The
PACMANUS and Jade deposits are two active hydrothermal fields that are forming
massive sulphide deposits at the
Pacific rim
margin. Both are associated with
felsic submarine volcanic rocks and both occur in back-arc settings. However,
PACMANUS is associated with almost sediment-free dacites and lies on the crest
of a volcanic ridge, whereas Jade occurs in a heavily-sedimented tectonic
depression associated with a basement of pumiceous rhyolite. The PACMANUS deposit is richer in Cu
and Au, and Jade in Pb. Zinc, Ag, As and Sb contents are similar. Sulfide
textures at PACMANUS suggest formation of Cu and Zn sulfides by simultaneous
double-diffusion involving inwards-moving seawater mixing with outwards-moving
hydrothermal fluid. By contrast, at Jade there is a more complex history of
formation where initial growth of barite on the sediment surface, probably
associated with microbial activity, formed an impervious cap which trapped
higher temperature hydrothermal solutions.