The development of volcanic hosted massive sulfide and barite–gold orebodies on Wetar Island, Indonesia

Wetar Island is composed of Neogene volcanic rocks and minor oceanic sediments and forms part of the Inner Banda Arc. The island preserves precious metal-rich volcanogenic massive sulfide and barite deposits, which produced approximately 17 metric tonnes of gold. The polymetallic massive sulfides are dominantly pyrite (locally arsenian), with minor chalcopyrite which are cut by late fractures infilled with covellite, chalcocite, tennantite–tetrahedrite, enargite, bornite and Fe-poor sphalerite. Barite orebodies are developed on the flanks and locally overly the massive sulfides. These orebodies comprise friable barite and minor sulfides, cemented by a series of complex arsenates, oxides, hydroxides and sulfate, with gold present as <10 lm free grains. Linear and pipe-like structures comprising barite and ironoxides beneath the barite deposits are interpreted as feeder structures to the barite mineralization. Hydrothermal alteration around the orebodies is zoned and dominated by illite–kaolinite–smectite assemblages; however, local alunite and pyrophyllite are indicative of late acidic, oxidizing hydrothermal fluids proximal to mineralization. Altered footwall volcanic rocks give an illite K–Ar age of 4.7±0.16 Ma and a 40Ar/39Ar age of 4.93±0.21 Ma. Fluid inclusion data suggest that hydrothermal fluid temperatures were around 250–270C, showed no evidence of boiling, with a mean salinity of 3.2 wt% equivalent NaCl. The d34S composition of sulfides ranges between +3.3& and +11.7& and suggests a significant contribution of sulfur from the underlying volcanic edifice. The d34S barite data vary between +22.4& and +31.0&, close to Miocene seawater sulfate. Whole rock 87Sr/86Sr analyses of unaltered volcanic rocks (0.70748–0.71106) reflect contributions from subducted continental material in their source region. The 87Sr/86Sr barite data (0.7076–0.7088) indicate a dominant Miocene seawater component to the hydrothermal system. The mineral deposits formed on the flanks of a volcanic edifice at depths of ~2 km. Spectacular sulfide mounds showing talus textures are localized onto faults, which provided the main pathways for high-temperature hydrothermal fluids and the development of associated stockworks. The orebodies were covered and preserved by post-mineralization chert, gypsum, Globigerina-bearing limestone, lahars, subaqueous debris flows and pyroclastics rocks

Arc–continent collision : the making of an orogen

There is no one model, no paradigm, that uniquely defines arc–continent collision. Natural examples and modelling of arc–continent collision show that there is a large degree of, and variation in, complexity that depend on a number of key first-order parameters and the nature of the main players; the continental margin and the arc–trench complex (the arc–trench complex includes the arc and the subduction zone). Although modelling techniques can be used to gain insights into these, they cannot and do not aim at reproducing the messiness of nature. In natural examples, identifying the nature of the main players involved, such as the age, physical properties, and pre-existing structure of the margin and the arc is just a beginning. Once this is done, parameters such as time, convergence velocity and vector need to be taken into account when determining the tectonic processes that were operative in any one arc–continent collision. In active examples, such as those in the southwest Pacific, some of these first-order parameters can be readily determined, and the nature of the main players easily assessed. Fossil arc–continent collisions, however, have commonly undergone post-collision deformation, erosion, and possibly partial dispersion to be left outcropping in the middle of a forest, with many of the key ingredients missing or hidden. This leaves the geologist to resort to comparison with other natural examples and with models that are mechanically constrained and simplified reproductions of the process to reconstruct and explain what may have been there and, importantly, what processes may have been operating and when. We attempt to show that this is not an easy task that can be put into one simple model. In this chapter we do not present a model for arc–continent collision. Instead, we begin with the main players involved, highlighting the characteristics of each that likely have a major influence on an arc–continent collision. Then, we investigate a range of possible processes that could take place once an intra-oceanic volcanic arc collides with a continental margin.17 page(s