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

The Chilean Iron Belt is an important location for Fe and Cu(Fe)-Au deposit s and includes the recently developed Candelaria deposit, which is located some 20 km south of the city of Copiapo in northern Chile. This mine is now a major Cu producer in Chile but its discovery in the late 1980s was relat ively fortuitous. The exploration programme included a ground magnetic surv ey from which lessons can be learnt in the search for further such deposits . Palaeomagnetic studies throughout the northern part of the Chilean Iron B elt indicate major crustal rotation of the region, probably, related to obl ique convergence and transpression at the Andean Margin. The application of palaeomagnetic techniques to a magnetite-apatite deposit, Mina Fresia, ind icates that magnetite-rich ores in this area are capable of maintaining a s ignificant remanence component which will contribute to their magnetic anom aly. As with the volcanics, intrusives and sediments of the region this, re manence is clearly rotated clockwise. Using 2.5D and 3D magnetic modelling, it is demonstrated that the magnetic anomaly associated with the Candelari a deposit is also dominated by a remanence component which is significantly rotated but of reversed polarity. Recognition of clockwise-rotated, remane nce-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 s howing this key feature of a clockwise-rotated remanence component dominati ng over the induced component is presented. Modelling of this anomaly indic ates a pair of sources which are either ten times as big or ten times as ma gnetic as the Candelaria deposit. It is suggested that the low cost of pala eomagnetic study of an exploration target should be a prerequisite to magne tic anomaly interpretation of targets in tectonic areas where vertical axis crustal rotations may be a significant element of the overall deformation pattern.