- 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 fromjasper which is a chemical sediment.Sulfidic jasperoids are typical examples of
silica -sulfide metasomatism ofdolostone s, and are found inNevada andIran . They are hard, dense purple-black rocks with considerable content ofpyrite . The bodies in Nevada are quite thin (seldom greater than 8 m) and stratabound. ( [http://pangea.stanford.edu/research/ODEX/willi-field-photos.htm Nevada Jasperoid photos; Stanford] )Hematitic jasperoids
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While contentious, these jasperoids are thought to form by extreme alteration of wall rocks within a
shear zone , and may occur in sediments,andesite s,trachyte s andbasalt s. 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 ofgold 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 orspilite . 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
metasomatism of the wall rocks to a shear zone. The presence of carbonate alteration, talc-carbonated high-magnesian basalts andultramafic rocks and the remnant mineralogy being restricted to hematite, silica, and sulfides indicates oxidising fluid chemistry. Relict volcaniclastic textures in some jasperoids indicate that aluminosilicates have been replaced pervasively by silica + hematite.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 andgersdorffite are encountered in the footwall of the Miitel nickel mine.Geographical distribution
True jasperoid is found in dolomitic formations in Nevada, USA, and within dolomitic limestone sequences in Iran, Iraq and Kurdistan.
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.
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