Vanouver — In 1995, after a lengthy search for an undervalued, world-class project,
The results of early work programs were encouraging, but by 1998, Campo Morado was frozen once again, partly because of low metal prices, but mostly because of complex mineralogy and low recoveries from conventional metallurgical test programs.
Farallon didn’t throw in the towel, given the grades and size of known deposits and the exploration potential of the surrounding land package. Instead, the company shifted its emphasis to cracking the metallurgy, and recent progress on that front appears to justify the approximate $35 million spent to bring the project to the prefeasibility stage.
The breakthrough came when a subsequent $2.1-million metallurgical program showed that average recoveries of 80-85% for copper and up to 97% for zinc can be achieved using a partial oxidation, solvent extraction-electrowinning process (SX-EW) for the selective removal of copper and zinc. Gold and silver are then extracted using standard cyanidation methods.
President Dick Whittington notes that the newly patented Partox process is not only technically viable, but well-suited to mineralization at Campo Morado. The results of the program will form the basis of the prefeasibility scheduled for completion early next year.
“The results vindicate the effort and initiative of those involved in the planning, design and operation of the program,” Whittington adds. “We are breaking new ground with this process [which also] removes a major element of risk from the overall project.”
The three-phase metallurgical program was the first major initiative carried out since 1998. It was followed by a drilling program aimed at enhancing the potential of the project, where four main deposits have been defined to date. The largest of these, El Largo, hosts an indicated resource of 4.13 million tonnes grading 1.12 grams gold and 128 grams silver per tonne, plus 0.4% copper, 5.76% zinc, and 1.33% lead. A further 1.26 million tonnes of 1.54 grams gold, 122 grams silver, 0.85% copper, 3.99% zinc, and 0.84% lead are classified as inferred, which (combined with other deposits) brings total inferred resources for Campo Morado to 8.98 million tonnes of 3.42 grams gold, 179 grams silver, 0.81% copper, 4.03% zinc, and 1.32% lead.
The deposits host finely disseminated mineralization that had previously thwarted the ability to produce marketable selective flotation concentrates using conventional processing. Farallon says its recent “Proof of Principle” pilot-plant program changed all that by confirming the technical viability of the new hydrometallurgical flow-sheet proposed for Campo Morado.
The company plans to market the Partox technology to companies with mineral projects suited to the treatment process.
The flowsheet calls for flotation bulk rougher concentrates to be subjected to conventional medium-temperature/pressure oxidation in an autoclave. This process is designed to oxidize about 20% of the sulphides, selectively extract zinc and copper into solution, and partially oxidize precious metal-bearing sulphides, leaving most of the barren pyrite in the concentrate, un-oxidized. Copper and zinc metal cathodes of commercial purity are then recovered from solution by normal SX-EW processes, while the washed-solids residue from pressure oxidation is subjected to cyanidation to recover gold and silver.
The metallurgical program began last year with a bulk-sample drilling program consisting of 14 holes totaling 4,200 metres. Ten holes were drilled at Reforma, three at Naranjo, and one at El Largo.
Flotation programs were then initiated to produce bulk rougher concentrates for bench-scale, scoping-level pressure oxidation tests at SGS Lakefield. The company says these tests “demonstrated the amenability of the mineralization to low temperature/pressure oxidation.”
In the third phase, a pilot-plant incorporating all elements of the flowsheet was designed and built to process Campo Morado mineralization at SGS. About three tonnes of bulk rougher concentrates were shipped to SGS for the phase-three pilot-plant, commissioned in May of this year.
The company says all unit processes in the hydrometallurgical circuit were successfully tested in continuous mode. In addition to the copper and zinc recoveries mentioned above, gold recoveries ranging from 57% to 66% were obtained from partially oxidized solids residues. Silver grades ranged from 76% to 88%, again depending on the source deposit. A program is under way to examine ways to enhance precious metals recoveries and optimize reagent consumption.
The program was supervised by two independent consultants, David Dreisinger (hydrometallurgy) and Peter Taggart (flotation test-work).
Buoyed by results from the hydrometallurgical program, Farallon plans to spend $10.4 million to complete the current drill program and the prefeasibility study being conducted by Bateman Engineering.
Results are in hand from 45 exploration holes, with more than 30 returning no significant results or massive sulphides. Several holes did return significant results, with highlights including: 23.5 metres grading 3.68 grams gold and 195 grams silver, 0.89% copper, 0.72% lead, and 12.16% zinc; 16.38 metres of 6.56 grams gold, 545 grams silver, 1.38% copper, 3.85% lead and 6.69% zinc; and 17.7 metres of 3.19 grams gold, 242 grams silver, 1.51% copper, 1.28% lead and 2.22% zinc.
The best results, including the high-grade intervals (all true widths) mentioned above, were from the newly discovered G-9 deposit. Assays from 12 more exploration holes drilled at G-9 are pending. Massive sulphides have been intersected in five of these holes, with the longest continuous intersection being 50 metres.
On the corporate front, Farallon has dropped plans to merge with
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