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 intersect the northwesterlies, as observed in the Siana pit. Such geological setting continues in the southern portion of the Siana MPSA, including the Alegria claim, where regional north-northwest to south-southeast structures transecting Siana extend southwards. The dominant rock types in the Alegria parcel, however, are volcaniclastic sequences and the intruding andesite porphyry.

MINERALIZATION
Siana is located along the Surigao Valley Fault, a major structure that is part of the Philippine Fault Zone. The area is known for various mineralization such as epithermal gold and porphyry-type copper-gold mineral deposits.

Mineralization in the Siana orebody occurs in both the limestone and basalt members of the Bacuag Formation, although earlier works describe mineralization to be confined only in the carbonates (UNDP, 1988). Ore in the limestone occurs as either high grade “massive sulfide” bodies consisting of pyrite, sphalerite, galena, chalcopyrite and quartz in cavities, or as disseminated gold in the clay or gouge portions and limestone. The massive sulfide bodies are as much as 7 meters long, with gold grades reaching 30 g/t Au.

Basalt-hosted mineralization to the east of the Siana pit occurs in soft, bleached, pyritic alteration zones, often 10 meters or more in width, over a lateral distance of approximately 140 meters. Minor mineralization also occurs within the volcaniclastics of the Mabuhay Formation. Mineralization progressively narrows to the south in both the carbonate and basalt hosts but is known to persist to approximately 400 meters below surface.

Reports indicate that the gold and sulfide mineralization present in the southern portion of the Siana tenement are hosted in andesite porphyry and intruded sediments where gold mineralization is more widespread. Exploration soil geochemistry indicates that a high level or near-surface epithermal system is possibly preserved in the area. A deeper copper-dominated type of mineralization reportedly appears to characterize the Alegria claim. Lenses of massive sulfides and hematite have been encountered in the immediate vicinity, prompting earlier workers to consider a skarn type origin for such mineralization.

In cross section, the central carbonate breccia varies between 20m to 80m in width below the current open pit floor (the main ore zone). Adjacent and west, the volcaniclastic sediments are often bleached and carbonate altered, and host some economic gold mineralisation. Distal chlorite altered sediments are barren. The basalt hosted mineralisation to the east occurs in numerous discrete soft, bleached, pyritic alteration zones, often 10m or more in width, over a lateral distance of approximately 140 metres. Hard fresh basalt between the lodes is barren. Mineralisation progressively narrows to the north and south in both the carbonate and basalt hosts, but is known to persist to approximately 400m below surface, but narrowing. The mineralisation is generally is sub vertical to steeply dipping to the east at approximately -70 degrees. The main mineralised zone strikes N-S to NNW-SSE and also trends to the NE in the northern section of the resource. The Siana resource has a strike length of approximately 500 metres with mineralisation down to approximately 500 metres below surface.

The majority of the Siana mineralisation occurs in what is referred to as the “Main Zone”. The Main Zone strikes at 330O to 360O and dips at -60O to -90O to the north-east. The strike length of the Main Zone is about 450m, known down dip extent is over 500m and the across strike horizontal thickness ranges up to about 60m. Main Zone includes six zones of internal waste, the largest of which has a strike length of nearly 400m, a down dip extent of about 400m and an across strike horizontal thickness of up to about 15m. The Main Zone extends to about 500m below the original ground surface.

The three footwall zones are relatively small compared to the Main Zone and strike and dip parallel to the Main Zone. Strike lengths range from 20m to 250m, down dip extents range from 50m to about 250m and across strike horizontal thicknesses are less than 10m. The footwall zones extend to about 500m below the original ground surface.

The nine hangingwall zones strike at about 360O and dip steeply to the east. Strike lengths range from 10m to 250m, down dip extents range from 100m to about 400m and across strike horizontal thicknesses are generally less than 15m. The hangingwall zones are known to about 450m below the original ground surface.

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 to the future underground portals.

Stage 6A
Stages 6A has been redesigned, mining of debris from the eat wall failure is necessary to gain access to ore at lower elevations, stabilise the east wall and to ensure the safety of plant and equipment.

Stage 7
Stage 7 has been redesigned to mine out debris from the north wall slip. The cut-back is necessary to gain access to ore at lower benches. The redesign maintains practical mining width whilst mining the debris from the slip. Several wider benches have been incorporated into the design to achieve this purpose and provide additional stability to the excavated wall. The design ramp used for mining the cutback will provide backup means of accessing future underground portals. The ramp is designed mainly as a single lane 12m ramp with passing bays at several points.

Stages 6B and 8
Stages 6B and 8 have been designed to link all 3 upper cutbacks and take the Siana open pit down to the 152.5 mRL. The main ramp is designed as an 18m double lane, narrowing down to 15m and narrowing even further down to a single lane 12m wide ramp from the -145mRL to the pit bottom. Stages 6B and 8 mine out all the silt material presently located in the pit. The two stages also leave behind a support buttress.

The ultimate open pit is the culmination of the five cuts to a final elevation of -152.5 mRL.

During the period of suspension, the water level in the open pit is being maintained at about -55 mRL. The volume of silt and slope failure debris in the open pit bottom, from -55 mRL to an elevation of -97.5 m RL.

To date about 11 million BCM of waste materials have been placed in the Tinang-an waste-rock storage facility (WSF), to a maximum elevation of +165 mRL. This translates to a maximum 90 m vertical height with an overall slope angle of about 4:1 (14°).

Underground
Careful consideration was given to the mine design. The stope designs were based on a geotechnical assessment of the relevant host rock mass and optimal cut-off grade considering both the hydrological setting and previous mining. Stope shapes were initially developed using a mine shape optimiser (MSO) to interrogate the underground resource. These designs were adjusted to ensure minability. The current plan is to use a cemented paste fill to maintain mine stability and maximise the resource extraction.

For the Siana pit in May 2020, Mining One has recommended 23 to 38 degrees in four sectors around the pit over the vertical 50 mRL to -10 mRL; 32 to 43 degrees in three sectors around the pit over the vertical range -10 mRL to -70 mRL and 32 to 40 degrees for three sectors from -70 mRL to pit bottom and angles flattened to 25 degree in upper elevation rocks to surface.

The mineralisation is invariably soft with a high clay content, requiring a much lower energy input into grinding from that at many other mines. The ore is crushed in a standard jaw crusher down to minus 150mm and either stockpiled or transferred directly to the 2.8MW SAG mill, with lime added prior to grinding. The SAG mill reduces the particle size to 80% passing 75 microns, prior to the slurry being discharged into the CIL tanks.

Material mined from the Siana open pit has been processed onsite at the Siana CIL gold processing plant that:
- has a design capacity of 1.1 mtpa;
- TVIRD understands has experienced processing recoveries of between 75 to 85% for gold and 40 to 45% for silver;
- is comprised of single stage crushing, SAG milling, gravity concentration and high intensity cyanidation, leaching and adsorption (CIL), followed by carbon elution and electrowinning to produce combined gold and silver doré;
- treats in a detoxification circuit tailings from the cyanide leach area to minimize cyanide concentration prior to discharge to the TSF.

TVIRD (TVI Resource Development (Phils.) Inc.) is currently reviewing the historical processing operations of its Siana Gold Project with the objective of testing and optimizing future performance and developing a plan to restart operations.

SIANA CIL GOLD PROCESSING PLANT
The gold and silver at Siana is extracted using a s ........