Gypsum & Anhydrite

Gypsum is an important industrial mineral, and has been mined and quarried in Manitoba nearly continuously since 1901. Gypsum is an evaporite mineral that was deposited in a shallow, inland sea that covered Manitoba during the Jurassic Period, approximately 200 million years ago.

The Western Edge of Lake Manitoba

Along the western edge of Lake Manitoba, and around Gypsumville, gypsum deposits occur very near to the surface. They create a unique landscape of sinkholes, caves, and karst topography. Gypsum is currently being quarried on the west side of Lake Manitoba, but since 1901 there have been multiple mines and quarries in a variety of localities across southern Manitoba.
 

Gypsum is extracted from the quarry using heavy machinery, transported to Winnipeg, and processed into wallboard or used to make cement.

Gypsum and Anhydrite

Gypsum is a calcium sulphate mineral, with its chemical formula being CaSO₄ • 2H₂O.
Gypsum is often white or grey, but may display a variety of colours. It can be fine to coarse grained, and grows several interesting crystal forms and textures. Gypsum is very soft, so soft that fingernails can scratch this mineral. The term gypsum can also be used to describe a rock composed primarily or entirely of gypsum crystals.

Possible replacement texture in gypsum; clays are interspersed through the rock.
Edge of rock hammer for scale.

Gypsum has undergone a complicated history of diagenesis (shallow, low-grade metamorphism) in Manitoba. Gypsum was originally deposited when a shallow, marine seaway that covered Manitoba in the Jurassic Period experienced increased rates of evaporation due to a hot, dry climate. Overtime, the gypsum was buried by younger rocks and sediments, and was subjected to hotter temperatures. The heat caused the water in gypsum to be driven out of the crystal structure, and the mineral turned into anhydrite, CaSO₄. When the anhydrite beds were exhumed and brought near the surface, where they were subjected to groundwater flow. This water re-hydrated the anhydrite, and some of the deposit has reverted back to gypsum.

Taking a Closer Look at Gypsum

Fractures and veins filled with clay and mudstone through the greyish-white gypsum.

Gypsum is an important industrial mineral in Manitoba. The quarried gypsum is hauled to Winnipeg, and processed into wallboard. It is also an ingredient of cement. Because gypsum is economically important, I will be examining known occurrences of gypsum and providing a comprehensive update on this commodity. This will involve reviewing previous studies done on gypsum, current and historical industrial activity, examining core logs, and completing stratigraphic sections and maps of quarries.




Several localities within the gypsum quarry, like this one, 
will be ideal for completing a stratigraphic section.

Vertical walls found in this active gypsum quarry provide an opportunity to complete a stratigraphic section. A stratigraphic section is a detailed look at how the geology of a unit of rocks changes upwards and laterally. This allows geologists to preform correlations with other occurrences of similar rocks. It also enables geologists to understand how the depositional environment of these rocks changed over time. Information collected for a stratigraphic section may include mineralogy, fossils, sedimentary structures, mineral textures, and veins/fractures. Representative rock samples are taken too.

Written by Kathryn Lapenskie