麻豆传媒

The Yava lead deposit at Salmon River, Nova Scotia, is a sandstone-hosted lead deposit situated in fluviatile clastic sediments, of late Carboniferous age. Ore reserves are estimated to be 5.6 million tonness at 5.3% Pb. During a 2 ¹/₂ year underground production operation (1979-1981), 388,000 tons were extracted.

The host rocks consist of cyclical fining-upward sequences of conglomerate, sandstone and shale, deposited in an upper meandering river environment. These rocks are grey-green in colour and contain abundant coaly matter.

The dominant sulphide is galena, occurring as concordant and discordant bands, clouds and disseminations. Pyrite and sphalerite are present, in association with galena, in minor amounts. These sulphides occur as a cement, infilling pore spaces in the clastic lithologies. The mineralisation is confined to the basal 15 m of the sandstones, above an unconformable contact with Windsor Group shales (Visean) and late Pre-Cambrian/early Cambrian basement rocks. Three ore zones have been outlined at Yava, and these are spatially related to palaeo-depressions in the basement landscape. In the West Zone, the ore horizon, which is up to 8 m thick, occurs in footwall contact with the Windsor unconformity.

Correlation of lead grade distribution, host rock stratigraphy and palaeotopographic features in 135 surface drill cores for the West Zone, in combination with underground mapping and channel sampling in the upper part of the same zone indicates the following features.

Regional pre-requisites to base metal concentration at Yava are: a porous sandstone host rock is a reduced state, an impervious footwall lithology, underlying basement of granitic composition, and an unconformity at the base of the mineralised sequence marking a period of emergence and sub-aerial weathering.]

Controls to ore localisation in descending order of importance are: negative topographic features in the pre-tectonic footwall relief, internal stratigraphic configuration and compositional variations within the clastic package, and the extent of sandstone diagenesis.

It is proposed that depressions in the buried Windsor landscape acted as basinal traps where metal-bearing solutions (derived from saline groundwater) pooled, while the porous organic-rich clastic host rocks provided the necessary conditions for fluid flow and sulphide precipitation.

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Pages: 258
Supervisor: Marcos Zentilli

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