|Zimapan S.A. de C.V. mined the Amantea deposit (266,500 tonnes grading 154 g/t silver, 1.0 g/t gold, 13.6% zinc, 2.6% lead and 1% copper) between 1967 and 1973. At today's metal prices 25% of the metal value is attributed to precious metals. Stratiform, banded mineralization of 4 to 8 m is exposed at Amaltea.
The mineralization was explored by drilling in 1975 and again between 1989 and 1990. Zimapan S.A. de C.V. calculated resources in 1990 for the San Rafael and San Pedro bodies based on 4,8698.6 m drilling in 58 holes (Table 2).
Table 2; El Desmoronado area historic resources*
El Desmoronado area covers La Sierra Cacoma with dendritic drainage and immature erosion features.
The area is underlain by volcano-sedimentary rocks with a lower sequence of flow and massive domes latites and dacites. The rocks are moderately silicified and seritezied with up to 8% pyrite. An intermediate sequence of lithic tuffs of latite to rhyolitic composition intercalated with andesitic tuffs, carbonate shales and agglomerates that are either silicified or argillic altered with less than 2% pyrite. The upper sequence is agglomerate with mainly rounded porphyric fragments in an aphanitic matrix that is intermediate silicified and less than 2% pyrite.
The volcano-sedimentary rocks are intruded by granite and diorite apophysis of the Puerto Vallarta batholith to the west.
The volcano-sedimentary generally strike east-west with variable dip depending on if they are related to the domes or the inter dome basins. Mineralization is related to the San Rafael, San Pablo and San Pedro domes.
The San Antonio fault is west of the mineralization and strike north-south with steep dip to the west and a lateral displacement of 80 m and 40 m vertically. The Amantea fault strikes northeast-southwest with a steep dip to the northwest and vertical displacement of 450 m. The Cuale fault is interpreted from the drilling to have northwest-southeast strike and dip to the northeast with lateral displacement of 30 m and 25 m vertically.
San Rafael has stockwork mineralization in a dacitic dome. The mineralization is veins and disseminated sphalerite, pyrite, galena and chalcopyrite with tetrahedrite and pearceite (silver bearing sulphides) in of quartz, barite, sericite and various clay minerals. The sequence of deposition is pyrite→sphalerite→pyrite-chalcopyrite-galena-tetrahedrite-pearceite→covelite.
San Pedro is a massive sulphide manto with east-west strike and dip of 10 to 20 degrees to the north. The host rocks are pyroclastic and carbonate shale of the intermediate volcano-sedimentary sequence. The sulphides are finely banded sphalerite, galena, pyrite and clacopyrite with variable amounts of silver suphosalts (tetrahedrite-tennantite).
San Pablo is fragmental massive sphalerite-galena with disseminated chalcopyrite and minor pyrrhotite in a gangue of quartz, chlorite, calcite, barite and sericite chaotic emplaced in a tuff. The mineralization has secondary hydrothermal enrichment. The mineralization is hosted by silicified and hydrothermal altered what appear to be a dacite.
Zimapan S.A. de C.V. planned to mine the resources by an open pit with a 4:1 stripping ratio (Figure 7) and truck it to the company's plant at Cuale. However, due to depressed metal prices at the time the plant was de-commissioned.
The drilling was done both from surface and underground along approximately 200 m of strike (Figure 8).
Drilling encountered long sulphides intersections with significant gold and silver values (Table 3 and Figures 9 to 11).
Table 3; Selected drill intersections
Hole 103 is one of the few holes below the 1,500 m level intersected high grade silver with gold (Figure 9).
Hole 145 at the southern edge of the drilling intersected silver and gold with high grade base metals (Figure 10).
Holes 116 and 139 at the southeastern corner of the drilling intersected high grade silver and gold (Figure 11). At present metal prices the precious metals in these holes account for approximately 55% of total value.
The results suggest that the mineralization extend to the south, east and depth beyond the present drill coverage and that specifically the gold grades increase to depth and the southeast.
The company is planning ground Electromagnetic (EM) and/or Induced Polarization (IP) geophysics surveys to delineate the mineralization for subsequent diamond drilling. The 10,000 m of planed drilling will be done on 50 m centres including the historic drill coverage to 200 m depth to upgrade the historic resource to NI-43-101 compliant resource. The budget for the program is $3.9 million with an estimated 12 months for completion.
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