81 Mile Vent, Ellendale, Derby-West Kimberley Shire, Western Australia, Australiai

Regional Level Types
81 Mile Vent	Group of Sills
Ellendale	- not defined -
Derby-West Kimberley Shire	Shire
Western Australia	State
Australia	Country

A sill complex consisting of a roughly circular ridge 200 metres in diameter, 15 metres high, and open to the south-east. The central depression is occupied by a core of phlogopite-rich lamproite, with pyroclastic surrounding the central complex, with more than half concealed by sand up to 5 metres thick.

The initial explosive phase of the vent excavated a large crater, now filled with well bedded lapilli tuff and tuff breccia. At the northern side the pyroclastics are coarser, and contain sandstone blocks of the Grant Group, and rare silicified wood. The southern side the tuffs are fine grained, and range from laminated quartzite tuff to well-bedded sandy lapilli tuff, consisting of phlogopite-leucite lamproite, lamproite ash, and quartz grains.

There is much small scale folding, fracturing, thin compaction and slumping.

Magmatic sills, 3 to 10 metres thick, intrude the pyroclastics. Three sills are on the northern side, separated by a thin screen of lapilli tuff. The lower sills are fine grained phlogopite-leucite lamproite, with an upper columnar jointed sill groundmass of poikilitic potassic richterite.

The central area contains phlogopite, with minor diopside and potassic richterite. The core was probably a lava lake, within the open vent of the volcano.

The reference makes no mention of diamonds, or the location of the site.

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References