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Geology
A Paleozoic basement and Meso-Cenozoic cover are exposed in the Cantón Aguas Blancas region. Regionally, the Paleozoic basement includes metasedimentary rocks in the Coastal Range and sedimentary to volcanic sequences, associated with coeval late Paleozoic intrusive complexes, in the eastern Cordillera Domeyko. The overlying Mesozoic-Cenozoic cover is represented by volcano-sedimentary, marine and volcanic sequences, intruded by a heterogeneous suite of plutonic bodies varying in size from huge composite batholiths in the Coastal Range to discrete, small-sized stocks in the Central Depression and the Cordillera Domeyko (Marinovic et al., 1995).
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The nitrate deposits occur mostly along the eastern side of the Coastal Range from Zapiga in the north to Taltal in the south, a distance of 700 km. Commercial deposits in Tarapaca and northern Antofagasta Provinces were confined to the Coastal Range, whereas to the south they spread out more widely into the Central Valley and lower slopes of the Cordillera Domeyko (Erickson, 1986). Many deposits occur along the margins of salars and ancient playas. Structural activity in the region may have destroyed early salars and playas through tilting and faulting, then infilling of the basins with alluvium.
The nitrate resources in the Aguas Blancas area occur on the upper slopes of large Miocene to Pleistocene age alluvial fans on the western side of the Atacama Desert. Two types of nitrate ores have been described in the region:
(1) Vein Ore - veins and irregular masses of introduced nitrate and other saline minerals in the bedrock.
(2) Alluvial Ore (the predominant type at the Property) - where saline minerals (including nitrate) act as cement to sediments in unconsolidated alluvium, such as alluvial fans.
The iodine-bearing caliche deposits at Aguas Blancas extend for over 10 km length (NW-SE) and 4 km width (SW-NE), occupying large flat areas and the hill slopes. The caliche layer commonly reaches up to 6 m, although in some areas it attains 11 m depth. It is overlain by 0.2 to 1 m of barren, weakly consolidated sand and gravel, which is commonly stripped prior to mining. The age of the salt deposits is considered as Pliocene to Pleistocene. The typical vertical zonation of the Chilean nitrate deposits is shown in the table below. The physical zoning corresponds to the classical nitrate/saline mineral zoning of the Chilean nitrate districts, and its terminology was developed over the decades by nitrate miners.
Vertical Zonation of the Caliche Beds
(after Erickson, 1981, and PAH, 1997b; local terms corrected by AMEC)
| Physical Zoning |
Chemical Zoning |
| Chuca (or Chusca)
| High sulfate and magnesium; moderate potassium; sparse nitrate; minor chloride. Low or
erratic iodine. High insoluble residue (80-90%).
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| Costra |
Upper Zone. High chloride; low nitrate; moderate sulfate, magnesium and potassium.
Strong iodine. Moderate insoluble residue (50-70%).
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| Caliche Layer |
Middle Zone |
| • Caliche |
• Upper subzone: High nitrate and chloride; low sulfate, magnesium and potassium. Strong iodine. Moderate insoluble residue (40-50%). |
| • Congelo |
• Lower subzone: High nitrate; low to moderate chloride; low sulfate, magnesium and potassium. Strong iodine. Moderate insoluble residue (40-50%). |
| Coba |
Lower Zone. Underlying sediments. Low nitrate, sulfate, magnesium and chloride. Iodine
low, erratic and decreases rapidly with depth. High insoluble residue (60-90%). |
Iodine, usually above 300 ppm, has higher concentrations on the southeastern half of the deposit, where the block model shows large zones with iodine grades above 500 ppm, even exceeding 1,000 ppm in some areas. For most of the area the nitrate grades range between 1% and 5%, although grades above 10% are not infrequent along the NW-SE axis of the deposit. Sodium sulfate has a conspicuous distribution, being present in the eastern half of the deposit with grades ranging from 20% to 40% Na2SO4, and sometimes but rarely exceeding these values.
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