3 research outputs found
Tectonic evolution, petrochemistry, geochronology and palaeohydrology of the Tampakan porphyry and high sulphidation epithermal Cu-Au deposit Mindanao, Phillipines
Magmatic-related porphyry copper and high-sulphidation epithermal copper-gold ore deposits
in continent-margin and intra-oceanic arcs of the Pacific Rim are spatially clustered in discrete
volcanic arc segments. Ore-forming episodes likewise occur within discrete time intervals and
are temporally associated with intervals of compression within arcs. Understanding the spatial
and temporal controls on fertility of magmas requires detailed multidisciplinary studies of young,
ore-productive districts where the tectonic evolution of the arc can be closely integrated with the
evolution of crustal stress, with the timing of mineralisation and with evolution of petrological,
petrochemical and magmatic physico-chemical properties. The late Miocene to Recent
magmatism of the Tampakan ore district of southern Mindanao, Philippines, provides this
opportunity. Porphyry copper and high sulphidation epithermal mineralisation within the giant
Tampakan Cu-Au deposit (2500 Mt@ 0.48% Cu) are hosted by a polygenetic volcanic complex
that was constructed over the past 7 Myr. This interval spans the pre-, syn- and late-collision
stages of arc-arc collision in the southern Mindanao segment of the Sangihe arc. Synthesis of
tectonic reconstructions and of plate motions from GPS data reveal that crustal compression in
southern Mindanao commenced at ~7 Ma and peaked at ~4-3 Ma as subduction waned at the
divergent Sangihe and Halmahera subduction systems, and during establishment of the nascent
Philippine Trench and Cotabato Trench subduction systems. Porphyry Cu mineralisation at
4.24-4.26 Ma (⁴⁰Ar- ³⁹Ar) and high-sulphidation Cu-Au mineralisation at 3.24-3.28 Ma (⁴⁰Ar-
³⁹ Ar; K/Ar) formed during peak compression. Crustal deformation was manifested by regional
folding and thrust faulting.
Laser-ablation ICPMS ²³⁸U-²⁰⁶Pb dating of detrital and rock-hosted zircon grains, together with
⁴⁰Ar-³⁹Ar and K/Ar radiometric dating and whole-rock chemistry define five magmatic cycles
which extended from the late Miocene to the present. These produced four stratovolcanoes that
were built and eroded in successive eruptive and erosional cycles. These semi-continuous
magmatic products recorded the covariation of magmatic physico-chemical variables as the arc
underwent a transition from normal subduction to cessation of subduction associated with
compressive crustal stress during arc-arc collision. The volcanic series evolved from water-poor
pyroxene-hornblende-phyric basaltic andesites to hornblende-pyroxene andesites and to waterrich
hornblende-biotite dacites. Major and trace element chemical data reveal progressive
advance of hornblende saturation, and retreat of plagioclase saturation in the crystallisation
sequence of successive magmatic cycles. High Sr/Y ratios are commonly attributed as a chemical
feature of adakites, which are ascribed an origin by partial melting of an eclogitic source where
refractory garnet retains Y. In contrast, high Sr/Y ratios in the Tampakan district are caused by
hydrous, high-pressure crystal fractionation in the lower crust, where suppression of plagioclase
crystallisation by high magmatic water activities allows Sr to accumulate during crystal
fractionation, and crystallisation of hornblende to form lower-crustal hornblende-augitemagnetite
cumulates depletes Y from the residual melt. This element ratio (Sr/Y) is identified as
a qualitative indicator of the magmatic water activity, and is a robust guide to the ore-forming
potential of a magmatic series.
Magmatic temperatures for the series range from 765°C to 909°C and volcanic log /02 varies
between NNO+l.53 and NN0+2.50. Sulphur abundances in the melt are low (312 to 57 ppm)
and decrease systematically with temperature. Data points for the series plot around the anhydrite-saturation curve in :ΣSmelt-temperature coordinates, consistent with sulphur speciation
calculations based on measured oxygen fugacity that indicate SO₃:H₂S abundance proportions
between ~200: 1 and ~6,000: 1 in the melt. The magmatic series were saturated with anhydrite
during much of their evolution. Magmatic water contents were calculated for the successive
magmatic cycles that erupted over the past 7 Myr. Magmatic water contents calculated using the
Housh and Luhr (1991) plagioclase-melt Na-Ca exchange geohygrometer reveal water contents
that increase from 4.1 % in the late Miocene to up to 8.2 % in the Pleistocene.
