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3582149 
Journal Article 
Geology of the Didipio Region and Genesis of the Dinkidi Alkalic Porphyry Cu-Au Deposit and Related Pegmatites, Northern Luzon, Philippines 
Wolfe, RC; Cooke, DR 
2011 
Yes 
Economic Geology and the Bulletin of the Society of Economic Geologists
ISSN: 0361-0128
EISSN: 1554-0774 
SOC ECONOMIC GEOLOGISTS, INC 
LITTLETON 
106 
8 
1279-1315 
English 
WOS:000296683600003 
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 bionte-amphibole-bearing Tunja monzonite and the Balut dike-a thin, variably textured and strongly mineralized clinopyroxene syenite pegmatite dike. The youngest phases oldie 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-nude saturated alkalic porphyry deposits. Stage 2 lacks quartz, contains high gold grades (2-8 g/t Au) and its sulfides have delta S-34 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 degrees C from an oxidized (sulfate-predominant) Na-K-Ca-Fe-rich brine.Intrusion of the quartz-saturated Quart porphyry and Bufu syenite led to the formation of the stage :3 quartzillite-calcite-chalcopyrite stockwork vein and alteration assemblage. The quartz stock-work 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) termed 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 degrees 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 similar to 430 degrees C and sulfate reduction by wall-rock interaction led to the precipitation of stage 3 sulfides with delta S-34 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 stock-work 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 mane 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 Be In syenite. Ultimately, the residual mafic melt was emplaced as a series of late-stage andesite dikes. 
Physical Sciences - Other Topics; new-south-wales, copper-gold deposits, epithermal transition, baguio; district, granitic pegmatites, recent enrichment, fluid inclusions,; magmatic fluids, stable-isotope, igneous rocks