Jasperoid

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.

The formation of hematitic jasperoids is considered to be the product of highly oxidised metasomatism of the wall rocks to a shear zone.

The presence of carbonate alteration, talc-carbonated high-magnesian basalts and ultramafic rocks and the remnant mineralogy being restricted to hematite, silica, and sulfides indicates oxidising fluid chemistry.

Transitional forms and poorly developed analogs are present in some gold camps within the Yilgarn craton where deep, reduced methane and carbonate bearing alteration fluids mix with shallower oxidised fluids, resulting in purple-pinkish carbonate flooding alteration within basalts and ultramafic rocks.

One example of this is the Widgiemooltha Dome, where carbonated hematite-bearing siliceous basalts containing native gold, arsenopyrite, nickeline and gersdorffite are encountered in the footwall of the Miitel nickel mine.