²³⁸U-²⁰⁶Pb geochronology on 471 zircon samples from the Tampakan volcanic succession were
used to parameterise time series in chemical compositions of volcanic rocks and phenocrysts,
and time-series in magmatic temperature, oxygen fugacity and wt.% H₂0 in the pre-eruptive
magma over the past 7 Myr. U/Ti, U/Ge and Th/Ti ratios in dated detrital zircon grains resolve
multiple million-year-scale magma recharge-and-crystallisation cycles within a long-lived lowercrustal
chamber. This deep reservoir resides at 18-22 km depth (~5-6 kbars; Al-in-hornblende
geobarometry). The cyclic ramp-up and drop of these element ratios coincides with a 7 Myr-long
"sawtooth" cyclic ramp-up in concentrations of volatiles and crystal incompatible trace elements
in erupted andesites and dacites. Water contents climbed from 4.1 wt.% to 8.2 wt.% as SiO₂
evolved from 57 to 67 wt.%, because the accumulation of volatiles in residual melt was passed
down through multiple cycles of magma-chamber replenishment, magma mixing and
crystallisation. The lower-crustal chamber was periodically tapped to form overlying subvolcanic
chambers and four overprinting stratovolcanoes within the late Miocene to Recent
Tampakan polygenetic volcanic complex.
A lower-crustal magma chamber having a long lifespan and slow crystallisation rate relative to
the frequency of recharge is required in order to generate the observed petrochemical trends and
cyclic climb magmatic water content relative to SiO₂ content of the melts. This longevity is
thermally and physically permissible where magma entrapment in the lower crust occurs in
compressive stress regimes beneath volcanic arc segments that undergo transient collision, or are
under-thrust by buoyant segments of the subducting plate. Calculated buoyancy forces of 1-3 km
thick basaltic to andesitic melt columns in the ductile lower crust are comparable to horizontal
tectonic stresses in orogenic zones, indicating that melt entrapment can be modulated by an
ambient stress regime that inhibits magma ascent by dyke propagation. Numerical thermal
models created using the 2-D graphical, user-interactive, heat flow program KWare HEAT
predict that lower-crustal sills that are entrapped in the lower crust cool extremely slowly, with
residual melt fractions remaining above the wet solidus for several million years, so
intermittently erupted magmas exhibit chemical continuity over the ~3-10 Myr period of crustal
compression in collisional volcanic arcs.
The results from this integrated study of the Tampakan district indicate that the spatial and
temporal clustering of magmatic Cu-Au porphyry ores in volcanic arcs is a product of shared
regional compressive stress which inhibits magma ascent by sub-vertical dyke propagation and
promotes development of sub-horizontal magma chambers in the lower crust, where the trapped
magma proceeds to crystallise cumulates until the residual melt evolves to sufficient buoyancy to
propagate sub-vertical dykes. Volcanics and epizonal plutons related to porphyry-Cu ore in the
Tampakan district display trace-element evidence that the melts segregated from high pressure
(lower-crustal) cumulates consisting largely of Al-rich augite and hornblende, but little or no
plagioclase. Magma chambers in hot lower crust cool very slowly and live long enough to undergo multiple, million-year-scale cycles of magma replenishment and fractional
crystallisation and tapping, over the course of which concentrations of "incompatible
components" such as H₂O, Cl and SO₃ are passed on through multiple cycles of chamber
replenishment and crystallisation and minor discharge and so accumulate to exceptional
concentrations relative to major elements (Si₂, AliO₃, Na₂O etc). Successive batches of
increasingly H₂O-rich melt leaving the lower crustal chamber began to exsolve a hydrothermal
fluid at successively greater depths. Hydrothermal fluids that exsolve at greater depths are denser
and more efficient in scavenging Cu from the melt, because the fluid-melt partition coefficient of
Cu is extremely pressure sensitive. This study suggests that the transition to metallogenic fertility
of magmas at convergent margins is ultimately modulated by compressional stress that induces
deep entrapment, build-up to anomalously high water contents and consequent magmatichydrothermal
fluid exsolution at deep mid-crustal depths in ascending magmas, and segregation
of Cu-rich brines to apical parts of the ascending magma body.
The superposition of both porphyry Cu and high-sulphidation-epithermal Cu-Au mineralisation
in the Tampakan deposit, and the partial preservation of the host stratovolcanic edifice, allows
investigation of the genetic relationship between these two deposit styles and study of the uppercrustal
palaeohydrology of a stratocone-centred, ore-forming magmatic-hydrothermal system.
