Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 82

Fig. 4.5 Photos of section cut through a chimney of a black smoker VMS-type deposit (Photo N.
Arndt); (b) and (c) photos of ore from deposits in the Yaman Kasy massive sulfide deposit in the
Urals. (b) is a fossilized tube worm dwelling tube and (c) contains fossilized monoplacopherans
and brachiopods (photos from Phil Crabbe)
vertically through highly altered host rocks. The proportions of metals vary within
the deposit: the upper massive sulfide is rich in Zn and Pb (in those deposits that
contain this metal) whereas the stockwork is enriched in Cu and Au. In many
deposits later deformation and metamorphic recrystallization has destroyed the
original ore textures, but well-preserved examples preserve bedding and other
sedimentary structures. In the remarkable deposits from the Urals and Ireland,
the chimneys of black smokers are beautifully preserved, to the extent that even
the dwelling tubes of tubeworms and other fossils, now replaced by sulfide, can be
recognised (Fig. 4.5 and Box 4.2).
The volcanic rocks that host VMS deposits erupt in a wide variety of tectonic
environments. Although the modern black smokers that provided the clues to their
origin are best known along mid-ocean ridges, many more recent discoveries, and
the locations of the ore deposits themselves, are in convergent margin settings.
From the recent classification based on the rock types associated with the deposits,
given in Table 4.2 , we can see that the setting varies from intra-oceanic arc and
backarcs through continental margins to mature epicontinental backarcs. The age
spans most of geological history: the Big Stubby deposit in the Pilbara of Australia
has an age of 3.5 Ga and is one of the oldest known ore deposits; large and important