Description: Variably altered dacite from the PACMANUS vent field in the eastern Manus back-arc basin, Papua New Guinea, was studied to determine the textural and mineralogical characteristics of hydrothermal alteration taking place in the immediate subsurface of this modern seafloor hydrothermal system. Detailed textural investigations show that fluid flow through the glassy dacite has been strongly controlled by the primary volcanic textures. Quench fractures and networks of interconnected perlitic cracks linking vesicles provided pathways for hydrothermal fluids flowing up to the seafloor. Hydrothermal alteration along these pathways resulted in the formation of pseudoclastic textures. Textural evidence suggests that alteration of the glassy dacite has not been sustained. The samples have been affected by incipient hydrothermal alteration that is typically not preserved in ancient volcanic-rock-hosted massive sulfide deposits. Interaction of the glassy dacite with hydrothermal fluids primarily resulted in the conversion of volcanic glass to dioctahedral smectite. Only minor amounts of trioctahedral smectite were formed. Destruction of the volcanic glass and the formation of smectite caused pronounced changes in the chemistry of the dacite samples, in particular a decrease in the SiO2 whole-rock content and the Na2O/K2O ratio. The two alkali elements behaved differently during hydrothermal alteration due to preferential incorporation of K into the interlayer position of the newly formed dioctahedral smectite. Smectite formation occurred under rock-dominated conditions although the addition of Mg was required to form trioctahedral smectite from the silicic volcanic glass. Primary plagioclase was resistant to hydrothermal alteration highlighting the fact that the destruction of volcanic glass and feldspar are not necessarily contemporaneous in massive sulfide forming hydrothermal systems. Incipient alteration of the glassy dacite close to the seafloor occurred at temperatures below 150 °C in an environment that allowed the development of steep temperature gradients. Comparison of the new data to the findings of deep drilling during ODP Leg 193 suggests that the smectite-rich alteration in the immediate subsurface of the PACMANUS hydrothermal vent field represents the low-temperature equivalent of illite- and chlorite-rich alteration associations forming in the upflow zones of the hydrothermal fluids in the deeper portion of the volcanic sequence.

Description: Metastable intermediate Na–K mica represents a product of hydrothermal alteration in volcanic rocks from the alteration halo of the Waterloo massive sulfide deposit, Australia. The XRD pattern of this solid solution between paragonite and muscovite is characterized by a rational series of basal reflections with d values intermediate between the end members. Transmission electron microscopy revealed that the intermediate Na–K mica forms thick stacks that belong to a two-layer polytype. Na-rich intermediate Na–K mica typically occurs together with paragonite whereas K-rich intermediate Na–K mica is intergrown with muscovite. The intermediate Na–K mica is interpreted to have formed as a result of the incomplete transformation of K-rich mica to Na-rich mica through dissolution and recrystallization processes driven by compositional changes of the hydrothermal fluids interacting with the volcanic rocks. Alteration must have proceeded under non-equilibrium conditions because the composition of the solid solution falls into the miscibility gap separating paragonite and muscovite.