Silangan, Boyongan, Bayugo
Super Porphyry Cu and Au|
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The Boyongan and Bayugo porphyry copper-gold deposits of the Silangan Project are located on the Surigao Peninsula in the provinces of Surigao del Norte and Agusan del Sur in northeastern Mindanao Island in the Philippines, some 20 km south of the provincial capital Surigao. Boyongan is ~2 km SE of Bayugo,
(#Location: 9° 33' 0"N, 125° 29' 18"E).
The Philippine archipelago currently lies within Philippine mobile belt that marks the juncture of the Philippine Sea oceanic plate to the east, and the Eurasia plate to the west. Sandwiched between these two converging plates, the Philippine mobile belt (Gervasio, 1966) is an actively deforming tectonic collage of oceanic crustal blocks surrounded on most sides by destructive plate margins. Four major subduction zones are currently consuming oceanic crust beneath the Philippines:
i). To the west, collision with the Eurasia Plate is driving east- and NE-directed subduction of South China, Sulu, and Celebes oceanic crust into the Manila (west of Luzon), Negros-Sulu (NW of Mindanao) and Cotabato (west of Mindanoa) trenches, respectively,
ii). East of the central and southern Philippines, the Philippine Sea plate is undergoing west-dipping subduction below the archipelago into the Philippine trench.
Convergence between the Philippine Sea and Eurasia plates is oblique at the Philippine trench, compensated by the major arc-parallel, sinistral, strike-slip Philippine fault zone, which extends longitudinally through the Philippine archipelago for more than 1200 km north to south, through the islands of Luzon, Masbate, Leyte and Mindanao (Braxton and Mathur, 2011).
Late Miocene to Recent arc magmatism related to the westward dipping subduction of the Philippine Sea plate into the Philippine
trench produced a number of volcanic and intrusive complexes in northeastern Mindanao, including the mid-Pliocene Boyongan Intrusive Complex of the Surigao Peninsula, with which the Bayugo and Boyongan porphyry copper-gold deposits are associated (Braxton and Mathur, 2011).
Boyongan and Bayugo lie within an old gold mining district that straddles the north-south trending Pliocene volcano-magmatic arc of eastern Mindanao.
The geology of the district enclosing the two deposits comprises a sequence of Pliocene carbonates, clastic sediments and basic to intermediate volcanics, volcanic breccias and volcaniclastics and coeval intrusions. The deposits, which are situated on the margin of a dormant volcano, are not exposed at surface, being obscured by 50 to 300 m of Quaternary volcanics and sediments.
At both Bayugo and Boyongan, hypogene copper and gold mineralisation, occurring as chalcopyrite, bornite and electrum, are accompanied by K silicate-stage copper-sulphide ± quartz vein stockworks in spatial and temporal association with early mineralised Pliocene diorite porphyry stocks (Braxton et al., 2006). At Boyongan, these progenitor stocks were emplaced into a silt-sand matrix diatreme breccia complex, while those at Bayugo intruded an earlier Pliocene diorite
complex and Oligocene to Miocene volcano-sedimentary rocks (Braxton, 2007).
At both deposits, hypogene copper and gold were concentrated in two distinct east and west zones flanking discrete, high-aspect ratio ('pencil-shaped') stocks that comprise inter- and late-mineralisation diorite intrusions which cut the early mineralisation stocks. Late, but limited, feldspar-destructive illite-pyrite alteration affected all intrusive phases at both deposits (Braxton and Mathur, 2011).
The diorite complex at Boyongan and Bayugo includes at least 12 discrete diorite phases, distinguished on the basis of texture and timing relationships to veining, alteration, and brecciation. Mineralising and brecciation events serve to subdivide diorite porphyry emplacement into temporally distinct episodes (Braxton et al., 2009):
i). Initial pre-mineralisation Intrusions, comprising an early diorite complex of at least three intrusive phases: a). birds-eye diorite porphyry, b). medium-grained diorite porphyry, and c). fine-grained
diorite porphyry. These intrusive events predated copper-gold mineralisation.
