The model of the primary halo that was created in the 1930s has not fundamentally changed. The primary halo of an ore deposit (ore body) is considered an environment of enrichment of ore-forming and associated elements as a result of ore genesis. However, extensive multi-element geochemical data show that the primary halo, like the positive anomaly, is only part of a complexly differentiated geochemical system. The system comprises the primary haloes of enrichment (positive anomalies) and haloes of depletion (negative anomalies) of ore-forming and other elements. However, negative anomalies have not been generally recognized for various reasons: they are combined with the background, taken to be background, or seen to be a random phenomenon, since they are not featured in the existing conceptual model of the primary halo.
The structure of geochemical systems is characterized by polar zoning, a fundamental feature of a system. Polar zoning can be defined as two separate areas (zones) within the system that are distinguished by the concentration or the composition of element or group of elements. The positive anomaly of deposit elements is the nucleus of the system. Polar zoning of the ferrous group (e.g. Sc, Ti, Cr) of elements is formed within the nucleus of the system: negative anomalies of this group of elements spatially coincide within mineralization and positive anomalies are recorded on the margin of the positive anomaly of ore-forming elements of system. The size of the geochemical ore deposit system varies, with a range between several and a few hundred square kilometers. The ore bodies are located in the nucleus of the geochemical system of the deposit.
The main criteria for the evaluation of a favorable area are based upon the linear dependence between the dimensions of the halo of depletion of the ore-forming elements and the metal reserves in the deposits. Larger ore bodies are found in systems with larger haloes of depletion. The linear dependency indicates a significant role of country rock as a source of metal.
- © 2003 AAG/The Geological Society of London