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 ........

Mining and processing activities are suspended at Siana and placed the site on temporary standby in April 2017 due to uncertainty surrounding the mining policy of the Philippines and difficulty obtaining environmental permit approvals. Ongoing activities through the current suspension have included dewatering of the open pit, infrastructure maintenance and monitoring of geotechnical issues.

The Siana deposit mined by open pit methods with a plan that included a transition to underground mining following completion of open pit mining. Underground mine development has commenced with 445m of development completed, three portals developed and the establishment of several critical surface infrastructures for the mining operations.

Open Pit Staged Designs
Stage 5
Over time, the southwestern sector of the Siana open pit has deteriorated, due to geotechnical conditions. To manage this a Stage 5 cut-back from surface (+50 mRL) to -60 mRL, covering the southwest sector is planned. The major zone of slumping is within the blue dashed lines. The yellow lines indicate major steeply dipping structures and red dashed lines show the locations of significant tension cracks. The pit water level at the time of the image being taken is at about -45 mRL. Stage 5 will require the decommissioning of Pond B and the re-routing of the western perimeter drain (WPD) to accommodate a 1:20 Annual Return Interval (ARI) for rainfall. The redesigned Stage 5 maintains access t ........

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