Geology of the Great Basin

13. Mineral Resources

13 mineral resources Wherever you travel in the Great Basin there is evidence of former or present mining activity. In some areas the land is so pockmarked with holes and scrapings that it has the appearance of having been the home of gigantic gophers . Some of the largest man-made pits on earth have been dug out of the Great Basin rocks in the search for minerals. Much of the human history of the region is traced through the discovery of a glint of silver on this ledge or a streak of gold through that vein. origin of mineral deposits Many mineral deposits are derived from magmas. Minerals have different temperatures at which they crystallize out of solution. As the magma cools, those minerals that crystallize at the highest temperatures form first. Thus, metallic oxides, chlorides, sulfides, and native metals often separate from the rock-forming minerals in the early phases of crystallization. Sometimes these early magmatic ore crystals form while the rock minerals are still liquid, and they settle to the bottom of the magma chamber as a segregated ore deposit. Other ore minerals crystallize at temperatures well below those necessary for the rock minerals to form. These late magmatic ore minerals remain molten after the rock minerals have crystallized and turned into rock. The late crystallizing ores may either solidify at the site of cooling or be injected into the surrounding rocks. The remaining fluids in the magma are concentrated as crystals form. Such brine fluids may contain carbon dioxide (CO2 ), hydrogen sulfide (H2 S), 166 Mineral Resources hydrochloric acid (HCl), or sulfur dioxide (SO2 ). These fluids often infiltrate and hydrothermally alter the surrounding host intrusive rock. If later solutions rich in metal chlorides encounter the hydrogen sulfide in hydrothermally altered rocks, the metal sulfides may be deposited. Such deposits are sometimes rich enough to be ore deposits. Thus are created syngenetic ore deposits, or deposits formed at the same time as the enclosing rock. Just before the complete crystallizing of the magmas, high vapor pressure fluids may be concentrated to such an extent that they shatter the area above the intrusive. This creates fractures into which the residual solutions may be drawn by the regions of lower pressure. These solutions, if heavily mineralized , may form epigenetic mineral deposits, or deposits formed later than the enclosing rock. Hydrothermal solutions that escape from the intrusive may be changed by contact with the surrounding rocks and become alkaline solutions. Or, if the temperature remains high enough so that the HCl remains intact, the solution may remain mildly acidic. The mineral substances in these solutions may replace the rocks through which they travel, thus creating replacement deposits. There are other possible methods for the concentration of elements into sufficiently localized deposits to be mined. Sometimes bacteria are the cause Evidence of mining activities is found throughout the Great Basin. Virtually every range has been scoured by prospectors. John Running. Mineral Resources 167 of the precipitation of metal sulfides. In other instances, submarine volcanic exhalations may concentrate ore. Such submarine deposits have been documented occurring today in the depths below the Red Sea. Frequently, these primary deposits are not sufficiently concentrated in the precious minerals to be minable. Sometimes nature helps to concentrate these otherwise uneconomic deposits. A secondary process, such as weathering, may free the ore minerals to be transported either in solution or mechanically. The erosion process sometimes sorts the minerals on the basis of their density and durability. Such deposits, called stream placers, usually form in drainages below the weathered ore deposits. In other cases, the weathering and solution remove the uneconomic minerals and concentrate the ore in the residual soil. Desert gold placers commonly form in this manner as residual deposits. Another secondary process, oxidation, may alter the nature of surface ore deposits. Minerals exposed to oxygen at the land surface or above the water table are often dissolved and carried downward below the water table. Here the different chemical environment can cause these oxidized minerals to precipitate or replace existing minerals. This process is called supergene enrichment, and the resulting deposits are often exceptionally rich. The many mineral deposits in the Great Basin result from a variety of geologic processes. The rich and varied geologic history of the region has created metallic and nonmetallic deposits of both great abundance and variety. age distribution of ore deposits The age of hydrothermal ore deposits can be determined...