The Mankayan Mineral District is located within a well-defined, 150 km long belt of porphyry copper deposits in the Central Cordillera of northern Luzon, Philippines, approximately 250 km north of Manila, and embraces a mineralised complex composed of four related significant gold bearing deposits. These are the Lepanto high sulphidation epithermal enargite-luzonite-gold Au-Cu orebody; the Victoria and Teresa intermediate sulphidation Au-Ag veins; and the concealed Far South-east porphyry Au-Cu deposit.
Four main units have been recognised in the Mankayan district, namely:
(i) Late Cretaceous to middle Miocene basement consisting of the Lepanto metavolcanic, Apaoan volcaniclastic and Balili volcaniclastic rocks. The Lepanto metavolcanic rocks are the lowermost stratigraphic unit found within the Mankayan district, and are only exposed in the west. They are inferred to be of Cretaceous- Paleogene age, comprise tightly packed andesitic to basaltic lavas with minor turbiditic sedimentary rocks, and are cut by mafic dykes which have subsequently undergone greenschist facies metamorphism. The overlying Apaoan volcaniclastic rocks are present in the northeast and consist of green and red thin-bedded siltstone-sandstone. The Balili group, which contains fossils indicative of a late Oligocene to middle Miocene age, unconformably overlies both the Lepanto metavolcanic and Apaoan volcaniclastic rocks, and consists of mostly matrix- supported polymictic volcanic conglomerates;
(ii) the Miocene tonalitic Bagon intrusive complex, which has been dated at 12 to 13 Ma and intrudes the Lepanto metavolcanic rocks;
(iii) the Pliocene Imbanguila dacitic to andesitic porphyry and pyroclastic rocks, which predate the Far Southeast porphyry Cu-Au mineralisation, and host much of the Lepanto enargite Au deposit and the Victoria veins.
(iv) postmineralisation cover rocks, including the Pleistocene Bato dacitic to andesitic porphyritic lava and pyroclastic flow units, and the Recent Lapangan tuff.
Both the Imbanguila and Bato dacitic units, which are difficult to distinguish, are characterised by complex sequences of volcanic breccias, pyroclastic horizons, and massive porphyritic rocks, as well as dyke emplacement during deposit formation. The Imbanguila host rock returned ages of 2.19±0.62 to 1.82±0.36 Ma, while the Bato rocks are 1.18±0.08 to 0.96±0.29 Ma. The Imbanguila unit overlies basement rocks, and was apparently issued from two identified vents that are located above the subsequent Far Southeast porphyry alteration and mineralisation, which are centred on quartz diorite porphyry dykes that intruded to about 300 to 400 m below these vents. The distribution of the Imbanguila unit is spatially related to, but offset by the Lepanto fault, a splay of the major north-south Philippine fault system that extends across the length of Luzon. The Lapangan tuff forms a thin and discontinuous cover of poorly consolidated dacitic air-fall tuff, which is mainly present in the centre of the Mankayan district.
The Lepanto enargite-luzonite-gold orebody produced Cu and Au as early as the 14th century, with organised mining from 1865 and by Japanese companies during WWII. Total production from 1948 to its closure in 1996 was 36.3 Mt @ 3.4 g/t Au, 2.9% Cu, 14 g/t Ag, when the remaining resource was 4.4 Mt @ 2.4 g/t Au,1.76% Cu.
The distribution of the Lepanto epithermal ore is controlled by the NNW-SSE trending Lepanto fault and its intersection with the generally flat-lying unconformity at the base of the Imbanguila dacite. Mineralisation extends along the NW-SE trending Lepanto Fault over a strike length of >3 km to the northwest of the Far South-east porphyry orebody, and outwards along the unconformity over widths of almost 1 km. This mineralisation persists over a thickness of up to 100 m and >200 m in the core of the deposit. The high sulphidation orebodies at Lepanto are hosted within a composite advanced argillic alteration zone composed of hypogene alunite, dickite, kaolinite and pyrite, locally including diaspore and pyrophyllite at depth. Alteration generally comprises an upper layer of dickite-kaolinite, underlain by a core of silica±energite, underlain in turn by quartz-alunite, grading downwards to further dickite-kaolinite on the periperies and pyrophyllite-diaspore-kandite at depth within the Lepanto Fault zone. This argillic alteration zone has a strike length of >7 km and width that varies from ~0.5 to >2 km, persiting over vertical thicknesses of generally ~100 m, but following the Lepanto Fault to >400 m below the unconformity. Within the argillic zone, the ores are closely associated with the silicic alteration, comprising vuggy residual to massive residual quartz. The Lepanto fault hosts ~70% of the ore, with strong brecciation of the silicic ore and offsets of alteration zones possibly reflecting syn-mineral movement. The balance of ore is contained in the subhorizontal blanket of the lithocap that follows the unconformity, hosted by the lower Imbanguila dacite, Balili volcaniclastic rocks and upper Lepanto metavolcanic rocks.
