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Baguio District - Acupan, Antamok, Black Mountain, Ampucao, Kelly, Thanksgiving
Luzon, Philippines
Main commodities: Au

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The Baguio gold district is located near Baguio City, ~220 km north of Manila in the southern foothills of the Central Cordillera of Luzon, near the major sinistral Philippines Fault.

The district has an estimated combined historical production and remaining potential resource of >1250 t of Au and 5 Mt of Cu (Malihan and Ruelo, 2009). It contains a diverse array of epithermal (including the giant Acupan and Antamok and numerous smaller intermediate sulphidation vein systems and the Kelly high sulphidation veins), porphyry (e.g., Ampucao or South Acupan, Black Mountain) and skarn deposits (e.g., Thanksgiving), together with a large, broadly strata bound, advanced argillic lithocap.

The Cordillera has an ophiolitic basement, overlain by a 7 km thick volcano-sedimentary pile, extending, with breaks from the Cretaceous to late Quaternary. The sediments are mainly marine, while the volcanics are basaltic to andesitic and comprise three main successions, the Eocene to Early Miocene Pugo Formation, the Early to Middle Miocene Zig-Zag Formation and the Late Miocene Klondyke Formation. These are intruded by a series of mid Tertiary to Quaternary gabbroic to dioritic and granodioritic bodies. Intrusive activity in the district commenced in the Miocene, with the composite Agno Batholith, a narrow, northerly trending belt to the east of Baguio City, composed of the 21 to 8 Ma Twin Rivers Complex, a low-K calc-alkaline series of 4 phases, together comprising the Kadang trondhjemite, Itogon quartz diorite, Antamok diorite and Liang gabbro. Two younger stocks within the batholith are dated at 7.6 and 5.2 Ma repsectively and comprise the Virac Complex (described below). The Zig-Zag formation, which comprises marine sedimentary rocks, including some limestones, with minor basaltic and andesitic lavas and breccia is intruded by the Twin River Complex and is unconformably overlain by the Klondyke Formation. The Klondyke Formation post-dates the Twin Rivers Complex, is composed of conglomerates, grits, sandstones and mudstones with interbedded andesitic lavas, breccias and tuffaceous beds, and is intruded by the Virac Complex.
  Gold mineralisation is localised within an 8 km wide corridor that extends over a north-south elongated interval of tens of kilometres, bounded to the east by Agno Batholith and to the west by a volcanic pile.
  The gold mineralisation of the district is associated with both porphyry and epithermal fluid types. The porphyry mineralisation is related to sub-volcanic dacitic porphyries of Pleistocene to Pliocene age, accompanied by K-silicate biotite-magnetite-anhydrite alteration, fringed by white sericite and propylitic zones. The superimposed low sulphidation epithermal mineralisation occurs as up to 1 m thick quartzose veins with associated adularia-sericite-silica alteration. Calcite, telluride and base metals are found in the latter phases of mineralisation, followed by barren anhydrite. The most recent manifestation of the hydrothermal system is the multiphase 200 x 300 m Balatoc diatreme.
  The Baguio gold district has produced approximately 800 t of Au and 900 t of Ag from several epithermal and porphyry systems. The Acupan mine has produced over 200 tonnes of gold from over 460 interconnected veins and breccias and veins mined over a length of 2 km and to a depth in excess of 900 m. In 1988, the average grade mined was ~6.1 g/t Au at a 0.4 g/t Au cut-off.

