Porphyry deposits and oxidized magmas

カナダ鉱山冶金学会 (Canadian Institute of Mining and Metallurgy) の憲章と定款

We reconstructed Philippine Sea and East Asian plate tectonics since 52 Ma from 28 slabs mapped in 3-D from global tomography, with a subducted area of ~25% of present-day global oceanic lithosphere Slab constraints include subducted parts of existing Pacific, Indian, and Philippine Sea oceans, plus wholly subducted proto-South China Sea and newly discovered “East Asian Sea” Mapped slabs were unfolded and restored to the Earth surface using three methodologies and input to globally consistent plate reconstructions Important constraints include the following: (1) the Ryukyu slab is ~1000 km N-S, too short to account for ~20° Philippine Sea northward motion from paleolatitudes; (2) the Marianas-Pacific subduction zone was at its present location (±200 km) since 48 ± 10 Ma based on a >1000 km deep slab wall; (3) the 8000 × 2500 km East Asian Sea existed between the Pacific and Indian Oceans at 52 Ma based on lower mantle flat slabs; (4) the Caroline back-arc basin moved with the Pacific, based on the overlapping, coeval Caroline hot spot track These new constraints allow two classes of Philippine Sea plate models, which we compared to paleomagnetic and geologic data Our preferred model involves Philippine Sea nucleation above the Manus plume (0°/150°E) near the Pacific-East Asian Sea plate boundary Large Philippine Sea westward motion and post-40 Ma maximum 80° clockwise rotation accompanied late Eocene-Oligocene collision with the Caroline/Pacific plate The Philippine Sea moved northward post-25 Ma over the northern East Asian Sea, forming a northern Philippine Sea arc that collided with the SW Japan-Ryukyu margin in the Miocene (~20–14 Ma)

Philippine Sea and East Asian plate tectonics since 52 Ma constrained by new subducted slab reconstruction methods

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 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

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

Oligocene to early Miocene volcanic rocks are preserved in the Central Cordillera range and Cagayan Valley
of northern Luzon, Philippines. Basaltic and andesitic rocks of the Pugo Formation in the Baguio district
of the Central Cordillera were intruded by the ~27 to 20 Ma calc-alkaline Central Cordillera Diorite Complex.
In the southern Cagayan Valley the subalkaline to alkaline late Oligocene Mamparang Formation overlies the
Cretaceous Caraballo Formation and has been intruded by the Didipio Igneous Complex, the Cordon Syenite
Complex, and the Palali batholith. The Didipio complex comprises an early suite of diorites, which were
intruded by the strongly mineralized stocks of the Dinkidi Cu-Au porphyry deposit. Whole-rock geochemical
data for intrusive and extrusive rocks of the Baguio district range from low K calc-alkaline to shoshonitic basalts
to dacites with rare earth element (REE) and high field strength element (HFSE) characteristics of suprasubduction
zone magmas and are all interpreted to have been sourced from the same parent melt. Samples from
Didipio display higher alkali contents but similar trace element characteristics. New age dates for the Didipio
area range from 25.7 to 24.8 Ma.
The potassic magmas of the Cagayan Valley are interpreted to have formed in a back-arc coeval to the mainarc
sequence that is preserved in the Baguio Miocene rocks. This contradicts earlier models, which invoke an
early Miocene arc reversal in the northern Luzon archipelago with the switch from early westward subduction
to later eastward subduction attributed to a variety of causes. The lack of a single compelling trigger for arc
reversal combined with the coeval emplacement of arc magmas in the west and back-arc magmas in the east
in northern Luzon is best interpreted as the result of eastward subduction since the late Oligocene. The presence
of ~20 Ma adakitic magmas in the Baguio district may indicate that flattening of the downgoing slab
resulted in a hiatus in magmatism and termination of back-arc rifting.

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?

The Baguio district of the Philippines is one of the world’s premier mineral provinces, containing >35 million
ounces (Moz) of gold and 2.7 million metric tons (Mt) of copper in epithermal, porphyry, and skarn deposits
that formed in the last 3.5 m.y. Pliocene and Pleistocene magmatic rocks of the Baguio district that are
spatially and temporally associated with mineralization can be broadly subdivided into an intermediate to felsic
suite of mineralized low to medium K intrusions, some of which have adakitic affinities and a suite of mafic
to intermediate, medium K to shoshonitic hornblende-phyric dikes. The geochemical and isotopic characteristics
of the dikes are consistent with primitive mantle-derived melts that underwent minimal crustal contamination
as they ascended through the arc crust. In contrast, the intermediate to felsic suite has been contaminated
by young arc crust, suggesting ponding and fractionation within shallow-crustal magma chambers.
The Philippine arc has formed in a complex tectonic environment and is currently sandwiched between two
active subduction zones. Eastward-directed subduction of the Scarborough Ridge along the Manila trench is
currently associated with flattening of the downgoing slab. The formation of the Mafic dike complex is broadly
coeval with the onset of subduction of the Scarborough Ridge and slab flattening. The extinct Scarborough
Ridge would have been younger than the downgoing plate and consequently more susceptible to melting.
These melts can account for the isotopic recharge of the Pliocene subarc mantle as well as the generation of
the primitive melts and adakitic rocks found within the Baguio district. The interaction between primitive
mafic melts and the more felsic calc-alkaline rocks has

Igneous Geochemistry of Mineralized Rocks of the Baguio District, Philippines: Implications for Tectonic Evolution and the Genesis of Porphyry-Style Mineralization

The Boyongan and Bayugo porphyry copper-gold deposits are part of a belt of gold-rich copper deposits in the Surigao district of northeast Mindanao, Philippines. The detailed age relationships described in this study provide insight into the geologically short life cycles that characterize porphyry formation in dynamic arc environments. Since their late Pliocene emplacement (2.3-2.1 Ma; SHRIMP [sensitive highresolution ion microprobe] U-Pb zircon dating) at depths of 1.2-2.0 km, these deposits were exhumed, deeply weathered, and buried. Weathering of these deposits led to the development of the world's deepest known porphyry oxidation profile (600 m thick) at Boyongan, and a modest (30-70 m) oxidation profile at adjacent Bayugo. This earlymiddle Pleistocene supergene event followed a period of rapid uplift and exhumation in northeast Mindanao (2.5 km/Ma; [U-Th]/He apatite age-elevation spectrum). Subsequent rapid subsidence (≥0.34 km/Ma; radiocarbon age-elevation spectrum) and burial of these deposits are attributed to a mid-Pleistocene shift from transpressional tectonics to the present-day transtensional setting in northeast Mindanao. During this period, debris flows, volcanic material, and fluviolacustrine sediments accumulating in the actively extending Mainit graben covered the weathered deposits, preserving the supergene profiles beneath 50-500 m of cover. This detailed geochronological study documents the geologically short (<2.3 Ma) time scales over which these major intrusion-centered mineral deposits evolved from emplacement, exhumation, deep oxidation, and burial, highlighting the dynamism of tectonic processes in environments such as the Philippine Mobile Belt. © 2012 Geological Society of America.

Patrick J. Waters

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

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?

Igneous Geochemistry of Mineralized Rocks of the Baguio District, Philippines: Implications for Tectonic Evolution and the Genesis of Porphyry-Style Mineralization

From crucible to graben in 2.3 Ma: A high-resolution geochronological study of porphyry life cycles, boyongan-bayugo copper-gold deposits, Philippines