Alteration systematics and mineralising potential of the Palinpinon geothermal field, Negros Island, Philippines

Palinpinon geothermal field (Negros Island, Philippines) is a high temperature, liquid dominated geothermal system in an active volcanic island arc setting. Previous workers have made parallels between the geological setting, mineral deposition and hydrothermal alteration styles recognised at Palinpinon with those that characterise several types of magmatic hydrothermal ore deposits (e.g., porphyry, skarn, and high and low sulphidation epithermal systems). Igneous rock formations on southern Negros Island have medium K, calc-alkaline, basaltic to dacitic compositions. The Middle Miocene Lower Puhagan Volcanic Formation is part of a volcanic sequence that is traceable throughout the Visayas region. It formed during southeasterly-directed subduction of the Sulu Sea oceanic basin beneath the Sulu arc. Late Miocene to Early Pliocene times marked a period of regional subsidence and marine sedimentation during which a thick sequence ( < 1500 m) of calcareous sediments was deposited (Okoy Formation). Locally in the southern Negros region, magmatism during Early Pliocene to Recent times produced a thick (< 2600 m) sequence of volcanic and volcaniclastic rocks (Southern Negros and Cuemos Volcanic Formations). During this time, diorites to quartz diorites of the Puhagan dikes (\\(^{40}Ar/^{39} Ar= 4.2-4.1 Ma\\)) and the Nasuji Pluton (\\(^{40}Ar/^{39} Ar= 0.7-0.3 Ma\\)) intruded the Middle Miocene, Late Miocene and Early-Late Phocene rock units. The igneous rocks at Palinpinon have adakitic geochemical signatures and are associated with Nb-enriched basalts. This is interpreted to indicate that magmatism in this region has been influenced by the melting of subducted oceanic basalt. Considering the regional tectonic history, the most likely scenarios for the generation of slab melts are considered to be: (1) melting of relatively young (< 20 Ma) oceanic crust during the Middle Miocene; (2) initiation of east-directed subduction along the Negros-Sulu Trench during Early Pliocene times; and (3) melting of young (< 10-20 Ma) oceanic crust during Late Pliocene times. Hydrothermal alteration at Palinpinon is spatially associated with the Puhagan dikes and the Nasuji Pluton. Only incipient calc-silicate alteration is spatially associated with the Puhagan dikes. In the Nasuji-Sogongon region, the hydrothermal alteration assemblages spatially associated with the Nasuji Pluton are K-silicate (biotite), calc-silicate, hypogene advanced argillic, propylitic and distal ilhte assemblages. Fluid inclusion evidence indicates that magmatic-hydrothermal fluids associated with the final stages of magma crystallisation and the formation of a biotite alteration assemblage and associated veins had homogenisation temperatures of 267¬¨‚àû to> 600¬¨‚àûC and salinities of 26 to 56 eq.wt.% NaCl. PIXE analyses show that these fluids were endowed with base metals (e.g., up to 0.2 wt.% Cu). Age determinations on hydrothermal biotite (\\(^{40}Ar/^{39}Ar= 0.7-0.6 Ma\\)), alunite (K/Ar = 0.9-0.8 Ma) and illite (K/Ar = 0.7 Ma) demonstrate that these assemblages all formed contemporaneously with the Nasuji Pluton, implying a genetic link between intrusion emplacement and the formation of porphyry, high sulphidation epithermal and low sulphidation epithermal alteration assemblages. The Early Pliocene age of the Puhagan dikes establishes they are not the heat source to the current geothermal system, which must be a much younger 'blind' intrusion situated beyond depths drilled in the Puhagan area. The emplacement of this intrusion, at depths greater than 2.5 km, occurred within the last 0.8 Ma and provides the heat source for present-day geothermal activity. Distinctive high temperature hypogene hydrothermal alteration types have developed at depths greater than 2 km and include calc-silicate and biotite alteration zones. Parts of the biotite alteration zone are in thermal equilibrium with the present-day geothermal system. The lack of a hypogene advanced argillic alteration zone in the Puhagan region is interpreted to indicate that the intrusion has been emplaced at depths great enough for lithostatic confining pressures to hinder or prevent magma degassing. With ongoing hydrothermal convection, areas of propylitic alteration have developed as halos surrounding the Puhagan magmatic-hydrothermal alteration zones. At shallow crustal levels (< 2 km), low sulphidation (illite) alteration