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Zentilli et al. (1984) have pointed out that most metalliferous deposits within the Cambro- Ordovician Meguma Group have a spatial relation to the Goldenville - Halifax Formation transition (GHT). The GHT is generally more finely bedded than regular Meguma rocks, and is locally calcareous, manganiferous, and rich in carbonaceous material. The Eastville deposit also exhibits these characteristics and is of special interest as the only significant zinc-lead deposit in the Meguma Group.

The deposit is anomalous compared to the Goldenville Formation in manganese (up to 11 Wt% MnO), and zinc and lead (1-3 Wt% Zn+PB for up to 10 m sections). The stratigraphy is marked by a finely laminated, locally calcareous, manganiferous unit at the top of the Goldenville which is overlain by black, pyrrhotitic slate comprising the base of the Halifax Formation.

The manganese predominantly is contained in spessartine garnets, which appear to have derived the Mn from carbonates and sulphides during metamorphic growth. The carbonates appear to have formed diagenetically due to the oxidation of the resident carbonaceous material. Stratiform zinc and lead are present in the rocks, but appear to have been largely remobilized by low oxygen fugacity fluids into structurally controlled concentrations.

The concentration of some metals can be ascribed to the abundance of minerals such as smectites and jarosite in the original sediments. The features of the deposit are consistent with at least some of the manganese and zinc being concentrated from the available sea water by diagenetic processes. Lead could have been fixed in the original sediments by absorption to the abundant organic material. However, it is possible that an outside source, such as hydrothermal sedimentary exhalative fluids, has been involved in the concentration of Mn, Zn, and PB in the original sediments at Eastville.

A better sedimentological understanding of the GHT at Eastville and possibly analyses of rare earth elements would give hints on the sea water chemistry at the time of deposition. Similarly, the discovery of synsedimentary feeder zones would strengthen arguments for a sedimentary exhalative contribution to the deposit.

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Pages: 146
Supervisor:听听 Marcos Zentilli