Vancouver company to mine Boleo copper — Historic Santa

To passing tourists, the tiny seaside town of Santa Rosalia is a place for a quick bite or a tank of gas as they make their way south, down the Baja California peninsula, to sport fishing and holiday grounds.

From the late 1800s to the mid-1900s, however, the town was the site of a bustling mining and smelting complex supported by the Boleo copper deposit, three kilometres inland. Underground production from 1886 to 1947 totaled 13.6 million tonnes of low-sulphide ore grading 4.81% copper, all of which was fed directly to a smelter at Santa Rosalia.

Vancouver-listed International Curator, which has an option to buy Boleo from the Mexican government, hopes to bring the property back to its former glory. The purchase agreement features three payment options. The least expensive of these includes buying a 10% interest by making staged payments totaling $1.2 million by Feb. 15, 1995, followed by a final payment of $20 million for the remaining 90%. The vendor retains a 3% net operating profit interest during the first three years of production, which increases to 9% in the fourth to sixth year and 18% thereafter.

Property logistics are excellent. They include a deep-water port and facilities for bulk-loading in Santa Rosalia, as well as electrical power from the Mexican national grid.

The Boleo copper deposits are sediment-hosted and stratiform. They occur in relatively impervious, moist, soft, dark-colored tuff beds in the Boleo formation. They are believed to have formed when favorable beds were replaced by hydrothermal fluids which ascended along faults and fractures in the underlying Comondu volcanics. The mineralized fluids were laterally distributed along conglomerate aquifers and the metals precipitated when they contacted the overlying reducing beds.

Five principal ore zones occur in the formation, each of which is underlain by conglomerate. Two of the beds host nearly all the ore produced to date and all the current known reserves. The thickness of the generally flat-lying beds ranges from 0.5 to two metres.

Mining grades in previous operations averaged about 4.8% copper, ranging from 8% in the early years to 3.5% in later years. Since the ore was fed directly to the smelter with no pre-concentration, anything grading less than 3% copper was considered waste, explained Project Manager Jan Christoffersen. As a result, it was either not mined or backfilled into old stopes. The old workings are extensive, totaling about 10 sq. km over an area measuring about 10 km parallel to the shoreline by up to three kilometres wide.

Grades tend to decrease toward the Gulf of California and, as a result, much of the old workings are inland, closer to the Comondu volcanics. With names such as “Purgatorio” and “Infierno,” the old working areas are somewhat less than inviting. The Dantesque nomenclature is justified, however, as The Northern Miner discovered on a recent descent to the Boleo underworld. The area has an elevated geothermal gradient which, when combined with the high moisture content of the clay beds, makes for a very hot, moist environment capable of opening every pore. On return to surface, temperatures in the mid- to high-30s seem relatively cool.

International Curator is concentrating on the potential for mining ore blocks which did not meet the historic 3% cutoff grade, in areas peripheral to the mined-out stopes.

Proven, probable and possible reserves are estimated at 12.1 million tonnes grading 3.32% copper and 0.10% cobalt over an average width of about 1.2 metres. Based on a 30% dilution at 0.5% copper and 0.015% cobalt, a mining reserve is estimated at about 15.7 million tonnes grading 2.67% copper and 0.08% cobalt in three main areas. Christoffersen pointed out that there is tremendous potential for mining backfilled stopes, which would add tens of millions of tonnes to possible reserves.

International Curator regards the San Guillermo area as the prime target for initial mining. It has the most detailed drill information, with proven and probable reserves totaling 4.8 million tonnes grading 2.99% copper over an average width of 1.9 metres.

The company is to study the economics of mining the narrow, flat-lying zone and also conduct metallurgical studies on the material. Previous operations used labor-intensive, thin-seam, pick-and-shovel coal mining techniques, but this resulted in production of only 45 tonnes per man-year.

As an alternative, International Curator will investigate the possibility of using longwall underground coal mining techniques. The ore beds are softer than coal and tremendous production and cost advantages could be realized if longwall mining proves feasible. A lot will depend on the continuity of the ore bed since large, unfaulted and continuous blocks of ore would be required for longwall mining.

The Boleo deposits are metallurgically complex and, so far, no process has been developed to recover economically both copper and cobalt. In the sulphide material, about half the copper and all the cobalt is contained in sulphide minerals, with the remaining copper ionically bound within the clay lattice. In the oxide ore, the dominant copper mineral is chrysocolla accompanied by lesser amounts of azurite, malachite and atacamite. Previous tests using flotation on the sulphide material resulted in low recoveries. These tests did not, however, take into account the fact that half the copper is in the clay lattice and, therfore, not available to flotation.

Preliminary metallurgical studies indicate that benefaction techniques used in the clay industry may be applicable to Boleo material, and the company plans to study this further.

Notwithstanding the risks, there is definite potential in the property. The company hopes to vend a piece of the project to a third party, Christoffersen said, noting that Curator’s management group do not pretend to be miners nor do they plan to be.