|Multi-Million-Year Cyclic Ramp-Up of Volatiles in a Lower Crustal Magma Reservoir Trapped Below the Tampakan Cu-Au Deposit by Mio-Pliocene Crustal Compression in the Southern Philippines|
Bruce D Rohrlach, Research School of Earth Sciences, Australian National University, Canberra, Australia, Consultant, Philippines, and Robert R. Loucks, Research School of Earth Sciences, Australian National University, Canberra, Australia, Geoscience Research, Lakewood, Colorado, USA
in - Porter, T.M. (Ed), 2005 - Super Porphyry Copper & Gold Deposits - A Global Perspective; PGC Publishing, Adelaide, v. 2, pp. 369-407.
Magmatic-related porphyry copper and high-sulphidation epithermal copper-gold ore deposits are spatially and temporally clustered in arc segments that underwent crustal shortening during magmatic differentiation. In the ductile lower crust or uppermost mantle, geodynamically induced horizontal compression inhibits propagation of subvertical dikes and can keep buoyant magmas trapped in sheet-like subhorizontal chambers where layered ultramafic-mafic cumulates crystallise until the regional stress regime relaxes or until further magmatic differentiation generates the buoyancy needed to overpower the stress field and permit magma ascent. In the hot lower crust or uppermost mantle, magma chambers tend to last long enough to experience multiple episodes of replenishment by primitive magma, partial mixing of arriving mafic and resident evolved melts, and fractional crystallisation of those hybrids. Over a succession of replenishment and partial-crystallisation cycles, the melt's concentration of incompatible chemical components (H2O, Cl, SO3, etc.) follows a "sawtooth ramp-up" time series. Multi-cycle magma chambers thereby become exceptionally fertile for magmatic-hydrothermal Cu metallogeny.
In this case study of the late Miocene to Recent magmatism in the Tampakan ore district of southern Mindanao, Philippines, we integrate the regional tectonic history with local geological, geochronological, petrochemical, and crustal stress data and with magma chamber thermal modelling to formulate a model for arc-scale porphyry Cu-Au metallogenesis in compressional arc segments. Porphyry copper and high-sulphidation epithermal mineralisation within the giant Tampakan Cu-Au deposit (2500 Mt @ 0.48% Cu and 0.2 g/t Au) is hosted by a cluster of four overlapping stratovolcanoes constructed over the past 7 Myr by intermittent emissions from a lower-crustal magma chamber having the same lifespan. This time interval spans the pre-, syn- and late-collision stages of the Sangihe arc in southern Mindanao. Crustal compression there commenced at ~7 Ma and peaked at ~4-3 Ma. The Tampakan porphyry Cu ore (4.24-4.26 Ma) and high-sulphidation Cu-Au ore (3.24-3.28 Ma) formed during peak crustal compression.
Geochronologic and geochemical studies of the Tampakan volcanic complex identify five mafic-to-felsic differentiation cycles, punctuated by major mafic-magma replenishments into the basal chamber between the late Miocene and present, as the arc underwent a transition from relatively non-compressive subduction to collision. 238U/206Pb geochronology on 471 zircon samples parameterise time series in chemical compositions of volcanic rocks and phenocrysts, and time-series in magmatic temperature, oxygen fugacity and wt % H2O in the pre-eruptive melt over the past 7 Myr. The time series of U/Ti, U/Ge and Th/Ti ratios in hundreds of dated detrital zircon grains resolve multiple, million-year-scale magma recharge-mixing-and-crystallisation cycles within a long-lived, lower-crustal magma reservoir near the arc Moho at 18-22 km depth (~5-6 kbars, by Al-in-hornblende geobarometry). The "sawtooth" cyclic ramp-up of these element ratios in zircons and their parent melts coincides with a 7 Myr-long sawtooth cyclic ramp-up in concentrations of volatiles and crystal-incompatible trace elements in erupted melts of basaltic andesite to dacite composition. At ~6.3 Ma, silicic andesite (62% SiO2) of the early mafic-to-felsic differentiation cycle had only ~3 wt% H2O in the melt and U/Ti ~20 in zircon, whereas by ~1.3 Ma the late-cycle silicic andesite (62% SiO2) had ~7.5 wt% H2O in the melt and U/Ti ~80 in zircon, due to inheritance of H2O and U and other incompatible components from prior cycles.
Porphyry-Cu-ore-related volcanics and epizonal plutons display trace-element evidence that the melts segregated from high pressure (lower-crustal) cumulates consisting largely of olivine, Al-rich augite and hornblende, but little or no plagioclase. Higher Sr/Y ratios in later cycles of the Tampakan igneous complex represent differentiation of increasingly hydrous hybrid melts in the lower crust. Retardation and suppression of plagioclase crystallisation by high magmatic water activities and by high load pressure allowed residual melt fractions to avoid depletion of Sr and Eu during fractionational crystallisation. Higher pressure and H2O activities in melts foster prolific crystallisation of hornblende earlier in the crystallisation sequence of later cycles, thereby depleting the melt in Y and heavy REE, and driving up its Sr/Y and Eu/Yb ratios to produce the distinctive chemical signatures characteristic of porphyry-copper ore-forming magmas worldwide throughout Phanerozoic time.
The paper also describes the deposit-scale alteration, mineralisation and magmatic history at Tampakan.