Picture this - element distribution and association models to improve exploration

The Broken Hill Domain is the site of the world's largest zinc, lead and silver deposit and has produced over 250 million tonnes of high-grade ore, worth an estimated A$144 billion in today's money.

There are many small Broken Hill-type deposits in the Broken Hill Domain and it is unlikely that such a large deposit is an isolated occurrence. Finding and exploring this type of mineralisation with current techniques is still a significant and expensive challenge, particularly for deep deposits.

Associate Professor Chaoshui Xu, Institute for Mineral and Energy Resources (IMER), and his colleagues are working to improve exploration efficiency and accuracy by using spatial modelling.
"Sampling is a very expensive exercise, and even if you have hundreds of thousands of samples they are still like a drop in the ocean when you compare them to the ore body itself. To use such a small amount of data to create a picture of the relationships within the whole ore body - that's really challenging," said A/Prof Xu.

"No two mines or ore deposits in the world are identical, but some of them are formed by the same geological processes - therefore their ore genesis might be similar. If their ore genesis is similar then it's likely that some of the relationships between elements will also be similar."

A/Prof Xu and his colleagues are using samples taken from the Western Mineralisation in the Broken Hill ore deposit to determine the spatial distribution of a range of elements within the ore body. 'Haloes' are created from the data showing the geochemical signatures of elements within the ore body and the relationships between different elements. This is the first time that spatial modelling has been used to identify the geochemical zonation haloes for any of the Broken Hill ore bodies.

"We try to find the spatial relationships between different elements based on existing data. If we can express these relationships mathematically, then we can use them to help explore similar types of deposits," A/Prof Xu said.

Indices such as the Zonality Coefficient (ZC), the product of the multi-element concentrations in the lower part of the ore body over those of the upper part of the ore body, have been developed as part of the research. ZCs can then be used to:

• estimate the degree of denudation level of an anomaly
• evaluate the exposure level of geochemical anomalies
• recognise the upper anomalous halo from the lower anomalous halo
• predict the presence of deeply buried blind or hidden mineralisations of lead or zinc.

The research on the Western Mineralisation has shown that cadmium, lead, sulphur and zinc tend to concentrate in the upper part of the mineralised zone, while silver, arsenic, copper and bismuth tend to concentrate in the lower part of the mineralisation.

Antimony and bismuth had the most pronounced geochemical haloes in all cross sections and their orientation and distribution conforms with those of lead and zinc haloes. These elements can be considered as possible pathfinder (indicator) elements for the exploration of lead and zinc in the Western Mineralisation and other similar types of deposits.

Geostatistics makes spatial modeling and resource estimation of an ore body more objective and less reliant on personal interpretation. The indices developed in this research, together with some special deep electrical geophysics (e.g. Zeus method), could potentially be a powerful combination for discovering ore bodies like the Broken Hill-type deposit that, due to their burial depth, can't be detected with current geochemical, geological and geophysical methods.

To find out more about this project, read A/Prof Xu's recent paper or contact him on:

Associate Professor Xu
Institute for Mineral and Energy Resources
School of Civil, Environmental and Mining Engineering
University of Adelaide, Adelaide SA 5005 Australia
Ph: +61 8 8303 5421
E-mail: chaoshui.xu@adelaide.edu.au 
Lotfolah Hamedani, M., Plimer, I.R., Xu, C. (2012) Orebody Modelling for Exploration: The Western Mineralisation, Broken Hill, NSW. Natural Resources Research 21 (3), pp. 325-345.

Broken Hill Western Mineralisation: The longitudinal sections of 10 geochemical haloes between local threshold and anomalous levels at E = 9357 m.