EVIDENCE FOR MAGMATIC FLUIDS IN THE FORMATION OF MODERN AND ANCIENT VOLCANOGENIC MASSIVE SULFIDE DEPOSITS

The importance of a magmatic fluid in the formation of volcanogenic massive sulfide (VMS) deposits is not well constrained and remains debatable. We evaluated this issue by conducting a melt inclusion study on the volcanic rocks that host the active PACMANUS hydrothermal sites of the eastern Manus back-arc basin, western Pacific, and on the rhyolites that are associated with the large (e.g., Brunswick #12, 236 x 106 t) Ordovician VMS deposits in the Bathurst mining camp, New Brunswick. A volatile-rich magma is indicated at PACMANUS by high concentrations of volatiles (0.9 to 2.5 wt% H₂O, up to 0.45 wt% Cl and 0.24 wt% F) in the mafic melt inclusions in phenocryst minerals of the volcanics. The presence of a fluid phase in the melt inclusions indicates that the pre-erupted magma was saturated with volatiles. The fluid is CO₂-dominated in the mafic magma and becomes a mixture of CO₂ and H₂O in the fractionated felsic magma. The ore metals in the degassed fluid, as inferred from the compositions of the metallic precipitates found in the vesicles of melt inclusions and matrix glass, progressively change from Ni+Cu+Zn+Fe in basalt and basaltic andesite, to Cu+Zn+Fe in andesite, Cu+Fe in dacite and Fe+Zn (+Pb?) in rhyodacite and rhyolite. Similarly, metal-rich melt inclusions are found in quartz of the rhyolites from the Bathurst mining camp. They contain precipitates of Cu, Fe, S and minor Cl and Zn on the walls of the coeval bubbles. Hypersaline inclusions consisting mainly of KCl and NaCl are observed on the periphery of some melt inclusions as well as in igneous zircon and F-rich apatite (5.3 wt% F), suggesting a magmatic fluid in a volatile-saturated felsic magma.

A significant amount of magmatic fluid (>1.7 wt% H₂O) is estimated to have exsolved from the magma at PACMANUS. The fluid, if concentrated in a magma chamber and discharged along a favorable structural zone that extends to the sea floor, could contribute large quantities of volatiles and ore metals directly to a seafloor hydrothermal system, a process demonstrated for the geothermal systems of subaerial volcanoes. The focused discharge of as little as 1 wt% of a magmatic fluid as a result of pre-eruptive degassing particularly in a fractionated felsic magma, if mixed with heated sea water, could provide >85% of the ore metals and S for the formation of large or super large VMS deposits