In many ways, the past year for
After gaining undisputed title to the promising Cerro Samenta copper porphyry prospect in northern Argentina a year ago, the junior saw its shares rise to a high of $2.25. However, with the release of uneconomic drill assays in June, the shares traded down to a low of 30. This setback was then reversed with the announcement of encouraging trench results by
With 21.8 million shares outstanding, or 26.8 million fully diluted, Mansfield is trading at the 55 level. The company has $3.7 million in cash.
The company specializes in grassroots exploration in northwestern Argentina, where it has focused activities since 1994, while maintaining an office in the northern city of Salta. Its portfolio includes eight early-stage Argentine properties and a lone property in British Columbia.
The company started off in Argentina by looking for Carlin-style sediment-hosted gold deposits. “In the first month of exploration, we realized that the structure was there, but the host rocks for that type of deposit were not present,” says President Gordon Leask. “In the early days, we started developing our own geologic models and metallogenic maps. When we discovered the Samenta porphyry, it opened the door for other ideas. The discovery of the Rio Grande property was a real breakthrough in our understanding of the geology of the area.”
The Rio Grande property is 250 km due west of Salta and can be reached by 435 km of all-weather road. Mansfield outlined the 13.5-km property while conducting a $1.2-million regional exploration program in 1999. The program was funded 50% by Teck through a private placement. This entitled Teck to a right of first refusal on any property acquired under the 1999 program that Mansfield wished to joint-venture.
In December 1999, Mansfield acquired title to the property, which centres on a zone of disseminated and veinlet-controlled chalcopyrite and magnetite mineralization hosted by intense, potassically altered volcanic rocks. The Main zone has a surface footprint measuring 700 by 200 metres. An initial 36 grab samples returned values of copper ranging from 0.1% to 3.7% for an average of 1.28%, and gold ranging from 0.06 to 7.3 grams per tonne for an average of 1.25 grams. The zone extends 100 metres vertically in exposed topography.
In March, Teck elected to acquire an initial 55% interest in the property by spending $4 million over four years and paying a total of $1 million. Teck can earn an additional 10% by financing the property through to the feasibility stage.
During initial work, Teck excavated three hand trenches on the discovery showing, which were channel- or chip-sampled at intervals of 1 metre. Two cross-trenches were completed in a northerly and easterly direction. The easterly striking trench averaged 0.58% copper and 0.98 gram gold over 96 metres, whereas the north-south leg averaged 0.72% copper and 1.1 grams gold over 30 metres.
A further 450 metres to the southwest, the third trench averaged 0.31% copper and 0.22 gram gold over a 19-metre section.
Mineralization within the area is extremely subtle, notes Douglas Leishman, a mining analyst with Yorkton Securities, who recently toured the project. Copper minerals consist of copper oxides and chalcopyrite. “It is a testament to the ability of Mansfield geologists in identifying this project,” Leishman says. “Copper oxide, while relatively abundant (upon cracking open the rocks), is not obvious. Copper sulphides are extremely fine-grained.”
Teck recently completed 52 line km of surface geochemical sampling for a total of 1,045 collected samples. In addition, a geophysical magnetic survey comprising 45 line km was completed. Mechanized trenching is under way on several targets. All data will be evaluated by year-end in preparation for drilling in early 2001.
The geochemical signature, along with petrographic studies of the alteration mineralogy, suggests that the Rio Grande prospect shows characteristics of an Olympic Dam-style iron-oxide/copper/gold deposit. Leishman says this is a relatively new classification and was developed as a result of the recognition of a large copper-gold deposits that did not conform to previous models. This model evolved from Western Mining’s Olympic Dam deposit in Australia, the Ernest Henry deposit (167 million tonnes grading 1.1% copper and 0.5 gram gold) in Australia and Phelps Dodge’s Candelaria deposit (400 million tonnes grading 1% copper, 0.2 gram gold and 4.5 grams silver) in Chile.
While these Olympic-Dam-type deposits are still poorly understood, they are characterized by several factors, as outlined below:
– Mineralogy — The mineralization is generally dominated by iron oxides (either magnetite or hematite).
– Metal suite — Iron is the dominant metal, while copper occurs as the next most abundant metal. Individual deposits may contain significant gold, silver, uranium and/or rare earth elements. Significant trace metals may include cobalt and molybdenum.
– Alteration — The host rocks are generally intensely altered. The alteration assemblage depends on host lithology and depth of formation, but there is a general trend from sodic (albite-actinolite) alteration at deep levels to potassic (potassium-feldspar-biotite) alteration at intermediate-to-shallow levels to sericitic (sericite-limonites) alteration and silicification at very shallow levels.
– Tectonic setting — The deposits lie in areas that were cratonic or continental margin environments, and most occur along major structural zones.
Though many of the deposits occur within silicic-to-intermediate igneous rocks of anorogenic type, the host rocks can be sedimentary.
