Platinum and palladium are not typically associated with Mexico, but that hasn’t swayed
Since 1998, the partners have been exploring a layered basic intrusion, 20 km north of the coastal city of Mazatlan. Their efforts recently caught the eye of Sumitomo Metal Mining of Japan, marking the mining conglomerate’s first foray in the country.
“It’s primarly a copper property, but with a real significant platinum-palladium kick,” says Morgan Poliquin, spokesman for Fairfield. “That, along with its location, made it a good fit to [Sumitomo].”
Petrograhically, the complex consists of anorthosites, pyroxenites and gabbros, some of which are cumulates. It stretches for 19 km in length and 2-4 km in width, similar in size to the complexes of northern Ontario. Poliquin says the tectonic setting at Tropico is comparable to that of the Lac des les region in northwestern Ontario, where
Regional deformation has left the complex with a northeasterly trend and near-vertical dip. Erosion has stripped most of the Tertiary-aged volcanic cover to expose the complex and the quartz-rich sandstones, mudstones and shales it intrudes.
Magma Copper explored the area for copper-porphyry deposits prior to its 1996 takeover by BHP Minerals (now
In 1998, Fairfield nabbed the property, only to deal an option to Santoy a year later. Since then, the partners have outlined several soil anomalies along an 8-km-long portion of the complex. One was trenched this past summer to expose mineralized bedrock beneath.
Contact zones
The trenched anomaly, known as Maricela, covers an area measuring 1,600 metres long by 500 metres wide. About 1,500 metres of bedrock was exposed in all.
Mineralization is characterized by disseminated chalcopyrite and bornite in pyroxenite, both fresh and metamorphosed, and mixed pyroxenite-gabbro. Generally, mapping at Tropico shows the best areas of mineralization to coincide with contact zones between gabbro and pyroxenite, where weak-to-moderate silicification occurs.
The best values came from trench 1, which averaged 0.95 gram palladium, 0.57 gram platinum and 0.36 gram gold per tonne, plus 1.33% copper, over 16 metres. That section was part of a longer interval of mineralization that averaged 0.53% copper, 0.35 gram palladium, 0.23 gram platinum and 0.17 gram gold over 160 metres.
Two shorter trenches were branched off trench 1 to the west to check for mineralized continuity. Trench 1A ran 1.01% copper, 0.58 gram palladium, 0.42 gram platinum and 0.27 gram gold over 16.5 metres, while trench 1B averaged 0.73% copper, 0.48 gram palladium, 0.34 gram platinum and 0.25 gram gold over 17 metres.
Copper values in the remaining trenches varied from 0.13% to 0.94%, and those for each of the precious metals were less than half a gram. Trench 8, the westernmost trench, sits nearby an old reverse-circulation drill hole that averaged 0.24% copper and 0.07 gram gold over the first 36 metres, and 0.14% copper and 0.06 gram gold over the next 81 metres. Another old hole, collared 200 metres to the southwest, came back dry.
Encouraging results
Although the other areas have received less attention, preliminary results are nevertheless encouraging. For instance, each soil anomaly is comparable in size to Maricela, and known outcrops contain both copper and precious metals.
At the Cupale area, seven grab samples of coarse-grained gabbro and pyroxenite yielded up to 4.75% copper, 5.21 grams palladium, 6.61 grams platinum and 5.21 grams gold. The best values seem to be restricted to the core of the soil anomaly.
At the Santa Fe area, two historic drill holes returned 0.18% copper over 32 metres (from 53 to 85 metres) and 0.22% copper over 75 metres (from 9.1 to 83.8 metres). Both carried minor palladium, platinum and gold, and each had higher-grading sub-intervals.
The anomaly sits in a narrow, east-west-trending ridge. Grab samples there run up to 1% copper and 1.14 grams palladium. Malachite (copper carbonate hydroxide) and minor chalcopyrite are visible in both gabbro and pyroxenite.
Outcrops found elsewhere on the property mostly assayed less than 1% copper and half a gram each of palladium, platinum and gold. Higher values occur in soils.
According to the recent agreement, Sumitomo can earn a 51% interest in the Tropico property by spending US$3 million on exploration over three years. The remaining 19% can be acquired in exchange for a bankable feasibility study and development financing. If half of this comes by way of debt, Sumitomo must begin construction within 18 months.
Both Fairfield and Santoy are carried through to production. The former would be left with 12%, and the latter, with 18%, if Sumitomo acquires its full share.
Drilling planned
Poliquin says ground crews are attempting to extend the anomalies in all directions. A 2,000-metre drill program will begin shortly at the Maricela prospect to test mineralization down-dip of the trenches.
Trenching at the other anomalies is planned as well.
As part of the deal, Sumitomo must spend US$600,000 by April 2002.
In related news,
Exploration at the 1150-sq.-km property has resulted in the discovery of three distinct areas of alteration and mineralization. The Central Grid target, which was drill-tested in 1998, displays porphyry-style copper-gold mineralization and related high-grade gold-silver veins. Exploration at the Highway and Northern targets has been limited to prospecting, mapping and geophysics. The former is also considered a porphyry target, whereas the latter is a high-sulphidation gold system. A float sample taken from the Northern zone returned up to 11 grams gold per tonne.
Noranda can earn a 75% interest in the project by paying U$2 million and completing a feasibility study within seven years. The major is also required to cover all property costs.
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