Abstract

Lead-zinc sulfides were deposited on the sea floor in a deep-water setting during the Middle to Late Devonian at the Jason deposit, Yukon Territory, Canada. Galena, sphalerite, and barite, among other minerals, precipitated from hydrothermal fluids expelled along faults. Stratiform mineralization is laminated to thickly bedded and massive. Underlying the stratiform deposits in two areas, there are zones of silicified, carbonatized, and brecciated rock containing veins with hydrothermal minerals (stockwork); the stockwork represents alteration in and around hydrothermal conduits. Mineralization is interstratified with, and cuts across, clastic rocks of the Lower Earn Group.

Lower Earn Group sedimentation was locally controlled by the graben and by movement on the graben-bounding faults. The graben center is represented by a thick sequence (as much as 1,500 m) of conglomerate, sandstone, and shale that was derived from a western provenance. These clastics were deposited by sediment gravity flows in channels and in channel-flanking areas, mostly within the graben. Graben-floor channels are estimated at being up to a few tens of metres deep. Conglomerates are lenticular, commonly imbricated, and locally normally or inversely graded. A very few beds have faint parallel stratification. Most sandstones are more evenly bedded, many show flutes and grooves, and almost all can be described using Bouma-sequence terminology. The lack of burrows in the Lower Earn Group at Jason suggests anoxic conditions on the sea floor. Abrupt stratigraphic thinning and the abundance of laterally discontinuous, locally derived, mass failure deposits indicate the existence of syndepositional faults in the mineralized area. The fault zone formed the graben margin and appears to have focused discharge of metalliferous fluids. The local tectonic setting during the Late Devonian was extensional and apparently related to transform motion between the North American cratonic plate and an ocean plate to the west.

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