Acupan, one of the major deposits of the district, occurs within an intrusive centre occuppied by the Virac Complex, which comprises the Lacbuban gabbro, Virac granodiorite (5.2±0.3 Ma), Ampucao dacite-porphyry (ADP - 2.4±0.5 Ma), Harwell plug (300 x 450 m, dacitic porphyry with brecciated margins, cut by epithermal veins), Itogon plug (a diatreme-dacite complex, with no significant mineralisation) and multiphase Balatoc diatreme (in 3 phases, the 'Old plug' containing high grade epithermal mineralisation, the 'Young plug' which is 80% of the diatreme, and the 'dacite plug' dated at 1 Ma).   Porphyry-style mineralisation occurred during the final crystallisation of the ADP during the Late Pliocene. The ADP is a sub-volcanic dacite porphyry stock, and does not outcrop at the surface.
  Paragenetic and fluid-inclusion studies indicate two contrasting styles of mineralisaiton and hydrothermal fluid types, each characterised by distinctive fluid-inclusion populations, ore and gangue associations and alteration, representing early porphyry and subsequent epithermal ore-forming events.
  Two porphyry vein stages (stages I and II) are related to emplacement of the subvolcanic ADP porphyry dacite. Stage I (quartz-magnetite-chalcopyrite) veinlets are associated with K-silicate alteration, with a dominant biotite, magnetite and anhydrite assembage. ‘White sericite’ (characterised by K-mica-quartz-pyrite-anhydrite-chlorite) and propylitic alteration halos of anhydrite-quartz-pyrite-chlorite veinlets envelope stage II , cross-cutting the earlier stage I mineralisation. Propylitic alteration is dominated by chlorite-pyrite-anhydrite, with epidote being notably absent. Porphyry related veinlets and alteration zones assay from 0.2 to 5 g/t Au.
  A prolonged and varied geothermal event resulted in the formation of a series of composite banded epithermal veins with an average width of 1 m that extend to depths of at least 1 km (relative to the present-day surface) and over strike lengths of up to 3 km. This event was characterised by periods of hydrothermal brecciation interspersed with more quiescent intervals when delicate to coarsely banded vein material was precipitated. Five distinct mineral assemblages have been defined in the epithermal veins. This epithermal mineralisation is represented by paragenetic stages III to V which occur together as up to one metre wide veins, superimposed on porphyry-style mineralisation. The highest precious metal concentrations are present in fine-grained 'grey quartz' (stage III), recognised predominantly along vein margins in the upper levels of the deposit and as clasts enclosed by later paragenetic stages. Fragments of altered wall rock are hosted by stage III gangue. Stage IVa mineralisation is coarse-grained and typified by ‘white quartz’ together with K-mica, adularia, rhodochrosite and mangano-calcite, accompanied by pyrite, sphalerite, chalcopyrite, galena, free gold and gold-silver tellurides with a gangue in which vuggy, colloform and brecciation textures are all recognised. Later-stage IVb gangue infills central portions of the veins and is dominated by coarse-grained calcite and quartz. Stage V (anhydrite-pyrite-quartz) veins reopen and crosscut those of stages III and IV. Stage IVb and V veinlets locally develop intense propylitic envelopes where they are isolated from the larger veins. Sericitic alteration (K-mica-quartz-pyrite-illite) is in general symmetrically disposed about the epithermal veins in envelopes up to a metre in width, and is surrounded by a large, weakly altered propylitic zone characterised by the mineral association epidote-chlorite-pyrite-carbonate-magnetite.

The larger Antamok gold deposit, hosted by the Zig-Zag Formation, is similar in character and occurs 5 km to the north of Acupan. It comprised a series of sub-horizontal and moderately to steeply pitching ore shoots within a pattern of northwest and east-west striking veins that dip at 70°S and northeast-trending, 70° north dipping veins. The entire vein system is characteristically composed of vein-breccia fillings and consist of (1) highly angular, brecciated, crushed grey quartz, lesser white quartz and sulphides, found in the hanging wall or footwall of the structures and make up 20 to 100% of that structure; (2) massive, coarse to medium grained, banded, ribboned and sheeted, partly brecciated calcite veins which occupy the central part of the vein structure or locally the hanging wall or footwall (opposite to the brecciated veining); (3) massive transgressive anhydrite veining, leaving only isolated islands of older gold bearing vein material, composed of an early anhydrite, which may in places be brecciated, and a late, banded, ribboned and crustiform variety. In the first group, gold occurs in a free form in quartz, with younger auriferous sulphides and tellurides. In the calcite veining, gold is found in sulphides and as free gold in some quartz rich sections. Gold is also found in the anhydrite veins in a similar fashion to the other two styles.

