Geology Reference
In-Depth Information
Box 4.1 Examples of Modern and Ancient VMS Deposits
TAG sulfide deposit. This deposit is currently forming on the rift valley of
the Mid-Atlantic ridge at 26
N, the site of an field of active black and white
smokers (hydrothermal springs). The deposit is located on the seafloor above
pillow basalts of the oceanic crust. It has the form of a classic VMS deposit,
comprising an upper lens of massive and semi-massive sulfide underlain by
a vertical pipe-like stockwork. Anhydrite, chert and red clay co-precipitated
with the sulfides. The deposit contains 3.9 mt of ore, 2.7 mt of massive and
semi-massive sulfide (~2% Cu) and 1.2 mt of mineralized breccias (~1% Cu)
in the stockwork
Cyprus VMS deposits. The Troodos ophiolite contains clusters of VMS
deposits in pillow basalts that probably erupted in a Cretaceous back-arc
basin. All deposits are Cu-rich (1-4%) with similar Zn tenors, as is normal for
deposits hosted by mafic volcanic rocks. They are made up of a massive
tabular cap overlain by sandy-textured and brecciated ore, in which the
pyrite-rich massive core is cemented by chalcopyrite and sphalerite, and an
underlying stockwork formed of a mixture of quartz and pyrite, with minor
(continued)
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