Most large skarn deposits are zoned in both space and time relative to
associated intrusions. Zonation occurs on scales from kilometers to micrometers,
and reflects infiltrative fluid flow, wallrock reaction, temperature variations,
and fluid mixing. The most spectacular examples of skarn zonation usually occur
at the skarn-marble contact, where transitions between monomineralic bands can
be knife sharp. Other small-scale examples occur in zoned veins and individual
mineral crystals. Although, visually striking and scientifically interesting, in
mineral exploration these small-scale variations are less useful than deposit-
or district scale zonation. In most skarn systems there is a general zonation
pattern of proximal garnet, distal pyroxene, and vesuvianite (or a pyroxenoid
such as wollastonite, bustamite, or rhodonite) at the marble front. As well,
individual skarn minerals may display systematic color or compositional
variations within the larger zonation pattern. Such patterns are reviewed for 14
well-studied examples of Cu, W, Sn, Au, and Zn-Pb skarns. In addition, many
deposits have endoskarn or other alteration of the associated intrusion, and
recrystallization or other subtle changes have occurred in the surrounding
wallrocks. Copper skarns, such as Mines Gaspé in Quebec and Big Gossan in Irian
Jaya, have high ratios of garnet:pyroxene and are zoned outward from the
intrusion, to garnet, to pyroxene, to massive-sulfide replacement and vein
deposits. Garnets in Cu skarn are Fe-rich and change from dark red-brown near
the intrusive contact to paler brown, green, or yellow in distal locations.
Pyroxenes in Cu skarns are pale and diopsidic near the intrusion, and become
darker and more Fe- and Mn-rich away from the intrusion. Tungsten skarns, such
as Salau and Costabonne in France and Pine Creek and Garnet Dike in California,
have intermediate ratios of garnet:pyroxene, are more extensive vertically and
along strike than perpendicular to the intrusive contact, and have zonation
patterns commonly complicated by overprinting of metamorphic lithologies. In W
skarns, garnet is commonly subcalcic and the pyroxene is Fe-rich, reflecting
particularly reducing wallrocks or great depth of formation. Tin skarns, such as
Dachang in China and Moina in Australia, also can have subcalcic garnet and
Fe-rich pyroxene, but this reduced mineral assemblage typically is due to an
association with reduced S-type granites. Tin skarns differ from most other
skarn types in having a late greisen stage that may replace earlier Sn-bearing
calc-silicate minerals, thus liberating Sn to form cassiterite. Many high-grade
Au skarns, such as Hedley in British Columbia and Fortitude in Nevada, have low
ratios of garnet:pyroxene and are associated both with reduced plutons and
reduced wallrocks. Goldrich zones occur in Fe-rich, pyroxene-dominant, distal
skarn. Zn-Pb skarns, such as the Yeonhwa- Ulchin district in Korea and Groundhog
in New Mexico, have low ratios of garnet:pyroxene and generally form distal to
associated intrusions. These skarns also are zoned from proximal garnet to
distal pyroxene and pyroxenoid (bustamite-rhodonite), with significant zones of
massive sulfides within and beyond skarn. Manganese enrichment of most mineral
phases, particularly pyroxene, is characteristic of distal zones. Fundamental
controls on skarn zonation include temperature, depth of formation, composition
and oxidation state of associated plutons and wallrocks, and tectonic setting.
Most W skarns form at relatively great depth, 5 km to 20 km, with extensive
high-temperature metamorphic and metasomatic mineral assemblages. In contrast,
most other skarn types are relatively shallow, <10 km and mostly <5 km,
with limited, lower temperature metamorphic aureoles. Differences in oxidation
state correlate well with different skarn zonation patterns, particularly
garnet:pyroxene ratios and compositions, and can be used in both classification
of and exploration for skarn deposits. Zonation models, especially where
quantified, can be used predictively in exploration both for known and blind
targets.
