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Zebra rock (zebra stone) deposits, Lake Argyle area, Wyndham-East Kimberley Shire, Western Australia, Australia

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Location is approximate, estimate based on other nearby localities.
 
Latitude & Longitude (WGS84): 16° South , 128° East (est.)
Margin of Error:~3km
Locality type:Group of Deposits
Köppen climate type:BSh : Hot semi-arid (steppe) climate


Zebra rock is a siltstone, rather than a mineral specimen, although the difference may be lost on many collectors. It is arguably the most commonly collected rock related material from Western Australia, and sold widely internationally. There are several retail outlets for the material also in Kununurra. This entry, and specific localities under it, is to try and address requests for information on Mindat in the recent past about zebra rock.

Zebra rock is a marketing name for a decorative stone showing brown banding, and similar patterns against a white background, found at several sites south of Kununurra, near Lake Argyle. Ribbon stone, okapi stone,
primordial stone, rainbow stone, and astronomite, are also marketing terms for siltstones found in the area, showing different patterns to zebra rock.
These will be described in 'other siltstone deposits' sub-locality.

The fine grained, clay rich siltstone is soft enough to allow precise cutting and carving with hand tools, finished with using ultra fine wet and dry emery paper, to produce a stone with a silky smooth and semi gloss finish. The stone is then sealed using talcum powder or warm vaseline, followed by several coats of polyurethane solvent to enhance the colour and gloss.

These siltstones are found in the Ranford Formation located to the south and south-east of Kununurra, composed of fine grained, thin bedded, micaceous, iron rich siltstone.

It was first discovered in 1924 by T. Blatchford, near the old Argyle Downs homestead. The damming of the Ord River, formed a very large water body called Lake Argyle, which drowned this deposit, and a number of others. There appears to be enough left above the water line to meet world demand for some time.

Zebra rock is found in the upper parts of the Ranford Formation from Snappy Gum Ridge in the south, extending in a north north-easterly direction for over 50 kilometres to the Northern Territory border. As of 2013, there are three openpit zebra rock mining operations in the area, mined during the dry season, and when demand dictates.

Zebra rock shows regularly spaced, ferruginous brown bands on a white to pale brown clay rich matrix. The width of the bands vary depending on the thickness of the beds they come from. Patterns also include rods and irregular blebs, extending through the rock in parallel rows, similar to the near vertical alignment of bands.

Zebra rock is composed largely of fine grained quartz and sericite, with minor alunite, kaolinite clay and its polymorph dickite. The colour banding
is thought to have resulted from the rhythmic precipitation of well defined, hematite rich liesegang bands in selected parts of the siltstone, caused by the percolation of warm hydrothermal fluids or other chemical activity during a period of alteration of the rock. (Loughman F.and Roberts F.,1990, Composition and origin of the zebra rock from the East Kimberley region of Western Australia, Australia Journal of Earth Sciences, v37, p201-205). One of the miners state the patterns are caused by cyanobacterial mats infilling ripple marks in low energy shallow marine environments, based on his own observations.


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References

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Fetherston, J.M., Stocklmayer, S., Stocklmayer, V.C.(2013): Gemstones of Western Australia- mineral resources bulletin 25, Geological Survey of Western Australia (2013): 285, 288
Mindat messageboard August 2012: http:www.mindat.org/forum.php?read,5,267822,page2

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