On the Viability of Granitic and Doleritic Rocks for Dimension Stone in the west Kimberley area, north Western Australia

A number of efforts over a few decades have been made to extract several stone varieties from the Kimberley district of W.A. Some granitic and metadoleritic rocks have been targeted because of their unusual and appealing character. For various reasons the activities were eventually abandoned. The granitic rocks occur as large whalebacks and a number of faces and quarries have been worked. Feedback from all the stone processors indicated that the stone was damaged by blasting or by stress, be it natural or man-made. More limited damage was also done to the granite immediately adjacent to the thermal channeling although there is a possibility that fracture propagation may have extended beyond - along the foliation plane and also because of the presence of considerable inherent stress in this granitic rock variety. Stressed rock will relieve its stress in a number of ways and is a serious consideration in terms of stone integrity and quality.

The metadoleritic rocks were also examined to gain some indications of extractability and therefore viability as a high-class product. These metadolerites occur as dykes that are intrusive into metasedimenats and gneissic granitic rocks. There have been about 30 locations is this region where some form of quarrying activity has taken place. Most operations have been unsuccessful commercially for a variety of reasons, the most common being stone quality and consistency.

An examination of the dykes focused on joint frequency, joint spacing, healed joints, veining, colour, and general textural homogeneity. A serious consideration to the commercial viability of the dolerite dykes is the joint spacing. Clearly, prolonged exposure to the elements has resulted in the formation of numerous vertical and horizontal joints. Their immediate effect is to broaden the dyke with displaced material and give a false impression of the true width.

It has been suggested numerous times (mainly by geologists) that the dykes should broaden with depth just below the point of exposure. When drilling and excavating has been done this premise has been shown to be totally erroneous. Any suggestion of broadening is nonsensical and extremely irresponsible when the overall environment of their formation is considered. These dykes have intruded through possibly 8km or more of granite and other country rocks and they are now exposed only because the landforms have reached a certain stage of denudation. The attitude of the dykes may well (and should) vary in their shape and form over a kilometer scale but not on the scale of say 50 to a maximum of 100 meters involved here. 

It must be emphasized that there are many more dolerite dykes that can be evaluated. Perhaps with more specialized expertise some dolerite can be found that are more amenable to quarrying and have fewer mineralogical problems. Similarly, the region has a wide spectrum of intrusive and metamorphic rocks that could be of special interest to the world dimension stone industry.