BASE METALS SPECIAL — International Curator aims to revive historic mining camp in Baja California

Vancouver-based International Curator Resources (IC-T) is resurrecting this historic mining town, on the eastern coast of Baja California, Mexico.

Mining in the area dates back to 1886, when a French company with ties to the Rothschild family operated an underground mining and surface smelting complex.

International Curator is taking a different approach, working toward the development of an open-pit, vat leaching, electrowinning operation capable of producing copper and cobalt metal on site.

Except for the old smelter buildings and smoke stack, which still stand, the historic phase of mining is not immediately evident.

Underground workings are extensive, however, spanning some 10 sq. km along the coast of the Gulf of California and extending several kilometres inland to surround Santa Rosalia.

The French processed 13.6 million tonnes of ore grading 4.8% copper from 1886 to 1947, smelting the material on site.

Copper mineralization is hosted in relatively flat-lying sedimentary beds, and the historic mining only took the higher-grade portion of the mineralized beds, leaving anything under the 3% cutoff grade.

After acquiring an option on the property in late 1992, Curator concentrated on developing additional underground minable reserves within, and peripheral to, the old mining areas.

Since previous operations did not test for or recover cobalt, Curator was also keen to determine cobalt grades on the property.

Curator soon shifted its focus to the open-pit potential at Boleo after encountering mineralized beds much closer to surface than in most of the historic workings.

By the end of 1995, Curator had completed 466 diamond drill holes, testing an underlying area of roughly 40 sq. km to outline a geological resource of 374 million tonnes grading 0.96% copper and 0.071% cobalt (based on an arbitrary 1% copper-equivalent cutoff).

Michael McInnis, president, estimates that close to 200 million tonnes of the resource is minable by open-pit methods, at a similar grade.

Current work is focusing on the Saturno area, where an estimated open-pit resource of 40 million tonnes grading 0.72% copper and 0.095% cobalt has been outlined, with a stripping ratio in the order of 4-to-1.

McInnis envisions a simple, open-cast mining operation.

The sedimentary beds at Boleo are relatively unconsolidated, and the company believes future mining will require little, if any, blasting.

“Previous underground operation did not use a stick of powder in the ore beds,” says McInnis.

Some blasting was done in the underlying conglomerate, which is almost always present at the footwall of the ore beds.

Studies project mining costs in waste at US35 cents per tonne, while mining costs in ore (where more selective mining methods are required) are estimated at US90 cents per tonne (due to more selective mining requirements).

“We think we’ve got a project that’s going to attract a lot of interest,” says McInnis in reference to the company’s plan to seek a buyer later this year.

“There are already a number of groups watching,” McInnis adds.

The metallurgy of the Boleo ore is of principal importance to the project’s future viability, and Curator is confident it has a workable process.

A majority of the copper and cobalt minerals at Boleo are soluble in sulphuric acid.

Curator’s proposed process would involve agitated acid leaching in seawater, first under oxidizing conditions and then under reducing conditions to bring both the copper and cobalt into solution (cobalt reacts better to leaching under reducing conditions).

Leaching would be followed by counter-current decantation to separate out the barren solids. The solids would then be neutralized with lime and limestone before being discharged into a tailings impoundment.

Pregnant leach solutions would then be selectively stripped of their respective copper and cobalt using solvent extraction and then electrowon into cathode copper and cobalt.

Leach solution would also contain a small amount of zinc which, if left in solution, would interfere with the solvent extraction of the cobalt. As a result, the zinc will be selectively removed using solvent extraction and then precipitated into a zinc carbonate before the cobalt is extracted.

Curator expects the proceeds from the sale of the zinc precipitate to cover the cost of zinc recovery.

Preliminary leach tests have returned copper and cobalt recoveries of 93% and 91%, respectively, and continuous pilot-scale tests are under way in Colorado.

Processing alternatives

Curator is also investigating other processing alternatives, including an LPF (leach-precipitate-float) process.

The copper and cobalt would be leached into solution using sulphuric acid as in the current process but would then be precipitated using a sulphate and floated to a concentrate.

The use of an LPF plant would lower capital costs by roughly US$100 million, down from the US$350 million estimated for the solvent-extraction electrowinning operation.

McInnis points out that operating costs would likely drop by US$5 per tonne, to $20 per tonne using the LPF plant, principally due to its lower power costs.

To lower the capital payback period, Curator is studying the viability of mining higher-grade material in the first eight years of operations.

The 40-million-tonne open-pit resource in the Saturno area includes both the No. 2 bed and the No. 3 bed, though the majority of the resource comes from the No. 3 bed, which averages 8 metres in thickness.

Grades in the No. 3 bed are typically higher (being closer to the contact with the footwall conglomerate), and the grade in the No. 2 bed is lower than that of the No. 3 bed.

As a result, Curator is looking at the possibility of stockpiling the No. 2 bed and the upper portion of the No. 3 bed, and processing only the higher-grade portion of the No. 3 bed in the initial years of operation, at a rate of roughly 5 million tonnes per year.

“The nice thing about this deposit is that you can increase the grade by mining the lower portion of the bed,” explains Richard Bailes, vice-president.

Based on an average process grade of 1.5% copper and 0.1% cobalt, the recoverable value of the ore is more than US$57 per tonne (assuming recoveries of 85% for copper and 75% for cobalt, and prices of US$1 and US$17.50 per lb., respectively).

That gives the project a good margin over projected operating costs of US$20-25 per tonne, Bailes says.

Infill drilling

Curator is now finishing infill drilling, the results of which will be incorporated into a mining plan and a revised reserve estimate.

A feasibility study will begin in September.

Curator has 26.3 million shares outstanding, or 32.6 million shares on a fully diluted basis.

The company holds a 20% direct interest in the Boleo project and can buy the remaining 80% interest by paying US$12 million by August 1996, or US$15.7 million by August 1997. The underlying vendors of the property retain a net revenue interest of 2.1% in years one to three, increasing to 6.3% in years four through six, and to 12.6% thereafter.