应用地球化学10年地化课件复习幻灯片.pptVIP

13.1 Dispersion train（分散流） 13.1.1 Concept of dispersion train Orebodies, primary halos and secondary halos may be eroded and dissolved by surface water and underground water, as a result, part of the elements are moved into river and re-precipitated in the bed sediments of rivers and streams under special conditions. The area with increased contents of some elements is called dispersion train. 13.1.2 Features of dispersion train 1. Indicative elements: same as those of primary and secondary halos (1) Contents of indicative elements: usually lower than those of primary and secondary halos but with smaller content range of variation. E.g., Cmax/Cmin ratio of primary halo may be up to n×10, whereas Cmax/Cmin ratio=1-2 in dispersion train. (2) Relation of contents to grain size of river-sediments: e.g., Cu, Zn, Co, Ni are enriched in muds, while Cr, W, Sn, Nb, Ta in coarser sands. (3) Relation of contents to sampling depth: usually higher toward the surface of sediments. (4) Variation of contents with locations of rivers (Fig. 13-1): ? 1——Steams；2——ore body；3——contents of thr elements Fig.13-1 Variation of element content in stream 2. Length of dispersion train The length of dispersion train near metallic deposits is mainly 1-2 km, with the shortest 300-500 m and longest up to tens of km. The length of dispersion train formed from solution transport is usually longer that that from from mechanic transport. 13.1.3 Controlling factors 1. Nature of primary minerals: (1) Minerals with high resistance to weathering are transported mainly in mechanic particles, thus elements contained in them tend to be enriched in fine sands, such as W, Sn, Be, Na, Ta, Au, Pt, Cr, Hg etc. (2) Minerals with weak resistance to weathering are easily dissolved and thus elements are transported in the form of soluble ions, complexes and colloids and tend to be enriched in muds, such as Cu, Zn, Co, Ni, Mo. 2. Other factors: Including physico-chemical conditions of media (composition, pH,