Porphyry deposits and oxidized magmas

The Mankayan mineral district of northern Luzon, Philippines, hosts several significant ore deposits and prospects of various types within an area of similar to 25 km(2), including the Far Southeast porphyry Cu-Au deposit, the Lepanto high sulfidation epithermal Cu-Au deposit, the Victoria intermediate sulfidation epithermal Au-Ag vein deposit, the Teresa epithermal Au-Ag vein deposit, the Guinaoang porphyry Cu-Au deposit, and the Buaki Old Palidan porphyry Cu-Au prospects, all having formed in a period of about 2 m.y., from similar to 3 Ma. The geologic units include (1) a basement composed of Late Cretaceous to middle Miocene metavolcanic rocks volcaniclastic rocks; (2) the Miocene 12 to 13 Ma tonalitic Bagon intrusive complex; (3) the Pliocene, similar to 2.2 to 1.8 Ma, Imbanguila dacite porphyry and pyroclastic rocks; and (4) postmineralization cover rocks, including die similar to 1.2 to 1.0 Ma Bato dacite porphyry and pyroclastic rocks and the similar to 0.02 Ma Lapangan tuff. Extensive advanced argillic alteration crops out for similar to 7 kin along the inconformity between the basement rocks and the Imbanguila dacite formation consists of quartz-alunite +/- pyrophyllite or diaspore, with local zones of silicic alteration and a halo of dickite +/- kaolinite. The alteration and its subhorizontal geometry indicate that it is a lithocap or coalesced lithocaps. The northwest-striking portion is similar to 4 km long and hosts the Lepanto enargite Au ore deposit, also controlled by the Lepanto fault. The Lepanto epithermal deposit is related to the underlying Far Southeast porphyry; the quartz-alunite alteration halo of Lepanto is contemporaneous with the similar to 1.4 Ma potassic alteration of the porphyry. There are also silicic-advanced argillic alteration patches similar to 600 m above the Far Southeast orebody at the present surface; these are interpreted to be perched alteration. There is no systematic mineralogical or textural zoning in the Lepanto lithocap that indicates direction to the intrusive source. Most surface samples of the lithocap contain less than 50 ppb Au, despite many being less than a few hundred meters from underground Cu-Au ore. This study found that several characteristics of the Lepanto lithocap change systematically with distance from the causative intrusion. The alunite absorption peak at similar to 1,480 nm in the short wavelength infrared (SWIR) spectrum shifts to higher wavelengths where the sample is closer to the intrusive center, due to higher Na and lower K content in the alunite; published experimental studies indicate that high Na/(Na + K) is related to higher formation temperature. High Ca alunite, including huangite, also occurs at locations proximal to the intrusive center. Alunite mineral composition analyzed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) indicates that the Pb content decreases toward the intrusive center, whereas Sr, La, Sr/Ph, and La/Pb increase markedly. Whole-rock compositions, using only nonmineralized (taken as Cu similar to 350 m above the upper extent of the veins. An airborne geophysics survey indicates that the Far Southeast orebody is associated with a wide zone of demagnetization due to extensive magnetite-destructive phyllic alteration. Such low magnetic anomalies on the margin of a large lithocap elsewhere may, deserve attention. The directional indicators and mineralization signatures found in this study have the potential to indicate direction to the intrusive center during exploration of similar porphyry-epithermal districts.

Exploration Tools for Linked Porphyry and Epithermal Deposits: Example from the Mankayan Intrusion-Centered Cu-Au District, Luzon, Philippines

Data-driven predictive mapping of gold prospectivity, Baguio district, Philippines: Application of Random Forests algorithm

The Baguio district contains a diverse array of epithermal, porphyry and skarn deposits, together with a large, broadly strata bound, advanced argillic lithocap. Magmatism, mineralization, and alteration occurred in response to subduction of the South China Sea plate and the Scarborough Ridge beneath northern Luzon over the past 3 m.y. Rapid uplift and exhumation resulted in epithermal veins overprinting several porphyry Cu-Au deposits. Most of the epithermal Au-Ag deposits of the Baguio district (including Antamok and Acupan, the two largest Au deposits) are intermediate sulfidation state quartz-carbonate-adularia-illite-base metal sulfide veins that contain electrum and minor Au-Ag tellurides. In contrast, high sulfidation mineralization at Kelly includes enargite, tennantite, electrum, and precious metal tellurides and is associated with advanced argillic alteration.

