STOP #8   SUMMARY TRAVERSE ACROSS TYPICAL COBALT AREA LITHOLOGIES

This stop best illustrates the most common lithologies associated with the silver veins in the camp.

The traverse is 90 over bedrock and serves to re-emphasize many of the features observed at earlier stops.  Recent detailed mapping of the area, particularly with regard to sedimentological characteristics has been carried out by Rainbird (1980), Legun (1981) and Scammel and Mustard (1983). The mapping has resulted in a detailed fades division within the Coleman sequence.  Subdivisions include two types of paraconglomerate, pebbly sandstone, laminated "varved?" siltstone and mudstone, non-cyclic siltstone, orthoconglomerate and basal breccia (Mustard, pers. comm., 1983).  However, despite this segregation, a confusing mixture of these sediment-types remains at even smaller scales.  Vertical contacts, laminite-draped depressions of short lateral extent, variable bedding attitude, recumbent folds and rapid changes in matrix composition and clast density (Legun, 1981) all.contribute to this problem.
Coarsening-upward sequences, ripple marks, cross-bedding, sand dikes and other sedimentary structures are present and suggest a sub-aqueous out-wash or glacio-lacustrine delta environment of deposition (Legun, 1981).
Basement rocks are variable in composition and include pillowed and massive mafic flows and intermediate to felsic volcanics.  The unconformity is relatively non-descript.  Huronian sediments are often "pasted" onto the basement rocks and are preserved as irregular, paper-thin outliers.  Basal breccia occurs sporadically along the unconformity in thin narrow zones.
The eroded base of a Nipissing diabase sill is inferred to have been within 50 m of the outcrop surface,  A down-faulted remnant of the diabase is exposed immediately north of Cobalt.

Points of Interest 

8a:  Contact between unaltered and altered pillowed mafic volcanics
"Chlorite spotting" is sporadically developed within the volcanic rocks.  The "spotting" is believed to form as a result of fluid migration associated with emplacement of the Nipissing diabase.  In this particular instance, a fairly sharp, irregular contact demarcates the boundary between "spotted" and unspotted flows.  The alteration is exceptionally intense here and illustrates a number of characteristics not commonly observed in spotted Huronian rocks. 

8b:  Chlorite alteration in mafic volcanic rocks
Chlorite occurs in euhedral, rectangular shapes, in coalescing trains in fractures and stringers and as massive replacement within pillow selvages.  The shape of the "spots" suggests that the chlorite has retrograded from previously formed, porphyroblastic growths of some mineral.  Thin section analyses and microprobing provide few clues as to the original mineral composition.  Spots are composed mostly of chlorite with accessory sphene, quartz, albite, biotite, fibrous amphibole, illmenite and pyrrhotite.  Donaldson and Munro (1982) have suggested that the chlorite has grown as pseudmorphs after amphibole.
A narrow, pre-Algoman, biotite lamprophyre dike has intruded the altered, pillow-volcanic flows.  These dikes are common in the area and characteristically have very gentle dips. 

8c:  Basal breccia and orthoconglomerate
The basal breccia is composed of angular fragments of the underlying basement lithologies.  Fragments appear to have been tectonically brecciated.  Fractured clasts have moved only slightly from original positions and can be pieced together.  The matrix is composed of a mixture of the underlying Archean lithologies and the Coleman sediments. The top contact of the unit is often indistinct and gradational into the overlying rocks (Mustard, pers.  comm., 1983).  Fragments decrease in size upwards.  The breccia is found only at the unconformity.  It has short, lateral distribution and maximum thicknesses of 1.5 m (Rainbird, 1980).  At this location, fragments from an underlying lamprophyre dike comprise part of the basal breccia.  They trail to the north for a maximum distance of 2 m and indicate that minimal transport has taken place.
Lindsey (1969) proposed that the breccia originated by glacial plucking or frost cracking.  Donaldson and Munro (1982) suggest that the breccia represents a paleoregolith produced by mechanical weathering.  Patterson (1979) described it as a paleosoil horizon and Rainbird (1980) suggested an origin due to slight gravity transport of frost heaved fragements and subsequent deposition as a talus-regolith in Archean depressions.
Note the extensive development of chlorite alteration in the underlying pillowed volcanics and the distribution of the alteration within clasts and matrix of the overlying basal breccia and orthoconglomerate.  Most "spotting" here is confined to fragments of the underlying rocks.  Alteration may be controlled by varying pebble compositions and susceptibility to fluid migration or alteration may reflect processes active within the volcanic cycle rather than with the intrusion of Nipissing diabase. 

8d:  Cross-bedded sandstone
Observe the cliff face to the east where good planar and trough cross-bedding is developed within medium to coarse-grained sandstone.  Paleocurrent directions, as determined from these cross-beds, indicate transport in a southwest direction (Legun, 1981).

 8e:   Unconformity
This exposure best illustrates the character of the unconformity between Huronian sediments and the underlying Archean basement.  The basement is comprised of a variety of felsic volcanic rocks (possibly very silicified mafic volcanics), including bands of stretched siliceous and sericitic fragmental material.  Skins of argillite and paraconglomerate occur as isolated patches.  Numerous fractured wedges, with sharp, angular boundaries within the basement are infilled by these sediments. 

8f:  Cyclically laminated siltstone/mudstone
These sediments are correlative with similar rocks exposed at stop #3. Thin parallel laminae «4cm) of green/grey siltstone and argillite display climbing ripples (Scammell, Mustard et al, pers.  comm,.  1983), soft sediment deformation features and dropstones.  The dropstones and varves are indicative of subaqueous, fresh-water deposition under floating ice (Legun, 1981).
The return route illustrates many of the rapidly changing fades common to the Coleman Member unit.  Problems encountered in mapping and interpretation of the original  
depositional environment are made apparent. 

8g:  Pragmental lamprophyre dike
Post-Algoman intrusions of biotite lamprophyre are quite common.  Contacts are often chilled and occasionally vesicular.  In many instances, the wallrock has been ripped up and incorporated within the dike to form fragmental bodies.  At this location, differential weathering of softer minerals creates a distinct pock-marked terrain.