Gold expert urges caution on Pilbara exploration rush

Nuggets discovered at the DOM's Hill prospect

Surbiton Associates director Sandra Close believes the excitement generated by the gold exploration rush gripping the Pilbara in Western Australia should be kept in perspective.

More than 25 publicly-listed companies are exploring for gold in the region, according to the Melbourne-based gold mining consultants.

These companies are looking for gold that occurs in conglomerates – sedimentary rocks composed of rounded pebbles in a finer grained matrix. Rocks of a similar type and age are also found in the Witwatersrand basin in South Africa.

Close said it was good to see such interest in gold exploration in the region but real caution was needed.

“It is very early days and despite the publicity, there is a great difference between an exploration play and a successful mining venture. Exploration is a risky and expensive business,” Close said.

She added the use of the Witwatersrand model, in the search for gold-bearing conglomerates in the Pilbara, was nothing new, although many seem to be unaware of this.

“Mark Creasy, one of Australia’s pre-eminent mineral explorers, spent the late 1970s and first half of the 1980s in the Pilbara specifically searching for Witwatersrand-style gold deposits,” Close said.

“Although he was technically successful at that time and found conglomerates which carried high gold values, the overall grades were too low to support economic development.”

In South Africa, the Witwatersrand basin is an ancient sedimentary basin about 400km by about 200km. Its gold deposits are not uniform, rather they occur where some of the ancient rivers and streams carried gold along with the sediments as they emptied into the basin around some three billion years ago.

Today in South Africa, the gold is found associated with uranium, pyrite (iron sulphide) and small particles of carbon and bitumen.

“Despite the parallels, the latest search in the Pilbara should be kept in perspective,” Close said.

“In addition to the usual uncertainties of exploration, these types of occurrences involve one of the classic challenges of sampling, the problem of the ‘nugget effect’.”

The nugget effect refers to the great influence that the presence a single nugget can have on an assay result if it happens to be contained in a sample. Conversely, if the sample fails to contain what is a representative gold content, the area could be wrongly dismissed as being of little value. Thus, careful and adequate sampling is essential in such circumstances.

“Dealing with the ‘nugget effect’ is an expensive and time consuming business,” Close said. “Such exploration involves collecting and processing a large number of large samples in order to obtain reliable results.”