Although the mineralizing fluids that formed the porphyry and epithermal deposits had distinct temperatures and salinities, stable and radiogenic data provide evidence for direct magmatic contributions into each deposit type. The epithermal mineralizing fluids were dilute (generally, 600°C) hypersaline brines (30 to >70 wt % NaCl equiv) and low-density vapor. Sulfur isotope compositions of sulfides in the porphyry, skarn and intermediate sulfidation epithermal veins of the southern and central Baguio district are mostly between +1 and +6 per mil, consistent with a predominance of H2S in the mineralizing fluids (i.e., reducing conditions). In contrast, sulfides from the high sulfidation, porphyry, and intermediate sulfidation deposits located adjacent to the Baguio lithocap mostly have negative sulfur isotope values (−6.9 to +0.8‰), consistent with oxidizing (SO42−-predominant) mineralizing fluids.

Intermediate sulfidation epithermal veins at Acupan have crosscut a well-mineralized porphyry Cu-Au stock-work at Ampucao. The two deposits cannot be distinguished on the basis of radiometric age determinations (Ampucao: 0.51 ± 0.26 Ma; Acupan: 0.65 ± 0.07 Ma), and are interpreted to be cogenetic, with telescoping of the two environments caused by the rapid uplift and exhumation associated with ridge subduction. Measured δ 34Ssulfide (+1.1 to +6.6‰), δ 34Ssulfate (+10.4 to +31.8‰) values and initial strontium ratios of anhydrite (0.70378–0.70385) are consistent with identical and predominantly magmatic sources of these components for the Ampucao porphyry and Acupan epithermal veins. Helium isotopes provide further evidence of mantle-derived components in the epithermal veins (R/Ra values of 6.0 and 6.7). Oxygen, deuterium, and carbon isotopes provide evidence for predominantly magmatic water at Ampucao and for hybrid magmatic-meteoric waters at Acupan that precipitated precious metals due to boiling. The proportion of magmatic water relative to meteoric water and precious metal grades both decreased with time during epithermal vein formation at Acupan. The common observation of cross-cutting relationships between porphyry and epithermal veins observed throughout the Baguio district imply that the evolution of porphyry-style to intermediate sulfidation-style mineralization was a common phenomenon in this region, and contributed significantly to its rich metal endowment.

Evidence for Magmatic-Hydrothermal Fluids and Ore-Forming Processes in Epithermal and Porphyry Deposits of the Baguio District, Philippines

The Yao’an porphyry Au system, Machangqing porphyry Cu-Mo system, and Beiya porphyry-skarn Au system, are spatially and temporally associated with potassic felsic intrusions emplaced during the Eocene to Oligocene epochs at 37 to 33 Ma in a postcollisional intracontinental setting in western Yunnan, western Yangtze craton, China. The Yao’an monzonite and quartz monzonite porphyry intrusions are alkaline and potassic with high K 2 O/Na 2 O ratios (1.1–1.5). They have Sr-Nd-Pb isotopes similar to coeval lamprophyres and are characterized by uniform zircon ɛ Hf (−6.4 to −8.7) and δ 18 O values (6.6–7.0‰). They are interpreted as products of fractional crystallization of lamprophyre-like potassic mafic magma derived from ancient metasomatized lithospheric mantle, a scenario similar to the mid-Cretaceous postcollisonal Scheelite Dome gold system in Yukon, Canada. The Machangqing granitic intrusions are high K calc-alkaline and show high Sr, Sr/Y, and La/Yb, but low Y and Yb geochemical signatures. They have Sr-Nd-Pb isotope compositions similar to amphibolite xenoliths hosted by potassic felsic intrusions in western Yunnan. The zircon ɛ Hf values of the Machangqing granitic intrusions are positive (0.3–4.7), and the zircon-depleted Hf mantle model ages are 1.1 to 0.8 Ga. They also have mantle-like zircon δ 18 O values (5.5–6.4‰). The Machangqing granites were most likely derived from partial melting of Neoproterozoic lower crust. The Beiya granitic intrusions are alkaline, with high K 2 O/Na 2 O (1.9–2.7), Sr/Y and La/Yb ratios, high Sr contents, and low Y and Yb contents. They contain abundant zircon inheritance and have variable magmatic zircon ɛ Hf (−4 to +4) and the highest magmatic zircon δ 18 O values (6.6–7.8‰). The Beiya felsic intrusions are interpreted to be derived from partial melting of a K-rich mafic source mixed with a metasedimentary component. The Eocene-Oligocene intracontinental potassic intrusions and associated mineralization in western Yunnan are located proximal to the Mesozoic Jinsha suture, suggesting that this Mesozoic lithospheric boundary may have provided a first-order control on localization of Cenozoic mineral systems. These potassic felsic intrusions are coeval with regional potassic mafic magmatism in western Yunnan and were emplaced between 37 to 33 Ma, after the collision between India and Asia at ca. 60 to 55 Ma. It is therefore postulated that continental collision may have preferentially thickened the continental lithospheric mantle (CLM) adjacent to the Jinsha suture, in which overthickened lower continental lithospheric mantle was subsequently removed during 37 to 33 Ma, inducing melting of residual metasomatized lithospheric mantle as well as lower crust. The gold-rich Yao’an and Beiya intrusions are alkaline and potassic, characterized by high zircon δ 18 O values (>6.5‰), which is consistent with supracrustal contributions. In contrast, the Cu-Mo-rich Machangqing intrusions are high K calc-alkaline with mantle-like zircon δ 18 O values ( ɛ Hf signatures, indicating negligible supracrustal recycling. Empirically, source compositions played an important role in determing the metal endowment among intrusions formed under the same tectonic setting with similar ages in western Yunnan. In western Yunnan, gold tends to be associated with alkaline and potassic melts with a supracrustal contribution, whereas Cu-Mo mineralization seems to be more related with juvenile crustal sources with little supracrustal influence.

