Jasperoid

Jasperoid is a rare, peculiar type of metasomatic alteration and occurs in two main forms; sulfidic jasperoids and hematitic jasperoids. True jasperoids are different from jaspillite, which is a form of metamorphosed chemical sedimentary rock, and from jasper which is a chemical sediment.

Sulfidic jasperoids are typical examples of silica-sulfide metasomatism of dolostones, and are found in Nevada and Iran. They are hard, dense purple-black rocks with considerable content of pyrite. The bodies in Nevada are quite thin (seldom greater than 8 m) and stratabound.

Hematitic jasperoids are examples of advanced silica-hematite alteration, and are known only from the Proterozoic rocks of the Glengarry Basin in Australia. These jasperoids are hard, purple to dark purple rocks composed primarily of amethyst quartz and fine disseminated hematite and some magnetite.

While contentious, these jasperoids are thought to form by extreme alteration of wall rocks within a shear zone, and may occur in sediments, andesites, trachytes and basalts. These bodies are often discordant to stratigraphy and are quite podiform in nature. The bodies in the Glengarry Basin are up to 120 m thick and over 3 km in length. These jasperoids are an important source of gold ore within the region.

Some hematitic jasperoids may be sourced from metamorphosed and altered jaspillite, and are located above areas identified as submarine basalt vents. These, therefore, may represent a type of exhalite chert or spilite. These are subordinate in volume to the shear-hosted forms and are usually quite thin (less than 3 m).

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