A metallurgical test work program following the maiden mineral resource estimate (MRE) was completed by SGS Laboratories. (ASX Announcement 25 February 2022) Building on Mithril’s excellent high-grade gold and silver maiden mineral resource estimate (ASX Announcement 17 November 2021), the high recoveries of gold and silver from the flotation and leaching test work reinforce attractive project metrics for the district.
The sample used for the test work is a composite sample from El Refugio crushed drill core. The average grade of the composite is similar to the average grade of the maiden resource estimate. The calculated composite grade from the flotation test work is 4.52 g/t gold and 124 g/t silver.
Flotation test work has been completed using the same reagent scheme determined for the Palmarejo deposit (located in the Sierra Madre Trend and north of the Copalquin District) test work, due to the similarities in geology and mineralogy between Palmarejo and El Refugio at Copalquin.
Intensive cyanide leaching of the flotation concentrate and low-level cyanide leaching of the flotation tailings allows high recovery of silver to be achieved. Cyanide leaching of silver is slower under the typical conditions used for gold only leaching. Recovering a high proportion of the silver into a concentrate (by flotation or gravity) allows the concentrate to leached under intensive conditions resulting in higher silver recovery. The overall recovery to solution is 96.5% for gold and 91% for silver. Gold and silver would be extracted from solution using the widely used Merrill-Crowe zinc precipitation method and then smelted to produce metal gold-silver ingots.
The process flowsheet for El Refugio-La Soledad resource material being tested is as shown below in Figure 2. This is a similar flowsheet used at similar deposits in the Sierra Madre Trend, Mexico including Coeur Mining’s Palmarejo and SilverCrest Metals’ Las Chispas (under construction).
| Description 48 hr Leaching Time | Test | Duplicate Test | |||
|---|---|---|---|---|---|
| Au % | Ag % | Au % | Ag % | ||
| Flotation Concentrate (% recovered) | A | 94.2 | 87.6 | 92.9 | 87.9 |
| Leaching of Flotation Concentrate | B | 97.3 | 93.4 | 97.4 | 92.9 |
| Flotation Tailings (% remaining) | C | 5.8 | 12.4 | 7.1 | 12.1 |
| Leaching of Flotation Tailing | D | 85.8 | 76.5 | 82.9 | 75.7 |
| Total Recovery to solution (B x A)+(D x C) | 96.6 | 91.3 | 96.4 | 90.8 | |
Further metallurgical test work will allow the crushing, grinding, thickening, flotation and filtration equipment sizes to be determined.