Paragenetic studies of Lepanto mineralisation indicate early coarse pyrite generated during the largely Cu- and Au-barren leaching event, which produced a core of residual vuggy quartz surounded by the halo of advanced argillic alteration. This was followed by the high sulphidation state minerals enargite and luzonite with fine pyrite, largely hosted by the silicic core, and finally by tennantite, chalcopyrite, sphalerite, galena and tellurides plus selenides. The post-enargite sulphides are associated with the introduction of gold, accompanied by anhydrite plus barite gangue minerals. The tennantite of the gold event may be of intermediate- or high sulphidation state, although the presence of chalcopyrite indicates the former.
Far South-east (FSE) is a concealed, deep seated, bell-shaped porphyry Au-Cu deposit hosted by volcaniclastic rocks, centred on a late Miocene quartz-diorite intrusive complex. The geological resource at FSE (calculated in 1995; Lepanto Mining website, 2011) at a cut-off grade of 0.7% Cu equiv., is estimated at 657 Mt @ 0.94 g/t Au, 0.65% Cu (and at a 1.5% Cu equiv. cutoff is 180 Mt @ 1.70 g/t Au, 0.80% Cu). The top of the porphyry-type mineralisation is >650 m below the surface, and the deposit is generally elongated parallel to the regional NW-SE structural trend. It has approximate dimensions of >1000 m east-west, 800 m north-south and persists over a vertical interval of >900 m. Cu and Au mineralisation grade shells are concentric around the dykes and irregular intrusive bodies that comprise the melanocratic quartz-diorite porphyry complex, which was emplaced into the basement Lepanto metavolcanic rocks. The porphyry system has a core of potassic alteration, which consists of a biotite-magnetite±K feldspar assemblage associated with veins of vitreous, anhedral quartz, and accompanied by bornite-chalcopyrite-magnetite mineralisation. This alteration is partially to pervasively overprinted and fringed by alteration assemblages of chlorite+hematite and/or white mica sericite-clay-chlorite accompanied by chalcopyrite-pyrite. No definitive paragenetic evidence links Cu sulphide minerals to the early veins of vitreous, anhedral quartz veins, although petrographic evidence shows that Cu sulphides are mainly associated with a later event characterised by euhedral quartz crystals and anhydrite, and the early anhedral quartz has been shown to be overgrown by euhedral quartz, the latter associated with sulphide deposition and mineral inclusions of illite. Bleached centimetre- to metre-wide halos of illite accompany euhedral quartz-anhydrite-white mica-hematite-pyrite-chalcopyrite-bornite veins, both of which cut sericite-clay-chlorite alteration. Gold occurs as free grains of electrum associated with chalcopyrite and bornite, locally accompanied by Bi-Te-bearing tennantite. Upward and outward from the core of the mineralised porphyry system, the pervasive sericite-clay-chlorite assemblage grades from white mica-dominated, with minor local pyrophyllite, to an assemblage containing abundant pyrophyllite, variably accompanied by quartz, anhydrite and kandite minerals (dickite, nacrite, and kaolinite). This pervasively altered rock is overlain and locally cut by a silicic zone with local alunite that hosts the southeastern extent of the Lepanto ore deposit. The alunite halo includes a variable assemblage of anhydrite, diaspore, dickite, and/or pyrophyllite.
A late intra-mineral hydrothermal breccia, with altered and mineralised porphyry fragments cemented by sulphide minerals, outcrops directly above the northeastern portion of the FSE deposit. This breccia changes from fragment to matrix-supported with depth and is steeply dipping with an anastamosing form. It has a north-south elongated long axis of up to 400 m, and cuts Imbanguila dacites, Balil volcaniclastics and quartz-diorite dykes, passing through the centre of the porphyry deposit.
A well-defined diatreme breccia outcrops above the Lepanto orebody, ~1 km northwest of the surface projection of the FSE deposit where it cuts Imbanguila dacite. The breccia contains lithic fragments with biotite and magnetite alteration, and bornite-chalcopyrite mineralisation, suggesting derivation from an underlying porphyry deposit.
