Tilting of the Balabag Epithermal Gold Vein System, Zamboanga Del Sur: 
Arguments for a Regional Tectonic Event

Aurelio, M.A.1, Angeles Jr., C.A.2, Hernando, J.3 Elviña, J.C.3,
Lasconia, J.J.3, Lobino, F.3 and the TVIRD Balabag Exploration Team

1National Institute OfGeological Sciences, University Of The Philippines
2Consultant, Cainta, Rizal
3TVI Resource Development(Phils.), Inc.


The Balabag low-sulphidation epithermal quartz-gold-silver veins in Bayog, Zamboangadel Sur, are hosted by Late Oligocene – Early Miocene volcano-sedimentary sequences built over a basement mix of Eurasian-affinity continental rocks and Mesozoic ophiolites. These arc sequences hosting the mineralization consist mainly of andesite flows intercalated with clastic sequences of conglomerates, sandstones, siltones and minor shales. Limestones are seen occasionally intertounging or interlayered with the clastic sequences. Induration in the clastic deposits is generally light to moderate.

The Oligo-Miocene volcano-clastic sequences are affected by regional folding and thrusting, forming WNW-ESE structural axes, and bounded occasionally by NNE-SSW trending transfer fault zones. This thrust-fold deformation is associated to a major tectonic event occurring towards the end of Middle Miocene, involving the docking of the central Mindanao arc terrane into the Zamboanga continental terrane. In Balabag, the folding component of this tectonic event resulted in the formation of the Unao-Unao anticline, forming beds moderately dipping (~60°) to the south-west and north-east on either bank of the easterly trending Unao-Unao Creek. The folding of the host volcano-clastics caused the tilting of the Balabag epithermal veins, giving rise to their present-day gentle (~30°) northerly dips.

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