Geology and Mineralization of the Porphyry and Epithermal Cu-Au Deposits at Maalinao-Caigutan-Biyog, Batong Buhay, Pasil, Kalinga, Philippines

¹LL Subang, ¹CJC Manipon, ¹OA Briola, ²CJB Ascaño, ²JN Lulu, ³MADA Celiz, ⁴JRR Taningco

¹Senior Exploration Geologist,²Exploration Geologist,³GIS Geologist, ⁴President
PDEP Inc.
4^th Floor La Paz Centre
211 Salcedo St., cor. V.A. Rufino St.
Legaspi Village, Makati City 1229

Abstract

The Maalinao-Caigutan-Biyog Cu-Au deposits are located in barangay Balatoc, municipality of Pasil, province of Kalinga, Northern Luzon. These deposits, representing porphyry Cu-Au mineralization partly overprinted by high-sulfidation epithermal Cu-Au and intermediate-low sulfidation epithermal Au-Ag mineralization, constitute a potentially large under-evaluated Cu-Au resource subjacent to the Dickson porphyry and epithermal Cu-Au deposits of Batong Buhay Gold Mines Inc (BBGMI).

The Kalinga Cu-Au district lies at the eastern side of the Early-Late Miocene (13.7ma-26.3ma) Luzon Central Cordillera Batholith. The Cu-Au occurrences are developed within zones of intersecting NW and NE trending strike-slip faults and NS and EW trending tensional faults. The Maalinao-Caigutan-Biyog Cu-Au deposits are strongly controlled by and localized within a 1km wide NW trending fault zone, complemented by subsidiary NE, EW and NS faults and fractures. Within the fault zone, the deposits are contiguous along a strike length of about 1.5km.

The porphyry Cu-Au mineralization is largely hosted in a Late Miocene-Pliocene polyphase intrusive complex made up of fine-medium grained porphyritic-porphyry stocks of quartz diorite and dacite porphyry. The later high-sulfidation epithermal Cu-Au and intermediate-low sulfidation epithermal Au-Ag mineralization, while partly overprinted on the porphyry deposits, are developed extensively at the margins of the intrusive complex within faults and diorite-dacite intrusive hydrothermal breccias cutting the basalt country rocks.

The Maalinao-Caigutan-Biyog Cu-Au system starts with an early phase porphyry mineralization. It is expressed at surface by quartz-oxide-sulfide stockwork veining mostly in medium grained moderately mafic quartz diorite, and to a much lesser extent, in fine grained to porphyritic basalt with a dominant chlorite-sericite alteration overprinted by silica-sericite (phyllic) alteration. Geochemically, the early porphyry mineralization is characterized by moderate-high Cu and Au values. This is followed by a late phase porphyry mineralization characterized at surface by anhydrite+quartz+oxide+sulfide stockwork veining with silica+chlorite+sericite selvage alteration hosted mainly in a more felsic dacite porphyry emplaced as wide dikes at the margins of the intrusive comlpex. Geochemically, the late porphyry mineralization is characterized by moderate-high Cu, low Au and elevated Mo values.

High sulfidation (HS) epithermal Cu-Au mineralization is also developed at the margins of the intrusive complex. It partly overprints the early and late porphyry mineralization and, going further away from the contacts with the porphyry intrusives, occurs as <1m wide quartz-sulfide veins and 10-60m wide silica-sulfide hydrothermal breccia zones hosted in the basalts. At the core of the diorite complex, the HS overprint is limited along narrow faults and pebble breccia dikes. Pyrite+bornite±chalcopyrite±covellite mineralization occurs in clay dominated zones while pyrite+enargite±luzonite±chalcopyrite is found in silica dominated zones occurring as fills with chalcedonic quartz in fractures and breccia matrices in silica+alunite replaced rocks. Molybdenite is found to occur in both clay-dominated and silica-dominated zones. Majority of the molybdenite occurrences, however, are seen associated with late chalcedonic quartz veinlets-veins as bands and as druse and selvage fills with pyrite and occasional enargite. Chalcedonic quartz veining becomes thicker and more numerous towards the outer margins of the Cu-Au system.

Intermediate-low sulfidation (IS-LS) epithermal Au-Ag mineralization is the last stage of the Maalinao-Caigutan-Biyog Cu-Au system, occurring at the edges across the width of and distal ends along the strike of the system. The IS-LS Au-Ag deposits are characterized by early quartz-sulfide breccia veins and late quartz veins made up of drussy crustiform banded fine quartz and chalcedony that are associated with silica+illite+chlorite±smectite alteration. Associated sulfides are mostly pyrite with some sphalerite and occasional fine enargite, molybdenite. So far, the most extensive development of these IS-LS Au-Ag veins is hosted in a dacite porphyry hydrothermal breccia (diatreme) body at the eastern margin of the system. The breccia body is over 300m at its widest and has a strike length of at least 1.2km, extending in a NS-NNW direction from Caigutan at the south to Biyog at the north.

These surface exposed features of the Maalinao-Caigutan-Biyog Cu-Au system show a central porphyry Cu-Au member that is slightly eroded, exposing largely only its uppermost lithocap section and a limited area of intermediate argillic-phyllic window. The surface evidences also show that this porphyry stockwork mineralization is at least 450m wide and 900m long. An exploratory drilling program to assess this porphyry Cu-Au target is scheduled on the last quarter of 2006.