The Victoria intermediate sulphidation quartz-gold-base metal tensional vein system is located to the southwest of, and passes within a few hundred metres of the FSE porphyry. Veins generally trend NNE with a dip to the southeast, although some have arcuate strikes, curving from northeast to southeast. They are predominantly hosted by Imbanguila dacite porphyry and pyroclastic rocks, although some extend downward into the Balili volcaniclastic and Lepanto metavolcanic units. The Victoria veins are not exposed and pinch out upwards at ~200 to 250 m below the surface, with more than 8 mineralised zones having been identified. Grades of 3 to 9 g/t Au are continuous over a 400 m vertical interval, although high-grade >30 g/t Au ore is more restricted within up to 250 m vertical intervals. The main veins are being mined over a 300 m vertical interval, with widths of 2 up to 8 m, and strike extents of up to 600 m. The paragenesis of gangue and ore minerals in the Victoria veins comprises: (i) early quartz veins, associated with an intermediate sulphidation-state assemblage including chalcopyrite, tetrahedrite and low Fe sphalerite, as well as pyrite and galena; (ii) a carbonate stage of rhodochrosite, with similar sulphides; and (iii) a late, sulphide-poor, sulphate stage of anhydrite. Bornite and hematite are restricted to the early quartz veining, while gold was introduced during the later part of the quartz vein development and through the carbonate stage, ending during the sulphate phase. The northwesternmost of the Victoria veins (anhydrite+quartz+pyrite±illite) cut and overprint advanced argillic alteration and enargite (quartz±alunite±pyrophyllite±diaspore±dickite assemblages) related to the Lepanto deposit. Similar epithermal quartz veins have also been recognised cutting enargite mineralisation in the main Lepanto deposit, near its base.
At Teresa, 2 km south of the FSE porphyry deposit, and just to the southwest of the southern margin of the Victoria vein cluster, veins trend north-south and mineralisation tends to occur over wider intervals, at lower grades, in breccia zones. These veins are mined over an ~250 m vertical interval (although narrow widths persist at greater depths), and over similar strike extents to those at Victoria.
Both the Teresa and Victoria veins are reported to be enveloped by halos of illite, or locally chlorite, and are rarely in direct contact with propylitic-altered wall rock.
The initial resource estimate (1997) at Victoria was 11 Mt @ 7.3 g/t Au representing ~80 t of contained Au. Mining commenced in 1997. As of January 2011, the remaining proved + probable reserve at Victoria was 2.87 Mt @ 4.64 g/t Au; and at Teresa was 0.13 Mt @ 4.30 g/t Au for a total of 14 t of contained Au in the two vein systems (Lepanto Consolidated Mining, 2011).
Biotite from the FSE porphyry-related potassic alteration and alunite in the halo to the silicic host of the Lepanto high sulphidation orebody returned ages of 1.41±0.05 (n = 6) and 1.42±0.08 Ma (n = 5), respectively (Arribas et al., 1995), demonstrating the coeval age of potassic alteration in the porphyry deposit and its overlying advanced argillic alteration lithocap. Phyllic alteration overprint of the porphyry ore body followed, with illite ages of 1.37 to 1.22±0.04 to 0.10 Ma (n = 10; Arribas et al., 1995). Illite samples from the Victoria veins were dated at 1.31±0.02 Ma (Ar-Ar; Sakakibara et al., 2001) and 1.14±0.02 and 1.16±0.02 Ma (Hedenquist et al., 2001). Although alteration around the Victoria and Teresa veins appears similar, a sample of illite from the 900 m level of the Teresa vein was dated at 2.22±0.05 Ma (Ar-Ar; Chang et al., 2011) indicating that this portion of the vein system is older.
Shinohara and Hedenquist (1997) and Hedenquist et al. (1998) argued that the synchronous alteration events of the main porphyry (FSE) and lithocap (Lepanto) were related to a coupled hypersaline liquid and a low-salinity vapour, which formed by separation as the solvus was intersected by critical fluid at depth. The hypersaline liquid remained at depth and caused the potassic alteration, as evidenced by fluid inclusions, while the buoyant vapour ascended to shallower depths to form an acidic condensate that leached the rock and created the residual quartz (silicic) and quartz-alunite alteration. The phyllic alteration overprint of the porphyry ore body followed and has been demonstrated to also be of magmatic in origin (Chang et al., 2011). Stable isotopic studies of the biotite, alunite and illite indicated that all were formed from aqueous fluids with a dominantly magmatic origin, although the alunite-stable fluid was an acidic condensate of magmatic vapour with a variable meteoric water component, the latter progressively and regularly increasing with distance from the porphyry, to around 4 km to the northwest (Hedenquist et al., 1998).
The Guinaoang porphyry Cu-Au deposit (500 Mt @ 0.4% Cu, 0.4 g/t Au) is located 3 km southeast of FSE along an extrapolation of the Lepanto Fault. Mineralisation is largely hosted by an altered quartz-diorite intrusion 200 to 1000 m below surface, concealed by post-mineralisation rocks and a shallow-level advanced argillic alteration (quartz-alunite) lithocap which overprints phyllic alteration and copper sulphides of the upper sections of the porphyry system.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2011)