Rigs to drill other targets at Venezuelan property — Placer

A feasibility study will be performed on the Venezuelan gold property known as Las Cristinas.

Placer Dome (TSE), which owns 70% of the project, approved the US$13-million study. (The remaining interest is held by a state-controlled Venezuelan corporation.)

The study is based on a measured and indicated resource estimated at 189.7 million tonnes grading 1.26 grams gold per tonne. This resource, hosted in weathered greenstones of Proterozoic age, is estimated to contain 7.7 million oz. gold, of which Placer Dome’s share would be 5.4 million oz. “The exploration potential elsewhere on the property is still reasonably good,” said Eliseo Gonzalez-Urien, Placer Dome’s senior vice-president of exploration. “We plan to have rigs drilling other targets this year.” The updated numbers represent a significant increase from last year’s estimate of 4.8 million oz., based on a partial geological resource of 123.9 million tonnes grading 1.32 grams gold per tonne (at a cutoff of 0.7 grams). Exploration drilling is continuing, in an attempt to identify areas with above-average grade within the study area and to explore several other targets outside the area. At least seven mineralized areas have been identified on the 6,000-hectare property.

A production decision will be considered once the feasibility study is completed, in the first quarter of 1995. If the decision is positive, Las Cristinas would likely become the largest gold mine in the South American country.

Such a decision would also usher in a new era of mining for the property, which has a production history dating back to the turn of the century. It was originally a small operation focused on vein targets, but it did not attract much attention at the time and the area was relatively dormant until the early 1970s.

Gonzalez-Urien said the discovery of Las Cristinas resulted from the local miners’ systematic pursuit of panning major drainages for gold. During the 1970s, these miners flocked to Las Cristinas and began mining the gold-rich, lateritic layer until it was virtually exhausted.

Today, little remains of this material which, overall, is estimated to have averaged between 6 and 8 grams gold per tonne. Some areas had better values, in the order of 10-15 grams.

Underlying the laterite is a unit of clay saprolite, which then became the target of the local miners. But because grades were lower and recoveries of the microscopic gold more difficult, syndicates were formed to buy the necessary equipment for larger-scale, pump-mining operations. Large amounts of mercury were used to amalgamate and recover the gold contained in the oxide saprolite unit. This layer was denuded in some parts of the property, along with the vegetation, down to the hard-rock surface. By this time, however, environmental problems became evident as the mercury and clays began making their way down the river systems. The syndicates began to shut down because mining was becoming more difficult and because of pressure from the government.

Gold recoveries in those times were poor — as little as 20-30%. Placer Dome tested a small portion of the estimated 13 million tonnes of tailings still on the property. This limited sampling returned values of about 2 grams per tonne.

Many millions of ounces of gold were extracted by local miners during those roughly 15 years. Indeed, official records of 2-2.5 million oz. are considered conservative, as it is believed not all the gold was reported. Placer Dome arrived in 1991 and has since mapped most of its 70%-owned concession. Work has been focused on two main targets: Conductora and Cuatro Muertos.

“Soil geochemistry has been an effective exploration tool,” said Gonzalez-Urien, adding that this is typically followed by auger drilling on a grid as well as magnetic and very-low-frequency geophysical surveys. Initial drill-testing was widespread, on a 400-metre spacing. This was followed by closer-spaced drilling, so that 295 holes totaling 40,000 metres have been drilled.

The deposit is 2,400 metres long, 1,800 metres of which has been drilled on 100-metre centres. No other dimensions were provided.

Because the laterite top layer has been stripped off the property, existing mineralization is largely hosted within three remaining units: oxidized saprolite and sulphide-stable saprolite (both still part of the total lateritic profile); and the underlying, altered metavolcanics. The oxidized saprolite contains clays and weathered, gold-bearing quartz veins. Nuggets are still occasionally found within this unit, including a 1-kg specimen recently found by a local miner. The sulphide-stable saprolite contains clays, pyrite grains ranging from 3-10%, microscopic gold and some chalcocite.

Gonzalez-Urien said the grades of the three units do not vary dramatically, although, in a few places, the contact between the last two units will contain some higher-than-average gold values, along with copper values of no more than 5%.

“The metallurgy is excellent,” Gonzalez-Urien said. “We expect recoveries will be in the range of 92-95%.”

Placer Dome’s exploration team is continuing to define the geological setting of Las Cristinas and the overall Guyana Shield. Similarities to the Abitibi greenstone belt in Eastern Canada have been noted, although it is speculated that Las Cristinas has other controls than those typically associated with the Canadian model.

“We still have much to learn about this property,” Gonzalez-Urien said.