Sauvé and Bergeron (1965) used the informal name of « Iron-bearing member » to designate a unit of iron-bearing rocks and phyllites, in lesser amount, forming the middle part of the Baby Formation. Subsequently, different terms have been used to refer to this unit: middle (or median) member of the Baby Formation (Clark, 1988; Wares et al., 1988; Wares and Goutier, 1989 and 1990; Goutier and Wares, 1991); Baby Iron Formation (Clark, 1978, 1979, 1980, 1988), Middle Baby or Median Baby unit (Goulet, 1995; Clark and Wares, 2004), Median Baby Formation (Clark and Wares, 2004, Figure 33, page 55); Middle Baby Formation (Clark, 2019). In this compilation, the middle member of the Baby Formation is referred to as the Middle Baby member.

Rocks of the Middle Baby member have been the subject of detailed geological and metallogenic studies (Fournier, 1982; Clark, 1988; Wares et al., 1988; Wares and Goutier, 1989 and 1990; Goutier and Wares, 1991; Goulet, 1995).

Middle Baby member, Undivided (pPbb(m)): Iron Formation, Metachert

This undivided unit consists of silicate facies, oxide facies, carbonate facies, silicate-carbonate facies and sulphide facies iron formation, grunerite schist, pyritic mudrock and metachert interstratifications (Sauvé, 1956 and 1959; Sauvé and Bergeron, 1965; Bélanger, 1982). These rocks are present in variable amounts, but carbonate and silicate-carbonate facies iron-bearing rocks represent the most abundant lithologies. According to Sauvé and Bergeron (1965), carbonate facies rocks are locally pyritic. Iron-bearing rocks are generally finely stratified and form laminae ≤1 mm thick. They vary considerably in appearance and composition and change gradually from one to another. Grunerite schist and pyritic mudrock occur in minor amounts.

In thin sections, silicate facies iron formation is generally composed of quartz, carbonate (dolomite-ankerite or siderite) and magnetite, with variable amounts of stilpnomelane, grunerite, minnesotaite and chlorite (Dressler, 1979; Clark, 1988). Minor amounts of actinolite and anthophyllite are also observed. Metachert comprises 10 to 15% dolomite-ankerite. Sedimentary laminae are characterized by variations in the proportion of silicates and carbonates (Clark, 1988).

Middle Baby member 2 (pPbb(m)2): Carbonate Facies and Silicate-Carbonate Facies Iron Formation; Interbedded Metachert or Ferruginous Metachert; May Include Ferruginous Dolomite

Unit pPbb(m)2 consists of carbonate facies and silicate-carbonate facies iron formations. The thickness of these layers is ~30 m (Clark, 1978, 1988). Carbonate-facies iron formation consists of alternating ferruginous carbonate having a reddish brown, purple-brown or orange alteration patina, and metachert or ferruginous metachert having a grey or reddish grey alteration patina. In fresh surface, ferruginous carbonate is grey and metachert is grey or greenish grey. The thickness of carbonate beds varies from 1 cm to several metres, but rarely exceeds 0.3 m. Carbonate may also occur as lenses or occurrences in metachert. Locally, the latter seems to have been partially replaced by carbonate. The silicate-carbonate facies consists of alternating rocks from the carbonate and silicate facies (pPbb(m)1) (Clark, 1988). The carbonate facies typically contains quartz veins that cut beds at a steep angle. These are absent from silicate-carbonate facies rocks.

Metachert occurs as laminae or thin beds that can form ≥50% of the rock. Locally, metachert beds contain disseminated grains of ferruginous carbonate (Clark, 1978, 1988). Unit pPbb(m)2 may also include ferruginous dolomite (Clark, 1978). Intraformational breccia beds are common at the base of the unit near the south bank of the Koksoak River (Clark, 1988). This breccia is thought to be the result of syn-sedimentary slides. It is formed by centimetric, angular tabular fragments of metachert or carbonate in a carbonate matrix.

Thickness and distribution

The Middle Baby member belongs mainly to the Gerido and Howse lithotectonic zones, as defined by Clark and Wares (2004). It extends >260 km from Feuilles Lake (sheet 24K13) to Dupoisson Lake (sheet 24C16). Its thickness generally ranges from 30 to 45 m north of latitude 58°N (Sauvé and Bergeron, 1965; Goulet, 1995). In the Hérodier Lake area, SW of Kuujjuaq, the Middle Baby member thickens from west to east and reaches at least 50 m in thickness (Clark, 1979, 1988; Wares et al., 1988). The true thickness of the unit is difficult to estimate due to the presence of thrust faults, folds and gabbro sills (Clark, 1988; Wares et al., 1988). Clark (1988) notes, however, that the thickness of the Middle Baby member is much less than that of the Sokoman Formation, which is estimated to be at least 113 m thick in this area.

Dating

None.