Baryte, Calcite and Marcasite Mineralization at Aguathuna, Newfoundland & Labrador

By David K. Joyce

Background

The limestone quarry near Aguathuna, located on the Port-au-Port Peninsula along the west coast of Newfoundland, has become well-known in recent decades for producing excellent crystal specimens of baryte, calcite, and marcasite. In addition to these striking mineral finds, the site also yields a variety of interesting fossils, reflecting its geologically rich history.

My first visit to this site to collect minerals and fossils was in the late 1970s. Since then, it has remained a memorable location for me. I’ve been fortunate enough to revisit several times over the years.

Location and History

The quarry was originally developed in 1911 by the Dominion Iron and Steel Company of Sydney, Nova Scotia. The cove at the site was initially named "Jack of Clubs Cove," but local residents opted for the name "Aguathuna," believed to be a Beothuk word meaning "white rock." This choice rejected the less distinctive proposed name, "Limeville," for the quarry and adjacent village. Limestone extracted from the quarry was transported by water to the Dominion steel mills in Sydney, where it was utilized in the steel-making process. The quarry operated until its closure in 1964 and has since served as a modest local source of rock and aggregate.

Geology

The limestone in the Aguathuna Quarry spans several geological periods, represented by three distinct units. The oldest is the Lower Ordovician Limestone, which formed approximately 470–485 million years ago on an ocean floor. This is overlain by Middle Ordovician Limestone, which, in turn, is topped by Mississippian Limestone, formed roughly 325–360 million years ago. Each of these units is separated by an unconformity, indicating periods of erosion or non-deposition between formations.

The mineral deposits at Aguathuna are seafloor carbonate mounds formed by the accumulation of calcium carbonate from chemosynthetic bacteria that thrived around warm water vents in the early Carboniferous period (Von Bitter, Scott, and Schenk, 1992). These bacteria derived energy from chemicals like sulphur and methane emitted from the vents, creating carbonate structures on the deeper (~300m), relatively quiet ocean floor. At least two prominent carbonate mounds are identifiable within the quarry complex.

The carbonate mounds at Aguathuna served as habitats for a variety of filter-feeding invertebrates, such as brachiopods, tube worms, and conularids, which likely fed on the chemosynthetic bacteria living around the mounds. Fossil evidence of these organisms is preserved within the mounds.

Warm water from beneath the Ordovician limestone transported elements like calcium, barium, iron, and sulphur upward through vents and feeder veins within the carbonate mounds. These elements precipitated in open spaces, veins, and fossil casts as barite, marcasite, and calcite.

Most of the limestone at the Aguathuna Quarry is barren and free of mineral impurities, boring to us collectors, but ideal for the quarry's industrial purpose of supplying limestone to steel mills. Quarry operators avoided mining the carbonate mounds, as their content of barium and sulphur would have contaminated the limestone. As a result, the mounds were left protruding from the quarry floor.

Mineralogy

Baryte

Baryte is arguably the most popular and sought-after mineral at the Aguathuna occurrence, where it forms well-developed crystals in open spaces. These crystals are typically tabular to prismatic in shape and can reach sizes of up to 30 mm along the edges. They are generally colourless to very pale blue, often featuring prominent thin white zones on their prism faces.

Interestingly, baryte crystals enclosed entirely within calcite lack the white zoning visible when the calcite is etched away, suggesting the zoning may be influenced by environmental exposure. While baryte often appears as individual crystals, it can also form semi-parallel or radiating aggregates. Occasionally, baryte crystals are found within hollow fossils.

Calcite

Calcite is found at the site as well-formed, twinned scalenohedral crystals within cavities. These crystals often grow large enough to bridge or partially fill the openings. Their colour varies from golden to colourless, depending on their specific formation conditions.

Calcite is also frequently observed within hollow fossil casts, such as brachiopods and fossilized tubes.

Marcasite

Marcasite, an iron sulfide mineral, occurs as crusts and cavity linings within the limestone at Aguathuna. The crystals are generally small and unremarkable in appearance. Over time, exposure to atmospheric oxygen and water often causes marcasite to oxidize to a brownish hue, reflecting its susceptibility to environmental conditions.

Fossils

The Aguathuna carbonate mounds preserve a fascinating array of fossilized marine life:

Fossil Tubes: These are believed to be remnants of serpulid or spirorbid worms. Tubes can be relatively long, though specimens exceeding 8 cm are uncommon. They may appear hollow, partially filled with calcite or baryte, or completely replaced by limestone.

Brachiopods: Abundant in the mounds, brachiopods occasionally form coquina, a rock composed primarily of their shells. The hollow interiors of these brachiopods are often lined with crystals of calcite, baryte, or marcasite, making them particularly intriguing for collectors.

Conularids: These peculiar fossils have a rounded yet square cross-section and are relatively rare at Aguathuna. Most are found as short segments or incomplete specimens, with well-formed examples being uncommon but highly prized.

Mineral Specimen Availability

Use the following link to view available Aguathuna mineral specimens: https://djoyceminerals.com/?s=aguathuna&post_type=product

For collectors visiting the Port au Port Peninsula, specimens may also be available from Cody and Jay Renouf (codyrenouf @ outlook.com), local experts who reside in the area. In recent years, they have become the primary collectors at Aguathuna and are known for their excellent specimens. They are also quick to offer valuable collecting advice to those exploring the region.

Summary
The Aguathuna Quarry, located on the Port au Port Peninsula in Newfoundland and Labrador, is renowned for its excellent crystallized barite and calcite, as well as intriguing fossils found within the carbonate mounds. These features provide valuable insights into the region's unique geological and paleontological history.

References
Von Bitter, P.H., Scott, S.D., & Schenk, P.E. (1992). Chemosynthesis: An Alternate Hypothesis for Carboniferous Biotas in Bryozoan/Microbial Mounds, Newfoundland, Canada. Society for Sedimentary Geology.