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CHAPTER ONE
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Introduction
Lead has been known since the beginning of history. It has been commonly used for thousands of years because it is wide spread, easy to extract and easy to work with. Lead is a native element, highly malleable and ductile as well as easy to smell.
Lead water pipes found in Pompey show that its present-day use for plumbing was known to the Romans empire in fact, it has been suggested that the fall and decline of the Roman empire may have resulted from lead poisoning of the aristocratic due to intake of it through the use of it as drinking vessels by Roman officials. The Chinese used it for money and debasing coins before 2008 BC.
Ancient silver bed deposits were worked in the Mediterranean countries, India, China, Persia and Arabia. The fumed Larium deposits of Greece were worked in 120BC. Lead was used by the ancients for ornaments, coins, solders, bronze, vases and pipes.
Lead is a native element, its highly malleable and ductile. It is very soft. Dense, durable, malleable. It has a low melting point, long-life span(in milder climates, lead roofs have been known. Generally corrosion-resistant, has little to no reaction most compounds and solutions.
It’s resistant to corrosion by most acids including chromic, sulfuric, sulfurous and phosphoric acids. Its corrosive to alkalis (such as lime mortar, Portland cement and uncured concrete), tannic acid found in wood and radiation. Also corrosive to hydrochloric, hydrofluoric acetic, formic and nitric acids.
Lead is a bright and silvery when freshly cut but the surface rapidly tarnishes in air to produce the more commonly observed dull luster normally associated with lead. It is a metal that has poor electrical conductivity. Lead can be toughened by addition of a small amount of antimony or other metals. All except 204Pb, is the end product of a complex radioactive decay. Lead is also poisonous as are its compounds.
Table 1: Chief Producer Of Lead in Order of Rank
| Lead | Percentage |
| 16 |
| 12 |
| 12 |
| 12 |
| 10 |
| 3 |
| 4 |
| 4 |
CHARACTERISTIC OF LEAD
PHYSICAL PROPERTIES OF LEAD
All minerals posses certain physical properties which are considered insome details. Character depending upon light, such as colour, streak. Lustre etc, characters depending upon certain senses such as those of taste, feel, etc then characters depending upon the atom ic structures and state off aggregation such as form, hardness,cleavega etc.
Lead has light gray to a slight bluish gray colour, hardness of 1.5, streak: light gray and has a shiny streak. Transparency: opaque. Specific gravity: 11.3. lustre:metallic. Cleavage cleavage. Fracture: hackly. Density: 11.4qmcm3. Tenacity:malleable,ductile and sectile
1.1.2 CHEMICAL PROPERTIES OF LEAD
The chemical properties of lead include, melting point: 600.65k. Boiling point: 2013k. Heat of fusion: 4.799kjmol. Heat of vapour: 177.7kjmol. Specific heat: 013jqmk.
ATOMIC PROPERTIES OF LEAD
The atomic properties of lead include, atomic number: 82. Atomic mass: 207.2u. Atomic radius: 1.47A. Covalent radius: 1.81A. Atomic volume: 18.17cm3mol. Stable isotopes:4. Electronegativity: 2.33. Electrical resistivity (20c) 208nohmsm. Magnetic ordering: diamagnetic. Crystal structure: face-centered cubic. Ocidation state: 4,2,-4(amphoteric oxide).
1.2.0 ORE GENESIS PROCESSES
Evans (1993) divided ore genesis into the following main categories based on physical process. These are internal process, hydrothermal process, metamorphic process and surficial process.
1.2.1 INTERNAL PROCESS.
These process are integrated physical phenomena and chemical reaction internal to magmas, generally in plutonic or volcanic rocks. These include:
1. Fractional crystallization:- either creating monominerli cumulate ores or contributing to the environment of ore minerals and metals.
2. Liquefaction or liquid immiscibility:- Between melts of differing composition usually sulfides segregations of nickel-copper-platinoic sulfides and silicates.
1.2.2 HYDROTHERMAL PROCESS
The process are the physicochemical phenomena and reactions caused by movement of hydrothermal waters within the crust after as a consequence of magmatic intrusions or tectonic upheavals. The foundations of hydrothermal process are the source-transport-trap mechanism.
Sources of hydrothermal solutions include seawater and meteoric water circulation through fractured rock, formational brines (water trapped within sediments at deposition) and metamorphic fluids created by dehydration of hydrous minerals during metamorphism.
Metal sources may include a plethora of rocks. However most metals of economic importance are carried as trace elements within rock-for ming minerals and so may be liberated by hydrothermal process. This happens because of:-
1. Incompatibility of the metal with its host mineral for example zinc in calcite, which favors aqueous fluids in contact with the host minerals during digenesis.
2. Solubility of the host minerals within nascent hydrothermal solutions in the source rocks for example minerals salts(halite),carbonates(cerussite),
3. Elevated temperature causing decomposition reactions of minerals.
Transport by hydrothermal solutions usually requires a salt or other soluble species which can form a metal-bearing complex. These metal-bearing complexes facilitates transport of metal within aqueous solutions generally as hydroxides, but also by processes similar to chelation.
The process is especially well understood in gold metallogeny where various thiosulfate, chloride and other gold carrying chemicals complexes(notably tellurium-chloride/sulfate or antimony-chloride/sulfate). The majority of metal depostics formed by hydrothermal process include sulfide minerals, indicating sulfur is an important metal-carrying complex.
Sulfide deposition;
Sulfide deposition within the trap zone occurs when metal-carrying sulfate, or other complexes becomes chemically unstable due to one or more of the following.
1. Falling temperature, which renders the complex unstable or metal insoluble.
2. Loss of pressure, which has the same effect.
3. Reaction with chemical reactive wall rocks usually of reduced oxidation state such as iron bearing rocks, mafic or ultramafic rocks or carbonate rocks.
