1984 Volume 34 Issue 184 Pages 115-129
The Mamut deposit, which has ever produced about 43 million tons of crude ore at 0.6% Cu, 0.7 gm/ton Au and 4 gm/ton Ag, is of typical gold-rich porphyry copper type of the southwestern Pacific island arc setting. The gold-silver mineralization as well as copper is genetically associated with adamellite porphyry intrusives emplaced in serpentinite and siltstone. Alteration zones exhibit a concentric arrangement around the major intrusive of adamellite porphyry successively from the centre to the margin, namely, a broad zone of potassic alteration from the innermost core slightly enriched in K-feldspar to an intensive quartz-biotite zone, a phyllic zone chara cterized by quartz-sericite and an argillic zone with a peripheral kaolinite-montmorillonite assemblage.
Microscope and electron probe studies have revealed that gold-silver minerals, which are closely associated with chalcopyrite and to a lesser extent with pyrite, cubanite and pyrrhotite, comprise native gold and electrum with an average grain size of 30 to 35 μ in the gold-rich quartz veins and 3 to 6 μ in the normal ore-grade zones.
Based on the geological information from the evaluation of diamond drills and daily blast hole samples, the attitude of gold-silver mineralization is summarized as follows. The gold mineralization as well as copper is best developed in the magnetite-poor quartz-biotite type of potassic alteration giving rise to a high grade ore shell which occurs near the intrusion contact straddling both the host intrusive of adamellite porphyry and wall rocks. However, a relative intensity of gold mineralization in terms of gold to copper ratio is also found within the core of adamellite porphyry characterized by weak potassic alteration, whereas the gold grade, compared with copper, tends to sharply decrease outward in the quartz-biotite zone. Thus, an overall concentric distribution pattern is recognized in the gold to copper ratio which becomes progressively less with increasing distance from adamellite porphyry. Notwithstanding the close association of gold and silver deposition, the best silver grades are mainly formed in the near-surface upper half of the ore shell. The gold to copper ratio of sulfide-poor granodiorite porphyry dykes postdating adamellite porphyry is much higher than that of adamellite porphyry, which may imply a gradational bias of metal contents in ore-forming fluids from copper-rich to gold-rich types.
The study results could offer valuable suggestions for investigation on improvement of gold recovery in flotation processes and lead to the establishment of an ore target criteria of gold-rich zones in further exploration drilling around the present ore body.
