Palaeomagnetism applied to magnetic anomaly interpretation: a new twist to the search for mineralisation in northern Chile

The Chilean Iron Belt is an important location for Fe and Cu(Fe)-Au deposits and includes the recently developed Candelaria deposit, which is located some 20 km south of the city of Copiapó in northern Chile. This mine is now a major Cu producer in Chile but its discovery in the late 1980s was relatively fortuitous. The exploration programme included a ground magnetic survey from which lessons can be learnt in the search for further such deposits. Palaeomagnetic studies throughout the northern part of the Chilean Iron Belt indicate major crustal rotation of the region, probably, related to oblique convergence and transpression at the Andean Margin. The application of palaeomagnetic techniques to a magnetite-apatite deposit, Mina Fresia, indicates that magnetite-rich ores in this area are capable of maintaining a significant remanence component which will contribute to their magnetic anomaly. As with the volcanics, intrusives and sediments of the region this, remanence is clearly rotated clockwise. Using 2.5D and 3D magnetic modelling, it is demonstrated that the magnetic anomaly associated with the Candelaria deposit is also dominated by a remanence component which is significantly rotated but of reversed polarity. Recognition of clockwise-rotated, remanence-dominated anomalies should provide a new key to the search for deposits in this part of Chile and elsewhere. An example of an unexplored anomaly showing this key feature of a clockwise-rotated remanence component dominating over the induced component is presented. Modelling of this anomaly indicates a pair of sources which are either ten times as big or ten times as magnetic as the Candelaria deposit. It is suggested that the low cost of palaeomagnetic study of an exploration target should be a prerequisite to magnetic anomaly interpretation of targets in tectonic areas where vertical axis crustal rotations may be a significant element of the overall deformation pattern. Received: 9 April 1999 / Accepted: 5 October 1999