Natural lead in the terrestrial environment

Lead is generally associated with massive sulphide deposits; it is a 'chal-cophile' element similar to copper, cadmium, zinc and silver. It occurs in the crystal structures of rock forming silicates (e.g. K-feldspar) and oxides of common crustal rocks [Klein and Hurlbut (1999); Krauskopf and Bird (1995)]. With respect to the mineralogical forms, the primary mineral of lead in nature is galena (PbS, Fig. 2) and its oxidation products like plat-tnerite (PbO2), cerussite (PbCO3), and anglesite (PbSO4) are important.

Most lead is found in the lithosphere (soils and sediments: ~ 5 x 1019 g), followed by the hydrosphere 1016 g) and biosphere 1012 g). The main

Fig. 2 An example of coarse, cubic galena (PbS), the principal ore mineral of lead, intergrown with pyrrhotite (iron sulphide). Sample taken from Black Mountain Pb-Zn-(Cu-Ag) mine in the northern Cape Province, South Africa.

natural source of lead in sediments and soils is dust from rock weathering. Weathering of igneous and magmatic rocks results in accumulation of lead particularly in the clay fraction and in ferruginous components. A review of the basic geochemical processes and mechanisms affecting the chemistry of lead in soils (in particular with respect to bioavailability and solubility) is given elsewhere [Hettiarachchi and Pierzynski (2004)].

Important insights with respect to the isotope systematics of lead in soils during weathering were gained from field studies [Hansmann and Koeppel (2000); Teutsch et al. (2001)] and dissolution experiments of fresh granite and soil samples [Harlavan and Erel (2002)]. The latter suggested (see Fig. 3) that during the early stages, lead is preferentially released from accessory phases (i.e. allanite, sphene, apatite) which results in higher 206Pb/207Pb values and different REE patterns in solution compared to rock values [Harlavan and Erel (2002)]. A very recent intriguing study used the lead isotopic composition of river sediments from the earth's major river basins, from old cratonic to young orogenic areas and from subarctic to tropical climates, to estimate the lead isotopic composition of the average upper crust, the source of 'natural' soil dust. The calculations based on flux weighted averages of particulate lead gave values of 19.07, 15.74 and 39.35 for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb [Millot et al. (2004)].

Fig. 3 206Pb/207Pb versus leaching time (in hrs) of acid leach fractions of a granite (sample Kec/TF-418) along with the average ratios of the total digest (TD) and isotopic trends for monazite, sphene, and apatite (taken from [Harlavan and Erel (2002)].

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