Unidirectional Solidification Textures, Miarolitic Cavities and Orbicles: Field Evidence for the Magmatic to Hydrothermal Transition in Intrusion-Related Mineral Deposits

Douglas Kirwin¹

¹Ivanhoe Mines Ltd., 999 Canada Place, Vancouver V6C3E1, Canada
 

Unidirectional solidification textures (UST’s), 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 UST’s 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 UST’s 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 –bearing 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 UST’s, 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.