EXSOLVED MAGMATIC FLUID AND ITS ROLE IN THE FORMATION OF COMB-LAYEREDQUARTZ AT THE CRETACEOUS LOGTUNG W-MO DEPOSIT, YUKON-TERRITORY, CANADA

Comb-layered quartz is a type of unidirectional solidification texture found at the roofs of shallow silicic intrusions that are often assoc iated spatially with Mo and W mineralisation. The texture consists of multiple layers of euhedral, prismatic quartz crystals (Type I) that h ave grown on subplanar aplite substrates. The layers are separated by porphyritic aplite containing equant phenocrysts of quartz (Type II), which resemble quartz typical of volcanic rocks and porphyry intrusion s. At Logtung, Type I quartz within comb layers is zoned with respect to a number of trace elements, including Al and K. Concentrations of t hese elements as well as Mn, Ti, Ge, Rb and H are anomalous and much h igher than found in Type II quartz From Logtung or in igneous quartz r eported elsewhere. The two populations appear to have formed under dif ferent conditions. The Type II quartz phenocrysts almost certainly gre w from a high-silica melt between 600 and 800 degrees C (as beta-quart z), in contrast, the morphology of Type I quartz is consistent with pr ecipitation from a hydrothermal solution, possibly as alpha-quartz gro wn below 600 degrees C. The bulk compositions of comb-layered rocks, a s well as the aplite interlayers, are consistent with the hypothesis t hat these textures did not precipitate solely from a crystallising sil icate melt. Instead, Type I quartz may have grown from pockets of exso lved magmatic fluid located between the magma and its crystallised bor der. The Type II quartz represents pre-existing phenocrysts in the und erlying magma; this magma was quenched to aplite during fracturing/deg assing events. Renewed and repeated formation and disruption of the pe ckers of exsolved aqueous fluid accounts for the rhythmic banding of t he rocks.