ii). At Boyongan, a large, pre-mineralisation, upward flaring, generally cylindrical, 900 x 1000 m, polyphase, silt- and sand matrix-rich breccia pipe complex has partially fragmented the early diorite complex and surrounding wall rock. Wall-rock clast dominated facies occupy the margins of the breccia complex, while breccia facies dominated by intrusive clasts comprise the breccia complex core. The breccia complex varies between matrix-supported and clast-supported and generally lacks stratification or other internal organization. The altered and weathered breccia matrix is interpreted to be composed of finely comminuted rock fragments. In Bayugo, only localised brecciation is observed along contacts between intrusive lithologies, especially in the early diorite phases. Breccias in Bayugo are attributed to the emplacement of the diorite stocks and are limited in dimensions. They are classified on the basis of the dominant clast type.
iii). A series of early-mineralisation diorite porphyry stocks, which intruded the diatreme at Boyongan, have a spatial relationship to elevated copper-gold grades and quartz-vein stockworks. These composite stocks intruded the diatreme in two places, ~200 m apart, producing the discrete East and West high-grade zones. Quartz-magnetite cemented breccias were formed in spatial and temporal relation to these intrusions in the East high-grade zone. At Bayugo, the early-mineralisation diorite porphyry is the intrusive phase associated with quartz-vein stockworks and elevated copper-gold grades.
iv). An Inter-mineralisation Diorite Porphyry, a distinctly coarser-grained, crowded, plagioclase- and hornblende-phyric diorite stock followed emplacement of the early mineral intrusive. This represents the deepest intrusion encountered in the Boyongan intrusive complex that truncated the Early Mineralisation diorite porphyry series. The counterpart at Bayugo, was encountered at depth in the East zone where the Early Porphyry grades into a coarser porphyry. Quartz veining is locally abundant in this intrusion, although copper-gold mineralisation is significantly lower than in both Early Mineralisation porphyries (Oliveros, 2011).
v). Late-Mineralisation Diorite Porphyries are evident at both Boyongan and Bayugo as narrow, <1 to 15 m thick dykes intermittently cutting the earlier phases. These dykes are seriate to crowded, plagioclase- and horneblende-phyric with limited quartz veining. The distinct texture and low quartz veining distinguish these from other intrusive units. Xenoliths of quartz veins are also found in abundance. An age determination for the Late Diorite Porphyry (zircon U-Pb SHRIMP; Braxton, 2007) indicates a late Pliocene crystallisation age of 2.1 ±0.2 Ma (Anglo American, 2008; Oliveros, 2011)).
vi). A Post-Mineralisation Coarse-Grained Diorite is also intersected at depth in Bayugo. This unit is petrographically similar to the pre-mineralization intrusive, but much more crowded and coarser-grained (Oliveros, 2011).
At both the Boyongan and Bayugo deposits, a pervasive potassic alteration is observed within the early-mineralisation diorite porphyry stocks, whilst illite-smectite-pyrite alteration, usually associated with argillic alteration over prints the potassic zone to varying degrees. Skarn alteration is observed along the contacts between carbonate-rich units and intrusive rocks. Propylitic alteration is well developed along the periphery of the diorite stocks and dissipates outwards. Mineralisation is concentrated on the early-mineralisation diorite porphyry stocks, with copper and gold grades decrease systematically outside the early stock towards the wall rock (Oliveros, 2011).
Elevated copper and gold hypogene mineralisation grades in Boyongan-Bayugo are associated with the early mineralisation diorite porphyries. In Boyongan, hydrothermal breccias spatially related to the early mineralisation diorite porphyries also carry significant copper-gold. Mineralisation in Bayugo is also correlated with the pre-mineralisation intrusions as these serve as host to the early mineralisation intrusion. Hypogene mineralization is present both as sulphide dissemination and sulphide in quartz veins. Chalcopyrite dominates as the primary copper mineral in the sulphide zone with rare to weak bornite. Hypogene gold occurs as pure gold, electrum and as Au-Ag tellurides locked in pyrite, bornite and rarely in chalcopyrite. Pyrite is widespread although it is more common in Bayugo than in Boyongan (Oliveros, 2011).
At Boyongan, bornite and chalcopyrite are the principal hypogene copper sulphides, in and around the early-mineralisation diorite porphyry stock in the East high-grade zone. Bornite is largely absent in the intermineralisation diorite stock and late-mineralisation diorite porphyry dykes, where chalcopyrite is the only significant hypogene copper sulphide. In the West high-grade zone at Boyongan there is near-complete oxidation of the hypogene sulphides (Braxton et al., 2009).
