Alan V. Morgan
Not so very far from the University of Waterloo are a number of large stone quarries working the dolostones at the top of the Niagara escarpment near Dundas, Ontario. Our older magazine "WAT ON EARTH" described a visit to this quarry a number of years ago (Wat On Earth, volume 14, number 2, 2001).
Background
The Dundas Quarry is working well-bedded sedimentary rocks of the Lockport Formation of middle Silurian age (about 425 million years). These sequences show some interesting mineralization that we will come to shortly. The principal product being quarried is dolostone, a calcium-magnesium rich sedimentary rock. The end use is for aggregate, building stone (Figure 1), facing blocks and a component that is used in fluxes in the metallurgical industry. The working faces have exposed a number of interesting geological features, two of which are mentioned here.
ABOVE: Figure 1. Dundas Quarry
Faults
Several small faults cut parts of the sequences. One of these can be seen in Figure 2. The student is looking at a curved face that cuts through the well-bedded dolostones. The curved face is the fault plane and the grooves, technically called "slickensides", show that this fault has moved horizontally rather than vertically. High horizontal stresses have been documented from the Hamilton region and these might also be expressed in the phenomenon known as:
Pop-ups
Southwestern Ontario has a large number of features known as "pop-ups". These are breaks in the bedrock sequences (both Ordovician shales and Silurian carbonates) where the flanks of the pop-up dip sharply away from a central crestline. In Figure 3 Peter is standing on such a crestline in the Dundas Quarry. These "heaves" or "bumps" are frequently encountered when mining has relieved some of the confining pressure and the stresses disrupt a quarry floor or mining adit.
ABOVE: Figure 2."Slickenslides" on the side of a fault.
BELOW: Figure 3. Peter is standing on the crestline of a pop-up in Dundas Quarry.
Mineralisation at Dundas
Given the nature of the bedrock (Calcium and Magnesium carbonates) it is not surprising that calcite, CaCO3, is abundant, infilling many fractures in the rock mass. These angular chunks are part of a breccia (a sedimentary rock with angular fragments) that has calcite cementing the blocks together. Elsewhere it is not uncommon to see small voids or "vugs" that are lined with calcite crystals. These whitish masses are "nail-head" calcite crystals in the more massive grey-buff coloured dolostone. The calcite crystals can also form more interesting shapes. We call these well-formed crystals, scalenohedrons. In more easy terminology they are known as "dog-tooth spar".
ABOVE: Figure 4. Celestite.
Sometimes the voids are lined with a greenish or yellowish brown mineral. This is a variety of iron sulphide known as marcasite. Here individual calcite crystals are sitting on marcasite. Iron pyrite (FeS2) and is commonly found as "films" - rather thin dustings that encrust joint surfaces and fractures in the rock. A rarer blue coloured mineral is celestite (SrSO4) or strontium sulphate. It is the source of strontium used in fireworks and sugar beet refining.
Perhaps the most interesting commonly found mineral is sphalerite (ZnS), the principle ore of zinc. Lead and zinc are frequently found in sedimentary sequences especially those of the American mid-west. These are known as "Mississippi type" ores. A more massive form (with white calcite) is seen below. Typical views of sphalerite show the traditional brownish-yellow (amber) colour on some of the lighter surfaces.
Other minerals that can be found at the quarry are the halide mineral, fluorite (CaF2), and lead sulphide, galena (PbS), the chief ore of lead.
Figure 5. Calcite scalenohedra, also known as dog-tooth spar.
Figure 6. Galena on marcasite found in a vein exposed in the 1960's.
Figure 7. Calcite on marcasite found lining vugs in the dolostone.
Figure 8. Sphalerite and calcite.