Geological setting of the Rapu Rapu gold-rich volcanogenic massive sulfide deposits,Albay Province,Philippines

Gold-rich Fe–Cu–Zn volcanogenic massive sulfide deposits occur within strata of probable Jurassic age on Rapu Rapu Island in Albay Province, Philippines. Massive sulfides at the Ungay Malobago and Hixbar deposits are spatially associated with dacitic volcanic rocks within a highly-deformed sequence of mafic volcanic and quartzofeldspathic sedimentary rocks. The massive sulfide deposits formed at the stratigraphic contact between footwall dacites and hangingwall mafic volcanic and quartzofeldspathic rocks. The deposits and their host strata have undergone regional metamorphism with strong penetrative deformation. Metamorphic mineral assemblages and textural evidence suggest that peak metamorphism was upper-greenschist to lower-amphibolite grade and syn-D₁ deformation. Based on the age of regional metamorphism, deformation is inferred to be mid-Tertiary in age. Deformation at Rapu Rapu resulted in reorientation of the strata into a broad antiform with strong shallow-plunging elongation fabrics, overturning of the volcanic sequence that hosts the Ungay Malobago deposit, and complex folding of the mineralized zones. The present highly linear form of the Ungay Malobago deposit is mainly a product of this ductile strain.Immobile element ratios for a given lithology generally remain constant in saprolitic samples, and thus provide an effective identification tool even in strongly weathered rocks. Lithogeochemical data define a bimodal volcanic suite that is comparable to bimodal assemblages that occur in several modern back-arc basins in the southwestern Pacific Ocean, including those behind the Vanuatu and the New Britain arcs. On Rapu Rapu, the dacitic rocks are enriched in light REE and have high Zr/Y ratios, which indicates a calc–alkaline affinity and suggests a mature island-arc setting. The quartzofeldspathic sedimentary rocks are more widespread than the dacites and have notably lower Zr/Y ratios; they may have been derived from erosion of a distant volcanic arc. The mafic volcanic rocks are dominantly low-K arc tholeiites of basaltic to andesitic composition, but with modest enrichment in the light REE; comparable rocks can be found in the Vanuatu and New Britain back-arc basins.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00126-003-0349-0An erratum to this article can be found at Editorial handling: O. Christensen     

The Acoje Block Platiniferous Dunite Horizon, Zambales Ophiolite Complex, Philippines: Melt Type and Associated Geochemical Controls

Abstract: The Zambales Ophiolite Complex, a supra-subduction zone ophiolite, is made up of the mid-ocean ridge-related Coto block and the island arc-related Acoje block. This crust-mantle sequence hosts platinum-group elements (PGE) in the Acoje block. The melts responsible for the PGE-bearing nickel sulfide and chromitite deposits are of magmatic origin characterized by high-MgO basalt to boninitic composition which, being second or third-stage melts, carry higher PGE budgets. Metal ratio diagrams, utilizing base and precious metals, reveal that the distribution and deposition of the PGE in the Acoje block are affected by olivine, chromite and sulfide crystallization. The generation, accumulation and segregation of the PGE, oxide and sulfide minerals from the melts are governed by the combined factors of high degrees of partial melting, multiple melt replenishment with concomitant magma mixing and fractional crystallization. Although previous sulfide segregation events could have occurred below the PGE-bearing nickel sulfide horizon as shown by the Ni/Cu (>1), the Cu/Pd and Ni/Pd strongly suggest that the main platiniferous zone is confined within the Acoje block transition zone dunite.     

Explaining State‐Environmental NGO Relations in the Philippines and Indonesia

This paper explains how the notion of “critical engagement” has been a popular basis for NGO‐state relations in the Philippines and Indonesia. It has been used, moreover, in both liberal democratic (post‐1985 Philippines) and authoritarian (late 1960s to late 1990s Indonesia) political regimes. For both states and reform‐minded NGOs, critical engagement has represented a flexible means by which to pursue assorted political, economic and social ends. In a context of regional political and economic upheaval, it is likely to remain popular with both states and many NGOs for the foreseeable future.     

