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The Dinkidi Cu-Au porphyry : geology of the Didipio region and paragenesis of the Dinkidi Cu-Au porphyry deposit

Wolfe, RC 2002 , '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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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/t 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 minor 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 plutons 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 clinopyroxene-gabbro-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 emplacement of the early diorite phase; (2) K-silicate magnetite-biotite alteration, and subeconomic Cu-Au 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 quartz-undersaturated 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 (1-2g/t Au) at Dinkidi and is typical of silica-saturated alkaline porphyry systems. A coarse-grained assemblage of quartz-actinolite-perthite (the 'Bugoy Pegmatite') formed as an apophysis on the Bufu Syenite, and was subsequently brecciated by late-stage faulting. High-level argillic and late-stage fault-related 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 mineralisation. 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 Cu-Au mineralised 'pegmatitic' dykes should therefore also be considered as a viable exploration target in alkaline porphyry systems.

Item Type: Thesis - PhD
Authors/Creators:Wolfe, RC
Publisher: Royal Society of Tasmania, University of Tasmania Library Special and Rare Materials Collection
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Copyright 2001 the author

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