EXPLORATION ACTIVITIES
Placer Mine

Intrusive Rocks

An important key to understanding the various mineralization episodes in the Placer Mining area lies on the identification and characterization of the various intrusive rock phases. There are at least four (4) phases of intrusion with the Placer Mineral District, which from oldest to youngest are as follows:

Boyungan Diorite - Little is known of this diorite as Philex-Anglo only mentions it as diorite porphyry with no further detailed description. In the Boyongan Porphyry Cu-Au deposit, several phases of stocks and intrusive breccias characterize this diorite unit. Philex-Anglo makes no mention of the age of the Boyongan diorite. UNDP (1987) infers a Middle Miocene age for the diorites of eastern Mindanao, since diorites younger than Middle Miocene are extremely rare in the Philippines.

Bird's Eye Porphyry - Andesite porphyry stocks characterized by large plagioclase phenocrysts up to 2cm long by 1cm wide in a fine matrix of plagioclase and hornblende. The occurrence of quartz eyes in some areas indicates gradation into a dacitic facies. This intrusive is widespread south of the Placer Mine at the areas of Legaspi, Amuslog and Tagbongabong, where it intrudes Bacuag basalts and limestone and Mabuhay Clastics. At Legaspi, the Bird's Eye Porphyry is host to vein type epithermal gold. A Middle Miocene age is inferred for this intrusive unit.

Andesite Porphyry - Stocks of the same composition as the bird's eye porphyry, but with smaller (coarse to medium grained) plagioclase phenocrysts. In the Placer Mines, the andesite porphyries are clustered north of Mapaso towards Banban, and are intimately associated in space with majority of the most productive epithermal gold deposits. This intrusive unit cuts the Bacuag basalts and limestone, the Mabuhay Clastics and Bird's Eye Porphyry. Samples of the andesite porphyry stock immediately south of Heine sent by MMC for radiometric dating gave Middle Miocene age.

Andesite/Dacite - Small Andesite and Dacite Porphyry plugs are found at Bad-as, Bugas-bugas, Placer, Cog Point and Ban-ban. These minor intrusions are characterized by their very fresh state and the occurrence of moderate primary magnetite, as opposed to the Bird's Eye Porphyry and Andesite Porphyry stocks which are widely chloritized and containing only very little primary magnetite due to destruction by varying degrees of alteration. An andesite porphyry plug has been mapped intruding the Tinabingan Breccia so that a Pliocene age is inferred for these minor intrusions.


Geology and Mineral Characteristics of Manila Mining's Placer Mine
Volcanic, Volcaniclastic & Sedimentary Rocks
Structures

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Placer Mine
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Additional Information
Geology and Mineral Characteristics of Manila Mining's Placer Mine
Volcanic, Volcaniclastic & Sedimentary Rocks
Intrusive Rocks
Structures
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