REGIONAL GEOCHEMICAL SECONDARY NEGATIVE ANOMALIES AND THEIR SIGNIFICANCE

Research involving secondary negative anomalies and their application in regional stream sediment surveys has been scarce. Conventionally, n egative anomalous threshold values have been calculated in the same wa y as positive anomalous threshold values. But the conventional methods all have drawbacks which hinder their application. In this paper, we have chosen to delineate negative (and/or positive) anomalies using co ntrast values in order to overcome the drawbacks. Regional stream sedi ment surveys at a scale of 1:200 000 have been carried out in western Jungger, Shanxi, Kunlun, northwestern Jiangxi, and other areas in Chin a. The geochemical data were processed using System RESMA. On the basi s of the distribution of negative and positive single-element anomalie s, three possible arrangements may occur: (1) negative anomalies accom panied by positive anomalies; (2) only positive anomalies occurring wi th no negative anomaly nearby; (3) only negative anomalies, with no po sitive anomaly nearby. These situations reflect different geological s ettings and different mineral forming processes. Basically, two differ ent distribution patterns of regional negative anomalies in relation t o the backgrounds - low background (LB) and high background (HE) - may be observed in different geological environments: (1) regional negati ve anomalies are distributed only around the positive anomalies in the LB area; (2) regional negative anomalies can exist on the periphery o f positive anomalies in both LB and HB areas. Two kinds of patterns fo r regional multi-element negative and positive anomalies reflecting di fferent geological processes have been noted: (1) coincident positive anomalies for one group of elements and negative anomalies for another group of associated elements can be used to uniquely define ore depos ition; (2) regional positive multi-element anomalies of some elements (including ore and associated elements) occurring over a deposit are a ccompanied by negative anomalies on the periphery of the deposits. Two regional models of negative and positive anomalies are established fo r Au and Cu deposits. Integration of multi-element positive and negati ve composited geochemical anomalies are much more useful than positive anomalies or positive composited anomalies to delineate regional stru ctures.