The Black Mountain porphyry Cu-Au deposit is located ~5 km west of Acupan and is characterised by low- to medium-K calc-alkaline arc-related rocks that were emplaced from 6.39 to 2.81 Ma (Hollings et al., 2011, 2013; Waters et al., 2011), intruding the Virac Granodiorite and the Pugo and Zig Zag Formations and the Late Miocene to Early Pliocene Mirador Limestone. It consists of two orebodies hosted by the Black Mountain intrusive complex, and has a total resource of 65 Mt at 0.40% Cu and 0.38 g/t Au (Malihan and Ruelo, 2009). The Main (Kennon) orebody is found at the NW end of the Black Mountain intrusive complex and was block-caved from 1969 to 1983. It had a preproduction reserve of 47 Mt at 0.38% Cu, 0.35 g/t Au and 0.01% Mo (United Nations Development Programme, 1987). The Southeast orebody was block-caved between 1977 and 1983 and had a preproduction reserve of 15 Mt at ~0.37% Cu and 0.26 g/t Au (Sillitoe and Gappe, 1984; United Nations Development Programme, 1987). The Southeast orebody has a NW-SE elongated, 600 x 150 m surface expression. Sulphide mineralization is chiefly chalcopyrite and pyrite, with rare molybdenite and bornite (United Nations Development Programme, 1987; Waters et al., 2011).

The Ampucao or Acupan South porphyry Au-Cu prospect was first discovered by underground drilling on the southern fringe of the Acupan mine. Subsequent field mapping in the late 1990s identified outcrops of potassic alteration and mineralised intrusions associated with the 0.51±0.26 Ma (40Ar-39Ar) Ampucao porphyry complex, which comprises early- and intra-mineralisation quartz diorite porphyries and a late-mineralization dacite porphyry with adakitic affinities (Waters et al., 2011). It is the youngest known porphyry deposit in the Baguio district (Waters et al., 2011). No resources has been estimated, although one-metre composite samples mostly assayed between 0.2 and 5 g/t Au, with free gold only observed from one vein intersection in the ~300 m long drill hole. Average grades in the bottom 70 m of this drill hole were ~0.18% Cu and 0.95 g/t Au. Potassic alteration produced a secondary biotite-magnetite-quartz-anhydrite assemblage in the andesites of the Zig-Zag Formation, and orthoclase-quartz-anhydrite assemblages in the Ampucao porphyry complex and Virac granodiorite (Cooke and Bloom, 1990). Intermediate argillic and phyllic alteration assemblages overprinted the early-formed potassic assemblages. Early quartz-magnetite-anhydrite-chalcopyrite veins were deposited from high temperature (300 to >600°C) hypersaline (30 to >70 wt.% NaCl equiv.) magmatic-hydrothermal brines and low density vapors (Cooke and Bloom, 1990). Later stage phyllic alteration and related quartz–pyrite veins formed from low salinity, moderate temperature fluids (1 to 2 wt.% NaCl equiv.; 260 to 345°C; Cooke and Bloom, 1990).

The Kelly-Baco gold deposit is located in the northern part of the Baguio district, ~2 km SW of Antamok. It lies immediately to the east of, and 200 to 300 m below the Baguio lithocap, a large stratabound sheet of silicic and advanced argillic alteration that crops out at high elevations on the eastern side of Baguio city (Waters et al., 2011). Kelly is atypical of the Baguio epithermal veins, in that it contains tennantite, enargite, and bornite mineralisation, and hypogene advanced argillic alteration assemblages (pyrophyllite-diaspore-alunite; Comsti et al., 1990; Aoki et al., 1993), in addition to intermediate sulphidation illite + base metal ± calcite veins that are more typical of the Baguio district. As well as the high sulphidation enargite, tennantite assemblage, the deposit is distinguished by the presence of tellurium-rich minerals (calaverite and goldfieldite). An average grade of 4 g/t was reported by Mitchell and Leach (1991) for Kelly, although resource figures are not available. Hypogene advanced argillic alteration (pyrophyllite-illite) along the vein has an approximate age of <0.6 Ma, whereas alunite from the nearby Baguio lithocap has been dated at 1.4±0.3 to 0.9±0.1 Ma (Aoki et al., 1993). Mineralisation occurs in steeply north-dipping, east-striking quartz veins hosted by the Oligocene-Miocene Zig-Zag Formation and Pliocene Kelly diorite (K-Ar age of 3.1±1.1 Ma; Aoki et al., 1993). The major vein systems include the Kelly, Bungalow, Kelly 35 south split, Maptung, Comasom, Frank, Lost Herd, and Dapung veins (Comsti et al., 1990). These veins are oriented orthogonal to the major NE- and NW trending vein systems at Acupan, Baguio Gold, and Antamok. The Kelly veins are observed to cut relict biotite and actinolite alteration at surface in the southern (Baco) area of the Kelly vein system, and also on level 170 of the Kelly mine (Comsti et al., 1990), suggesting an underlying porphyry system.

