A. K. Chatterjee

The Precambrian rocks of the George River Group of Cape Breton Island include thick sections of quartzites, slates, limestone and dolomites and their metamorphic derivatives, together with lesser amounts of volcanic rocks and other sedimentary rocks. The wall rocks of mineralization can be grouped into two general types: (1) those containing moderately abundant calcium-magnesium silicates, and (2) those containing abundant feldspars and micas. The first group includes calcareous rocks of Meat Cove, Lime Hill, Whycocomagh and Boisdale Hills area, and the second group includes garnetiferous quartz-feldspar-biotite schists of Faribault Brooks, quartz-K-feldspar-biotite gneisses of Nile Brook and sericite-biotite schists of Second Gold Brook area.

At Meat Cove, the forsterite-rich unit of the magnesian skarn was the site of most metallization. Sphalerite is the main sulphide mineral but also included are pyrite-pyrrhotite-galena-chalcopyrite, pyrite-pyrrhotite-germanite-renierite-magnetite and pyrite-pyrrhotite mineral assemblages. At Lime Hill hypogene minerals include sphalerite, pyrite, pyrrhotite, galena, tetrahedrite, chalcopyrite and bornite in the magnesian skarn and scheelite and hydrotungstite in the diopside-wollastonite skarn. At Whycocomagh the hypogene minerals are magnetite, scheelite, molybdenite, chalcopyrite, pyrite and pyrrhotite and all of these are confined to the andradite-diopside unit of the magnesian skarn. In the Boisdale Hills veins of pyrite-pyrrhotite-chalcopyrite-sphalerite and "spotty" distribution of scheelite are restricted to wollastonite-diopside-andradite skarn.

In the Faribault Brook area, the base-metal mineralization occurs in three zones, designated as Galena Mines, Silver Cliff and Core Shack prospects. The host rock in all three deposits is garnetiferous quartz-feldspar-biotite schists. The hypogene minerals are pyrite, pyrrhotite, arsenopyrite, magnetite, lollingite, enargite, tetrahedrite, chalcopyrite, galena, sphalerite, chalcocite and covellite in a dominantly quartz gangue.

An alteration halo surrounds all known sulphides; exception to this have not yet been found. Wherever the entire system is visible, the halo is zoned, and the zones are always in the same sequence. In areas too large to see in their entirety the same zones are present and the same sequences are found. The width of the altered rock zones that bound the mineralized zone ranges from a few centimetres up to a few hundred metres. The lateral zonation adjacent to the mineralization appears to have developed contemporaneously, or nearly, with the deposition of the vein material.

The altered carbonate wall rocks adjacent to the mineralized zones are similar throughout the region (i.e. Meat Cove, Lime Hill, Whycocomagh and Boisdale Hills) in the talc-antigorite mineral association is adjacent to the mineralized veins and tremolite-antigorite association is further out, in detail the degree of alteration, the width of altered zones and, to some extent, the type of alteration vary because of differences in lithology and structure of the host rocks and differences in the intensity of the hydrothermal environment.

The wall alteration halo enclosing the mineralization in quartz-feldspar schists demonstrates convergent zonal alteration in that intense alteration has changed earlier metamorphic minerals to a uniform mineral association which consists of clays and sericite. Fresh garnetiferous quartz-feldspar-biotite schist of zone 4 grades into rock of zone 3 in which feldspars are altered incipiently to clay minerals. The clay mineral components in zones 3 and 2 consist of mixed assemblages of montmorillonite, illite and random mixed-layer montmorillonite, clay minerals. Further alteration results in the remainder of the biotite and the clay minerals being converted completely into sericite.

The relative abundances of Cu, Pb, Zn and their relationships of mineral deposits of the Faribault Brook area are remarkably similar to a discordant ore body ascribed to magmatic-hydrothermal processes.

The migration of heavy metals into, or within, the wall rocks was tested by direct chemical analyses of the zones in the alteration halo surrounding the mineralization. Such analyses have shown that the anomalous concentrations of heavy metals are confined to the zones of alteration and this would imply that the metallization galena from the Meat Cove zinc deposit yielded a model lead age of 415 Ma and a mica associated with the sphalerite yielded a K-Ar age of 409 Ma. This again suggests the time of metallization and the time of development of the alteration halo were synchronous.

The endogenic haloes of heavy metals in the wall rocks are extensive, and this parameter has considerable potential in detailed exploration programs.

The altered wall rocks are infallible guides to sulphide mineralization in the George River rocks of Cape Breton Island, for alteration was largely synchronous with metallization.

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Supervisor: G. C. Milligan