© Copyright, 1993 by R.A.Kanen, All Rights Reserved
The search for diamonds in the Kimberley reached boom proportions in 1978 when a pegging frenzy involving over 4000 claims began.
The attention of diamond explorers was drawn to the Kimberley district by a suite of minor igneous intrusions of lamproite. These were found at Ellendale on the Lennard shelf. They occur as pipe like bodies and have a genetic relationship with Kimberlite pipes in which diamonds are found. IN 1968 and 1969, indicator minerals of kimberlite pipes were found at Mount Abbot, a cryptovolcanic structure containing lamproites. In 1969, nine diamonds were recovered from alluvium in the Lennard River; however, follow up exploration failed to find any more.
In 1972, the Kalumburu Joint Venture was formed for diamond exploration in the Kimberley and in 1973 their first diamond was recovered. In 1976, the Ashton Joint Venture was formed, incorporating the Kalumburu Joint Venture. The following year five Kimberlite pipes were discovered and by 1979 diamondiferous pipes were discovered in the Northern, Eastern and Southern Provinces of the Kimberley district. This brought about a pegging frenzy by mining companies, joint ventures and entrepreneurs.
The Kimberley Basin forms the central tectonic feature of the area, forming a stable Proterozoic block flanked by the King Leopold and Halls Creek Mobile Zones. Although the mobile zones are believed to have formed at the same time as the Kimberley Basin, they have been subjected to several episodes of tectonic activity.
Faulting within the Kimberley Basin is widespread, forming linear features which can be traced for 50km. Faulting within the mobile zones is concentrated and extremely well developed.
Intrusions of leucite/lamproite enhance the region for kimberlite exploration. These are of Jurassic age and outcrop as a series of volcanic plugs, dykes and sills mainly in the Fitzroy Trough, but also on the Lennard Shelf near the Kimberley Basin margin. They are genetically related to kimberlite; wherein, lies their importance.
The cost of pegging claims in the Kimberley is very expensive due to its remote location. The average cost is about $4000-$6000 per day/pegging team. In addition, in an attempt to overcome access difficulties, helicopters hired at $500 per hour ferried claim pegging teams and exploration personnel. In order to spread the exploration costs, joint venture agreements were signed and South African diamond experts were called in to fill the gap in lack of local expertise in this field.
The most successful exploration group so far has been the Ashton Joint Venture which is managed by a subsidiary of Conzinc Riotinto of Australia Ltd. (C.R.A.). By far the most important discovery came in 1979 when the Smoke Creek kimberlite pipe was found a few kilometres south of Lake Argyle. This richly diamondiferous pipe and the associated diamond bearing gravels of Smoke Creek have been responsible for most of the production to date.
As an example, the cost of exploration during 1978 for C.R.A. was a total of 6.5 million dollars. This exploration was designed to identify the pipes and to determine which of them contain diamonds. About 50 million dollars would have been spent by C.R.A. proving reserves, setting up pilot plants and on further exploration. Much more was spent (approximately 30 million dollars) to set up full scale mining facilities. The annual value of gem quality stones would be in the order of $137 million dollars.
The exploration companies engaged in exploration for diamonds in the Kimberley are understandably reticent in explaining their exploration techniques. However, kimberlite offers the explorer with a limited choice of techniques, a combination of mineralogical sampling, satellite image and air photographic interpretation, geochemical sampling and airborne and ground geophysical techniques.
Exploration for diamonds invariably involves a search for indicator minerals (I.M.) within drainage channels or surface soil and sands. The mineral suite diagnostic of kimberlite includes pyrope garnet, which is often chromium bearing, magnesian ilmenite and chromium bearing diopside. Approximately three quarters of known kimberlite pipes have been traced by I.M. in Western Australia.
Colour air photographs and satellite images, where the aim is to identify small circular depressions displaying tonal and textural differences from the surrounding host rocks, are almost always used in the initial stage of exploration.
Geochemistry has been used to limited extent since kimberlite is rich in nickel and chromium.
Airborne and ground magnetic surveys have had success in the Kimberley; particularly airborne methods, which were responsible for the discovery of the Ellendale kimberlite pipe.
A large number of joint venture and mining companies have been involved in the exploration for diamonds in the Kimberley. The major advances in diamond exploration work have been made by the Ashton Joint Venture. Several kimberlite provinces have been identified in the Kimberley district by the Ashton Joint Venture. From north to south, these have been named the King George Province which incorporates the King George River drainage system; The Argyle Prospect with the richly diamondiferous Smoke Creek kimberlite pipe and nearby diamond bearing river gravels; the Eastern Province, including the country around Wilson River; and the Southern Province which includes the Ellendale Prospects.
Exploration results from the King George, Eastern and Southern Provinces have not been as spectacular as the Argyle deposits and so far the Southern Province has produced the most encouraging results. Ellendale pipes A and B of the Southern Province yield a high proportion of gem quality stones but testing suggests that the overall diamond grades are low by world standards (<10 carats per 100 tonnes). Several companies have expressed interest in alluvial diamonds. Numerous claims have been made on the deltaic accumulations of King Sound and in the lower reaches of the rivers which drain the Lennard River area but as far are known no diamonds have been reported from these areas as yet. The Ashton Joint Venture's Smoke Creek Kimberlite pipe and associated alluvial deposits are the most important diamond deposits in Australia. The Smoke Creek Pipe, or AK1 as it is known, has an area of 45 hectares and diamonds have been recovered to a depth of 200m. It is expected to produce 25 million carats of diamond, of which 10% are gem quality, 30% near gem quality and the remainder of industrial quality. The diamondiferous gravels, referred to as Upper and Lower Smoke Creek alluvial deposits, consisted of 500,000 cubic metres of gravel averaging 8 carats per cubic metre and 30 million cubic metres at about a carat per cubic metre respectively. At least 28 kimberlite pipes have been found by the Ashton Joint Venture. Ellendale pipes A and B are the other main economic deposits of diamond. These show a high proportion of gem quality stones but a low overall diamond grade. Recent discoveries of diamonds have been made in Central Western Australia and the Northern Territory but whether these are economic deposits remains to be seen.