EXPLORATION ACTIVITIES
Placer Mine

Volcanic, Volcaniclastic & Sedimentary Rocks

Chrono-stratigraphic wise, Placer mine is within a Tertiary-Quaternary sequence of rocks. The oldest rock suite is the 1.5 km thick, Oligocene to Upper Miocene Bacuag Formation. The lower section consists of a thick pile of basaltic pillow lava and basaltic andesite breccias. a) Conglomerates with basalt and limestone clasts, b) well-bedded calcirudite and calcisiltites and c) thin carbonaceous marls dominate the upper section. The Basalt lava is augite-phyric and contains amygdales of chlorite, zeolite, or quartz. Limestone widely distributed within the basaltic succession contains Upper Oligocene to Lower Miocene foraminiferal assemblages. K/Ar whole-rock determination on basalt from the middle part of the succession near the Siana Mine yielded an age of 23.11.1 Ma, or earliest Miocene (UNDP, 1987).



Forming a north-south trending belt unconformably overlying the older rock suite is the Mabuhay Formation rocks consisting mostly of andesitic detritus and rare interbedded lava. Volcaniclastic rock of the Mabuhay Formation is the predominant host rock to most epithermal veins and hydrothermal alteration in the Surigao Gold District. It forms hills and ridges rising above the surrounding calci-clastics or basaltic rocks. At Placer, the formation lies in steep angular unconformity on Bacuag basalt. The age of these andesitic clastics is placed at early Middle Miocene (UNDP, 1987). Determinable fossils are reported only in lime- stone clasts, of which two samples from the Placer Mine contained a Lower to Middle Miocene fauna, suggesting derivation from the underlying Bacuag FM. The Mabuhay Clastics is 500m thick.



Tinabingan Breccia - This rock unit, also known as the Briggs Pyroclastics is restricted to a semicircular diatreme vent approximately 1.5km in diameter, and hosts the Heine-Briggs-Colorado epithermal gold deposit at its south margin. The breccia is divided into an upper tuff apron part and a lower vent breccia part. The tuff apron varies from massive to coarsely bedded. Accretionary lapilli have reportedly been observed in this material, lending support to a base surge deposit. The vent breccia is exposed in the lower part of the Heine pit. It is massive, showing no evidence of layering, and consists of angular to rounded clasts of andesite porphyry, basalt, limestone, and bedded sediments supported by fine matrix derived from milling of clast materials. The vent breccia contains large blocks of limestone and andesite porphyry. The inferred age of the Tinabingan Breccia is Middle Miocene to Upper Miocene.

South of Placer Mineral District, the Mabuhay formation is overlain by the Pliocene Timamana Reefal Limestone while at Bamban area on the north coast of Placer mine, a lense of reefal limestone overlies the Briggs pyroclastics. The Timamana Limestone is thus age as Upper Pliocene to Quaternary.

Tugunan Formation - poorly consolidated interbedded conglomerates, wackes, and silty mudstones. The conglomerates contain clasts of peridotite, greenstone, amphibolite, phyllite, limestone, hornblende andesite, bird's eye porphyry, and andesite porphyry. They also contain silicified and pyritized andesite boulders, angular vein quartz blocks, and detrital gold. The Tugunan Formation is at least 600m thick and overlies andesite porphyry, Mabuhay Clastics, and Bacuag basalts and limestones. The Tugunan Formation, mostly eroded in the Placer Mine site area, is aged as Quaternary clastic sediments.



Maniayao Andesite - extensive andesite is interbedded with or overlies the clastic sediments of the Tugunan Formation. These include the Maniayao Andesite, forming the prominent dome of Mt. Maniayao rising to 644m, and ridges to the south, mostly consisting of flow-banded lava and massive flows. Andesite from Mt. Maniayao gave a K/Ar whole- rock age of 0.30.1 Ma placing it at Quaternary age. The presence of biotite in the Quaternary andesite distinguishes them from andesite of older age.


Chrono - Stratigraphy of Kalaya-An - Manila Mining Corporation Mine & Prospect Site
  1. Bacuag Formation: Basaltic pillow lava, basaltic andesite breccias, conglomerates with basalt and limestone clasts, well-bedded calcirudite and calcisiltites and thin carbonaceous marls. K/Ar whole-rock determination on basalt from the middle part of the succession near the Siana Mine yielded an age of 23.1 1.1 Ma, or earliest Miocene (UNDP, 1987).
  2. Mabuhay Formation: Clastic rocks with mostly andesitic detritus and rare inter-bedded lava. These mostly form hills and ridges rising above the surrounding turbidites or basalt. At Placer, the formation lies in steep angular unconformity on Bacuag basalt. Age of these andesitic clastics is placed at early Middle Miocene (UNDP, 1987).
  3. Boyungan Diorite: Philex-Anglo only mentions this as diorite porphyry. In the Boyungan Porphyry Cu-Au deposit, several phases of stocks and intrusive breccias characterize this diorite unit. UNDP (1987) infers a Middle Miocene age for the diorites of eastern Mindanao, since diorites younger than Middle Miocene is extremely rare in the Philippines.
  4. 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. This intrusive is widespread south of the Placer Mine at the areas of Legaspi, Amuslog and Tagbongabong, where it intrudes Bacuag basalt and limestone and Mabuhay Clastics.
  5. 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.
  6. Tinabingan Breccia: Also known as the Briggs Pyroclastics, is restricted to a semicircular diatreme vent approximately 1.5km in diameter, and hosts the Heine-Briggs-Colorado epithermal gold deposit at its south margin.
  7. Andesite/Dacite: Small Andesite and Dacite Porphyry plugs are found at Bad-as, Bugas-Bugas, Placer, and as small porphyry bodies at 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.
  8. Tugunan Formation: Poorly consolidated inter-bedded conglomerates, wackes, and silty mudstones. Tugunan Formation is at least 600m thick and overlies all the older rock units are considered Quaternary clastic sediments.
  9. Maniayao Andesite: These include the Maniayao Andesite, forming the prominent dome of Mt. Maniayao.


Geology and Mineral Characteristics of Manila Mining's Placer Mine
Intrusive Rocks
Structures

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Placer Mine
Kalaya-An Project
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Geology and Mineral Characteristics of Manila Mining's Placer Mine
Volcanic, Volcaniclastic & Sedimentary Rocks
Intrusive Rocks
Structures
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