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
Hematitic jasperoids
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).
Conditions of metasomatism
The formation of hematitic jasperoids is considered to be the product of highly oxidised
Transitional forms and poorly developed analogs are present in some gold camps within the
Geographical distribution
True jasperoid is found in
Hematitic jasperoid is only found within the Glengarry basin, Western Australia, although it probably represents an end-member state of a variety of alteration and metasomatism styles found elsewhere.