Infill drilling at the Alamo Dorado property in Sonora state, Mexico, is proving up what could become an open-pit, heap-leach silver mine.
The new holes all passed through mineralization of substantial widths and grades, with each containing a higher-grade core. Furthermore, all were drilled deeper than the earlier holes and consequently proved the system to be much wider than previously thought.
Silver grades varied from less than 20 grams to more than 222 grams per tonne, with gold averaging less than 0.56 gram over the intervals reported.
Generally, the grades appear to diminish with depth and downdip.
Of particular note are three holes collared in the southern area of the zone, on section 2+50 south. There, hole 29 intersected 229.5 metres averaging 120 grams silver and 0.25 gram gold, including 117 metres grading 222 grams silver and 0.41 gram gold; hole 25, 229.5 metres averaging 73 grams silver and 0.22 gram gold, including 64.5 metres grading 193 grams silver and 0.56 gram gold; and hole 24, 156 metres averaging 46 grams silver and 0.1 gram gold, including 70.5 metres grading 78 grams silver and 0.19 gram gold.
If Alamo Dorado proves feasible, Corner Bay would dig a starter pit in that area, picking the higher grades to repay capital more quickly. (A conceptual model developed by Mintec, an engineering firm from Arizona, envisaged a 200,000-tonne-per-month operation developed in three phases, as reported in T.N.M., March 15/99).
Similar results are reported for holes collared on the other two sections.
The highest grade came from hole 35, collared on 1+50 south, which returned 122 grams silver and 0.3 gram gold over 105 metres of a 142.5-metre interval that averaged 93 grams silver and 0.26 gram gold. Hole 33, collared 200 metres to the north, intersected the widest interval — 198 metres averaging 51 grams silver and 0.16 gram gold, including 127.5 metres grading 64 grams silver and 0.2 gram gold.
Also, two holes collared in the central portion of the zone show mineralization extending downdip: hole 30 intersected 27 metres averaging 72 grams silver and 0.03 gram gold, whereas hole 31 cut 51 metres averaging 29 grams silver and 0.25 gram gold.
Mineralization is associated with oxidized sedimentary rocks that appear highly schistose. Silver and gold are disseminated, with the former occurring primarily in the form of chlorargyrite, a silver chloride mineral known to form in secondary enrichment zones above silver veins subjected to intense weathering.
“Unlike argentite, which tends to be the principal silver mineral in most mines and in some heap-leach operations, chlorargyrite is highly soluble,” notes Peter Mordaunt, president of Corner Bay. “And that’s what really gives this project a leg-up over other silver projects out there today.” The deposit itself sits in a crescent-shaped ridge that rises 150 metres vertically above the valley floor and slopes steeper than the deposit dips.
Overburden is negligible in the north but deepens southwards, averaging 40 metres along the entire zone.
Meanwhile, recent surface exploration has led to discovery of geochemical anomalies to the south and east. Similarly, a large anomaly was discovered on an adjacent ridge and traced for 750 metres in length and 100 metres in average width.
Also, two “mini column” tests of drill cuttings indicate that recovery rates of 90% for gold and 89% for silver are possible. Earlier bottle-roll tests on four composite samples gave similar recoveries after 24 hours of leaching in a 1% cyanide solution.
Currently, Corner Bay is extending the 10 delineation holes drilled previously and will soon begin a separate 1,200-metre, larger-diameter drill program for additional metallurgical tests. Concurrent with the latter program will be a resource-reserve update.
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