Arsenopyrite, Dolomite, Gold & Quartz

Arsenopyrite is a sulphide similar to pyrite but contains arsenic as well as iron. It has a more silvery colour than pyrite and forms blocky or tabular crystals rather than cubes. Its surface often has striations, or stripes. Arsenopyrite is one of the main ore minerals for arsenic, which is used in wood preservatives and insecticides. In its oxide forms, arsenic is toxic so is not widely used.

Gold is a valuable, highly prized mineral used in everything from jewellery to electronics and dentistry. Gold is desirable due to its special properties, such as malleability and resistance to tarnishing. Gold is commonly microscopic or embedded within or around sulphide grains. Free visible gold occurs as disseminated grains, or rarely as crystals. Crystals of gold commonly form within or around quartz. In its natural mineral form, gold is commonly alloyed with silver. Gold is distinguishable by its characteristic golden yellow colour and extreme heaviness.

Quartz is the second most abundant mineral on Earth, occurring in many different types of rocks. Although usually clear or milky white in colour, quartz can be found in a variety of colours because of impurities in the crystal structure. Pure quartz is made up of silicon and oxygen only, but atoms of other elements, like iron or titanium, often make their way into the quartz crystal structure. Some varieties of quartz, like purple amethyst, are considered to be semi-precious gemstones and have been used since ancient times to make jewellery and decorative objects.

Dolomite is an anhydrous carbonate mineral composed of calcium magnesium carbonate. The term is also used for a sedimentary carbonate rock composed mostly of the mineral dolomite. An alternative name sometimes used for the dolomitic rock type is dolostone.

This sample comes from the Whitewater Mountains near the post-producing Tulsequah Chief mine in northwestern BC, about 100 km southwest of Atlin, BC and 65 km northeast of Juneau, AK. It is made up mainly of dolomite and quartz with very fine arsenopyrite and gold.

The area is known to host several volcanogenic massive sulphide (VMS) type deposits. These are found in ancient volcanic arcs and usually form as pods or lenses of sulphide minerals like pyrite, chalcopyrite, sphalerite and galena. They can also contain gold and silver.

The nearby Tulsequah Chief mine was active from 1950 to 1957, and produced gold, silver, lead, zinc and copper from several sulphide lenses. The deposit contains large amounts of pyrite and lesser amounts of arsenopyrite, both of which are known for their role in generating acid rock drainage (ARD). As these minerals weather, their sulphide component is released and reacts with water to produce sulphuric acid, which is potentially harmful to the environment. The Tulsequah Chief mine ARD is currently being studied to assess its impact on salmon and other aquatic life in the nearby Tulsequah and Taku rivers.

Between 1950 and 1957, the mine produced over 580,000 tonnes of ore. This ore was processed with ore from the nearby Big Bull mine at the Polaris Taku mine facility. Average ore grades for the mines combined were 3.77 grams per tonne gold, 126.5 grams per tonne silver, 1.59 % copper, 1.54% lead and 7.0% zinc. The mine and surrounding area has been the focus of ongoing exploration work as recently as 2013.

Specimen Information

Store:  Association for Mineral Exploration (AME)

Collection: –

Accession #: AME 502

Primary Mineral: Arsenopyrite, Dolomite & Gold

Secondary Mineral: Quartz

Site Locality: Whitewater Group

Location: Tulseqauh, British Columbia

Special Features: n/a

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