Geology Reference
In-Depth Information
Description
: The distinctive feature of a VMS deposit is its association with
volcanic rocks. Depending on the particularly setting, these can be mafic (basaltic)
or felsic. Invariably they were deposited under water, either in a mid-ocean ridge
setting or more commonly in island arcs or during arc-continent collision. Sedi-
mentary rocks form an important part of the host sequence in certain classes of
VMS deposits.
Most VMS deposits are relatively small, usually containing only a few million
tons of ore. Exceptions are the large Kidd Creek deposit in Ontario, Canada
(160 mt) and even larger deposits in Spain and in the Russian Urals (Box 4.2).
The grades of the ores are high, however, which makes this type of deposit an
attractive exploration target, particularly for small or “junior” mining companies.
Typical grades are 1-5% of Cu, Zn, and/or Pb with minor quantities of Au and Ag.
The mineralogy is relatively simple. As with almost all sulfide deposits, with the
exception of those in purely sedimentary settings, iron sulfides predominate. In
VMS deposits, pyrite or pyrrhotite make up about 90% of the sulfide assemblage,
which may also include chalcopyrite, sphalerite and galena, and in some cases and
in minor amounts, bornite, arsenopyrite, magnetite, and tetrahedrite.
Most deposits have the very distinctive structure illustrated in Fig.
4.4
. A tabular
or mound-shaped body of stratiform, banded, massive sulfide overlies a crudely
pipe-shaped discordant “stockwork”, a zone of mineralized veins that cuts
Debris apron and
metalliferous sediment
Black smokers
White smokers
Anhydrite cone
Sulfide talus
anhydrite
pyrite
quartz
Sealed
zone
Zn-rich marginal
facies
gradational contact
Approxim. limit of
demagnetized zone
Silicified, pyritic stockwork
Chloritized + / - hematized
basalt
Alteration pipe
Fig. 4.4 Diagram of a typical VMS deposit, from the example of “TAG sulfide mound” on the
media-Atlantic Ridge (Modified from Hannington et al.
1998
)
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