Ongoing drilling at
El Sauzal is a high-sulphidation, epithermal, oxidized gold deposit that lies in steep, rugged mountainous terrain in the Sierra Madre Occidental. It is accessible by a 40-km all-weather road that connects to a paved highway running 250 km northeast to Chihuahua city.
In 1998, Mine Reserves Associates of Wheat Ridge, Colo., prepared a resource estimate based on 124 diamond drill holes totalling 20,300 metres. It was estimated that El Sauzal hosts 3.6 million contained ounces of gold within a drill-indicated resource of 50.6 million tonnes grading 2.21 grams gold per tonne, based on a cutoff of 0.5 gram gold.
Mineralization occurs at and below a regional unconformity separating a lower sequence of stratified dacite and andesite rock units from an upper sequence of bi-modal, syn-mineral volcanic and clastic rocks.
Three distinct pockets of mineralization have been structurally and topographically identified: the West, West Lip and East zones. Fault displacement and erosion have caused each zone to have unique structural, stratigraphic and mineralization characteristics.
Gold mineralization in the West zone is hosted in a thick, gently east-dipping sheet coinciding with a stratigraphically controlled transition between vuggy, residual silica and an underlying hematitic, argillic-altered zone.
In the East zone, mineralization is localized within a continuous northeasterly dipping zone of vuggy silica formed along the unconformity dividing the two stratigraphic sequences. Both zones contain downward extensions of high-grade mineralization along a series of northeasterly striking feeder structures.
The West Lip contains a wide zone of low-grade gold mineralization intercalated with narrow, high-grade, structurally controlled feeders. Francisco says additional drilling is required to determine whether the lower-grade mineralization is part of a peripheral envelope outside a higher-grade core.
Recent diamond drilling has further tested the southwestern extension of the West zone. Francisco targeted high-resistivity geophysical anomalies that it believes represent the extensions of structurally controlled feeder zones underlying the main body of mineralization to the east. Holes 141 and 142 stepped out on one of the targeted structures and intersected gold mineralization at the projected depth, extending the mineralized zone a further 150 metres west of the previously drilled area.
Hole 141 intersected 26 metres grading 0.7 gram gold per tonne, starting at a down-hole depth of 27 metres, whereas hole 142 encountered 32 metres averaging 2.6 grams (including a 6-metre interval of 10.3 grams), beginning at 16 metres down-hole.
Drill hole 143 tested a separate high-resistivity anomaly farther to the north and intersected 54 metres averaging 3.21 grams right from surface, extending the zone another 50 metres to the southwest.
Four holes stepped out on the West zone’s southeastern margin, where surface sampling returned high gold values within vuggy quartz and underlying hematitic alteration.
All four holes encountered mineralization starting at surface:
- hole 144 averaged 4.24 grams over 18 metres;
- hole 145 averaged 3.03 grams over 25 metres;
- hole 148 averaged 2.45 grams over 36 metres; and
- hole 149 averaged 6.82 grams over 42 metres, including a 27-metre interval averaging 10.12 grams.
The continuity of a zone of gold mineralization associated with a low-angle, northwesterly dipping fault in the West Lip zone was confirmed by holes 146 and 147, which stepped out 80 and 120 metres, respectively, from previously drilled holes 51 (96.7 metres averaging 1.34 grams) and 129 (18 metres of 1.82 grams). Hole 146 encountered 16 metres averaging 1.87 grams (including 7 metres of 3.54 grams) at a down-hole depth of 122 metres, whereas hole 147 intersected a 13-metre interval of 1.33 grams, starting at 79 metres down-hole.
Further drilling is planned for both the West and West Lip zones. Currently, the eastern downdip extension of the East zone is being drill-tested, with several holes also scheduled to test a northwestern fault block.
A 1998 scoping study by Denver-based Behre Dolbear recommended El Sauzal be developed as an open-pit mine, with a 10,000-tonne-per-day gravity and carbon-in-leach mill that would produce more than 300,000 oz. annually for an initial life of seven years. A minable open-pit resource was estimated at 23.5 million tonnes grading 2.98 grams, representing a contained 2.25 million oz. gold, at a stripping ratio of 1.58-to-1.
Metallurgical testwork by METCON Research of Tuscon, Ariz., was carried out on a representative 2.3-tonne bulk sample averaging a head grade of 4.13 grams. Gold recovery was greater than 96% from a combined gravity separation and cyanidation treatment, with 40% of the gold recovered by gravity circuit alone.
The potential of heap leaching as a processing alternative is being evaluated. Initial column leaching tests on a crush size of 3/8-inch yielded a gold recovery of 72% after 30 days, with low consumption of sodium cyanide and lime. After a 49-day cycle, gold recoveries rose to more than 80%.
Francisco contracted Tuscon-based M3 Engineering & Technology to undertake a prefeasibility study of El Sauzal, details of which were released in March. M3 examined the proposed open-pit mine and 10,000-tonne-per-day conventional milling proposal and estimated a capital cost of US$138.8 million for mine construction, which includes sustaining capital and a US$19.4-million contingency. Cash operating costs for a 300,000-oz.-per-year operation are estimated to average US$86 per oz., with total production costs coming in at US$150 per oz. During the mine’s first four years, the cash cost is forecast at US$68 per oz.
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