Metallurgy

Atacama Pacific has undertaken extensive metallurgical testing on the oxide-associated gold mineralization from the Cerro Maricunga deposit with over 200 bottle roll and 30 column leach tests confirming that the mineralization is amenable to heap leach processing. Gold recoveries have averaged approximately 80% for mineralization crushed to 19 mm (3/4 inch) with the majority of the gold being recovered within the first 2 weeks. Column leach test on mineralization crushed to 100 mm (4 inches) achieved gold recoveries of 77%.

It is significant to note that gold recoveries and reagent consumables are not dependent on the depth of the mineralization due to the very low levels of sulphide mineralization throughout the deposit. Composite samples composed of gold mineralization from the bottom on the proposed open pit, up to 400 metres below surface, have shown no significant variation in gold recoveries.

Test work has been carried out by Kappes, Cassiday and Associates (Reno, Nevada), Laboratorio Plenge (Lima, Peru) and AMTEL (London, Ontario).

Atacama has completed numerous metallurgical tests on gold mineralization from the oxide-associated Cerro Maricunga gold deposit and continues to conduct test work in order to optimize gold recoveries. Tests have been completed on composite samples of low to high grade gold mineralization, mineralization from various deposit depths and at various crush sizes. The results confirm the Cerro Maricunga gold mineralization is amenable to heap leach processing. The tables below summarize the key metallurgical results to date from bottle rolls and columns leach tests.

Column leach tests are used to project gold recoveries that may be achieved in a mining operation. Crushed mineralization and lime is place within a narrow vertical column and a NaCN solution is dripped through the column in a manner to replicate the conditions on a leach pad. In the case of Cerro Maricunga, the composite samples used in the tests vary in weight from 30 to 580 kg and the test period lasts 80 to 120 days.

Bottle / drum roll tests are used to determine the relative leachability of the mineralization rather than to predict actual recoveries. From 0.5 to 5 kg of crushed mineralization along with NaCN solution and lime is placed in “bottles” or “drums” and spun on rollers to expose the mineralization to the solutions and speed gold recoveries. The tests, which run typically run 4 to 6 day but in the case of coarse crushed mineralization were run up to 10 day, can quickly establish if there are any significant issues that could impact leaching such as excess consumption of NaCN or lime. Further, the testing can predict the impact of gold grade or crush size on recoveries.

Check the News Releases section of the web site for results from further metallurgical testing as well as details on the results summarized below.

Summary of Metallurgical Test Results

Crushing and grinding of ore is a significant capital and operational cost in mineral processing plants. As such it is important to fully determine the comminution parameters relevant to the crushing of the Cerro Maricunga gold mineralization in order to determine potential cut off grades and to assist in plant design.

Five samples of gold mineralized material have been tested to determine: 1) the work index, the amount of energy, measured in kilowatt hours per tonne (“kWhr/t”), required to grind the host rock; and, 2) the bond abrasion index, to determine abrasiveness of the mineralized material.

The Work Indices were low varying from 9.77 to 11.26 kWhr/t suggesting relatively low power requirements for crushing. The work index was determined from ball mill grindability tests conducted at 100 mesh (150 µm). Abrasion tests returned low (0.067) to moderate (0.124) Bond Abrasion indices, averaging 0.095. Material with a Bond Abrasion Index of less than 0.1 has relatively low abrasiveness to processing equipment. Abrasion indices were conducted on a -3/4” + 1/2” fraction of crushed samples