⁴⁰Ar-³⁹Ar dating reveals that the porphyry Cu mineralisation formed during the early Pliocene
(4.24 ± 0.02 Ma, 4.26 ± 0.02 Ma), whereas high-sulphidation-epithermal mineralisation formed
during the middle Pliocene (3.23 ± 0.03 Ma, 3.34 ± 0.05 Ma, 3.28 ± 0.06 Ma). The ~1 Myr age
difference requires their formation from separate magmatic-hydrothermal systems that were
established in the upper crust from different batches of melt. Petrochemical trends indicate that
both hydrothermal systems emanated from epizonal magma chambers fed from a shared, longlived
lower crustal magma reservoir. Erosion of Cycle 3 andesites during collisional uplift
exposed porphyry-Cu-stage quartz stockwork veins at the palaeosurface in less than ~ 350 Kyr
after porphyry mineralisation. After unroofing of the porphyry system, construction of the Cycle
4a middle Pliocene volcanic centre commenced at 3.93 Ma. A cryptic unconformity between
Cycle 3 and Cycle 4a andesites became the principal surface for a stratigraphic groundwater
aquifer that acted as a condensor for high-sulphidation-stage magmatic volatiles.
Three aspects of the Tampakan high-sulphidation-epithermal palaeohydrological system are
investigated: 1) the physical properties and hydrological transport mechanics of the magmatic
supercritical fluids along the "magmatic vapor plume" from the site of accumulation in the
carapace beneath the deposit to the meteoric- and magmatic-fluid mixing environment within the
deposit; 2) identification of the composition and thermal properties of the fluid end-members and
the geometry of mixing paths within the deposit; 3) the geometry and relative mixing ratios of
magmatic and meteoric groundwater in various regional alteration zones of the district and the
effect of topographic forcing of hybrid hydrothermal fluids along the western flank of the
volcanic complex.
The Tampakan high-sulphidation epithermal mineralisation formed from a dense vapor-like
supercritical fluid with a density of ~ 0 .15 to 0 .25 g/ cc that exsolved from a relatively mafic
andesitic melt emplaced at shallow depths of 2.6 km to 4 km. These melts had significantly less
magmatic water (~3-4 wt.% H20) than the more evolved andesitic melts associated with
precursor porphyry Cu mineralisation (~ 6.0 wt.% H20). The lower water content of highsulphidation-
stage melts allowed shallower crustal emplacement and fluid exsolution as a low-density vapor, whereas more water-rich porphyry-stage melts from the preceding cycle exsolved
a dense supercritical brine phase (0.3 to 0.45 g/cc) at deeper (~ 6 to 8 km) crustal levels.
Pressure and enthalpy constraints calculated for the high-sulphidation-stage magmatic fluid at
several points along its flow path provide substantial insights into the magmatic fluid transport
process. The magmatic vapor ascended along a nearly isochoric decompression path from the site
of exsolution to the site of fluid mixing. The density of the vapor increased from ~0.2 g/cc to
~0.3 g/cc over a vertical ascent distance of ~1.2 km. During transit, the vapor cooled
conductively by ~350°C. The nearly isochoric vapor transport mechanism through the
lithostatically pressured, ductile rock column requires propagation of fluid-filled, fine-scale,
migratory hydrofractures, with intimate contact between the vapor and the ductile wall rocks
during vapor ascent. This ensured substantial conductive cooling (~875°C to ~525°C) along the
ascent path and that thermal contraction of the vapor balanced the tendency to expand with
decompression. Instantaneous isoenthalpic decompression of the magmatic-vapor-charged
mobile hydrofractures at the lithostatic-hydrostatic interface (brittle/ductile transition) near the
base of the deposit, was associated with "instantaneous" cooling of the supercritical vapor from
~525°C to ~375°C. This pressure-temperature quenching efficiently condensed magmatic vapor
to a modestly saline (5 wt.% NaCl equivalent) condensate that concurrently mixed with ambient
meteoric water within a palaeo-aquifer at the base of the hydrostatic regime. Cooling of the dense
magmatic condensate liquid (~0.62 g/cc) by dilution in the mixing column was associated with
hydrolysis of SO₂ to H₂SO₄ , HSO₄ , SO₄ and to H₂S which in turn produced a vertical pH
gradient and a vertical textural zonation in alteration facies in the advanced-argillic lithocap.
