Extracting gold without the pain

Mining without mining sounds like an abstract concept but it is actually very close to becoming a reality.

Last year CSIRO Minerals Down Under Flagship scientists released research they had carried out on the Towards the Invisible Mine project.

The scientists identified a major drawback to using the traditional in-situ leaching process of injecting leaching solution into ore deposits and then extracting the resulting ore-rich liquid.

The drawback they realised was that the solution does not always penetrate fully into the ore deposit.

“This can be due to ‘short-circuiting’ where the solution takes a path of least resistance,” said CSIRO scientist Dr Guy Metcalfe upon the release of the Flagship’s research paper.

“In addition, the mineral can dissolve into the leaching solution too slowly.

“What is needed is a way to ‘stir’ the solution while it is underground and that’s where chaos theory comes into the equation.”

The method suggested by the research involves a carefully orchestrated switching on and off of the leaching solution flow between various inlet and outlet wells.

“While the method is simple, the best solution is different for every orebody so CSIRO has developed computational methods to optimise the solution for different orebody configurations,” Metcalfe said.

“These solutions have been demonstrated to work in laboratory settings.”

The new, environmentally friendly process developed by the CSIRO scientists, to extract gold without mining, is to be the subject of first-time field testing on a South Australian gold deposit.

The breakthrough in-situ process, which has the potential to reduce the need for conventional, expensive open-cut mining techniques, is being trialled through a collaboration agreement between the CSIRO and ASX-listed minerals explorer, Minotaur Exploration.

Minotaur will run a 15-month program of field trials to confirm the process concept at its Tunkillia gold deposit in the Gawler Craton, south of Tarcoola.


Gold projects in South Australia. Source: Minotaur Exploration.

“The process has the potential to revolutionise the way we normally mine near surface weathered (or ‘oxide’) gold deposits, while at the same time delivering potential massive cost savings for the owners of oxide gold resources,” Minotaur Exploration managing director Andrew Woskett, said in an announcement to the Australian Securities Exchange.

The full trial program at Tunkillia and associated exploration is expected to cost around $3 million, and is intended to lead to Proof-of-Concept for the in-situ gold extraction process.

Successful demonstration of the process at the field scale could open up the potential for any similar oxide gold deposits to be brought into production at low capital and operating costs, compared to traditional mining and processing methods.

The in-situ gold extraction process has been developed through intensive laboratory work by the CSIRO, with support from Minotaur and several other research project participants.

Minotaur said the Tunkillia oxide samples had performed well under CSIRO laboratory conditions testing the in-situ extraction process.

The company has elected to solely drive and fund the next development step for the process in the form of the field trials.

If these prove to be successful the company said it will be well positioned to take advantage of the technology’s inaugural commercial application, by implementing its use at the Tunkillia gold project.

The process involves installation of a conventional groundwater recovery bore field and injection of an appropriate solution (“lixiviant”) into the host rock.

The lixiviant dissolves the gold in the rock and the gold in solution travels through the rock mass to be recovered, to surface, via nearby extraction wells, as ‘pregnant’ solution.

This is a major departure from conventional heap leach gold operations which require the ore to be mined, crushed and stockpiled on surface, as ‘heaps’, prior to irrigation with a cyanide solution.

The CSIRO has developed a non-cyanide solution that, when injected into the host rock, is expected to cause minimal alteration to groundwater chemistry.

The field trials have been designed to focus on determining key objectives:

–    The transmission characteristics of the lixiviant through the host orebody;

–    Groundwater modification;

–    Gold recovery rates; and

–    Economic feasibility of the in-situ gold recovery process.

A range of environmentally acceptable lixiviants has been trialled in the laboratory by CSIRO, with promising results.

The most notable feature of the new process is that it negates the traditional reliance on cyanide in solution to liberate gold from ore, thereby minimising hazardous material consumption and potential site contamination.

It also circumvents the need for pre-stripping, drill and blast, open-cut mining, crushing and grinding circuits, cyanide leach systems, waste dumps and tailings dams.

The process is not dissimilar to the in-situ extraction methods successfully employed for over a decade by the uranium sector.

Until now that process has not been to be applied to gold as a non-cyanide based solvent had not been previously identified.

“Successful results from the lixiviant leach trials are expected to show that operating and capital investment costs are substantially lower, and implementation timelines much shorter, than for conventional gold mining and processing installations,” Woskett said.

“The in-situ recovery process could potentially make many low grade oxide gold deposits economic to extract.”

The day following Minotaur’s announcement its Joint Venture partner Helix resources released an announcement claiming it had not received a copy of the CSIRO report on the laboratory scale investigations.

“Helix has previously raised technical and commercial concerns regarding the potential use of in-situ leaching on the existing Tunkillia Resource with the JV Manager, Minotaur Ventures Pty Ltd,” Helix said.

“Helix has previously notified the JV Manager that it will not allow any in-situ studies to be conducted on the Tenements which have the potential to give rise to environmental liabilities and/or potential damage to the existing gold resource as a result of such studies.”

Helix also said it was not aware of Tunkillia JV approval regarding the proposed Collaborative Research Agreement with CSIRO to recover gold from the oxide zone of the Tunkillia gold deposit.

Nor was it aware of any agreement between Tunkillia JV and Minotaur Gold Solutions to complete activities as reported in the Minotaur release.