Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 84

amounts of base metal mineralisation. Layers of umber , Fe-, Mn-, and trace-
metal enriched mudstones of volcanic exhalative origin, cap the sulfide lens.
Archean VMS deposits. The Delbridge deposit is typical of VMS ore
bodies in the 2.7 Ga Abitibi belt in Canada. The deposit formed at the contact
between felsic pyroclastic rocks and andesitic volcanic lavas that were once
part of an ancient island arc. The diagram to the left, redrawn from Boldy
(1968), illustrates all the essential features of a VMS deposit and shows that
Canadian geologists understood how these deposits formed well before the
discovery of active black smokers in 1975. According to Boldy, the deposit is
of “ volcanic exhalative origin ”, an example of “ mineralization of a flank
fissure which was the site of solfataric activity within which the various
metals were rhythmically precipitated. ”
deposits also form in the late Archean and through the Proterozoic and Phanerozoic;
and as mentioned above, deposits continue to form on the modern ocean floor.
The types of ore metals are directly related to the geological setting and host
rocks. Ore bodies in mainly basaltic rocks are rich in Cu and contain only minor
amounts of Zn and other metals, which leads to a parallel classification in which
they are known as Cu-Zn deposits. Those in bimodal mafic-felsic settings are richer
in Zn than Cu (Zn-Cu deposits); and those in sedimentary settings contain Pb in
addition to the Cu and Zn (Zn-Pb-Cu deposits).
Origin : A genetic model for the formation of a VMS deposit, summarized from
Franklin et al. ( 2005 ), is illustrated in Fig. 4.6 . It has six main elements. (1) a heat
source to drive the hydrothermal convective system and potentially to contribute some
ore metals. In many deposits the source is a shallow-level intrusion of mafic to felsic
magma; (2) a zone of high-temperature reaction in which metals and other
components are leached from volcanic and/or sedimentary by circulating seawater;
(3) synvolcanic faults or fissures which focus the discharge of hydrothermal fluids; (4)
footwall, and less commonly, hanging-wall alteration zones produced by interaction