At Bayugo, chalcopyrite is the dominant hypogene copper sulphide, with virtually no bornite, although overall sulphide contents are low, with chalcopyrite generally <2%. A >250 m wide peripheral pyrite halo lies between 250 and 500 m from the edges of the early- and intermineralisation diorite stocks (Braxton et al., 2009).
Copper and gold grades correlate directly in the hypogene zone, where both metals are primarily associated with K silicate-stage chalcopyrite ± bornite ± quartz veins. Gold occurs principally as electrum (commonly as micron-scale blebs within bornite), and locally as calaverite (Braxton et al., 2009).
Pervasive zones of intense, texturally destructive illite-pyrite-alteration have overprinted the K silicate-altered rocks, occurring as narrow (0.2 to 10 m wide), localised, structurally controlled or breccia zones, with accessory pyrophyllite, kaolinite, chalcopyrite, tetrahedrite-tennantite, sphalerite, galena and locally siderite. The illite-pyrite-altered structures at both deposits may represent the root zones of widespread quartz-clay ± alunite-altered rock preserved as clasts within Quaternary debris flows that flank the Pliocene porphyry deposits (Braxton et al., 2009).
Exhumation and weathering during the late Pliocene-Pleistocene produced distinctive supergene profiles at both Boyongan and Bayugo. Mid-Pleistocene transtension in the Surigao Peninsula promoted subsidence and volcanism associated with the Mainit graben. Sunsequent burial of the deposits beneath volcanic and fluvio-lacustrine detritus ended oxidation and contributed to preservation of the supergene profiles (Braxton et al., 2009).
Supergene profiles are well developed at both Boyongan and Bayugo, although they differ in their mineralogy and vertical extent. Following the emplacement of all of the diorite phases, the system is tectonically exhumed exposing its upper portion to supergene processes. During this stage, the upper portion of the deposit was eroded and the hypogene mineralised zone was subjected to supergene oxidation and enrichment. A thin 10 to 50 m remnant leached cap formed over an oxidised zone. A supergene enriched zone with secondary chalcocite and native copper formed below the oxidised zone, occurring along the contact between the oxide and sulphide zones. In Boyongan, oxidation extends to ~600 m depth in the the centre of the deposit, creating an oxidized zone and a localised zone of secondary sulphide enrichment. Bayugo, has a shallower oxidation profile but a more pronounced enriched zone of secondary sulphide as chalcocite and/or native copper (Oliveros, 2011).
At Boyongan, copper oxides and carbonates (mainly cuprite, malachite and azurite) predominate in the supergene profile and persist throughout a vertical interval of >600 m below the palaeosurface on the western end of the weathering profile. At the base of oxidation, a zone of mixed copper oxides, native copper, chalcocite and chalcopyrite ± bornite occurs, particularly on the eastern side ofthe deposit (Braxton et al., 2009). Within this lower sulphide zone at th etop of the hypogene mineralisation, chalcocite, digenite, djurleite and covellite occur as overgrowths on, and as fractures filling within, a variety of hypogene sulphide phases (chalcopyrite, bornite, and pyrite), consistent with a supergene origin (Ignacio, 2005; Braxton, 2007).
Drilling at Boyangan has outlined a 600 x 300 m area within the oxide zone carrying >1% Cu equivalent grades embraced by a larger 1000 x 500 m area containing more than 0.5% Cu equivalent. The deposit is composed of an east and a west zone, each representing a pre-Quaternary cover basement high, with supergene mineralisation from as shallow as 50 m below surface (Braxton and Mathur, 2011).
At Bayugo, a 10 to 50 m thick goethitic leached cap overlies a discontinuous, 10 to 50 m thick oxide copper zone. A thicker (50 to 140 m) zone of enriched copper sulphides (chalcocite with minor digenite, djurleite and covellite) lies between the oxide and hypogene sulphide zones, where the supergene copper sulphides grade downwards into quartz stockworks and/or related K silicate alteration assemblages with lower copper grade and an exclusively hypogene chalcopyrite ± bornite assemblage. A tabular zone of exotic supergene copper sulphides and oxides extends southeast from the early mineralisation stock at East Bayugo (Braxton and Mathur, 2011).
