User:ChrisCosby/McDonald Feldspar Mine

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Background

The MacDonald Feldspar Mine is located in the meta sedimentary Belt of the Grenville Province of the Canadian Precambrian Shield 16 kilometers north of Bancroft in Hybla, Monteagle. The Mineral deposit is a fracture-filling, coarsely crystalline, zoned granitic pegmatite dike . Its pegmatitic nature allows this deposit to be viable for economic minerals. Pegmatites generally have large euhedral crystals due to disequilibrium conditions generated by the presence of water, which acts as a flux causing fractional crystallization.

The country rock at the mine is amphibolite. This unit was intruded by the feldspar pegmatite, containing both plagioclase and potassium feldspars. This pegmatite contains very large feldspar crystals that can be up to 1 meter in size. There are both potassium feldspar (Microcline) and sodium feldspar (Albite) crystals at the mine. The quartz that is present at the mine is a smokey variety due to the presence of radioactive minerals (mainly Allanite).

History

In 1919 the Canadian Crystal Spa Corporation and Donald McAllister, owners of the property, established the Macdonald Mine and ran the mine until 1935. The Macdonald Mine is a traditional mine with large open cuts and adits driven into the face of a large hill. The largest open cut measures 168 x 21 x 37 meters and largest adit has been driven down 53 meters . According to Ontario Minerals this was the largest producer of feldspar in the Bancroft district and for some time in all of Canada with a total output of approximately 32 000 tons of Feldspar .

Feldspar is used in the glass and ceramics industries, and quartz used in silica glass and silicon microchips . Feldspar and quartz are the predominant minerals found in the mine but there are notable quantities of others as well including albite, allanite, andradite, apatite, betafite, biotite, calcite, chalcopyrite, chlorites, fluorite, galena, garnets, goethite, gypsum, hornblende, ilmenite, magnetite, marialite-meionite series, amazonite, molybdenite, perthite, pyrite, pyroxenes, pyrrhotite, smoky quartz, uranothorite, thorogummite, titanite, uranpyrochlore and zircon .

Regional Geology

Approximately 1.0 to 1.3 billion years ago , giant continental landmasses moved together and put a great stress upon the continent of North America. The mountains that were created by this major orogeny were known as the Grenville Mountains, and what remains today is the root of these mountains known as the Grenville province. Millions of years later it was in this Grenville province that many batholiths began to form at the bottom of the crust and began to rise through it. When one of these plutons were rising up and forming in the ground; the pressures was so great that the magma branched off into weaknesses in the rock to create dikes. It was in one of these minor intrusions that the MacDonald Feldspar Mine pegmatite was formed.

Formation of the Pegmatite

The formation of the visible minerals at the McDonald Feldspar mine is due to a combination of different conditions related the the crystallization.

Water Content

For the crystal sizes that areseen at the MacDonald Feldspar mine there is a need for a flux to be present. In this case it was most likely water. The effects of a flux are:

1. Lowers the viscosity
2. Lowers the temperature of liquidus to solidus
3. Suppresses the nucleation of minerals

Lowering the viscosity of the magma increases the rate of crystallization. The rate of crystallization is increased because elements in the melt are diffused and have a higher mobility. As such they will be able to be plucked out of the melt and bonded to the crystal much easier, increasing the rate of crystal growth. The flux lowers the solidus temperature, allowing the pegmatite to cool slower and push its way farther into the crust . The presence of flux will suppress the nucleation of the minerals. With a lack of nucleation, there will be fewer crystals competing and fewer, larger crystals will create a pegmatic texture. These giant crystals are feldspars because feldspars have to ability to grow in both an elongate fashion, but also laterally as well. Due to this property, these giant feldspar crystals grow at the expense of other minerals like quartz .

Zonation

A pegmatite can be broken into three main zones. The first to crystallize is the border zone where the pegmatite is in contact with the local rock. In this area the composition is more complex with finer grained crystals due to the faster rate of cooling. Since the temperature will drop the quickest in this zone, it will have most of the minerals in the continuous solution series of Bowens Reaction Series. This area will have plagioclase feldspar, along with mica, quartz and garnets.

The next region in the pegmatite is the intermediate zone which is indicated by its high composition of feldspars. The subzones within the intermediate zone can be derived by the main mineral varying between plagioclase and potassium feldspar. The percent composition of quartz stays relatively steady throughout.

The last zone is the core, which is identified by its high compositions of quartz, albite and potassium feldspar. As well there is a lot of waste material left in the core. This is caused by the flux materials creating a barrier in front of the crystals which only allows the mineral building components through and forces the components ahead until there is only the core left. In the core there can be found most of the rare ores and impurities.

Subsolvus Crystallization

Subsolvus Crystallization occurs when the flux lowers the melting temperature to a point where both Potassium Rich and Sodium rich feldspars form seperate crystals. Under unfluxed conditions the two feldspars would crystallize together, and the difference between them would be only distinguishable through a microscope.

External links

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