Vancouver — Minefinders (MFL-T) reports that ongoing metallurgical and geochemical tests indicate that silver grades are significantly higher than previously estimated at the Dolores deposit in Chihuahua, Mexico.
Column-leach tests conducted by McClelland Labs indicate that ores from Dolores contain an average of 30% more silver than estimated by the original geochemical analysis of drill material.
A check analysis by ALS-Chemex Labs in Vancouver tested 307 five-foot samples from 12 holes representing different portions of the deposit. When total digestion techniques were used, the results showed significant increases in silver grades. Of the 307 samples, 52 were re-assayed as check samples. Results for 21 high-grade samples compared favourably with previous fire assays.
Using aqua-regia digestion techniques on 119 samples of oxide material, the average grade was 26 grams silver per tonne. When multi-acid digestion techniques were used, the average grade for the same samples was 38.8 grams silver per tonne. This was a 49% increase.
Similarly, 105 samples of mixed oxide and sulphide material were subjected to aqua-regia and returned an average grade of 24.5 grams silver per tonne. These same samples averaged 28.7 grams per tonne when subjected to multi-acid digestion. This was a 16.8% increase.
Sulphide material from 62 samples was subjected to the same tests. Aqua-regia returned an average grade of 34.2 grams silver and multi-acid returned a grade of 33.6 grams silver. This was a 1.7% increase. A total of 21 high-grade samples was subjected to multi-acid digestion and compared to fire assay results. The multi-acid tests averaged 188.75 grams silver and the fire assay tests returned 186.29 grams silver. This was a 1.3% increase.
The economic impact of the increased silver content and grade is expected to be substantial. In-pit resources are effectively moved from waste to ore categories, resulting in reduced strip ratios, additional silver production and accelerated cash flow. The recovery rates and percentage recoveries predicted by column leach tests were based upon total digestion analysis procedures and would not reflect the increased silver grades and content of the deposit.
Dolores hosts a measured, indicated and inferred resource of 100.1 million tons grading 0.76 gram gold and 40.3 grams silver per tonne. This is equivalent to 2.4 million contained ounces of gold and 129.7 million contained ounces of silver, using a cutoff grade of 0.3 gram gold-equivalent per tonne. All gold assays from Dolores used fire assay techniques and therefore are not impacted by the aqua-regia digestion.
Aqua-regia digestion (with atomic absorption analysis) was used for all previous Dolores samples. Steve Armstong, manager of Bondar Clegg Laboratories in Mexico, says: “There are some areas, especially in Mexico, where the silver content is significantly higher in total digestion assays because partial digestion by aqua-regia results in reporting lower grades of silver mineralization.”
At Dolores, incomplete silver dissolution in aqua-regia is believed to be caused by the presence of silver halide minerals which occur in substantial quantity within the oxidized and mixed oxide-sulphide material.
Minefinders says an extensive check assay program was performed by ALS Chemex to confirm the new assay numbers. Additional total digestion assays will be completed on mineralized drill intercepts from Dolores. The results will be incorporated into the block model.
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