Intracontinental eocene-oligocene porphyry Cu mineral systems of Yunnan, Western Yangtze Craton, China: Compositional characteristics, sources, and implications for continental collision metallogeny

The Baguio district is located in the Central Cordillera of northern Luzon, Philippines. It contains numerous
mineralized porphyry copper-gold, epithermal gold-silver and skarn gold-lead-zinc deposits. The district is
floored by Cretaceous-Eocene metavolcanic and metasedimentary rocks, which are overlain by marine to terrestrial
sedimentary and volcanic rocks of early Miocene to Pliocene ages. Tertiary arc magmatism related to
east-directed subduction of the South China Sea plate along the Manila Trench beneath northern Luzon produced
a major batholith, the Central Cordillera Intrusive Complex, which defines the eastern boundary of the
Baguio district. The intrusive complex was emplaced in the early Miocene based on 40Ar/39Ar age determinations
for the Lucbuban gabbro (22.6 ± 0.5 Ma) and Virac granodiorite (20.23 ± 0.38 and 20.2 ± 0.7 Ma).
Recent exploration has led to the discovery of several new porphyry copper-gold and skarn prospects in the
western Baguio district. Mineralization was preceded by the intrusion of a suite of hornblende megacrystic
andesite dikes in the central part of the district from 4.55 ± 0.15 to 3.45 ± 0.19 Ma. Porphyry copper-gold and
skarn deposits at Black Mountain and Mexico, on the western side of the district, formed between 3.09 ± 0.15
and 2.81 ± 0.24 Ma. The Santo Tomas II Cu-Au-(Pd) porphyry deposit was emplaced into the central southern
part of the district at about 1.5 Ma, based on secondary biotite 40Ar/39Ar ages of 1.48 ± 0.05 Ma and 1.47
± 0.05 Ma. On the eastern side of the district, porphyry-style mineralization at the Hartwell and Ampucao porphyry
Cu-Au prospects returned 40Ar/39Ar ages of 1.09 ± 0.10 and 0.51 ± 0.26 Ma, respectively.
Quartz-carbonate-base metal sulfide style epithermal gold-silver veins are well-developed on the eastern
side of the Baguio district, and crosscut porphyry-style mineralization at the Acupan, Baguio gold and Nugget
Hill deposits. Epithermal veining is inferred to have occurred in the past million years, based on a reported
K-Ar age determination of 0.65 ± 0.07 Ma from the Acupan gold mine. A large advanced argillic alteration zone
(the Baguio lithocap) crops out in the northwest part of the district, and formed between 1.4 and 0.9 Ma, based
on previous K-Ar dating.
Pliocene-Pleistocene mineralization in the Baguio district was triggered by the east-directed subduction of
the Scarborough Ridge. Ridge subduction caused the subduction angle along the Manila Trench beneath
northern Luzon to decrease, which facilitated crustal thickening and exhumation. Ridge subduction also promoted
the development of northwest-trending faults in the upper plate that interacted with major arc-parallel
north- and arc-normal northeast-trending fault sets associated with the Philippine fault system. Fertile magmas
were emplaced into transtensional strike-slip relay basins, resulting in the accumulation of more than 35 Moz
of gold and several million tonnes of copper over approximately 3 m.y

Rene I. Gonzales

Papers

Evidence for Magmatic-Hydrothermal Fluids and Ore-Forming Processes in Epithermal and Porphyry Deposits of the Baguio District, Philippines

Porphyry and Epithermal Deposits and 40Ar/39Ar Geochronology of the Baguio District, Philippines