EXPLORATION ’93 — Geological tour of copper, gold deposits

Sixteen geologists from Canada, the U.S., Australia and Chile investigated copper and gold deposits of the Tucson-La Caridad-Hermosillo triangle of Arizona and Mexico last February. What follows is a summary of their observations:

Our morning visit to the Mission/Pima operation of Asarco (NYSE) was blessed by clear skies and a crackerjack tour by the company’s geologists and engineers. The porphyry copper deposit (with no gold) is of the skarn type. It formed in Paleozoic carbonates, resulting in spectacular intergrowths of wollastonite and barite in places, all related to a Laramide porphyry quartz monzonite.

The group next visited Cyprus Minerals’ (NYSE) Sierrita mine, which may represent the “guts” of the system from which Mission/Pima slid northwest about 10-19 million years ago. Mineralization is dominantly intrusion-hosted. Field studies were cut short by the arrival of a storm. Notwithstanding the elemental excesses, the rocks told their story well, and an end-of-the-day visit to the San Xavier Mission provided a welcome postscript to a thought-provoking first day.

The next day the group visited Asarco’s Silver Bell operation, which is interesting in that annual production (with only 12 employees) from leaching of dumps is 7 million lb. of copper. Silver Bell also hosts the only remaining (currently known) leached capping. The hypogene grade is increased from between 0.1% and 0.3% copper to 0.8% copper by the leaching effect as a result of at least two cycles of leached blanket development. The group then visited Arimetco International’s (TSE) Johnson Camp operation, east of Tucson. Oxide copper is being mined here, leached on typical sprinkler pads. The pregnant solution is then treated by the solution exchange-electrowinning process. This is introduced to a kerosene-bearing solution in carefully controlled convective cells, constrained within stainless-steel-lined concrete vats. Copper ions exchange from the pregnant solution into the kerosene. The copperless solution is then recycled to the leach pad, and the copper-bearing solution is introduced to steel cathodes upon which copper is plated by electrowinning.

Our first major stop in Sonora, Mexico, was Mexicana de Cobre’s Cananea mine, where the group received an informative and mind-boggling tour of a deposit that still contains more than 2.2 billion tonnes of reserves at a copper grade of 0.6% (1.3 billion tonnes minable at 0.66% copper) still open on three sides. The stripping ratio is 1.2-to-1 and the cutoff is 0.35%. Material mined with grades between 0.1% and 0.35% copper is leached; reserves in this category are more than one billion tonnes.

One of the more spectacular orebodies at Cananea was the La Colorada breccia pipe from which Anaconda mined seven million tonnes of 6.0% copper, 0.4% molybdenum, with silver and gold credits. The breccia pipe topped at 300 metres below surface; the surface expression was minimal except for some glassy quartz veins and was discovered by systematic drilling by Anaconda. Mineralization consists of chalcocite and chalcopyrite. One of the earlier pits, with a steep 55 wall, is now used as a holding pond for the SX-EW plant.

After lunch, the party visited the Maria mine of Cominco (TSE) and Empresas Frisco, an impressive copper deposit apparently “fed” by a quartz-rich pegmatite. The Marie deposit is peripheral to a small porphyry body (six million tonnes, 0.9% copper) and, put simply, is a giant pegmatoidal cavity (about 200 metres in long dimension) filled by large chalcopyrite crystals and several interesting gangue minerals such as apatite and “hairy” tourmaline.

After spending the night in Douglas, Ariz., it was on to the La Caridad porphry copper project, Mexico’s second-largest copper producer. Together with Cananea, they provide 10% of the world’s copper. La Caridad is also owned by Mexicana de Cobre, which, it seems, is owned in turn by an individual in Mexico City. The leached capping is well-exposed and is 100 to 150 metres thick, comprising “live limonite” after chalcocite, and thin quartz-tourmaline veinlets.

One of the lessons of the visit, especially as demonstrated by tour leader Spencer Titley, is the necessity of recognizing various secondary oxides in all their different forms.

La Caridad was found in the 1970s and current reserves are 1.2 billion tonnes at 0.45% copper. Cutoff for material shipped to the mill is 0.3% copper; material between 0.1% and 0.3% copper is leached.

Peripheral to the La Caridad system, the Pilares breccia orebody was a 3,000-tonne-per-day operation in 1900. Phelps Dodge (NYSE) mined 50 million tonnes at 1% copper. At surface today, one sees brecciated, silicified volcanic rocks in a hypogene (yes!) specular hematite matrix. Clasts of volcanic rock are french-fry-shaped and often aligned parallel to one another, the origin of which engendered much discussion, especially with the Australians.

Two days were spent based out of Hermosillo. We first visited Eldorado Mining’s La Colorada project where test-leaching of a 4-million-tonne orebody at 1.35 grams gold per tonne is in progress. Plans are in place to mine 2,000 tonnes per day of mineralized rock at a stripping ratio of 2-to-1, subject to permitting.

The next deposit visited, San Antonio de La Huerta, is a breccia body virtually identical to Pilares, which is near the huge La Caridad copper deposit and which begs the question, Is there a “La Caridad” near San Antonio De La Huerta?

En route to Magdalena De Kino, a stop was made at Exall Resources’ (TSE) Verde Grande property, northeast of Hermosillo. This is an oxidized skarn system, formed near the contact between the Sonoran batholith and limestone. The skarn is dominated by garnet and epidote, mineralization by chrysocolla veinlets, silica and hematite. Grades are typically 2% to 3% copper, with up to three grams gold per tonne, 50 grams silver and 1.5% zinc. Some 35 km south of Magdalena de Kino, the Fresnillo Co. is exploring the San Francisco system where reserves are 4.3 million tonnes at 1.76 grams gold per tonne.

The final deposit visited was Minera Roca Roja’s Amelia project (owned by Walhalla of Australia) near Phelps Dodge’s Santa Gertrudis mine (which we were unable to visit). Mineralization at Amelia is present as two auriferous mantos (stratabound, stratiform) hosted by an Ostrea-bearing limestone. Average grade is 1.5 grams gold. Two pits are planned, the largest of which is 200 metres in diameter and 120 metres deep; estimated capital cost is $2.5 million. Nevada-types consider Amelia and perhaps Santa Gertrudis to be gold analogues for sediment-hosted (Carlin) gold deposits in that state. A number of observations were made after the tour. The key to finding another Mission/Sierrita/Silver-Bell/Cananea/La Caridad-type deposit is an understanding of secondary oxides in leached cappings, a skill that comes with considerable experience. Valuable high-grade breccia or pegmatite bodies can develop peripherally, or within these bulk tonnage low-grade deposits, and successful delineation of one of these orebodies could alter the economics of an “average” porphyry. Problem is, these starter bodies have little surface expression and demand diamond drilling. In reverse, the presence of various breccia and pegmatite bodies may provide clues to the whereabouts of associated porphyry systems.

Deposits in Sonora at the exploration and pre-production stages appear well-suited to junior mining companies. We wish them all “muy buena suerte.” Our sincere thanks to Prof. Spencer Titley (University of Arizona) and John-Mark Staude (USGS, Tucson) for their excellent leadership of the Mineral Deposits Division’s 1993 field trip.

— James Macdonald is a research associate with the Mineral Deposit Research Unit, University of British Columbia, and chairman of the Mineral Deposits Division of the Geological Association of Canada.