What mineral deposits were mined on Redonda?

The island of Redonda, a dependency of Antigua and Barbuda, was mined for phosphate found within the bird guano that covered the volcanic rock.1

Mining operations began in the 1860s despite the island's difficult terrain and poor anchorage. Laborers from nearby Montserrat extracted the mineral, reaching an annual production of 3,000 to 4,000 tons during the 1890s. These operations eventually ceased following the outbreak of World War I.1