Super Porphyry Cu and Au|
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The Dinkidi porphyry copper-gold deposit, at Didipio, is located near the boundary of Nueva Vizcaya and Quirino Provinces on the island of Luzon, ~270 km NNE of Manila and 100 km east of Baguio City, Philippines.(#Location: 16° 51' 30"N, 120° 47' 0"E).
The deposit was formed at the southern tip of the ~200 x 50 km, north-south elongated, southward-propagating Cagayan Valley basin in a late Oligocene to early Miocene back-arc setting. The Cagayan Valley basin is bounded by the Northern Sierra Madre and Caraballo Mountains, and the Central Cordillera to the east, south and west respectively. The Caraballo Mountains and Central Cordillera are in turn bounded to the SW and west by the major north-south Philippines Fault, a sinistral strike-slip fault complex that longitudinally divides the Philippine archipelago. This structure lies midway between the western (east-dipping) and eastern (west-dipping) subduction zones that straddle the archipelago, and roughly parallels them in strike. It is believed to have been active since the Late Pliocene (~4 Ma).
The basement to the Cagayan Valley basin to the east and south comprises tectonically amalgamated schists and Cretaceous ophiolites. These are ovelain by the Caraballo Group, a thick pile of primitive Eocene island-arc volcanic (MMAJ-JICA, 1977). These volcanic rocks were subsequently intruded by the tonalitic Eocene Coastal batholith (MMAJ-JICA, 1977; Knittel et al., 1988), and by the Oligocene tholeiitic to calc-alkalic diorite Dupax and Northern Sierra Madre batholiths. Shoshonitic lavas of the Upper Mamparang Formation and silica-undersaturated alkalic intrusions were then emplaced along the rifted western margin of the Cagayan Valley basin, associated with the commencement of rifting (MMAJ-JICA, 1977, 1987; Knittel et al., 1988; Hollings et al., 2011).
Three late Oligocene to early Miocene alkalic intrusive complexes, the Didipio (Wolfe et al., 1999; Wolfe, 2001) and Cordon Syenite complexes (Knittel, 1987; Knittel and Cundari, 1990) and the Palali batholith (Albrecht and Knittel, 1990) are known within the region. These units are unconformably overlain by Oligocene to middle Miocene shallow-water carbonates of the Santa Fe, Aglipay, Ibulao, Callo and Columbus Formations (Corby, 1951; Durkee and Pederson, 1961; Christian, 1964; MMAJ-JICA, 1977; Caagusan, 1980), which are intercalated with the youngest volcanic units preserved in the region, the alkalic Palali Formation (MMAJ-JICA, 1977).
The Cagayan Valley basin began to subside in the late Oligocene and was filled by a thick succession of Miocene to Plio-Pleistocene carbonates and clastic sediments, that included the Maddela, Matuno and Pantabangan Formations (Christian, 1964; MMAJ-JICA, 1977; Caagusan, 1980).
During the interval 25 to 23 Ma, three contemporaneous magmatic suites were emplaced into Northern Luzon:
i). the Dupax and Northern Sierra Madre batholiths, representing a waning arc-tholeiitic-calc alkalic suite;
ii). the Didipio and Cordon Syenite complexes and the Palali batholith alkalic suites at the tip of the Cagayan Valley basin, and
iii). the initial phases of the Central Cordillera Diorite Complex (Knittel and Defant, 1988; Wolfe, 2001; Hollings et al., 2011; Waters et al., 2011).
Although there are compositional differences between these suites, radiogenic isotope data suggests they shared a similar mantle source (Knittel et al., 1988; Hollings et al., 2011).
Regionally, the volcanics and sediments are folded about north-south anticlinal and synclinal axes, and are cut by prominent, steeply dipping, north-west and north-trending faults sub-parallel to the major Philippine Fault zone. A set of later, steeply north dipping, ENE trending faults are associated with the batholitic intrusions.
The oldest rocks in the immediate Didipio district are the Mamparang Formation, which comprises a calc-alkalic to shoshonitic sequence of andesites and related volcaniclastic rocks that outcrop to the SE. These include volcanic facies of alternating andesitic lavas and volcaniclastic breccias, while sedimentation produced volcaniclastic arkoses preserved at the top of some breccia beds. The immaturity of the breccias, indicate derivation from a proximal volcanic source.
