Introduction to Ore-Forming Processes. Laurence Robb

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СКАЧАТЬ as raw materials for the manufacture of vehicles and in construction, whereas others (the rare earth elements, for example) are needed in very much smaller amounts for use in the alloys and electronics industries. Only two elements appear at the present time to have little or no commercial use at all (Figure 4). These are francium (Fr, atomic number 87), and protactinium (Pa, atomic number 91). Francium is radioactive and so short‐lived that only some 20–30 g exists in the entire Earth's crust at any one time! Astatine (At, atomic number 85) is another very unstable element that exists in vanishingly small amounts in the crust as a decay chain by‐product or is manufactured synthetically. Astatine has been manufactured in particle accelerators and is occasionally used in various nuclear medical applications.

The useful elements can be subdivided in a number of different ways. Most of the elements can be classified as metals (Figure 4), with a smaller fraction being non‐metals. The elements B, Si, As, Se, Te, and At have intermediate properties and are referred to as metalloids. Another classification of elements, attributed to the pioneering geochemist Goldschmidt, is based on their rock associations and forms the basis for distinguishing between lithophile (associated with silicates and concentrated in the crust), chalcophile (associated with sulfides), siderophile (occur as the native metal and concentrated in the core), and atmophile (occur as gases in the atmosphere) elements. It is also useful to consider elements in terms of their ore mineral associations, with some preferentially occurring as sulfides and others as oxides (see Figure 4). Some elements have properties that enable them to be classified in more than one way – iron is a good example, in that it occurs readily as both an oxide and sulfide.

      Common Ore and Gangue Minerals

      It is estimated that there are about 3800 known minerals that have been identified and classified (Battey and Pring 1997). Only a very small proportion of these make up the bulk of the rocks of the Earth's crust, as the common rock forming minerals. Likewise, a relatively small number of minerals make up most of the economically viable ore deposits of the world. The following compilation is a breakdown of the more common ore minerals in terms of chemical classes based essentially on the anionic part of the mineral formula. Also included are some of the more common “gangue” minerals, which are those that form part of the ore body, but do not contribute to the economically extractable part of the deposit. Most of these are alteration assemblages formed during hydrothermal processes. The compilation, including ideal chemical formulae, is subdivided into six sections, namely native elements, halides, sulfides and sulfosalts, oxides and hydroxides, oxysalts (such as carbonates, phosphates, tungstates, sulfates), and silicates. More detailed descriptions of both ore and gangue minerals can be found in a variety of mineralogical texts, such as Deer et al. (1982), Berry et al. (1983), Battey and Pring (1997), and Wenk and Bulakh (2017). More information on ore mineral textures and occurrences can be found in Craig and Vaughan (1994) and Ixer (1990).

      Native Elements

      Both metals and non‐metals exist in nature in the native form, where essentially only one element exists in the structure. Metals occurring in the native form include copper, silver, gold, and platinum which are all characterized by cubic close packing of atoms, high densities, and are malleable and soft. The carbon atoms in diamond are linked in tetrahedral groups forming well cleaved, very hard, translucent crystals. Sulfur also occurs as rings of eight atoms and forms bipyramids or is amorphous.

      Metals

      1 Gold – Au

      2 Silver – Ag

      3 Platinum – Pt

      4 Palladium – Pd

      5 Copper – Cu

      Non‐metals

      1 Sulfur – S

      2 Diamond – C

      3 Graphite – C

      Halides

      The halide mineral group comprises compounds made up by ionic bonding. Minerals such as halite and sylvite are cubic, have simple chemical formulae, and are highly soluble in water. Halides sometimes form as ore minerals, such as chlorargyrite and atacamite.

      1 Halite – NaCl

      2 Sylvite – KCl

      3 Chlorargyrite – AgCl

      4 Fluorite – CaF2

      5 Atacamite – Cu2Cl(OH)3

      Sulfides and Sulfosalts

This is a large group of minerals in which bonding is both ionic and covalent in character. The sulfide group has the general formula A_MX_P, where X is typically S but can be As, Sb, Te, Bi, or Se, and A is one or more of the metals. The sulfosalts, which are less common than sulfides, have the general formula A_MB_NX_P, where A is usually Ag, Cu, or Pb, B is commonly As, Sb, or Bi, and X is S. The sulfide and sulfosalt minerals are generally opaque, dense, and have a metallic to sub‐metallic luster.

      Sulfides

      1 Chalcocite – Cu2S

      2 Bornite – Cu5FeS4

      3 Galena – PbS

      4 Sphalerite – ZnS

      5 Chalcopyrite – CuFeS2

      6 Pyrrhotite – Fe1–xS

      7 Pentlandite – (Fe,Ni)9S8

      8 Millerite – NiS

      9 Covellite – CuS

      10 Cinnabar – HgS

      11 Skutterudite – (Co,Ni)As3

      12 Sperrylite – PtAs2

      13 Braggite/cooperite – (Pt,Pd,Ni)S

      14 Moncheite – (Pt,Pd)(Te,Bi)2

      15 Laurite – RuS2

      16 Cobaltite – CoAsS

      17 Gersdorffite – NiAsS

      18 Loellingite – FeAs2

      19 Arsenopyrite – FeAsS

      20 Molybdenite – MoS2

      21 Realgar – AsS

      22 Orpiment – As2S3

      23 Stibnite – Sb2S3

      24 Bismuthinite – Bi2S3

      25 Argentite – Ag2S

      26 Calaverite – AuTe2

      27 Pyrite – FeS2

      Sulfosalts

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