The Gendering of Philippine International Labor Migration*

This paper provides an institutional analysis of the use of gender as an organizing principle for labor migration flows. Through a case study of the marketing and recruitment strategies of Philippine government and private institutions, I examine how gendered representations contribute to divergent patterns of male and female labor migrants from the Philippines. Representations, or controlling images, are used by institutions to provide explanations or justification of a policy. Findings indicate that the global marketing of workers is organized around specific gendered assumptions of male versus female occupations. Recruitment strategies are also influenced by gendered representations of occupations, locations, and workers. The use of gender by government and private institutions as an organizing device provides a gendered context in which migrants and their households must subsequently operate.     

Geochemistry of Adakites from the Philippines: Constraints on Their Origins

Abstract. We have identified in the Philippine Archipelago 230 samples of Late Miocene to Quaternary intermediate and evolved magmatic rocks or glasses, the compositions of which plot within the adakitic field defined by Defant and Drum-mond (1990) using Sr/Y ratios versus Y contents. These rocks belong to four different subductions, along the Manila Trench (Batan, Northern Luzon, Central Luzon), the Negros and Sulu Trenches (Negros and Western Mindano), the Cotobato Trench (Southern Mindanao) and the Philippine Trench (Eastern Mindanao). Lavas from Central Mindanao overlie the deep remnants of the Molucca Sea Plate, and were emplaced in a post-collision setting.
All these samples show a significant depletion in Y and HREE with respect to their "normal" calc-alkaline equivalents, suggesting that garnet was either a residual phase during partial melting or a fractionating mineral during differentiation or assimilation coupled with fractional crystallisation (AFC). However, only 19 samples out of our set (i.e., 8 %) display very high Sr/Y ratios (100–250). Our preferred model for the genesis of these "typical adakites" is ca. 20 % partial melting of subducted altered oceanic metabasalts converted to eclogite. This melting process could have been triggered by water from the underlying serpentinites. Most of the samples, termed "intermediate adakites", display major and trace element chemical features intermediate between those of the former group and those of normal calc-alkaline lavas. We show that magma mixing between slab-derived adakitic magmas and mafic mantle-derived melts accounts for most of the trends linking typical and intermediate adakites, although an additional contribution of mantle is required in some cases.     

Rare Earth Element Geochemistry of the Zigzag - Klondyke Sedimentary Rock Formations: Clues to the Evolution of the Baguio Mineral District (Luzon), Philippines

Abstract. Petrographic analysis of the Oligocene-Miocene Zigzag Formation and Miocene Klondyke Formation of the Baguio Mineral District reveals a transitional arc source for the former and an undissected arc source for the latter. Whole rock geochemistry of these sedimentary rock formations show affiliation of the Zigzag Formation to active continental margin whereas the Klondyke Formation appears to have been derived from an oceanic island arc source. A different aspect of the geology of the district is further gleaned from the whole rock rare earth element geochemistry of these rocks. Samples from the Zigzag Formation are characterized by higher REE concentrations compared to the Klondyke Formation samples. The REE data indicate that the sedimentary rocks of the Klondyke Formation had geochemically-less fractionated igneous lithologies as their progenitor whereas the older Zigzag Formation was derived from a more differentiated source. This new set of information contributes to the understanding of the evolution of this district from a marginal basin to an island arc setting.     

Occurrences of Ore Minerals and Fluid Inclusion Study on the Kingking Porphyry Copper–Gold Deposit, Eastern Mindanao, Philippines

The Kingking deposit is a gold‐rich porphyry copper deposit and the southernmost deposit at the eastern Mindanao mineralized belt, Philippines. It is underlain by Cretaceous–Paleogene sedimentary and volcanic rocks that are intruded by mineralized Miocene diorite porphyries and by barren Miocene–Pliocene dacite and diorite porphyries. The main alteration zones in the deposit are the inner potassic zone and the outer propylitic zone. The biotite‐bearing diorite and hornblende diorite porphyries are the primary host rocks of mineralization. Two dominant copper minerals, bornite and chalcopyrite, which usually occur as fracture fillings, are associated with fine crystalline quartz veinlet stockworks in the mineralized diorites. Minor secondary covellite, chalcocite and digenite are also observed. The primary Cu‐Fe sulfide phases initially deposited from ore fluids consisted of bornite solid solution (bnss) and intermediate solid solution (iss), which decomposed to form the bornite and chalcopyrite. Peculiar bornite pods that are different from dissemination and are associated with volcanic rock xenoliths in biotite‐bearing diorite porphyry are noted in a drill hole. These pods of bornite are not associated with quartz veinlet stockworks. Fluid inclusion analyses show three types of inclusions contained in Kingking samples: two‐phase fluid‐rich and vapor‐rich inclusions and polyphase hypersaline inclusions from porphyry‐type quartz veinlet stockworks. The liquid–vapor homogenization temperatures (T_H) and the dissolution temperature of halite daughter crystals (T_M) from the polyphase hypersaline inclusions predominantly range from 400°C up to >500°C. The wide range of T_H and T_M may be due to heterogeneous trapping of variable ratios of vapor and brine. For some inclusions, T_H > T_M and in some cases, T_H < T_M, indicating that some of the brine was supersaturated or saturated with NaCl at the time of entrapment. Calculated salinity of the polyphase hypersaline inclusions ranges from 40 to 60% NaCl equivalent. Temperature and vapor pressure of mineralized fluid were estimated to be 400°C and 16 MPa.     

