Summary:
Bloom Lake property mineralization style is a deposit typical of the Superior-Lake type.
The Bloom Lake deposits are about 24 km southwest of Labrador City and about 8 km north of the Mount Wright range. The western 6 km of this range contains very large reserves of specular hematite-magnetite iron-formation in a synclinal structure that is regarded as a southwest extension of the Wabush Lake ranges.
The iron-formation and quartzite are conformable within a metasedimentary series of biotitemuscovite-quartz-feldspar-hornblende- garnet-epidote schists and gneisses in a broad synclinal structure. This succession, following the first stage of folding and faulting, was intruded by gabbroic sills which were later metamorphosed and transformed into amphibolite gneiss with foliation parallel with that in adjacent metasediments. Two separate iron-formation units are present; these join northwest of Bloom Lake, but are separated by several dozen meters of gneiss and schist in the southern part of the structure. Quartzite, present below the upper member throughout the eastern part of the area, pinches out near the western end. Folded segments and inclusions of ironformation in the central part of the syncline that are surrounded by amphibolite, are in most cases thought to be part of an overlying sheet that was thrust over the main syncline during the first period of deformation. The large amphibolite mass in the central part of the area was apparently emplaced along the zone of weakness created by this early thrust fault.
Iron-formation in the western 5 to 6 kilometers of the structure is predominantly of the magnetitehematite-quartz facies that forms the major zones of potential ore. Hematite is distributed in two ways through the quartzite. The hematite is of the specularite type and has a silvery-grey colour and is non-magnetic. It is most often occurring as anastomosing to discontinuous stringers and bands less than 10 cm thick in a quartz or actinolite-quartz matrix. Bands tend to be folded and deformed but also can be regular and tabular. Quartz is milky and granular.
Magnetite typically occurs in narrow millimetric veinlets associated with quartz-carbonate veining material. The crystals are sub- to euhedral and demonstrate the typical dull to sub-metallic luster. When associated to hematite-enriched mineralization, the magnetite occurs as blebs of porous grains, often granoblastic, that may extend up to several centimetres. Enriched magnetite horizons are mostly found, but not always, in the upper portion of the iron formations in close contact with the amphibolite mass.
With the actual state of geological knowledge in the western sector of the Bloom Lake deposit, magnetite-rich IF are less important in volume than in the eastern half of the Bloom Lake pit area. The thickness of drillhole intercepts is lower than 10 vertical metres. Many drill holes did not return significant magnetite intersections. Very few actinolite or grunerite minerals associated with magnetite mineralization were described in the western holes.
A fairly abrupt change in facies takes place along strike east of a line passing northwest across Bloom Lake, east of which the grunerite-Ca-pyroxene-actinolite-magnetite-carbonate facies predominates. The oxide facies to the west is uniform.
The lower unit is less than 30 meters thick in some places and is considerably thinner than the upper unit. The iron content ranges from 32 to 34 per cent in this facies. In places the silicatecarbonate facies to the east contains more than 50 per cent cummingtonite, which in part is magnesium rich, and the manganese content ranges from 0.1 to more than 2.0 per cent. Mueller (1960) has studied the complex assemblage of minerals in this rock and has discussed chemical reactions during metamorphism in considerable detail. He has shown that a close approach to chemical equilibrium in the amphibolite metamorphic facies is indicated by the orderly distribution of Mg, Fe, and Mn among coexisting actinolite, Ca-pyroxene, and cummingtonite, and the restriction in the number and type of minerals in association with each other. Furthermore, a comparison between the composition of the silicates and the presence or absence of hematite shows that the Mg to Mg plus Fe ratio is increased, but is much less variable when hematite is present.
The iron formation forms a long doubly plunging syncline that is canoe-shaped but buckled across the centre to produce two distinct oval-shaped basins. Although this structure appears to be relatively simple in form, it seems to have been developed during two stages of deformation. Folding along northwest-trending axes and overthrusting of the upper iron formation during the first stage of deformation appear to have been followed by gabbro intrusion, folding along eastwest axes, faulting, and metamorphism during the Grenville orogeny.