Individual deposits occur as strongly discordant veins and breccias, to massive concordant bodies. Both morphology and the extent of mineralization and alteration appear to be largely controlled by permeability along faults, shear zones and intrusive contacts, or by permeable horizons in sequences of volcanic flows.
Madison says the Rio Grande prospect displays the typical mineralogy, metal suite and alteration styles characteristic of an iron oxide-copper-nickel deposit. According to Leask, these characteristics include magnetite breccias and stockwork, dominant potassic alteration, and disseminated copper-gold. The Rio Grande property lies 25 km southeast of Cerro Samenta. The copper-gold showing occurs near the top of a volcanic edifice in what appears to be a small caldera.
The area is characterized by a thick pile of porphyritic andesites that have been cut by post-alteration andesitic dykes and several andesitic-to-dioritic plugs. A several-metre-thick blanket of float and weakly developed soil covers most of the area. Outcrop in the area is poor. The prospect area consists of a large, zoned, hydrothermal alteration system that appears to be at least 2 km long and 1.9 km wide.
Geochemistry results, when compared with alteration styles, show that 90% of the copper-gold mineralization in samples running greater than 0.4% copper and 0.2 gram gold is associated with two styles of alteration: potassic, and sericitic with hematite-limonite veinlets.
Mansfield has identified a similar-style target on its wholly owned Arizaro property, 12 km southeast of Rio Grande. Copper-gold values occur in association with disseminated and veinlet-controlled chalcopyrite and magnetite within a zone of potassically altered volcanic rocks.
Assays from 93 grab samples taken from an area measuring 600 by 800 metres averaged 0.49% copper and 1.09 grams gold. Two short hand-dug trenches in the southeastern portion of the potassic zone yielded 22 metres of 0.59% copper and 1.18 grams gold, plus 8 metres of 0.66% copper and 1.44 grams gold.
A recent geoche
mical sampling program of talus fines outlined a copper-gold anomaly extending over an area of 900 by 800 metres. A second zone of mineralization, dubbed Lindero, has been discovered 3 km north of the main showing. There, grab sampling has yielded several samples in excess of 2 grams gold, with a high of 6.67 grams. Copper values range up to 0.62%.
Mansfield is trenching the central part of the Arizaro geochemical anomaly and has begun detailed geochemical sampling over the Lindero prospect. Teck holds a right of first refusal, which expires November 2001, should Mansfield choose to joint-venture the property.
Subsequent to the discovery at Arizaro, Mansfield and Teck reached an agreement to co-fund grassroots exploration in northwestern Argentina. This year’s budget is set at $1.2 million, and Mansfield is the operator. Any new properties acquired under this program will be jointly owned. Teck is required to spend the first $500,000 on each property. Follow-up expenditures will be shared equally.
A new zone of copper-gold-fluorite-specularite mineralization has been found on the southeastern portion of the Cerro Samenta property. Local fluorite, hematite and copper-gold mineralization occur sporadically in a 1-by-4-km area. A mineralized stockwork-breccia zone extends over an area of 600 by 300 metres.
Teck is reviewing this new showing in conjunction with all previous data to determine the future course for this property.
Mansfield acquired Cerro Samenta in 1994 and optioned it to Teck in late 1995. Initial geophysical work revealed a large induced-polarization (IP) anomaly measuring 2.5 by 1 km at the north end of the property. A second IP anomaly, measuring 1 by 4 km, was outlined several kilometres to the south. Teck began a comprehensive program of geological mapping and trenching. A surface exposure yielded grades of 0.5% copper over 250 metres, including 0.7% copper over 150 metres and 1.4% copper over 75 metres.
Eight trenches along the periphery of the north anomaly revealed a copper-depleted leached cap of variable thickness. Two additional trenches exposed a potassic core, and another trench cut intermittent supergene copper mineralization, with grades of 1.6% copper, at the southern tip of the anomaly.
Teck completed two holes and a partial third hole in 1996, before an ownership dispute between Mansfield and Jorge Daroca, an Argentine national who held neighbouring claims, brought all work there to a halt.
The dispute was finally resolved some three years later, in December 1999. Teck resumed drilling in May and completed 10 more holes. The program partially tested portions of the both the north and south IP anomalies. No significant grade intercepts were returned. The best hole cut 18 metres of 0.42% copper. It was determined that an outer pyrite shell was responsible for the strongest IP response.
Mansfield says a partially overlapping copper-molybdenum geochemical anomaly remains virtually untested.
Teck can earn a half-interest in Cerro Samenta by spending US$7.5 million and making cash payments totalling US$1.6 million over a period of five years. A US$250,000 payment is due to Mansfield in February 2001.
The three properties — Cerro Samenta, Rio Grande and Arizaro — all occur in the Puna region of the province of Salta at relatively high elevations ranging from 4,000 to 4,600 metres.
In related news, Idaho-based
Sunshine can earn a 60% interest by spending US$2.5 million and paying US$1.2 million over five years.
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