Abstract

The Dinkidi Cu-Au porphyry deposit, northern Luzon, Philippines, formed at the tip of the southward-propagating Cagayan Valley basin in a late Oligocene to early Miocene back-arc setting. The 110 Mt ore deposit is characterized by elevated hypogene grades (avg 1.2 g/t Au and 0.5% Cu) and is one of only a few alkalic porphyry deposits to have been discovered outside of British Columbia and eastern Australia.

Dinkidi is hosted by the Didipio intrusive complex, which intruded calc-alkalic to shoshonitic volcanic rocks of the Mamparang and Upper Mamparang formations in the late Oligocene. An early, composite diorite-monzodiorite pluton was intruded by the Surong monzonite and the Dinkidi stock. The initial phases of the Cu-Au mineralized Dinkidi stock were the equigranular biotite-amphibole–bearing Tunja monzonite and the Balut dike—a thin, variably textured and strongly mineralized clinopyroxene syenite pegmatite dike. The youngest phases of the Dinkidi stock are a plagioclase-phyric monzonite-syenite (Quan porphyry) and the Bufu syenite, a crystal-crowded leucocratic quartz syenite. Postmineralization andesite dikes cut the intrusive complex.

Emplacement of the Tunja monzonite was temporally and spatially associated with pervasive stage 1 biotite-magnetite-K silicate alteration, which primarily affected the diorite-monzodiorite pluton. Emplacement of the diopside-phyric Balut dike produced the stage 2 calc-potassic diopside-actinolite-K feldspar-bornite vein stockwork and a calc-potassic alteration assemblage typical of silica-undersaturated alkalic porphyry deposits. Stage 2 lacks quartz, contains high gold grades (2–8 g/t Au) and its sulfides have δ34S values of −3.5 to −0.7 per mil. The stage 2 calc-potassic assemblage is inferred to have formed at temperatures in excess of 600°C from an oxidized (sulfate-predominant) Na-K-Ca-Fe–rich brine.

Intrusion of the quartz-saturated Quan porphyry and Bufu syenite led to the formation of the stage 3 quartz-illite-calcite-chalcopyrite stockwork vein and alteration assemblage. The quartz stockwork hosts most of the lower-grade (1–2 g/t Au) mineralization at Dinkidi and is typical of silica-saturated alkalic 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 faulting late in stage 3. The stage 3 quartz stockwork was emplaced at high temperatures (mostly >600°C) from a quartz-saturated, oxidized (sulfate-predominant) Na-K-Fe brine (>68 wt % NaCl equiv) that contained up to 0.6 wt percent Cu and 4 wt percent Fe. Cooling to ~430°C and sulfate reduction by wall-rock interaction led to the precipitation of stage 3 sulfides with δ34S values of −4.2 to −0.2 per mil in the quartz stockwork. The quartz-bearing assemblage formed at paleodepths of 2.9 to 3.5 km. Periods of quartz growth from overpressured brines were interrupted episodically by brittle failure events that caused the system to depressurize to near-hydrostatic conditions, triggering vapor generation via boiling. Mineralization was followed by intermediate argillic and high-level advanced argillic alteration (stage 4), and by late-stage, fault-related zeolite-calcite alteration and veins (stage 5).

The hydrothermal mineral assemblages at Dinkidi reflect the composition and degree of fractionation of the associated intrusions. Extensive fractionation in a silica-undersaturated dioritic magma chamber is interpreted to have ultimately caused quartz saturation and the development of the late-stage syenite intrusions and related quartz stockwork mineralization. The calcic, silica-undersaturated pegmatitic Balut dike, which is 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 comagmatic mafic melt that underplated the siliceous magma chamber prior to formation of the Balut dike. A reversion to fractionation-dominated magmatic processes in the silicic magma chamber then led 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 dikes.

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