Deposit Type: Gold-Silver Replacement Type Deposit.
The Surigao Peninsula at the northeastern tip of Mindanao is characterized physiographically by a narrow north-northwest trending ridge, the Malimono ridge, and the wider and more geologically complex Diwata mountain range to the east. Intervening between these two topographic highs is a structurally-controlled valley, in fact a graben, from the Surigao lowlands in the north, to Lake Mainit and the Tubay Valley southwards. This topographically depressed corridor is interrupted by a young volcanic center, the Maniayao highlands, sitting north of Lake Mainit.
The oldest rock unit identified in Siana Project is the Bacuag formation, the limestone and marly sedimentary members of which mainly host the gold mineralization in the Siana mine. Unconformable on top of the Bacuag is the Middle-Late Miocene Mabuhay Clastics, observed southwest of the Siana pit, and the Late MiocenePliocene Timamana limestone, occurring as tower karsts mostly east of the Siana mine. The Bacuag and, probably, the Mabuhay clastics, are intruded by porphyritic andesite that is related to the Pliocene Maniayao volcanism. The andesite porphyry in the northern tip of the Siana pit is observed as post mineral intrusion to the Bacuag.
North-northwest trending faults are the major structures, several splays of which transect the parcel, including horse-tail splits in the north. Antithetic northeasterly faults are also observed to ........