4. Digenesis of the hydrothermal fluid into a gas and water system or boiling which alter the metal carrying capacity of the solution and even destroys metal-carrying chemical complexes.
Metal can also become precipitated when temperature and pressure or oxidation state favor different ionic complexes in the water, for instance the change from sulfide to sulfate, oxygen fugacity exchanging of metals between sulfide and chloride complexes, et cetera.
1.2.3 METAMORPHIC PROCESSES
Lateral secretion:
One deposits formed by lateral secretion are formed by metamorphic reactions during shearing which liberate mineral constituents such as quartz, sulfides, gold, carbonates and oxides from deforming rocks and focus theses constituents into zones of reduced pressure or dilation such as faults. This may occur without much hydrothermal fluid flow, and this is typical of pod form chromite deposits.
Metamorphic process also control many physical process which form the sources of hydrothermal fluids outlined above.
1.2.4 SURFICIAL PROCESSES
Surficial process are the physical and chemical phenomenon which cause concentration of ore materials within the regolith generally by the action of the environment. This involves placer deposits laterite deposits and residual or eluvial deposits. The physical process of ore deposits formation in the surficial realm include.
1. Erosion
2. Deposition by sedimentary process, including winnowing, density separation (e.g gold placers)
3. Weathering via oxidation or chemical attack of a rock, either liberating rock fragments or creating chemically deposited clays, laterite or manto ore deposits
4. Deposition in low-energy environment in beach environments.
1.3.0 CLASSIFICATION OF ORE DEPOSITS
Ore deposits are usually classified by ore formation processes and geological setting. For example, SEDEX deposits, literally meaning “sedimentary exhalative” ore a class of ore deposit formed on the seafloor(sedimentary) by exhalation of brines into seawater(exhalative)causing chemical precipitation of ore minerals when the brine cools mixes with seawater and loses it metal carrying capacity.
Ore deposits rarely fit snugly into the boxes in which geologists which to place them. Many be formed by one or more of the basic genesis processes above,creating ambiguous classification and much argument and conjecture. Often ore deposits are classified after examples of other types. For instance broken Hill type lead-zinc silver deposits or carlin type cold deposits.
Classification of hydrothermal ore deposits is also achieved by classifying according to the temperature of formation which roughly also correlate with particular mineralizing fluids, mineral association and structural styles. This scheme proposed by Waldemar Lindgren (1933) classified hydrothermall deposits sat hypothermal, mesothermal, epithermal and telethermal deposits.
The modern classification scheme of today also briefly discussed the following will be outlined.
1. The depth-temperature of formation scheme which was proposed by lindgren; and
2. The environment-rock association scheme as described by Stanton (1972)and later workers.
Depth-temperature of formation scheme:
This is divided into three major groups and two more minor one and these are now dealt with in turn.
1.3.1 HYPOTHERMAL DEPOSITS
Depth of formation is between 3000-15000m, temperature of 3000-600c)
These are commonly associated with acid plutonic rocks in old precambrian terrains and often associated with faulting. Replacement ore bodies are most common and often broadly tablular in shape but veins also occur. The elements found are Au, Sn, Mo, W, Cu, Ph, Zn and As and the important ore minerals are magnetite, pyrrhotite, cassiterite, arsenopyrite, wolframite and scheelite(with specularite, molybdenitebornite,chalcopyrite,pyrite, galena and gold also found). Common gangue minerals include garnet, biotite, muscovite, topaz, tourmaline,(high-temperature)quartz and Fe-chlorite (but plagioclaise, epidote and carbonates also occur). The wall-rocks may suffer albitization tourmalinization,perharps sericitizaiton in feldspar if present and chloritization and with development of rutile change with depth are gradual and the ore bodies are usually coarse-grained(Lindgren1933).
1.3.2 MESOTHERMAL DEPOSITS
Depth of formation is 1200-4500m and temperature is200-300c. these are associated with intrusive igneous rocks,regional fracturing and faulting. Ore bodies vary from massive to disseminated and include stock works pipes and tubular bodies. The element occurring include Au,Ag,Cu,As,Pb,Zn,Ni,Cu,N,Mo,U and others. The important ore minerals being.
Gold,chalcopyrite,bornite,sphalerite,galena,enargite,chalcocite,bournonite,pitchblende,niccolite,cobaltite,carbonate and siderite(and with pyrite, argentite and sulphosalt also found. The main gangue minerals include quartz, sericite, chlorite, carbonate and siderite (the high-temperature minerals found in the hypothermal deposits-garnet, tourmaline, etc-are not present), and with albite, epidote and smectite clays also present. The wall-rock suffer chloritizaton, seritization or carbonitization minerals changes gradually with depth, and the ore bodies tend to be variable in rain size.
1.3.3 EPITHERMAL DEPOSITS
Depth of formation near surface to 1500m, and temperature is 50-200oc. These are found in paliozoic or younger sedimentary or igneous rock, sassociated with high level intrusive/sxtrusive developed, and also in pipes and stock works. Element present include Pb,Zn,Au,Ag,Hg,Sb,Cu,Se,Bi, and U’ the important ore mineral being Ag-rich gold, copper, marcasite, sphalerite, galena, Cinnabar, stibnite, realgar, orpiment, ruby, silvers, argentite and selenides (and with silver, pyrite, chalcopyrite and the tellurides also found.
The main gangue minerals include chalcedonic silica, dickite clays and zeolites (and with quartz,fe-poor chlorite, epidote group minerals, carbonates, fluorite and barite also occuring). Wall rock alteration is rare, carbonates, fluorite and barite also occurring). Wall rock alteration is rare, mineralization varies quickly with depth and the grain size is very variable, with vugs and veins brecciating present.
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