mining operation, followed from re-samplinga relatively narrow epithermal quartz vein. This sampling returned a value of 89 g/t Au over 1.5 m,which was an order of magnitude higher in Au-grade than had been recorded in earlier sampling of thesame   outcrop.   Exploration   was   conducted   under   extreme   conditions,   during   a   time   of   escalatingsectarian violence, when many thousands of people are believed to have been killed on HalmaheraIsland.The Kencana ore body is about three times the size of the combined Gosowong and Toguraci orebodies and was discovered as a result of good science, but assisted by serendipity. Re-positioning of aninitially-planned  drill hole resulted in the  hole penetrating the edge of the  Kencana ore body andrecording a mineralised interval grading 20 g/t Au over 10.5 m. Compounding the serendipitous natureof this discovery, the Kencana ore body is hosted in a different fault to the one hosting the Gosowongore body and which was the target of the drilling. Discovery of Kencana will be the focus of the talk.