Oxygen-isotope and enthalpy balances indicate that sericite in the deep portions of the deposit
and pyrophyllite at higher and peripheral regions precipitated from hybrid magmatic-meteoric
waters which comprised ~50% magmatic condensate. The hot, hybrid fluids formed a thermally
buoyant plume due to transfer of heat from the high-enthalpy magmatic vapor into the meteoric
water regime. The plume ascended and became entrained into a stratabound aquifer system on
the west slope of the volcano. A substantial hydraulic head in the aquifer is implied by downstratigraphic-
slope deflections in the time-integrated proxy fluid isotherms identified by
calibration of PIMA II™ infrared spectral parameters with the chemical composition of potassic
white mica. These calibrations reveal chemical trends in the composition of potassic white micas
that can be tracked across several alteration environments. A central, and deep-seated, hightemperature
zone of nearly stoichiometric muscovite coincides with the locus of the inferred
magmatic vapour plume. This zone is transitional to shallower and peripheral regions where
there is an increasing replacement of K ions by neutral H₂O molecules in the potassic white mica
crystal structure, and decreasing Cu and Au grades. These trends reflect a central, deep-level
zone of high fluid temperatures, with cooling paths deflected down-palaeo-slope at shallower
levels in the volcanic edifice. Substantial magmatic fluid ascended into the hydrostatic regime
along a 5 km by 1.5 km wide NNE-trending fault zone that partly controlled mineralisation.
Lateral outflow of the hybrid fluids was controlled by regional dilational faults that transect the
volcanic centre. Zoning of hydrothermal mineral compositions and assemblages reveal a superb
example of hydrothermal plume-groundwater interaction and downslope dispersion. The plume
of heated meteoric water and admixed magmatic condensate in the hydrostatic environment was
centred within the Tampakan deposit. The deposit is located where gradients in the hybrid fluid's
temperature proportion of magmatic fluid are greatest. Mineralisation was localised in the zone
of steep temperature and pressure gradients associated with the interface between a deep
lithostatic-pressured plume and a shallow hydrostatic-pressured plume
Geochronology of the Tumpangpitu porphyry Au-Cu-Mo and high-sulfidation epithermal Au-Ag-Cu deposit: Evidence for pre- and postmineralization diatremes in the Tujuh Bukit District, Southeast Java, Indonesia
The Tumpangpitu porphyry and high-intermediate-sulfidation epithermal deposit is the largest deposit in the Tujuh Bukit district, southeast Java, Indonesia. The porphyry resource contains 1.9 billion tonnes @ 0.45% Cu and 0.45 g/t Au, for 28.1 Moz Au and 19 billion lbs of Cu. There are an additional 2.1 Moz Au and 72.9 Moz of Ag in oxidized high-sulfidation epithermal mineralization. Tumpangpitu is located along a NW-striking structural corridor covering an area of 12 × 5 km that hosts several Cu-Au-Mo mineralized tonalitic porphyries, each with varying degrees of metal enrichment. At least eight discrete intrusions spanning the alteration-mineralization sequence have been identified at Tumpangpitu. What is unusual, however, is the presence of both a premineralization, relatively dry volcanic breccia pipe (Tanjung Jahe) and a late-mineralization diatreme complex associated with a significant, large magmatic-hydrothermal system (Tumpangpitu) in the same district. Magmatism, mineralization, and alteration at Tumpangpitu occurred in response to north-directed subduction of the Indo-Australian plate beneath the Asian continental plate margin. The Tujuh Bukit district is floored by early to late Miocene sedimentary and andesitic volcanic rocks. Volcanic-hydrothermal activity at Tujuh Bukit began with the formation of the weakly altered Tanjung Jahe diatreme complex (U-Pbzircon ages of 8.78 ± 0.22-8.52 ± 0.21 Ma). Mineralization at Tumpangpitu was preceded by the intrusion of a large, equigranular, dioritic batholith (5.81 ± 0.20-5.18 ± 0.27 Ma). Hydrothermal activity associated with mineralization has been constrained by U-Pb age determinations from syn- to late-mineralization porphyries that were emplaced in the early Pliocene from 5.40 ± 0.46 to 3.94 ± 0.69 Ma. High- and intermediate-sulfidation Au-Ag ± Cu mineralization and associated advanced argillic alteration (part of a district-scale lithocap) has overprinted and significantly upgraded the top of the porphyry orebody. Ar/Ar dating of alunite (4.385 ± 0.049 Ma) and Re-Os dating of molybdenite (4.303 ± 0.018 Ma) have defined a short time period between the high-sulfidation epithermal and porphyry mineralization events. This suggests extreme rates of uplift, exhumation, and erosion in the vicinity of the Sunda-Banda magmatic arc. Volcanic-hydrothermal activity associated with the Tumpangpitu diatreme occurred during epithermal mineralization (breccia matrix zircon age of 2.7 ± 1.0 Ma with systematic errors). Clasts of high-sulfidation state mineralized rocks are a minor but significant component of the diatreme, and late-stage epithermal veins cutting the diatreme demonstrate an intermineralization timing with respect to epithermal activity in the district, implying that epithermal mineralization continued intermittently for 1 to 1.5 m.y. after porphyry mineralization ceased at Tumpangpitu