Many
low sulphidation epizonal gold and zinc-lead vein deposits within magmatic arcs
are situated on basement highs, and post-date both the main volcanic episode and
porphyry copper (gold) mineralisation. These features are best explained by
emplacement of a flat batholith a few kilometres beneath each mineral district,
as recently demonstrated by geophysical data in the Baia Mare district of
Romania. The batholith provides heat for the hydrothermal system, and results in
extensional faulting both through up-doming of the overlying rocks, and in some
arcs through incipient collapse of the thermally weakened crust under regional
extensional stress. More water-rich magmas emplaced at greater depths could
explain some deep epizonal and mesothermal gold deposits situated outside
magmatic arcs.