Use and implications of paleoweathering surfaces in mineral exploration inAustralia

Over much of Australia, considerable problems for bedrock exploration are c aused by a cover of weathered rocks, sediments and soils produced by long a nd intense periods of weathering. The ore deposits, like the bedrock sequen ces that host them, are highly weathered and their appearance, mineralogy a nd chemistry are radically altered. The same weathering processes, however, can result in geochemical dispersion patterns that, although weak, can for m characteristic target geochemical anomalies. Characteristic dispersion ha loes that relate to paleoweathering surfaces and surround different types o f ore deposits are presented. Gold deposits almost invariably show a disper sion halo of An, as well as some combinations of associate elements Cu, Pb, As, Sb, Bi, In, Mo and W. The dispersion haloes for base metal deposits co mmonly contain combinations of the previous elements with Ni, Zn, Co or Cr. For rare metal deposits, Sn, Ta, Nb, Li and rare earth elements can be dis tinctive geochemical fingerprints, in some cases with As, Sb and Bi. Diamon d exploration can use Cr, Ni, Nb, As and rare earth elements in the search for kimberlitic host rocks. Three conceptual models are presented for defin ing geochemical exploration targets for concealed ore deposits in distinctl y different terrains: (1) exposed lateritic weathering surfaces; (2) buried lateritic surfaces; and (3) beneath sedimentary basins. In the case of (1) , the preferred sampling media for geochemical analysis at the stages of re connaissance through to delineation of drill targets are: (most preferred) lateritic pisoliths or nodules, lateritic duricrust, ferruginous saprolite, ferruginous mottles, soil overlying any of the previous categories and iro n segregations within upper saprolite (least preferred). Lag can also be sa mpled but because the characteristics of lag vary with the immediate substr ate, for effective interpretation, lag sample sites need to be directly rel ated to mapped regolith-landform features. For (2), regolith stratigraphy s hould first be established by scout drilling before systematic sampling is carried out, concentrating on: (most preferred) pisolitic or nodular lateri tic residuum, lateritic duricrust, basal lateritic colluvium, ferruginous s aprolite, ferruginous mottles in upper saprolite and iron segregations in u pper saprolite (least preferred). In the case of (3) where a conceptual mod el is proposed for a partly weathered base-metal-gold deposit, geochemical sampling is directed at the buried weathering surface, hydromorphic anomali es in weathered bedrock, dispersion anomalies at the unconformity between b asin sediment and weathered bedrock, as well as Leakage geochemical haloes and iron segregation anomalies in the cover sequence. (C) 2000 Elsevier Sci ence B.V. All rights reserved.