The overlying shoshonitic Upper Mamparang Formation which occurs on the eastern, northern, and western margins of the district, is composed of an upper 25.66±0.22 Ma pale, coarsely porphyritic trachytes and related trachytic volcaniclastic rocks, underlain by andesite-dominated volcaniclastic units with trachyte clasts which increase in abundance upwards to the trachytic rocks. The andesitic and trachytic sequences together are at least 400 m thick. All observed contacts between the trachytes and the Didipio intrusive complex are faulted, although stratigraphic and other relationships indicate that the trachytes were erupted before emplacement of the intrusive complex. Nevertheless, the Upper Mamparang Formation is coeval with the 25 to 23 Ma Cordon Syenite Complex, Palali batholith and Didipio intrusive complex (Knittel, 1983; Albrecht and Knittel, 1990; Hollings et al., 2011).
The Didipio Intrusive Complex, which hosts the Dinkidi porphyry Cu-Au deposit, intrudes the volcanic and volcaniclastic rocks of the Mamparang and Upper Mamparang Formations, and includes a suite of rocks with alkalic compositions ranging from gabbro through syenodiorite to syenite (Wolfe, 2001; Hollings et al., 2011). It consists of four main intrusive phases:
i). an early composite diorite-monzodiorite pluton, comprising diorite and monzodiorite, with minor cumulate phase clinopyroxene gabbros which are exposed along the flanks of the pluton. The most primitive noncumulate phase is a fine- to medium-grained dark grey epidote-chlorite-altered clinopyroxene diorite, also on the outer margin of the Didipio intrusive complex. This diorite was intruded by a fine- to medium-grained equigranular to plagioclase-phyric clinopyroxene monzodiorite that is intensely K metasomatised adjacent to the subsequent Surong monzonite;
ii). The Surong monzonite, a weakly mineralised monzonite that intrudes the composite diorite-monzodiorite pluton and occupies the core of the Didipio Intrusive Complex. Its outer contact is characterised by a widespread monzonite-cemented breccia, with an associated broad zone of intense biotite-magnetite alteration and weak Cu-Au mineralisation that extends at least 200 m into the surrounding diorites. Hydrothermal magnetite and biotite selectively replace primary mafic minerals, while epidote, chlorite and actinolite have locally selectively overprinted primary clinopyroxene and hydrothermal biotite. The Surong monzonite is also a composite pluton, containing several phases, the most abundant of which are fine- to medium-grained equigranular clinopyroxene monzonites, which are cut by crystal-crowded biotite monzonites and thin (30 to 500 cm wide), weakly mineralised quartz monzonite and equigranular micromonzonite dykes. It has been dated at 25.15±0.20 Ma (U-Pb zircon; Hollings et al., 2011), and encloses isolated rafts of diorite up to 200 m long;
iii). The Dinkidi stock, which is located at the eastern margin of the Surong monzonite and is dominantly hosted by the composite monzonite stock. It comprises a strongly mineralised composite monzonite-syenite stock, the northern and southern ends of which are truncated by shear zones, while the core has been brecciated during late stage
faulting. The stock is 150 m wide and 600 m long and consists of four phases each of which has its own distinctive hydrothermal features:
Tunja monzonite, which hosts the bulk of the Cu-Au mineralisation, is the oldest unit in the stock. It is a 24.79±0.22 Ma (Hollings et al., 2011), medium-grained white- to pale pink-grey, equigranular biotite-amphibole to weakly plagioclase-phyric equigranular monzonite. This phase occurs as an elongate 150 to 200 m wide, 600 m long body that persists to at least 800 m deep, with small dykes extending at least 100 m laterally into the wall rocks. It contains xenoliths of clinopyroxene diorites, monzodiorites, and a xenolith-rich monzodiorite breccia.
Balut dyke, a ~10 to 30 m wide, and up to 600 m deep clinopyroxene syenite dyke intruding the Tunja monzonite. It displays extreme textural variations over short distances, with three main styles: a). fine-grained equigranular aplite; b). coarsely porphyritic pegmatite; and c). layered pegmatite. All consists predominately of perthite and diopside with accessory magnetite, titanite, apatite and plagioclase.
Quan porphyry, a light grey plagioclasephyric porphyry that varies from monzonite to syenite and has intruded the diorites, Tunja monzonite, and Balut dyke. It contains abundant miarolitic cavities (up to 1 to 2 mm) filled with medium-grained anhedral quartz.
Bufu syenite, the 24.81±0.28 Ma (U-Pb zircon) crystal-crowded syenite to quartz syenite core of the Quan porphyry, which also defines the core of the Dinkidi stock. The gradational contact between the Bufu syenite and the Quan porphyry indicates synchronous emplacement, although crosscutting dykes suggest emplacement continued after final consolidation of the Quan porphyry.
