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