UNIT-WISE AD,USTMENT OF GEOCHEMICALBACKGROUND DATA AND ITS SIGNIFICANCEIN GEOCHEMICAL ANOMALY DELINEATION
Zbou Di(South China Sea Institute of Oceenology,Academia Sinica)
: Conventional analyses of exploration geochemical data assume that the bank- ground is a constant or slowly changing value, equivalent to a plane or smoothly curved surface. Nevertheless, It is better to regard the geochemical background as a rugged surface, varying with changes in geology and environment. This regged surface can be estimated from observed geological, geochemical, and environmental properties by using robust multivariate statistics. A metbod of background adjustment was developed and applied to ground- water and stream sediment reconnaissance data collected from the Black Hills area, South Dakota, U. S. A. The Iithology of source rocks seems to be a dominant factor determining chemical composition of groundwater or stream sadiments. The most efficacious adjustment procedure is to regress uranium comcentration on sele- cted geochemical and environmental variables for each lithologic unit, and then to delineate anomalies of the combined residuals. This is called the "methed of unit-wise adjustment of geochemical background". Robust versions of regression and RQ-mode principal components analyds rather than ordinary techniques were used to guard against distortion caused by outliers. Anomalies delineated by this background adjustment procedure correspond with uranium prospects much better than anomalies delineated by conventional procedures do. The procedure should be applicable to geochemical exploration for other metals at different scales, and the adoption of this procedure may result in a revolution in geochemical data analyses.