Controls of Porphyry Copper and Epithermal Gold Mineralization in the Northern Surigao Peninsula
Fernando G. Sajona, Ph.D.
Mines and Geosciences Bureau
Surigao peninsula is one of the most prolific gold districts in Mindanao. The northern portion of the peninsula can be structurally divided into: (i) a narrow fault-controlled western ridge, the Malimono ridge; (ii) a relatively lower-lying eastern ridge, here called the Siana ridge; and (iii) a central valley defined by the Mainit graben.
The Malimono ridge exposes Cretaceous-Paleogene basement ultramafic and metavolcanic rocks intruded by a polyphased Late Miocene-Early Pliocene andestic/dioritic shallow intrusive. The same basement rocks in Siana are overlain by Oligo-Miocene volcano-sedimentary sequences, while Tertiary rocks in the Mainit graben are truncated by early Pliocene clastic sequences. Plio-Pleistocene volcanism is manifested as an extinct andesitic-dacitic stratovolcano in the Mainit graben and shallow dikes and plugs of the same composition in the Siana Ridge. Mineral exploration in Mainit graben and Siana ridge disclosed dioritic intrusives that may be correlated with the exposed intrusion in Malimono.
The Siana Ridge is famous for the occurrence of abundant epithermal to mesothermal gold (e.g., Siana, Placer, Mabuhay) and gold-copper (e.g., Ntina orebody of Placer mine) deposits as veins, fault breccias, stockworks and replacement deposits in calcareous rocks within mid-Tertiary units and younger permeable volcaniclastic sequences. There are possibilities of porphyry copper mineralization at depth. The Mainit graben hosts the Boyongan porphyry copper deposit which lies underneath a Plio-Pleistocene volcaniclastic cover, along with other possible occurrences of blind or buried porphyry copper mineralization (e.g., Bagacay). The Malimono ridge is the locus of several epithermal gold prospects and at least two telescoped porphyry copper prospects (Gacepan and Hanagdong) hosted in andesitic/microdioritic intrusive rocks. Structurally-controlled gold mineralization in serpentinite occurs as sporadically distributed prospects and small-scale workings along the eastern flank of the ridge. The copper prospects in Malimono appear to be affected by NNW structures related to the Philippine Fault.
Geologic and morpho-structural features would indicate that:
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a polyphased magmatic event commenced in the Late Miocene-Pliocene, developing porphyry copper mineralization and meso- to epithermal gold mineralization;
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the Gacepan and Boyongan deposits were exhumed and exposed to secondary enrichment, probably requiring differential uplift that decoupled Malimono and Mainit from Siana;
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the Philippine Fault was installed in the mid-Pliocene, downfaulting the Mainit graben to form a lake that inundated the Boyongan deposit, and dissecting the eastern Malimono Ridge;
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late Pliocene-Pleistocene volcanism covered the Boyongan deposit, thus preserving it from destruction;
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telescoping of epithermal deposits with porphyry copper prospects in Malimono requires either a second mineralizing phase, or that mineralization continued, during or after uplift and erosion.
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although epithermal gold mineralization is present in both the Siana and Malimono ridges, potential for gold mineralization in the latter is significantly limited due to the absence of more favorable host rocks, its deeper erosional level, tight faulting and the apparent weaker intensity of hydrothermal activity. Thus, gold mineralization in Malimono is limited into narrow fault-controlled veins and closely-spaced fracture systems amenable to small-scale operations, as opposed to rich vein and replacement deposits in the Siana ridge;
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the Gacepan prospect appears to be truncated to the west by the Philippine Fault. The existence, however, of possible extension or similar prospects beneath pyroclastic deposits to the east should be considered; and
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the Hanagdong deposit seems to be also structurally bounded at its eastern limits. It exposes only shallow alteration zones that might indicate a more important porphyry copper mineralization lying at depth.
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