The historic Thanksgiving mine lies on the eastern flank of the Black Mountain porphyry complex. Skarn alteration occurs along contacts between diorite-porphyry and limestone as irregular lens-like masses ranging up to 1.5 m in thickness and 15 to 20 m in length, and as narrow veins up to a metre in thickness and a hundred metres in length. The skarn mineralogy is predominantly garnet and clinozoisite and varies from garnet-rich to clinozoisite-rich, with local minor interstitial calcite and sphalerite. The garnet-rich variety is a compact brown rock composed of euhedral andradite garnets ranging up to about 2 mm in diameter and a little clinozoisite. Calcite and chlorite are generally present in the interstices between the andradite crystals. Where clinozoisite is abundant, it imparts a yellowish colour to the rock and generally occurs as clusters of grains up to several mm across, set in a groundmass of fine-grained clinozoisite and colorless chlorite. The mine exploited a number of small sulphide bodies mined for their gold, silver, zinc, copper, lead, sulphur and cadmium content. The ore occurs as massive sulphide, mainly composed of sphalerite and pyrite with minor chalcopyrite, galena, arsenopyrite, magnetite, hematite, chlorite, calcite, garnet and quartz within the skarn alteration halo. A fine grained assemblage of the tellurides altaite, hessite, petzite and sylvanite are scattered throughout the sulphides. Free gold is found as rare minute particles in the pyrite and in patches of telluride minerals. The massive sulphides occurs as veins and pods, mainly along contacts between diorite porphyry dykes and limestone, with a few veins entirely within the limestone. They vary from several cm up to about 2.5 m in thickness and a hundred metres or more in length. The average head grade of the ore from the mine in 1965 was 35 g/t Au, 170 g/t Ag, 12.11% Zn, 0.6% Cu, 0.36% Pb and 0.01% Cd (Callow, 1967).