Bugoy pegmatite, 1 to 5 m thick, coarse-grained, massive, quartz-perthite and actinolite apophyses above and along the walls of the Bufu syenite. The pegmatite has been almost completely brecciated, forming the quartz fragment-rich Bugoy breccia above the Bufu syenite. The pegmatite and breccia is associated with intense quartz-orthoclase flooding and quartz stockwork veining. The outer margin of the massive quartz-perthite zone occurs immediately below a domain of intense quartz veining and rare 5 to 50 cm thick dykes of the Bugoy pegmatite that have intruded the Tunja monzonite, Balut dyke and Quan porphyry.
iv). late-stage andesite dykes and sills - 1 to 5 m thick, un-altered, dark green-grey, fine- to medium-grained hornblende-bearing andesite porphyry dykes mark the final stage of magmatic activity in the Didipio Region. Locally extensive andesite sills and rare irregular intrusive bodies up to 200 m wide appear to be related to the late-stage dykes.
The Didipio deposit is a roughly elliptical alkalic gold-copper porphyry system, with surface dimensions of 450 x 150 m, occurring as a near vertical pipe that extends to at least 800 to 1000 m below the surface. The tabular composite intrusive and
associated alteration and mineralisation strike grid north-south and dip at 80 to 85° east. Higher-grade gold and copper mineralisation is closely associated with the Quan diorite and Bugoy breccia, both of which are elongated in plan view along the north-south trending, steeply east-dipping Tatts Fault Zone.
Alteration comprises the following stages:
Pervasive stage 1 biotite-magnetite-K silicate alteration was temporally and spatially associated with emplacement of the the Tunja monzonite, and primarily affected the diorite-monzodiorite pluton. The diorite wall rocks have undergone pervasive orthoclase alteration for up to 20 m from contacts with the Tunja monzonite. The orthoclase alteration assemblage has local associated igneous-cemented and magmatic-hydrothermal breccias and by coarse-grained biotite-K feldspar vein-dykes. Clots of biotite-magnetite alteration are found in the diorites up to 500 m from the Tunja monzonite contact. These clots grade outward to a broad selective halo of propylitic alteration (epidote-pyrite) with rare epidote-chlorite-pyrite veinlets. The Tunja monzonite has undergone extensive internal sodic alteration, resulting in the ubiquitous albitisation of plagioclase laths and accessory apatite, anhydrite, chalcopyrite and pyrite. This sodic alteration is interpreted to represent a late-stage magmatic alteration assemblage.
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, accompanied the emplacement of the diopside-phyric Balut dyke. This stage, which was the first high-grade Cu-Au mineralising event, is devoid of quartz, contains high gold grades (2 to 8 g/t Au) has sulphides with δ34S values of -3.5 to -0.7‰. The stage 2 calc-potassic assemblage is inferred to have formed at temperatures in excess of 600°C from an oxidised (sulphate-predominant) Na-K-Ca-Fe–rich brine. Stage 2 alteration has been divided into four substages:
Stage 2A coarse-grained diopside-perthite veins and breccias. Bornite-rich stage 2A veins are gold-rich, with veins from the stockwork zone typically containing 2 to 20 g/t Au.
Stage 2B pink perthite ± actinolite veins, which contain interstitial apatite, magnetite, bornite, chalcopyrite and titanite, are surrounded by an alteration halo of fine-grained granular orthoclase, and are barren or weakly mineralised.
Stage 2C comprises a more spatially extensive stockwork of green actinolite ±perthite ±bornite ±apatite veins, which also contain subhedral to euhedral perthite and actinolite, and are typically weakly to moderately mineralised, with stockwork zones locally associated with elevated but typically subeconomic Cu-Au mineralisation.
Stage 2D produced stockworks of irregular bornite-chalcopyrite veins with alteration halos of K feldspar, accompanied by orthoclase-sulphide-rich dykes and hydrothermal breccias. These stage 2D structures are characterised by distinctive equigranular mosaics of orthoclase and bornite + gold ± chalcopyrite. The sulphides and orthoclase contain interstitial titanite, apatite, magnetite and either actinolite or rare diopside.
The stage 3 quartz-illite-calcite-chalcopyrite stockwork vein and alteration assemblage accompanies the bulk of the ore-grade Cu-Au mineralization at Dinkidi, and is related to intrusion of the quartz-saturated Quan porphyry and Bufu syenite. The quartz stockwork hosts most of the lower-grade (1 to 2 g/t Au) mineralisation. The Bugoy pegmatite and breccia were emplaced at >600°C from a quartz-saturated, oxidised (sulphate-dominant) Na-K-Fe brine (>68 wt.% NaCl equiv.) that contained up to 0.6 wt.% Cu and 4 wt.% Fe. Wolfe and Cooke (2011) suggest cooling to ~430°C and sulphate reduction by wall-rock interaction led to the precipitation of stage 3 sulphides with δ34S values of -4.2 to -0.2‰ in the quartz stockwork. They also conclude that quartz-bearing assemblage formed at palaeodepths of 2.9 to 3.5 km, with periods of quartz growth from overpressured brines episodically interrupted by brittle failure events causing the system to depressurise to near-hydrostatic conditions, triggering vapor generation via boiling.