assemblages overprint the biotite and hypo gene advanced argillic alteration types associated with the Nasuji Pluton and are in thermal equilibrium with the present-day geothermal system. Therefore, it is possible that biotite and illite alteration assemblages in the Puhagan region either have a protracted history of formation, or else have formed during two discrete time periods in the past 0.8 Ma. Lastly, perched aquifers of steam-heated acid sulphate water have altered regions above the water table to an advanced argillic (steam-heated) alteration assemblage. The percolation of these waters down near-vertical permeable structures (i.e., faults and joints), results in steeply dipping zones of steam-heated advanced argillic alteration. The current geothermal hydrology at Palinpinon is grossly influenced by permeable zones related to faults and lithological boundaries. The upflow zone is situated at a fault intersection (Ticala and Lagunao Faults) and the main northeasterly outflow zone is parallel to NE-striking faults (Ticala and Puhagan Faults). The neutral chloride reservoir water chemistry is affected by boiling, mixing and conductive cooling. Boiling occurs close to the region of upflow and is mainly restricted to narrow zones of permeability. Mixing occurs peripherally to the upflow zone. The proposed end-member mixing solutions are steam-heated sulphate and meteoric waters. Steam-heated sulphate waters are sourced from perched aquifers in the vadose zone beneath areas of high elevation. Hybrid neutral chloride - sulphate waters have chemistries influenced by host rock dissolution and have not attained chemical equilibrium. In contrast, hybrid neutral chloride - meteoric waters have chemistries that are closer to chemical equilibrium with the host rock. Conductive cooling occurs in the northeasterly outflow zone and water chemistries influenced by this process are also close to chemical equilibrium with the host rock. The occurrence of base and precious metal scale deposits in several geothermal wells demonstrates that the present-day deep reservoir fluid is capable of transporting and depositing base and precious metals. However, trace metal analysis of the deep reservoir fluids shows they are undersaturated with respect to gold (1-4 ˜í¬¿g/kg). Chemical modelling of Palinpinon geothermal water predicts that boiling should be the most effective mechanism for base and precious metal deposition. Boiling of the neutral chloride water is predicted to produce sulphide assemblages similar to those seen in well scale deposits. Mixing with acid bearing sulphate waters can also produce these sulphide assemblages, but does so less efficiently, and the deposition of gold and silver is predicted to occur only at much lower temperatures (i.e., < 110¬¨‚àûC). In terms of the mineralogy and the sequence of mineral deposition, mixing with meteoric water is predicted to produce the same results as conductive cooling of the neutral chloride water. Based on fluid modelling results, neither can be considered effective deposition mechanisms for base and precious metal mineralisation. Despite indications that both the early magmatic-hydrothermal fluids and the modern hydrothermal fluids were, and are, capable of metal transportation, base and precious metal deposition is not strongly developed at Palinpinon. Geochemical assays of drillcore and drillcuttings show base and precious metal concentrations to be one to two orders of magnitude below ore grade (< 0.02 wt.% Cu, < 0.03 wt.% Pb, < 0.01 wt.% Zn, < 0.01 wt.% Mo, < 8 g/t Ag and <0.05 g/t Au). It may be that the geothermal wells dnlled to date have failed to intersect mmeralised ore zones. Alternatively, the lack of significant ore mineral deposition may imply that there has been, and still is, a lack of sufficient permeability to create a focus for fluid flow. High lithostatic pressures (i.e., > 0.6 kb) during emplacement of the intrusions may account for this apparent lack of permeability in the magmatic-hydrothermal domain. These elevated pressures would also hinder any metal-bearing magmatic brines from ascending to form ore at shallower levels. Therefore, for base and/or precious metal ore deposition to occur at Palinpinon, a resurgence of magmatism may be required, with an influx of metal-bearing brines and gases, and significant fault dilation and/or phreatic/phrcatomagmatic breccia formation to provide favourable permeability for these fluids to escape and form mineralised ore horizons at shallow crustal levels.