Gold is found in all of the supergene zones, occurring either as free pure phases or as discrete phases associated with pyrite and chalcocite. If not destroyed by oxidation, 23% of gold occurs as Au-Ag tellurides. Pure phases are common in the oxide zone while gold associated with pyrite and chalcocite is observed more abundantly in the enriched zones.
Published mineral resource estimates at July 2011 (Philex Mining Corporation, release, 2011) based on a cut-off of 0.5% Cu equiv., were:
Measured resource - 201 Mt @ 0.54% Cu, 0.78 g/t Au;
Indicated resource - 72 Mt @ 0.46% Cu, 0.57 g/t Au;
Measured+Indicated resource - 273 Mt @ 0.52% Cu, 0.72 g/t Au;
Inferred resource - 26 Mt @ 0.41% Cu, 0.49 g/t Au.
Measured resource - 99 Mt @ 0.64% Cu, 0.65 g/t Au;
Indicated resource - 26 Mt @ 0.76% Cu, 0.69 g/t Au;
Measured+Indicated resource - 125 Mt @ 0.66% Cu, 0.66 g/t Au;
Inferred resource - 7 Mt @ 0.77% Cu, 0.60 g/t Au.
The mineral resource estimate at a cut-off of 0.5% Cu equiv., for Boyongan and Bayugo as of August 1, 2019, are as follows (Philex Mining Corporation Annual Reporrt, 2020):
Measured+Indicated resource - 279 Mt @ 0.52% Cu, 0.70 g/t Au;
Inferred resource - 218 Mt @ 0.36% Cu, 0.49 g/t Au.
Sub-total - 497 Mt @ 0.45% Cu, 0.61 g/t Au.
Measured+Indicated resource - 172 Mt @ 0.57% Cu, 0.60 g/t Au;
Inferred resource - 4 Mt @ 0.27% Cu, 0.42 g/t Au.
Sub-total - 176 Mt @ 0.57% Cu, 0.59 g/t Au.
Bayugo-Kalayaan a joint venture with Manila Mining Corporation
Measured+Indicated resource - 120 Mt @ 0.44% Cu, 0.47 g/t Au;
Inferred resource - 2 Mt @ 0.81% Cu, 0.40 g/t Au.
Sub-total - 122 Mt @ 0.44% Cu, 0.47 g/t Au.
Boyongan and Bayugo combined
Measured+Indicated resource - 571 Mt @ 0.52% Cu, 0.62 g/t Au;
Inferred resource - 224 Mt @ 0.36% Cu, 0.48 g/t Au.
TOTAL - 795 Mt @ 0.47% Cu, 0.58 g/t Au.
Silangan Mindanao Mining Co., Inc., a wholly owned subsidiary of Philex Mining Corporation, plans to commence commercial production from the Silangan mine exploiting the Boyongan deposit in 2024 (Philex Annual Report, 2020). A 4 Mt per annum sub-level cave mine is planned.
This summary is drawn from the references listed below and from Oliveros, N.C., 2011 - Technical Report on the Boyongan-Bayugo deposit in Surigao Del Norte, Mindanao, Philippines; a technical report of exploration results and mineral resources, compliant with the Philippine Mineral Reporting Code, prepared for Philex Mining Corporation; 132p.
The most recent source geological information used to prepare this summary was dated: 2011.
Record last updated: 23/12/2012
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.
Braxton D and Mathur R, 2011 - Exploration Applications of Copper Isotopes in the Supergene Environment: A Case Study of the Bayugo Porphyry Copper-Gold Deposit, Southern Philippines: in Econ. Geol. v.106 pp. 1447-1463|
Braxton D P, Cooke D R, Ignacio A M, Rye R O and Waters P J, 2009 - Ultra-Deep Oxidation and Exotic Copper Formation at the Late Pliocene Boyongan and Bayugo Porphyry Copper-Gold Deposits, Surigao, Philippines: Geology, Mineralogy, Paleoaltimetry and their Implications for Geologic, Physiographic and Tectonic Control: in Econ. Geol. v104 pp 333-349|
Braxton, D.P., Cooke, D.R., Ignacio, A.M. and Waters, P.J., 2018 - Geology of the Boyongan and Bayugo Porphyry Cu-Au Deposits: An Emerging Porphyry District in Northeast Mindanao, Philippines: in Econ. Geol. v.113, p 83-131.|
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