Summary:
The Scotia Mine Deposit mineralization has long been considered a Mississippi Valley-type (“MVT”) lead zinc deposit (MacEachern and Hannon, 1974). Characteristics of sedimentary formations that host MVT lead zinc mineralization include shallow-water, shelf-type carbonate rocks with reefs around the peripheries of intracratonic basins, karst structures, limestone-dolomite interfaces, and proximity to a major hydrocarbon-bearing basin.
At the Scotia Mine Deposit, textures (including fossils) have been preserved; representing volume- for-volume replacement of original limestones by dolomite, and the sulphides are, in turn, replacements and porosity fillings within the previously altered host rocks. Kontak (2002) feels that petroleum in fluid inclusions in the Scotia Mine Deposit mineralization suggest a role of hydrocarbons in the mineralising process, like many MVTs, but Sangster and others (1998) point to basement rocks underlying the Palaeozoic sedimentary rocks as the source of the mineralising fluids.
Locally, the Scotia Mine is located in the Gays River Geological Formation, part of the Windsor Group of Carboniferous aged rocks.
The carbonates of this formation started growing in a warm sea of Carboniferous age when the ocean invaded the Maritime Basin 300 million years ago. The carbonate host is an algal bank-coral reef that grew along an anticlinal fold in the older Cambrian-Ordovician rocks, which were folded by compressional forces during the Taconic orogeny 440 million years ago. The carbonates formed a barrier between the two basins. To the Northwest was the Shubenacadie Basin and to the Southeast was the Musquodoboit basin. The Goldenville anticline’s slope is controlled by bedding and jointing. The base metal deposit is on the north side of the anticline, where the Goldenville rocks dip between 43° and 46° to the north. The strike of the beds is between 085° and 090°.
There are three main zones of mineralization within the deposit: the Main Zone (formerly Gays River deposit), the Getty Zone, and Northeast Zone. The Main Zone lies along the Southside of the Gays River, immediately east of the confluence with the Gays River South Branch. The Getty Zone lies just northwest of the zone on the western side of the Gays River. The two Zones are separated by less than one kilometre.
Main Zone Mineralization
The base metal is controlled by initial porosity. The mineralizing fluids chose the channel ways through the coral gardens. The most likely source of a channel way is a change in the direction at the reef crest, where joint controlled slopes meet. Galena is generally in veins and in fractures. There is some dissemination through the smaller openings in the rock.
Once over the coral mound, the mineralizing fluid travelled along sheets of skeletal debris and dolomite sand. Thus, in the reef proper, the mineralized beds are generally flat-lying to gently dipping (16°SE around the portal). Secondary mineralization is also open space-filling. The open spaces probably were caused by several uplift periods. A minor amount of marcasite and zinc oxide is present in fractures of the carbonate near the surface and in the outcrops near the confluence of the Gays River and South Branch Gays River.
Nesbitt Thomson Inc. (1991) describes high-grade mineralization as consisting of a massive sulphide zone in contact with the evaporite or Trench, ranging in thickness from 0.1 to 5.0 metres and locally containing up to 78% Pb and 57% Zn. On the footwall of the massive sulphide, there is a zone of disseminated material (>7% Zn equivalent) which, in places, is up to 12 metres in thickness. Locally disseminated mineralization (>2% Zn equivalent) extends up to twenty metres into the footwall. The parameters used to calculate “Zn equivalent” are unknown but would have reflected the prices of zinc and lead at the time.
A sinuous paleo coastline essentially controls the Main Zone. The main part of the deposit is shallow (generally <150 m deep), has a dip length of approximately 100 m and a strike length following the paleo coastline over a straight-line distance of 2 km (Nesbitt Thomson Inc., 1991).
The mineralization at the Main Zone consists of massive and disseminated ore hosted predominantly by the carbonate rocks, with extensions down into the basal breccia unit. Massive sulphide mineralization consists of fine-grained beige-coloured sphalerite and medium to coarse-grained galena (Kontak, 1998; Savard and Kontak, 1998). The massive mineralization is restricted to the carbonate-evaporite contact and is 1 to 3 metres thick.
Disseminated mineralization fills in primary porosity in the dolomitized carbonates and walls of primary cavities (Kontak, 1998).
Sphalerite and galena constitute about 99.5% of metallic minerals. Other sulphide minerals are marcasite, pyrite and chalcopyrite, while gangue minerals include calcite, dolomite, fluorite, barite and selenite (Patterson, 1993).
Getty Zone Mineralization
The Gays River Formation mineralization has long been considered a Mississippi Valley-type lead-zinc deposit. This type of deposit is carbonate-hosted, classified as a typical open space filling type, and hosted in a dolomitized limestone. The limestone developed as a carbonate build-upon an irregular pre Carboniferous basement topographic high where conditions allowed for growth of reef-building organisms.
The zinc/lead-bearing Gays River Formation trends in an east-north east direction across the Property. Locally, the mineralisation dips up to 45º on average, and up to vertical in places, to the north northwest which is the depositional slope of the front of the Gays River reef unit. The dip tends to be horizontal in the back reef area (south of the main trend). The mineralisation is present as sphalerite and galena and grades from massive Pb-Zn mineralized material in the fore reef to finely disseminated, lower grade material in the back reef. In the mine area, the Gays River Formation is overlain either by the evaporites of the Carroll’s Corner Formation and/or overburden.
Gypsum
Gypsum is an evaporite, a class of rocks that precipitate when water evaporation keeps pace with water input. Block faulting during the early Carboniferous lowered the land relative to the sea. Slowly, water invaded the area. The marine invasion led to carbonate-based life forms such as coral to establish colonies along the edge of the ancient sea. As the temperature and salinity of the water increased, gypsum, anhydrite and halite, and some other salts precipitated out as the sea dried up. Anhydrite is deposited directly from the evaporation of sea water at a temperature of 42°C or higher or at lower temperatures with increased salinity. At lower temperatures, gypsum is deposited. (Berry and Mason, 1959). Gypsum precipitates as a hydrated calcium sulphate (CaSO4 · 2H2O). Gypsum has a hardness of 2 on the Mohs scale, between talc and calcite, so that one can scratch gypsum crystals with a fingernail. Pure gypsum has a density of 2.3 tonnes per bank cubic metre and a bulk density of 1.6 tonnes per cubic metre. In the arid environment of the time, gypsum accumulated in thick beds over a long time. The sea water evaporated in cycles. Halite (NaCl common salt) and other mineral salts precipitated out as the sea dried up.