Total historic production from Spanish and German operations in the Bramador area is estimated at 500,000 tons, of which roughly 350,000 tons was oxide material that was processed by amalgamation using the patio method for its gold and silver contents. Galena-rich ores were processed used direct smelting, and sphalerite-pyrite ores were roasted using a reverberation furnace first (Fernandez-Valle, 1984). By 1873 there were seven mines and ten beneficiation plants (e.g. Fig 1). The most successful of these was Socorro with a hydraulic wheel of 8 meters in diameter and a stamp mill with the capacity of 10 tons per day (Fernandez-Valle, 1984). A sample of concentrate from Socorro contains 4% Pb, 9.3% Zn, 488 g/t Ag, 1110 ppm As, 318 ppm Cd, 664 ppm Sb, 14 ppm Bi, 195 ppm Hg and 170 ppm Se (sample 16265).
Fig. 1 Ruins of a historic beneficiation plant in the arroyo San Jeronimo near Bramador.
Review of the airborne electromagnetic data for the Bramador area shows a resistivity low between the Rio Coatante and the Rio Desmoronado. The area was selected for reconnaissance geological and structural mapping prior to the monsoon season of 2006 to better understand the geophysical data. Ground surveying of the anomaly area showed that the resistivity low is formational, and is co-incident with black argillites of Unit 1A that contain disseminated to banded sulfides. The argillites trend east-northeast, dip gently south, and are underlain conformably to the north by quartz-feldspar porphyritic rhyolite (Unit 10C, footwall to the Cuale Sequence). To the south, the argillites grade upwards into feldspar-crystal tuffs intercalated with pillow dacites (Fig. 2, Unit 10D). Within the argillites, there is an aphyric rhyolite flow-dome with associated volcaniclastic rocks that sits under the Bramador Plant. In general, the Cuale Sequence is much thicker at Bramador, with at least 1 kilometer of argillite between major volcanic Units 10C and 10 D (Fig. 3).
No systematic alteration mapping or lithogeochemical sampling has yet been done at Bramador. Nonetheless, radiometric mapping by Fugro Airborne (Smith, 2006), has highlighted some areas of high potassium counts that probably reflect sericitic alteration zones, mainly in Unit 10C rhyolite.
Most of the historic workings in the Bramador camp are in the San Jeronimo Gulch on the internally held Properties where erosion along the creek has exposed massive sulfides. Although the showings appear to line up along a northwest trend, resistivity mapping and EM anomaly profiles show that multiple horizons of massive sulfide mineralization trend easterly and dip to the south (Smith, 2006), essentially parallel to the formational trend.
No significant surface showings of massive sulfide have yet been identified on ZIM's Cabrel and Volantin's Properties. Nonetheless, the exploration data generated as part of the 2006 program are starting to delineate some specific targets that merit drill testing. Specifically, strong conductors south of the Socorro Plant and surrounding the altered QFP (dome?) at Ojo de Agua have geological and geochemical characteristics of VMS deposits.
Fig. 2 Regional geological interpretation map of Bramador based on field traverses and interpretation of the geophysical data. The resistivity map was used to help define contacts between black argillite (resistivity lows) and coherent rhyolite and dacite (resistivity highs). The hangingwall dacite to the south also has a characteristic magnetic signature. Soil geochemistry for zinc is markedly anomalous, particularly near Socorro and Bramador, both on ZIM's Cabrel Property.
Fig.3 Cross-Sections of Bramador (looking East) showing specific target areas implied by the airborne EM and soil geochemical surveys. SOIL=soil geochemical anomaly. SOLID RED LINES are bedrock conductors. DASHED red lines are resistivity lows or conductive zones.
Socorro is one of the largest historic mineral processing sites between Cuale and Talpa de Allende. However, no significant mine workings are known close to the plant site. Given that the ores from La Concha and Bramador were probably treated at the Valenciana and Bramador plants, respectively, there must have been a local source of ore for Socorro. Although grid coverage is incomplete, some of the most anomalous soil geochemical results to-date come from lines 200 meters and 1400 meters east of the Socorro hacienda, and provide an important clue as to the possible location of any historic workings. Specifically, these anomalies are:
Line 496 000 E between 2237975 N and 22378100 N (about 200 meters east of Socorro) with maximum values of 2606 ppm Zn, 1311 ppm Pb, 6.8 ppm Ag, 54.6 ppb Au, 30 ppm Sb, 9 ppm Hg and 121.8 ppm Cu with As, Cd, Ba (samples 67172 to 67177).
Line 497 200E between 2238000N and 2238200 N with values up to 671 ppm Zn, 245 ppm Pb, 67 ppm Cu, 1.2 ppm Ag and 2.8 ppm Hg with anomalous Ba, As, Cd and Sb (samples 66969-66978).
The high mercury and lead values suggest that the soil samples are not far removed from near-surface massive sulfides.
The Bramador plant is located on an aphyric rhyolite flow-dome complex intercalated with black argillite. Tailings from the site contain 1.1% Cu, 12.4% Pb, 8.4% Zn, 589 g/t Ag, 7.6 g/t Au, 0.21 % Hg, 0.2% Sb and 0.5% Se (sample 19065). Geochemically, the Bramador tailings are copper and gold rich compared to Socorro. Soil lines close to the Bramador plant are markedly anomalous for silver, gold and mercury as well as base metals. Specific soil geochemical anomalies near the plant site are:
Line 494 400 from 2234625 N to 2235050 N with maximum values of 4852 ppm Pb, 166 ppm Cu, 2333 ppm Zn, 63.5 ppm Ag, 11 ppm Hg and 320 ppb Au (samples 66193-66212).
Line 494 800 between 2234350 N and 2234750 N with maximum values of 483 ppm Pb, 694 ppm Zn, 78 ppm Cu, 6.7 ppm Ag, 31 ppb Au and 8 ppm Hg (samples 66015-66031).
Line 495 200E from 2234650 N to 2234775 N with values from 785 ppm Zn, 114 ppm Cu, 52 ppm Pb, 6.5 ppm Ag and 21 ppb Au (samples 66314-66319).
Fig. 4. Iron oxy-hydroxide crust from water seeping through mineralized argillite west of Bramador. Sample 16279 contains 45% Fe, 8.9 ppm Mo, 1880 ppm Pb, 456 ppm Zn, 0.6 ppm Ag, 1123 ppm As and 33.1 ppm Sb. The high Fe, As and Sb content suggest that the groundwater circulating through the argillites has interacted with massive sulfides.