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The Dinkidi Cu-Au Porphyry: Geology of the Didipio region and paragenesis of the Dinkidi Cu-Au Porphyry deposit

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Wolfe, RC (2001) The Dinkidi Cu-Au Porphyry: Geology of the Didipio region and paragenesis of the Dinkidi Cu-Au Porphyry deposit. PhD thesis, University of Tasmania.

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Abstract

The Dinkidi Cu-Au porphyry deposit is located in northern Luzon, Philippines. The 110
Mt deposit is characterised by high gold grades (ave. 1.2 g/i Au and 0.5% Cu) and is one
of only a few alkaline porphyry deposits to have been discovered outside British
Columbia.
Alkaline magmatism in northern Luzon is related to the Late Oligocene rifting event that
formed the Cagayan Valley Basin, and to the final stages of west-directed subduction
along the East Luzon trench. Subalkaline andesitic and trachytic lavas and rninor
volcaniclastic rocks of the Mamparang Formation were emplaced along the southwest
margin of the Cagayan Valley Basin, and have.been intruded by a series of alkaline
piu tons and stocks. The Dinkidi deposit is hosted within the multi phase Dinkidi Stock,
which is in turn part of a larger alkaline intrusive body, the Didipio Igneous Complex.
The Didipio Igneous Complex consists of: (I) an early composite clinopyroxenegabbro-
diorite-monzodiorite pluton; (2) the Surong clinopyroxene to biotite monzonite
pluton; (3) the Cu-Au mineralised Dinkidi Stock, which comprises an early equigranular
biotite-monzonite stock (Tunja Monzonite), a thin, variably-textured clinopyroxenesyenite
(the Balut Dyke), and a monzosyenite porphyry (Quan Porphyry) that grades, in
its core, into a crystal-crowded leucocratic quartz-syenite (Bufu Syenite); and (4) postmineralisation
andesite dykes.
Whole-rock chemistry indicates that the volcanic formations in the Didipio region
become progressively more alkaline up stratigraphy, indicating that Late Oligocene
rifting intermittently tapped an LILE-enriched mantle source that became progressively
more LILE-enriched over time. Whole rock and mineral compositions indicate that all
. intrusions in the Didipio region were sourced from a common magma chamber, and
were related by shallow level fractional crystallisation.
Five main hydrothermal events are recognised in the Didipio region: (I) contact
metamorphism and weak biotite-cordierite alteration is associated with ernplacement of
the early diorite phase; (2) K-silicate rnagnetite-biotite alteration, and subeconomic CuAu
mineralisation associated with the emplacement of the Surong monzonite pluton; (3)
intensely developed porphyry-style alteration and ore-grade Cu-Au mineralisation which
is spatially and temporally associated with emplacement of the Dinkidi Stock; (4) an
advanced argillic alteration assemblage, which has overprinted the Didipio Igneous
Complex and is associated with subeconornic high-sulphidation style Cu-Au
mineralisation; (5) late-stage unmineralised zeolite-carbonate veins, which are
associated with post-mineral strike-slip faulting.
. .
At Dinkidi, emplacement of the Tunja Monzonite was temporally and spatially
associated with the formation of a pervasive biotite-magnetite K-silicate alteration
assemblage in the pre-mineral diorites. Emplacement of the Balut Dyke was associated
with a calc-potassic style diopside-actinolite-K-feldspar-bornite alteration assemblage
and associated vein stockwork. This quartz-free mineral assemblage is associated with
high gold grades (2-8 g/t Au) and is typical of alteration assemblages found in quartzundersaturated
alkaline porphyry systems. Intrusion of the Bufu Syenite led to the
formation of a quartz-sericite-calcite-chalcopyrite stockwork vein. and alteration
assemblage, which has overprinted the calc-potassic assemblage. The quartz stock work hosts the bulk of low grade mineralisation Cl c2g1t Au) at Dinkidi and is typicaJ of silicasaturated
alkaline porphyry systems. A coarse-grained assemblage of quartz-actinoliteperthite
(the 'Bugoy Pegmatite') formed as an apophysis on the Bufu Syenite, and was.
subsequently brecciated by late-stage faulting. High-level argilJic and late-stage faultrelated
zeolite mineral assemblages have overprinted the porphyry-style hydrothermal
assemblages.
The calc-potassic assemblage is inferred to have formed at temperatures in excess of
>600°C from a silica-undersaturated K-Ca-Fe brine. Fluid inclusion studies indicate that
the quartz stockwork was emplaced at submagmatic temperatures (>600°C) from a
quartz-saturated Na-K-Fe brine (>68 wt. % eq. NaCI) that contained up to 0.6 wt. % Cu
and 4 wt. % Fe. Cooling to -420°C and neutralisation by wall rock interaction lead to
the precipitation of sulphides within the quartz stockwork. The quartz-bearing
assemblage was emplacement at 2.9 to 3.5 km paleodepth, and was associated periods
of overpressurisation and quartz growth disrupted by episodic depressurisation to near
hydrostatic pressure conditions.
The hydrothermal mineral assemblages at Dinkidi reflect the composition and degree of
fractionation of the associated intrusions. Extensive fractionation within a feldspathoid.
normative dioritic magma chamber is interpreted to have ultimately caused quartz
saturation and the development of the late-stage syenite intrusions and related quartz
stockwork inineralisation. The calcic, silica-undersaturated Balut Dyke (associated with
the calc-potassic stockwork) does not fit this fractionation trend, and is interpreted to
have formed by interaction between the late-stage syenitic melt and a co-magmatic
mafic melt that underplated the siliceous magma chamber prior to formation of the
Balut Dyke. A reversion to fractionation-dominated magmatic processes in the silicic
magma chamber then lead to the intrusion of the quartz-saturated Quan Porphyry and
Bufu Syenite. Ultimately, the residual mafic melt was emplaced as a series of late-stage
andesite dykes.
The Dinkidi porphyry Cu-Au deposit shows that it is possible for silica-undersaturated
and silica-saturated styles of alkaline porphyry mineralisation to form in the same
magmatic-hydrothermal system, given the right conjunction of geological processes.
Exploration models for alkaline porphyry systems therefore need to be flexible enough
to accommodate the possibility of silica-undersaturated and saturated mineralised zones
forming in the same deposit. The strongly mineralised, variably textured Balut Dyke
shares textural and genetic similarities with mineralised pegmatite dykes from midcrustal
granitic environments. Highly eu-Au mineralised 'pegmatitic' dykes should
therefore also be considered as a viable exploration target in alkaline porphyry systems.

Item Type: Thesis (PhD)
Publisher: Royal Society of Tasmania, University of Tasmania Library Special and Rare Materials Collection
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Date Deposited: 05 Jul 2011 01:53
Last Modified: 11 Mar 2016 05:54
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