Solidification Textures, Miarolitic Cavities and Orbicles: Field
Evidence for the Magmatic to Hydrothermal Transition in
Intrusion-Related Mineral Deposits
Unidirectional solidification textures (USTís), miarolitic cavities
and orbicles are features sometimes observed associated with or
within mineralized felsic cupolas. The UST zones are comprised of
irregular sub parallel layers of quartz crystals with orientated
c-axis terminations. They are products of rhythmic precipitation of
quartz and quartz plus feldspar during periods of fluctuating
volatile over pressuring within a relatively small degassing felsic
intrusion.. Such textures have been observed in porphyry copper
systems, tin, tungsten, molybdenum and gold-bearing intrusions.
Mineralized USTís suggest that in some cases metals began to deposit
at magmatic temperatures and pressures with subsequent transition to
hydrothermal conditions and main stage deposition of mineralization.
There also appears to be a close relationship between USTís and "A"
veins in some porphyry copper and molybdenum deposits, in that both
textures can sometimes be seen to coalesce. Miarolitic cavities are
microscopic to metre size irregular voids which often underly a UST
zone. They can be infilled with pegmatitic and metallic assemblages
which are considered to have been entrapped at magmatic temperatures
and pressures during late-stage crystallization of the intrusion.
Miarolitic cavity zones are relatively common in tin, tungsten,
molybdenum and gold intrusion-hosted deposits. They are also known
to occur in association with copper and gold Ėbearing systems.
Orbicles are ovoid to spherical, microscopic to centimeter-sized
concentrically zoned mineral assemblages which tend to occur as
aggregates in some granitic to noritic intrusions, whilst they are
not normally considered to be associated with mineralization, some
examples have been noted to contain copper sulphides. It is
concluded that the above textures, and in particular USTís, provide
important evidence for initial metal sulphide deposition at magmatic
temperatures and pressures in a large variety of intrusion-hosted
mineral deposits. The mineralizing fluids subsequently evolve into a
hydrothermal system as temperatures and pressures decrease.