- Narrow Vein Mining
East Mindanao Regional Geology
This richly mineralised region with a long mining history measures approximately 400 kilometres in length and the Company now controls about 20% of the strike length.
The Company’s tenements are centrally located along the richly endowed Diwata Ranges which form the East Mindanao Ridge (Figure 1). The regional geology comprises an Early Tertiary ophiolitic basement overlain by andesitic lavas and pyroclastic beds, sandstone, shale, conglomerate and limestone. The volcano-sedimentary sequence is intruded by Late Tertiary dacite and quartz diorite plutons.
The dominant structural feature is the Philippine Rift Fault, a major regional structure that extends for 1,200 kilometres in a north-northwesterly direction over the length of the Philippines from southern Mindanao to northern Luzon. The Diwata Ranges straddle the Philippine Rift Fault which provides the main source of Tertiary volcanism and mineralisation.
Mineralisation associated with the Philippine Rift Fault and Subduction System occurs as copper-gold porphyries, epithermal gold veins, and skarn and disseminated deposits associated with calcareous sediments. Locally, gold mineralisation is controlled by strike-slip faults parallel to the Philippine Rift Fault or splay structures off the Rift Fault and dilationary structures which develop orthogonally to the main structures as a result of strike slip movements on structures parallel to the Philippine Rift Fault.
The Company’s tenements are centrally located between the well known and exploited gold and copper districts of North Davao in the south and Surigao in the north. The Diwalwal Gold Mine located some 60 kilometres south of the Co-O Mine is reported to have contained seven million ounces of gold and represents a target type of epithermal vein mineralisation while the Boyongan Deposit in the Surigao District and the King King Deposit in the North Davao District represent porphyry copper-gold target types.
The Co-O Mine and the Tambis areas have been interpreted to be within truncated calderas which are identifiable from volcanic features known to be favourable for gold and base metal mineralisation.
Regional Mineralisation – Background
The East Mindanao Ridge has been a major mining region since before World War II. Previous mining has essentially been divided into two areas, the Surigao District in the north and the North Davao District in the south as shown on Figure 1.
Pre-WWII operations were mainly confined to the Surigao District with production from high grade veins at the Mindanao Motherlode (or Mabuhay) Mine, Tapian and Mapaso Mines, from veins at the Siana underground mine commencing in 1938, and from veins at the Placer Mine commencing in 1936. In later years Siana became an open pit mine and numerous open pit mines were established in the Placer Mine area where porphyry copper-gold bodies were found adjacent to and below the high grade vein systems.
The most notable recent discovery in the Surigao District is the large Boyongan porphyry copper-gold deposit which is undergoing further work.
In the North Davao District, the Maco (previously Masara) copper-gold deposits were discovered in 1938 and subsequently mined. The Amacan porphyry copper deposits were mined in the 1980s, and the large Kingking porphyry copper deposit was discovered as well as smaller porphyry copper deposits such as Mapula. Epithermal veins such as Hijo were mined as part of the Amacan operation, and approximately 20 years ago the large Diwalwal epithermal vein system was discovered with high grade mineralisation extending to approximately 600m from surface.