The medieval friar St. Anthony is occasionally invoked for things lost; but another San Antonio, this one about 150 km southeast of Hermosillo, Mexico, might be better appreciated for things found.
What has been found at the San Antonio property, held by
The 110-sq.-km property has three known gold deposits and a small, low-grade copper deposit, all hosted by sedimentary rocks brecciated by later igneous activity. The sediments are exposed in a “window” in younger volcanic cover.
Significantly, all the copper and gold occurrences are accompanied by hydrothermal hematite alteration. The association — an iron oxide with copper and gold mineralization — suggests, to those who like joining the dots, that the deposits may be of iron oxide-copper-gold affinity. That interpretation has implications well beyond the property boundaries: hematite alteration is common in many Sonoran sediment-hosted gold deposits.
Zaruma was formed out of two companies, Norwegian-domiciled Zaruma Mining and a Vancouver-listed junior, Laminco Resources. Laminco’s predecessor, Golden News Resources, had optioned San Antonio in 1993, and Laminco had advanced the property as far as a March 1997 feasibility study, which put capital costs for a small heap-leach operation at US$19 million.
Later joint ventures with
In 2003, as gold prices recovered and financing became available, Zaruma raised $1.7 million in two private placements, and started drilling some new targets on San Antonio, which had been selected from a compilation of geophysical and geochemical data that had not been available to earlier operators. The first area to be drilled was a prospect called Centenario, a short distance east of the Golfo de Oro deposit, where a resource of 2.8 million tonnes grading 3.4 grams gold per tonne had been outlined in the mid-1990s.
(Two other resource figures have been calculated, both from the same era: the California deposit holds 1.6 million tonnes grading 2.6 grams, and the Cerro Sapuchi deposit, 1.4 million tonnes at 2 grams gold per tonne. All the estimates pre-date National Instrument 43-101.)
The drilling encountered breccia mineralization resembling that found at Golfo de Oro, over a strike length of about 300 metres. Among the significant mineralized intersections were a 22.5-metre length grading 4.2 grams gold per tonne, a 1.5-metre length running 33.6 grams per tonne, and a 2.3-metre interval with an average grade of 3.1 grams.
A second phase of drilling in early 2004 put three holes into Centenario. One hole intersected 15.3 metres at a grade of 3.9 grams gold per tonne and a second cut two narrower zones, one of 3 metres grading 3.8 grams per tonne and the other of 1.4 metres grading 3.2 grams. The third, and northernmost, hole did not intersect any significant mineralization and is thought to mark the northern limit of the zone.
Northwest of Centenario, surface sampling at the Corina showing indicated a 1.5-metre width grading 4.3 grams gold per tonne, and two surface vein zones, one grading 7.3 grams gold across 4 metres and another grading 6.1 grams per tonne across 1.2 metres. Drilling at Corina encountered similar breccia material, though gold grades were only termed “anomalous.”
The Mina High Life area, 500 metres east of Corina, appears to be the western extension of the Corina structure, and samples from small prospect pits and road cuts returned gold grades of 1-5 grams per tonne, with widths ranging between 1.6 and 2.4 metres. Another sample in a prospect pit averaged 11.4 grams per tonne over 2.2 metres.
Road cuts to the east of High Life showed comparable grades and widths wherever veins outcropped at the surface.
Infill drilling at Golfo de Oro itself, which started early this year, is building the picture of a higher-grade core zone within the known resource. To date, 10 of 14 drill holes have intersected significant gold mineralization.
The most recent results come from hole C83-04, drilled to test up-section from known mineralization at Golfo de Oro. The hole intersected 91 metres of breccia material, including a 6.1-metre zone that averaged 2.7 grams gold per tonne, another of 6.9 metres grading 4.5 grams per tonne, and a third, of 6.8 metres grading 4 grams gold per tonne.
The results suggest that near- surface mineralization above the hole may be continuous with the main mineralization below the hole, though the company cautioned that such an interpretation is still premature.
East of the recent drilling, infill holes have returned mainly positive results. Three holes drilled along section 5E of the known deposit intersected gold mineralization at grades mainly at or above the resource grade of 3.4 grams per tonne. That included a 21.7-metre intersection that graded 5 grams gold per tonne. Earlier drilling on the section had intersected a 26.7-metre zone that ran 4.5 grams per tonne.
On section 7E, 200 metres to the east, two holes intersected relatively thick breccia zones at grades similar to the resource grade. Hole 72-04 cut an 8.9-metre length grading 3.4 grams gold per tonne, including a 3-metre interval that ran 5.8 grams. Hole 73-04 intersected a 25.3-metre zone averaging 3 grams per tonne, which included 2.4 metres of 5.6 grams gold per tonne and 3.4 metres of 5.4 grams per tonne.
San Antonio’s copper deposit, Luz del Cobre, is at the eastern end of the trend, about 1 km east of Sapuchi, and has not seen any new work since the feasibility studies of the mid-1990s. Luz del Cobre has a resource of 6.7 million tonnes grading 0.76% copper, which pre-dates National Instrument 43-101.
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