Palaeohydrology of the Tampakan High-Sulphidation Epithermal Cu-Au Deposit, Mindanao, Philippines

Dr Bruce D. Rohrlach, Dr Robert R. Loucks and Dr J. Michael Palin.

Abstract

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 allowed us to investigate the upper-crustal palaeohydrology of a stratocone-centred, ore-forming magmatic-hydrothermal system. Our ⁴⁰Ar-³⁹Ar dating together with geological constraints 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. Rapid erosion of an early Pliocene age stratocone during collision-related uplift in the district resulted in the exposure of deep-seated porphyry-Cu-stage quartz stockwork veins at the palaeosurface within a timeframe of <350 Kyr following porphyry mineralisation. This erosional surface was buried by the subsequent formation of a younger stratocone which commenced eruption at 3.93 Ma. The cryptic unconformity between the porphyry-stage volcanism and the high-sulphidation-stage volcanism became the principal surface for a stratigraphic groundwater aquifer that acted as a condensor for high-sulphidation-stage magmatic volatiles. Three components of the Tampakan high-sulphidation-epithermal palaeohydrological system were 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 stock 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 broad geometry of mixing paths within the deposit; 3) the geometry and relative mixing ratios of magmatic and meteoric groundwater in 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 two-phase supercritical fluid comprising a trace of dense hypersaline brine and >99 wt.% vapour with a density of ~0.15 to 0.25 g/cc that exsolved together from a relatively mafic andesitic melt emplaced at shallow depths of 2.6 km to 4 km. Pressure and enthalpy constraints were calculated for the high-sulphidation-stage magmatic fluid at several points along its flow path and provide insights into the magmatic fluid transport process across the brittle-ductile transition. The magmatic vapor+brine mixture ascended along a nearly isochoric decompression path from the site of exsolution to the site of fluid mixing. The density of the mixture increased from ~0.2 g/cc to ~0.3 g/cc over a vertical ascent distance of ~1100 m. During transit, approximately 3 wt.% of hypersaline brine condensed from the vapour, and the mixture cooled conductively by ~350°C. The nearly isochoric fluid 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 at high levels in the mixing column was associated with hydrolysis of SO₂ to sulfuric acid and to H₂S in a 3:1 ratio, 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 down-stratigraphic-slope deflections in the time-integrated proxy fluid isotherms identified by calibration of PIMA II^TM 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, high-temperature 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 also 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 magmatic-meteoric fluids was strongly controlled by regional dilational faults that transect the volcanic centre. Zoning of hydrothermal mineral compositions and assemblages and isotopic constraints reveal that the Tampakan high-sulphidation deposit is 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 and proportion of magmatic fluid are greatest. Epithermal mineralisation at Tampakan was localised in the zone of steep temperature and pressure gradients associated with the interface between a deep, lithostatic-pressured, magmatic-hydrothermal plume, and a shallow hydrostatic-pressured plume of hybrid waters.

Dr Bruce Rohrlach - 1. RSES, Aust. Nat. Univ., Canberra, Australia. 2. Unit 21 Legaspi Suites, 178 Salcedo St, Makati, Philippines.

Dr Robert Loucks - 1. RSES, Aust. Nat. Univ., Canberra, Australia.

Dr Michael Palin - 1. RSES, Aust. Nat. Univ., Canberra, Australia. 2. University of Otago, Dunedin, New Zealand.