Stage 3 alteration has been divided into three substages:
Stage 3A quartz-actinolite ±K feldspar-magnetite-sulphide veins, which contain clear to milky quartz and green-colored actinolite, with interstitial cavities filled by calcite, apatite, titanite, bornite, chalcopyrite and pyrite. Bornite contains small (<1 mm) inclusions of native gold and tennantite, and has been partially replaced by chalcopyrite. The margins of the Bufu syenite and the Bugoy pegmatite host an intense, barren to strongly mineralised domain of stage 3A quartz ±sulphide flooding, with associated disseminated chalcopyrite, bornite, actinolite, magnetite and rare titanite;
Stage 3B quartz ±perthite veins and quartz-sulphide stockwork, representing the main period of quartz veining, consisting of two assemblages: a). barren quartz-perthite veins with related K feldspar alteration, which formed along the Quan porphyry-Bufu syenite contact; and b). an extensive, well-mineralised quartz-sulphide stockwork, with associated quartz-illite-calcite alteration. The stockwork veins are typically 0.5 to 15 cm wide and consist of grey to milky quartz intergrown with calcite, anhydrite, chalcopyrite, pyrite and accessory actinolite, orthoclase and bornite. The stage 3B quartz-sulphide stockwork is associated with an intensely developed illite-calcite alteration assemblage that contains disseminated sulphide mineralisation, the bulk of which is hosted in the Tunja monzonite and Quan porphyry.
Stage 3C calcite-sulphide ±quartz-illite veins, breccias and calcite-illite alteration and widespread, selectively pervasive calcite ± illite alteration of actinolite and plagioclase. Miarolitic cavities in the Bufu syenite were partly filled with calcite, chalcopyrite and pyrite, while sphalerite-galena-pyrite trails cut the Tunja monzonite and Quan porphyry. Breccias are typically clast-supported and consist of >95% angular quartz fragments in an altered matrix which comprises 1 to 5% of the breccia, and has been replaced by stage calcite ±illite,
Stage 4 intermediate argillic and high-level advanced argillic alteration followed mineralisation, and
Late stage 5, fault-related zeolite-calcite alteration and veining.
The deposit is oxidised from the surface to a depth of between 15 and 60 m, averaging 30 m to form a blanket that largely comprises silicification, clay and carbonate minerals, accompanied by secondary copper minerals including malachite and chrysocolla.
Published NI 43-101 compliant ore reserves and mineral resources (Redden and Moore for Oceana Minerals, July, 2011) were:
Proven + probable reserves - 50.65 Mt @ 0.45% Cu, 1.03 g/t Au (included within resources);
(including probable underground reserves - 5.91 Mt @ 0.45% Cu, 2.25 g/t Au);
Measured + indicated resources - 70.17 Mt @ 0.41% Cu, 0.95 g/t Au;
Inferred resources - 30.73 Mt @ 0.23% Cu, 0.44 g/t Au;
Published mineral resources (Joyce and Thomson for Climax Minerals, 2004?) were:
Total resources - 121 Mt @ 0.39% Cu, 0.97 g/t Au (cut-off grade 0.5 g/t Au equivalent).
This summary is redominantly drawn from Wolfe and Cooke, 2011.
The most recent source geological information used to prepare this summary was dated: 2011.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Garrett S, 1996 - The geology and mineralisation of the Dinkidi porphyry related Au-Cu deposit: in Porphyry Related Copper and Gold Deposits of the Asia Pacific Region, Conf Proc, Cairns, 12-13 Aug, 1996, AMF, Adelaide pp 6.1 - 6.15|
Kamenetsky, V.S., Wolfe, R.C., Eggins, S.M., Mernagh, T.P. and Bastrakov, E., 1999 - Volatile exsolution at the Dinkidi Cu-Au porphyry deposit, Philippines: A melt-inclusion record of the initial ore-forming process: in Geology v.27, pp. 691-694.|
Wolfe R C and Cooke D R, 2011 - Geology of the Didipio Region and Genesis of the Dinkidi Alkalic Porphyry Cu-Au Deposit and Related Pegmatites, Northern Luzon, Philippines : in Econ. Geol. v.106 pp. 1279-1315|
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