Chalcophiles

Indium (0.24 ppm) is similar in abundance to Sb and Cd, whereas T1 (0.7 ppm) is close to Tm and somewhat less abundant than Mo, W and Tb (1.2 ppm). Both elements are chalcophiles (p. 648), indium tending to associate with the similarly sized Zn in its sulfide minerals whilst the larger T1 tends to replace Pb in galena, PbS. Thallium(I) has a similar radius to Rb and so also concentrates with this element in the late magmatic potassium minerals such as feldspars and micas. 

None of the three elements is particularly abundant in the earth s crust though several minerals contain them as major constituents. As can be seen from Table 13.1, arsenic occurs about halfway down the elements in order of abundance, grouped with several others near 2 ppm. Antimony has only one-tenth of this abundance and Bi, down by a further factor of 20 or more, is about as unabundant as several of the commoner platinum metals and gold. In common with all the post-transition-element metals. As, Sb and Bi are chalcophiles, i.e. they occur in association with the chalcogens S, Se and Te rather than as oxides and silicates. 

Figure 15.1 Position of the chalcophilic elements in the periodic table these elements (particularly those in white) tend to occur in nature as sulfide minerals the tendency is much less pronounced for the elements in normal black type.

Zinc (76ppm of the earth s crust) is about as abundant as rubidium (78 ppm) and slightly more abundant than copper (68 ppm). Cadmium (0.16 ppm) is similar to antimony (0.2 ppm) it is twice as abundant as mercury (0.08 ppm), which is itself as abundant as silver (0.08 ppm) and close to selenium (0.05 ppm). These elements are chalcophiles (p. 648) and so, in the reducing atmosphere prevailing when the earth s crust solidified, they separated out in the sulfide phase, and their most important ores are therefore sulfides. Subsequently, as rocks were weathered, zinc was leached out to be precipitated as carbonate, silicate or phosphate. 

Note It is sometimes convenient to describe the element distribution between sulfides and oxides as chalcophiles (occurring in the Earth s crust as sulfides) and lithophiles (predominating as oxides and halides in the Earth s crust) (see Fig. 1.5). This geochemical classification includes also the siderophiles (remaining as metals or alloys, especially in the Earth s core) and the atmophiles (which occurs largely in volatile form in the atmosphere and dissolved in the oceans). 

In the northern Yilgarn and surrounding areas, such as the Baxter deposit) chalcophile elements are enriched in groundwaters contacting with weathering sulfides (e.g., As, Mo, Ag, Sb, W, Tl, and Bi) and these may be more useful regional exploration tools than dissolved Au itself (Fig. 1). 

Occurrence. Copper is a typical chalcophilic element its principal minerals are sulphides, mostly chalcopyrite (CuFeS2), bornite (CusFeSyCujFeSj) and chalcocite... 

Occurrence. Thallium can be associated to heavy metals that occur in sulphidic ores (chalcophilic element behaviour) or to alkali metals in minerals such as car-nallite, sylvite, mica (the Tl+1 ion behaves as an alkali metal ion), or in true, but very rare, thallium minerals such as lorandite (T1AsS2), chalcothallite (Cu3T1S2). 

Occurrence. Selenium is occasionally found as native. Minerals are rare and occur together with the sulphides of chalcophilic metals. Sometimes the minerals are partially oxidized (MSe03 2H20 with M = Ni, Cu, Pb). 

Cu may be reduced to Cu", especially if soft bases such as halides and S are present to stabilize the Cu" " ion. All are chalcophiles and tend to form insoluble sulfides in anaerobic conditions (pKs = 21.3-25.6, 19.4-26.6 and 36.1, respectively). They therefore tend to have low mobilities in submerged soils, especially Cu +, and accumulate. 

Mercury occurs in soils predominantly in the +2 oxidation state. Elemental Hg in the atmosphere is oxidized to Hg + and deposited in rainfall. It is a strong chalcophile and under anaerobic conditions forms the extremely insoluble sulfide cinnabar (HgS, pK = 52.7). Nonetheless it is not entirely immobilized under anaerobic conditions because it is reduced to volatile Hg° or methylated to volatile methyl mercury compounds by microbial action, and so returned to the atmosphere. The methylation is mediated by various bacteria, especially methanogens, through the reactions ... 

Goodfellow, W.D., Jonasson, I.R. Morganti, J.M. 1983. Zonation of chalcophile elements about the Howard s Pass (XY) Zn-Pb deposit, Selwyn Basin, Yukon. Journai of Geochemicai Expioration, 19 503-542. 

We used a Thermo X7 ICPMS coupled to a New Wave Research 213 nm UV laser to determine the PGE and Au in the sulfides. Other chalcophile metals (Ag, As, Bi, Cd, Co, Cu, Fe, Ni, Pb, Re, Sb, Se, Sn, Te and Zn) were also monitored. Analytical conditions were beam size of 80 pm laser pulse rate of 10 Hz laser output power of 0.3 mJ/pulse to ablate the sulfide for 60s after a 20s gas blank was collected. Sulfide standards were used to... 

Barnes, S.J., Prichard, H.P., Cox, R.A., Fisher, P.C., Godel, B. 2008. The location of the chalcophile and siderophile elements in platinum-group elements ore deposits (a textural, microbeam and whole rock geochemical study) Implication for the formation of ore deposits. Chemical Geology, 248, 295-317. 

A sulfidic halo is characterized mostly by elevated S, Au, As and Sb. It extends further than suggested by previous alteration studies, and is defined by the development of disseminated hydrothermal pyrite and, to a lesser extent, arsenopyrite. Gold deposits of the Costerfield stibnite domain (i.e., Fosterville and Costerfield) can be differentiated from Au-As orogenic deposits by a greater primary dispersion of anomalous As and higher threshold values for Sb, as well as by the presence of slightly elevated concentrations of Hg (>0.01 ppm). Other chalcophile elements at variably elevated levels within the sulfidic alteration halo include Mo, Se, Bi, Pb and Cu. 

A cosmochemical periodic table, illustrating the behavior of elements in chondritic meteorites. Cosmic abundances are indicated by symbol sizes. Volatilities of elements reflect the temperatures at which 50°/o of each element would condense into a solid phase from a gas of solar composition. As in Figure 1.2, the chemical affinities of each element, lithophile for silicates and oxides, siderophile for metals, and chalcophile for sulfides, are indicated. Some of the most highly volatile phases may have remained uncondensed in the nebula. Stable, radioactive, and radiogenic isotopes used in cosmochemistry are indicated by bold outlines, as in Figure 1.2. Abundances and 50% condensation temperatures are from tabulations by Lodders and Fegley (1998). 

Distinguish between the following terms lithophile, siderophile, chalcophile, atmophile. 

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