![]() ![]() In addition, lead and zinc of this type are also found in large vein deposits at Keno Hill in the Yukon, Coeur d' Alene in Idaho, Hidalgo del Parral in Mexico, and Casapalca in Peru, all of which have important amounts of co-product silver (Beaudoin and Sangster, 1992). They are found in rocks as old as 3.5 billion years and are still forming today on the seafloor in several parts of the world ( Sangster et al., 2000). The most important deposits of this type are found in the Kuroko district of Japan ( Kesler, 1994). Sulfide precipitation takes place on the seafloor in response to abrupt changes in temperature, pressure, and pH when the debouching fluids are brought into contact with seawater. The hot buoyant metal-bearing fluids then find their way back to the seafloor along sub-sea fractures. Although several types of VMS deposits have been recognized according to their geological environment, all are considered to be developmentally related to the volcanic rocks in which they are found, with metals being leached from the surrounding sub-seafloor rocks by circulating seawater. These deposits are among the most abundant base metal deposits on earth, and lead and zinc deposits of this type are found largely in continents and island arcs. Volcanogenic massive sulfide (VMS) deposits are also related to seawater hydrothermal systems. The largest SEDEX deposits, including Broken Hill, Mt Isa, and McArthur River in Australia, and Sullivan in British Columbia, are in a system of 1.8 to 1.3 Ga-old rifts that cut the early continents ( Young, 1992). The SEDEX are recognized largely by their spectacular internal layering SEDEX deposits are almost invariably bimetallic Pb and Zn the Ag content may range from essentially zero in some deposits to several hundred grams per ton in others ( Sangster et al., 2000). In many cases, these solutions did not actually reach the sea floor and instead replaced lenses of sediment just below the water surface, usually shortly after the sediments were deposited ( Kesler, 1994 Sagster et al., 2000). The SEDEX were precipitated by submarine hot springs that flowed into basins filled with fine-grained, clastic sediments. Sedimentary exhalative (SEDEX) deposits consist of layers of lead-zinc-iron sulfides which, as with MVT deposits, are the products of sedimentary processes and are found within large ancient sedimentary basins. Fluid inclusion studies show that the hydrothermal fluid had salinities of 15% or more and temperatures of 90 to 180☌ ( Kesler, 1994). These deposits originally formed due to the tectonically related expulsion of high salinity metal sulfide-rich hydrothermal fluids from sedimentary basins into surrounding carbonate platforms ( Kesler, 1994 Sangster et al., 2000). The MVT deposits consist of galena, sphalerite, barite, fluorite, and other minerals that fill secondary porosity in limestone and dolomite. One of the most prominent MVT lead deposit is located in the tri-state district (Oklahoma–Kansas–Missouri) near the city of Joplin, Missouri as a result, a particular type of MVT deposits with distinct lead isotope signatures are also known as the J-type deposits. ![]() The MVT deposits are so named because they were first found in the midcontinent United States in the valley of the Mississippi River. Basinal hydrothermal systems formed the most important lead deposits, including Mississippi Valley-type (MVT) and sedimentary exhalative (SEDEX) deposits. They are found almost entirely in hydrothermal deposits which formed in three major geologic environments ( Sangster, 1990 Kirkham et al., 1993 Kesler, 1994 Eckstrand, 1995 Sangster et al., 2000). Lead is most commonly obtained from the ore mineral galena and occurs in several distinctly different types of base metal ore bodies. ![]() % Pb), sphalerite (ZnS), chalcopyrite (CuFeS 2), pyrite (FeS 2), and pyrrhotite (FeS). Metals are precipitated as the sulfide minerals galena (PbS 87 wt. Lead compounds are formed when the metals are precipitated from the ore fluids by various processes depending on specific local conditions the most common processes are cooling, mixing with other fluids, and pH change. Lead and zinc are often found together in ore deposits however, less so with copper and iron. Lead deposits are members of a class of ore deposits known as the “hydrothermal” deposits, in which metals were initially leached by hot highly saline aqueous fluids in the subsurface of the Earth's crust and transported to the site of deposition. Millette, in Environmental Forensics, 1964 4.3.1 Type of Lead Ore Deposits ![]()
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