MINING IN MEXICO — Metalurgica keen on copper property

Private Mexican company Metalurgica Costa Dorada hopes to prove up what could be one of the world’s largest sediment-hosted, stratiform copper deposits.

The find is centred on the small town of Cuatrocienegas, which lies on the highway between Monclova and Torreon in the northeastern state of Coahuila. Project manager John Goodwin says Costa Dorada has acquired a land position of more than 144,000 hectares. In the same area, Noranda Exploration has accumulated more than 50,000 hectares.

The main copper-silver mineralization occurs within a thick sedimentary sequence of Lower Cretaceous age, in a 10-20-metre layer of (reduced) siltstones and sandstones that range in color from green to grey-black, Goodwin says. The mineralization is sandwiched between underlying redbed (oxidized) shales, sandstones and conglomerates and an overlying thick pile of sandy dolomites, gypsum and massive marine limestones.

Goodwin adds: “This reduced arenaceous layer, which is known as the Main Favorable horizon, marks the transition between predominantly continental sandstone deposition and the overlying, shallow, water-to-marine, carbonate deposition.”

The horizon represents the product of a marine transgression that occurred over thousands of square kilometres. “The net result of the marine transgression is that a remarkably uniform layer of reduced sandstones was developed over great distances,” Goodwin says.

The problem with clearly delimiting the area of potential mineralization is that outcrops of the horizon are confined to the cores of a series of large, northwest-southeast-trending anticlines where these structures have been breached by erosion to expose the underlying redbed sandstones, Goodwin explains. He adds that within the breaches, which give rise to the steep-sided valleys, the horizon occurs high on the valley sides and is mainly covered by a thick boulder talus. However, prospecting and bulldozer stripping have demonstrated continuity of mineralization along the horizon. Estimating average thickness and grade is another matter. Surface hand trenching and bulldozer work have not been very helpful, nor have exposures worked previously by miners. The company’s El Granizo showing, for example, sampled along a 42-metre strike length, returned 2.11% copper and 37.9 grams silver per tonne over a thickness of 8.72 metres, but neither the top nor base of the mineralization is available for sampling.

Grab samples from the property have returned variable assays. Testing of the samples has returned soluble copper values that indicate 95-100% leachability, Goodwin says.

Little work has been done to probe for downdip extensions other than in a few isolated mining locations.

Costa Dorada has been negotiating possible financing to explore its property further, either alone or with a partner.