Vista Gold has initial results of automated sorting tests and is updating Mt Todd prefeasibility study

Vista Gold has completed automated sorting tests on the coarse fractions of the four 5-t bulk samples from its Mt Todd gold project. The tests, completed at the facilities of Tomra Sorting Solutions near Hamburg, Germany, demonstrate that plus 16 mm product from the HPGR crusher can be efficiently sorted to separate the non-gold-bearing particles from the gold-bearing sulphides and quartz/calcite veining, at production rates.

The tests confirmed that, consistent with previous test work, 8-10% of the run-of-mine feed to the crushing circuit can be removed from the process stream prior to the grinding circuit. This is expected to reduce grinding circuit operating costs and provide a modest increase in grade to the grinding circuit.  These results, combined with the favourable previously announced results of the HPGR crushing tests, provide support for key assumptions underlying the decision to commence an update of the Mt Todd PFS.

Vista’s President & CEO, Frederick H. Earnest, commented, “This technology is being used effectively where ores have distinct visual and/or density variations. Being present for the automated sorting of the bulk samples at production rates allowed us to observe the efficiency of this technology and its applicability to the Mt Todd gold project. The material balance results are consistent with previous test results and we expect to have assay results for the sorted material in August.

“With this phase of our metallurgical testing program complete, we are prepared to commence the grinding and leach recovery optimization testing once the samples are returned from Germany. The final results of this metallurgical testing program are expected to be available the middle of September. We have commenced the work of updating our PFS for Mt Todd, inclusive of flow sheet changes to integrate this sorting technology. We believe these changes will deliver a better technical project with improved economics.”

The bulk sorting tests confirmed the ability to selectively remove particles known to have minerals associated with the presence of gold in the Batman deposit at production rates. The plus 16 mm material from the HPGR crushing/screening tests at Thyssen Krupp Industries was shipped to the facilities of Tomra Sorting Solutions.  Three members of Vista’s management/project development team were present to observe the tests, which were conducted in a standard production-size sorter with a 60 cm wide belt.  Material from each of the four composites was sorted first using X-ray transmission (XRT) to remove particles containing sulphide minerals and subsequently using lasers to remove particles containing quartz/calcite.  The XRT sorting tests were conducted at three different sensitivity levels.    The material balance results of the sorting tests are summarized in the following table.

Plus 5/8″ Fraction Composite 1 Composite 2 Composite 3 Composite 4
   XRT Sorted Material 33-57% 38-54% 27-46% 38-57%
   Laser Sorted Material 2.2-1.4% 7.6-6.7% 7.7-5.1% 1.9-1.3%
   Sorting Reject 40-62% 38-54% 46-64% 40-59%
Fraction of Total Composite
   Sorting Reject 7.0-10.9% 6.8-9.6% 7.7-10.7% 7.4-11.0%


Each composite represents a different part of the Batman deposit with varying amounts of sulphide and quartz/calcite mineralization which provided the opportunity to sort material that represented: high grade core, low grade peripheral, average to high grade early production and average grade main deposit. The variation in the amount of material sorted by the XRT and laser sorting steps is a reflection of the relative mineral content and is not an indication of sorting efficiency.  Upon receipt of assay results, the company will determine the optimal settings, taking into consideration gold losses, grade of sorted product and total product rejected.

Based on the results of this testing, the company plans to apply a different sorting recovery/waste reject factor to each of the different major mineralization styles in the deposit.

Mt Todd in Northern Territory is one of the largest undeveloped gold projects in Australia.