Geology and geochemistry of epithermal precious metal vein systems in the intra-oceanic arcs of Palau and Yap, western Pacific

Abstract The Palau and Yap arcs are part of an intra-oceanic island-arc-trench system which separates the Pacific and Philippine plates in the western Pacific Ocean. The 350-km-long Palau arc consists of over 200 islands while the 400-km-long Yap arc located to the north has only four major islands exposed. Four of the largest islands in Palau are composed primarily of early Eocene to mid-Miocene volcanic rocks and the four islands comprising Yap contain only Miocene volcanic rocks. Basalt and basaltic andesites of the Babelthuap Formation are the oldest volcanic rocks in Palau and are characterized by high MgO, Ni and Cr and low TiO 2 and have a boninitic affinity. They form the central and southeastern parts of Babelthuap Island. Oligocene arc tholeiite flows having an age of 34–35.5 Ma comprise most of the three smaller volcanic islands in Palau and the western part of Babelthuap. The youngest volcanic rocks are dacitic intrusions having an age of 22.7–23.2 Ma. The Yap arc is unusual in that metamorphic rocks up to amphibolite grade form most of the islands. These are underlain by a melange composed of igneous and volcanic clasts as well as clasts from a dismembered copper-gold skarn deposit. Miocene volcanic rocks consisting of flows and volcaniclastic deposits overlie the melange and metamorphic complex. An epithermal precious-metal vein system hosted by flows and flow breccias of the Babelthuap Formation occurs in an area 1.5 km by 1 km on the southeast side of Babelthuap Island. Over 50 veins and mineralized breccias ranging up to 2 m in width and having a strike length up to 500 m contain from trace to 13.0 ppm gold. The veins consist of quartz with varying amounts of sulfides and iron oxides after sulfides and the mineralized breccias consist of brecciated country rock cemented by quartz and iron oxides after sulfides. The veins and mineralized breccias generally dip within 15° of vertical and have two preferred orientations, north-northwest and north-northeast. Hydrothermal alteration of the host rocks consists of a widespread weak to moderately strong propylitic alteration and a more restricted sericitic alteration adjacent to the veins and shear zones. Sulfide minerals in the veins consist primarily of pyrite accompanied by lesser amounts of sphalerite, chalcopyrite, galena, acanthite, native silver, cerargyrite, and iodyrite in partly oxidized parts of the vein system. Gold is typically fine-grained, 1–20 microns, and occurs as native gold, electrum and gold-silver-telluride. Elements correlated with high gold concentrations include molybdenum, tellurium, bismuth, lead, silver, copper, zinc and arsenic. In Yap a similar vein system to that present in Palau is hosted by the Miocene Tomil Volcanics on the islands of Maap and Gagil Tamil. The quartz veins and quartz-cemented breccias contain up to 3.7 ppm gold and trace elements associated with the gold include tellurium, copper, silver and vanadium. Within the mineralized area an unusually iron-rich (3–20%), 4-m-thick, hot-spring deposit contains up to 1.04 ppm gold and high concentrations of tellurium, copper and vanadium. Hydrothermal eruption breccia beds are present within the deposit. The presence of the hot-spring deposit and banded and comb textures of the vein quartz suggest that the vein system presently exposed formed at a shallow level. The vein systems in Palau and Yap have similar textures, geochemical suites and alteration assemblages. Both vein systems formed late in the volcanic evolution of the intra-oceanic arc. Media tested for their effectiveness in geochemical exploration in the tropical, deeply weathered environment of Palau and Yap included stream sediments, heavy-mineral concentrates from stream sediments, and sediment from the mangrove coastal environment which is well developed around most of the islands of Yap and Palau. Geochemical surveys in both Yap and Palau of mangrove sediment show that adjacent to areas of gold mineralization, gold and tellurium contents are anomalous, ranging from 10 to 100 ppb gold and from 100 to 2100 ppb tellurium. Away from areas of mineralization gold contents are less that 6 ppb and tellurium less then 40 ppb. Mangrove coastal environments are common throughout the western Pacific and geochemical sampling of mangrove sediment can provide an effective method to quickly evaluate the potential of land adjacent to the coast as a first step in exploration.