First-level liquefaction hazard mapping of Laoag City,Northern Philippines

During the 1990 Luzon earthquake (M_s 7.8), the central part of Luzon Island, Philippines suffered much from liquefaction-related processes. Examination of inventories shows that the affected areas lie on certain geological environments that are characteristically vulnerable to liquefaction. Based on this local experience and the findings of earlier workers correlating geological setting with liquefaction susceptibility, a first-level map of liquefaction hazard for Laoag City, Northern Philippines, was produced. Distinct micro-geomorphological units were identified within the mainly fluvio-deltaic setting of the study area. The liquefaction susceptibility of each unit was then ranked as high, moderate, low or non-liquefiable, taking also the geomorphological evolution of the area into account. The geomorphological model of the fluvio-deltaic basin was tested against the results of the georesistivity survey carried out in this study. Moreover, compatibility of the liquefaction susceptibility map with historical liquefaction records supported the validity of the proposed ranking. The study showed that microzonation based on geomorphological criteria is indeed very useful in less-developed countries like the Philippines, where funds for a more rigorous determination of liquefaction potential are limited and not always available.     

The oceanic substratum of Northern Luzon: Evidence from xenoliths within Monglo adakite (the Philippines)

Abstract   A 8.65 Ma adakitic intrusive sheet exposed near Monglo village in the Baguio District of Northern Luzon contains a suite of ultramafic and mafic xenoliths including in order of abundance: spinel dunites showing typical mantle-related textures, mineral and bulk rock compositions, and serpentinites derived from them; amphibole-rich gabbros displaying incompatible element patterns similar to those of flat or moderately enriched back-arc basin basalt magmas; and amphibolites derived from metabasalts and/or metagabbros of identical affinity. A single quartz diorite xenolith carrying a similar subduction-related geochemical signature has also been sampled. One amphibolite xenolith provided a whole-rock K–Ar age of 115.6 Ma (Barremian). We attribute the origin of this suite to the sampling by ascending adakitic magmas of a Lower Cretaceous ophiolitic complex located at a depth within the 30–35 km thick Luzon crust. It could represent an equivalent of the Isabela-Aurora and Pugo-Lepanto ophiolitic massifs exposed in Northern Luzon.     

Slab rollback and microcontinent subduction in the evolution of the Zambales Ophiolite Complex(Philippines):A review

New radiolarian ages show that the island arc-related Acoje block of the Zambales Ophiolite Complex is possibly of Late Jurassic to Early Cretaceous age.Radiometric dating of its plutonic and volcanichypabyssal rocks yielded middle Eocene ages.On the other hand,the paleontological dating of the sedimentary carapace of the transitional mid-ocean ridge-island arc affiliated Coto block of the ophiolite complex,together with isotopic age datings of its dikes and mafic cumulate rocks,also yielded Eocene ages.This offers the possibility that the Zambales Ophiolite Complex could have:(1)evolved from a Mesozoic arc(Acoje block)that split to form a Cenozoic back-arc basin(Coto block),(2)through faulting,structurally juxtaposed a Mesozoic oceanic crust with a younger Cenozoic lithospheric fragment or(3)through the interplay of slab rollback,slab break-off and,at a later time,collision with a microcontinent fragment,caused the formation of an island arc-related ophiolite block(Acoje)that migrated trench-ward resulting into the generation of a back-arc basin(Coto block)with a limited subduction signature.This Meso-Cenozoic ophiolite complex is compared with the other oceanic lithosphere fragments along the western seaboard of the Philippines in the context of their evolution in terms of their recognized environments of generation.