The most recent source geological information used to prepare this decription was dated: 2018.     Record last updated: 1/3/2018
This description is a summary from published sources, the chief of which are listed below.
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  References & Additional Information
   Selected References:
Bryner L  1969 - Ore deposits of the Philippines - An introduction to their geology: in    Econ. Geol.   v64 pp 644-666
Cao, M., Hollings, P., Cooke, D.R., Evans, N.J., McInnes, B.I.A. Qin, K., Li, G., Sweet, G. and Baker, M.,  2018 - Physicochemical Processes in the Magma Chamber under the Black Mountain Porphyry Cu-Au Deposit, Philippines: Insights from Mineral Chemistry and Implications for Mineralization: in    Econ. Geol.   v.113, pp. 63-82.
Cao, M.J., Evans, N.J., Hollings, P., Cooke, D.R., McInnes, B.I.A. and Qin, K.Z.,  2021 - Apatite Texture, Composition, and O-Sr-Nd Isotope Signatures Record Magmatic and Hydrothermal Fluid Characteristics at the Black Mountain Porphyry Deposit, Philippines: in    Econ. Geol.   v.116, pp. 1189-1207.
Cao, M.-J., Hollings, P., Evans, N.J., Cooke, D.R., McInnes, B.I.A., Zhao, K.-D., Qin, K.Z., Li, D.F. and Sweet, G.,  2020 - In Situ Elemental and Sr Isotope Characteristics of Magmatic to Hydrothermal Minerals from the Black Mountain Porphyry Deposit, Baguio District, Philippines: in    Econ. Geol.   v.115, pp. 927-944.
Cooke D R  1990 - The development of epithermal gold and porphyry copper style mineralisation within an intrusive centre: Acupan, Baguio District, Philippines: in   Proc Pacific Rim Congress 90 AusIMM, Melbourne    pp 521-528
Cooke D R, Bloom M S  1990 - Epithermal and sub-jacent porphyry mineralisation, Acupan, Baguio District, Philippines: a fluid-inclusion and paragenetic study: in    J. of Geochemical Exploration   v35 pp 297-340 =44 pages (copied 2 onto one)
Cooke D R, Deyell C L, Waters P J, Gonzales R I and Zaw K,  2011 - Evidence for Magmatic-Hydrothermal Fluids and Ore-Forming Processes in Epithermal and Porphyry Deposits of the Baguio District, Philippines: in    Econ. Geol.   v.106 pp. 1399-1424
Cooke D R, McPhail D C  2001 - Epithermal Au-Ag-Te mineralization, Acupan, Baguio district, Philippines: numerical simulations of mineral deposition: in    Econ. Geol.   v96 pp 109-131
Cooke D R, McPhail D C, Bloom M S  1996 - Epithermal gold mineralisation, Acupan, Baguio District, Philippines: Geology, mineralisation, alteration and the thermochemical environment of ore deposition, (Extracts): in    Econ. Geol.   v91 pp 243-247, 270-271
Cooke D R, McPhail D C, Bloom M S  1996 - Epithermal Gold mineralization, Acupan, Baguio district, Philippines: geology, mineralization, alteration, and the thermochemical environment of ore deposition: in    Econ. Geol.   v 91 pp 243-272
Cooke, D.R., Agnew, P., Hollings, P., Baker, M., Chang, Z., Wilkinson, J.J., White, N.C., Zhang, L., Thompson, J., Gemmell, J.B., Fox, N., Chen, H. and Wilkinson, C.C.,  2017 - Porphyry Indicator Minerals (PIMS) and Porphyry Vectoring and Fertility Tools (PVFTS) - Indicators of Mineralization Styles and Recorders of Hypogene Geochemical Dispersion Halos: in Tschirhart, V. and Thomas, M.D., (Eds.), 2017 Exploration 17: Sixth Decennial International Conference on Mineral Exploration, Toronto, Canada, October 22 to 25, 2017, Proceedings,   Geochemistry, Paper 32, pp. 457-470.
Deyell C L and Hedenquist J W,  2011 - Trace element geochemistry of enargite in the Mankayan District, Philippines: in    Econ. Geol.   v.106 pp. 1465-1478
Fernandez H E, Damasco F V  1979 - Gold deposition in the Baguio gold district and its relationship to regional geology: in    Econ. Geol.    pp 1852-1868
Hollings P, Cooke D R, Waters P J and Cousens B,  2011 - Igneous Geochemistry of Mineralized Rocks of the Baguio District, Philippines: Implications for Tectonic Evolution and the Genesis of Porphyry-Style Mineralization: in    Econ. Geol.   v.106 pp. 1317-1333
Hollings P, Wolfe R, Cooke D R and Waters P J,  2011 - Geochemistry of Tertiary Igneous Rocks of Northern Luzon, Philippines: Evidence for a Back-Arc Setting for Alkalic Porphyry Copper-Gold Deposits and a Case for Slab Roll-Back?: in    Econ. Geol.   v.106 pp. 1257-1277
Sawkins F J, ONeil J R and Thompson J M,  1979 - Fluid inclusion and geochemical studies of vein gold deposits, Baguio District, Philippines: in    Econ. Geol.   v74 pp 1420-1434
Waters P J, Cooke D R, Gonzales R I and Phillips D,  2011 - Porphyry and Epithermal Deposits and 40Ar/39Ar Geochronology of the Baguio District, Philippines: in    Econ. Geol.   v.106 pp. 1335-1363
White, N.C., Leake, M.J., McCaughey, S.N. andd Parris, B.W.,  1995 - Epithermal gold deposits of the southwest Pacific: in    J. of Geochemical Exploration   v.54, pp. 87-136.

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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