Exsolved magmatic fluid and its role in the formation of comb-layered quartz at the Cretaceous Logtung W-Mo deposit, Yukon Territory, Canada

Comb-layered quartz is a type of unidirectional solidification texture found at roofs of shallow silicic intrusions that are commonly associated spatially with Mo and W mineralization; it consists of multiple layers of euhedral, prismatic quartz crystals (type I) that have grown on subplanar aplite substrates, separated by porphyritic aplite containing equant phenocrysts of quartz (type II) which resemble quartz typical of volcanic rocks and porphyry intrusions. At Logtung, type-I quartz within comb layers is zoned with respect to some trace elements, including Al and K; concentrations of these elements, as well as Mn, Ti, Ge, Rb and H are anomalous and much higher than found in type-II quartz. The two populations seem to have formed under different conditions; the type-II quartz phenocrysts almost certainly grew from a high-silica melt at 600-800 degrees C (as beta -quartz), whereas the morphology of type-I quartz is consistent with precipitation from a hydrothermal solution, possibly as alpha -quartz grown at <<<$I> 600 degrees C. It is considered that type-I quartz may have grown from pockets of exsolved magmatic fluid located between the magma and its crystallized, type-II quartz representing pre-existing phenocrysts in the underlying magma quenched to aplite during fracturing/degassing events. Renewed and repeated formation and disruption of the pockets of exsolved aqueous fluid accounts for the rhythmic banding of the rocks.