CHARACTERISTICS OF THE ROOT OF Cu-Au PORPYRY SYSTEM: RESULTS OF STUDY FROM BATU HIJAU Cu-Au PORPHYRY DEPOSIT

PROCEEDINGS PIT IAGI RIAU 2006
The 35^th IAGI Annual Convention and Exhibition
Pekanbaru – Riau, 21 – 22 November 2006

CHARACTERISTICS OF THE ROOT OF Cu-Au PORPYRY SYSTEM: RESULTS OF STUDY FROM BATU HIJAU Cu-Au PORPHYRY DEPOSIT

Setyandhaka, D. & Arif, J.
Mine Geology Department, PT Newmont Nusa Tenggara, NTB - Indonesia.
ABSTRACT

Batu Hijau shares many similar characteristics to porphyry copper-gold deposits and the geology of Batu Hijau is an example of classic Cu-Au porphyry system. Compared to many typical porphyry deposits; it has very minor overprints of later alteration and mineralization. Copper and gold mineralization is directly related to quartz veining and wall rock alteration that developed with multiple tonalite porphyry intrusion.

Detailed geological logging, petrography analyses and assays results of the diamond drill cores from the upper level to the deep portion of world class Batu Hijau porphyry copper-gold system shows the high grade ores containing both copper and gold in deep portion are occupied within the host rock intrusion body of tonalites associated igneous breccia, and wall rocks of volcanics and quartz diorite. The ores from the bottom zone containe approximately 1% Cu and 2 g/t Au which are present consistently down to -600 m elevation. Hydrothermal alteration and mineralization in deeper parts show that the quartz veins are dominantly early A-veins and B-veins with the density ranges from 8-15 percent and bornite ± chalcopyrite mineralization.

Below the -600 m elevation, the mineralization is almost barren even though the quartz veins consistently are high in density (>5%) and consisted of B and D type veins. The total sulfide content of barren ore at depth is significantly decreased and it is dominated by pyrite-magnetite with trace of chalcopyrite. The magnetite and quartz grains tend to have coarser grain at barren ores.

This paper describes the characteristics of the root of Cu-Au porphyry system at Batu Hijau deposit and examined the distinction of geological characteristics in deep and shallow parts of the porphyry system by detail geological logging, petrography analyses, thin section and assays results work of the diamond drill cores.