Summary:
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.
Open pit
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.
Underground
The mineralisation at Siana occurs over broad widths up to 60m in the main zone in the upper levels and narrows with depth to approximately 6 metres below the - 300 mRl. Between -130 to -290 mRL the main zone averages approximately 30 to 40 m in width. The deposit envelope is orientated approximately north-south with the East Wall fault zone having an influence in the orientation of the mineralisation. The main resource is up to 400 metres in strike and to a maximum of 400 metres down dip. The dip above the -290 mRL is near vertical to -80 to the east with the mineralisation below -290 mRL changing dip to –65 degrees to the east. The sub parallel footwall and hanging wall lenses are significantly small in width, and shorter in strike length and down dip extension varying from less than 100 metres to 300 metres. The average widths for the footwall and hanging wall lenses vary from 2 to 4 plus metres.
The majority of the Siana mineralisation occurs in what is referred to as the “Main Zone”. The Main Zone occurs along a steeply dipping structural corridor between volcaniclastics on the footwall and basalt on the hanging wall. The structural corridor controls the strike, dip and shape of the Main Zone. In addition to the Main Zone, there are three smaller sub-parallel footwall lenses and nine sub- parallel hanging wall lenses which contribute to the resource.
The Main Zone strikes at 330° to 360° and dips at -60° to -90° 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 360° 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.