Magnetic mineralogy in the Chuquicamata mine, Northern Chile: Application to paleomagnetic data for validation of structural interpretation in hydrothermal systems

Rock paleomagnetic properties are directly related to their ferromagnetic mineral contents and types upon acquired magnetization . Characteristic remanent magnetizations carried ferromagnetic minerais crystallized during a primary rock forming process are determined by a detailed study of paleomagnetic properties and easily interpreted assuming these mineraIs formed in a single event. In the case of hydrothermally altered rocks the situation is more complex as ferromagnetic minerals are susceptible to change, destruction or neo-crystallization. In porphyry copper system s the complexities associated with multistage events , both intrusive and hydrothermal, imply a need for detailed mineralogical studies to help understand the nature and paragenesis of ferromagnetic minerais with regard to primary magmatic and overprinted hydrothermal events. Results of a paleomagnetic ­ mineralogical study of the Chuquicamata porphyry copper deposit, northern Chi le, are here presented . The objectives of this study are to relate paleomagnetic properties of hydrothermally altered rocks with both primary magmatic and hydrothermal mineralogy, and to determine the effects of different types of alteration on the original paleomagnetic properties , be it destructive or constr uctive. GeoIogical Setting The Chuquicamata deposit is hosted within thre e porphyries which, from early intrusive phases to postdating supergene alteration, show evidence of spatial and genetic relationship with the evolution of the West Fault System . The oldest is the East porphyry (35-34 Ma), host rock to the largest proportion of copper mineralization. The West and Banco porphyries are younger (34-3 1 Ma) and show the largest proportion of alteration (Ballard , 2001 ). The early post-magmatic alteration is represented by selective potassic alteration , with partial K-feldspar and albite replacement of plagioclase and biotite replacement of hornblende. A propilitic association (chlorite­ epidote) is superimposed on the biotitic alteration. A later fine-grained quarz-K-feldspar alteration obliterates biotite and is accompanied with cataclastic deformation. Quarz-molibdenite veins eut all previous alterations. The principal stage of hydrothermal alteration is related with phyllie alteration (quarz-sericite), overprinted on previous alteration associations , with evidence of partial remobilization of early mineralization. Late stages of hydrothermal alteration are represented by sulfide veins (enargite-pyrite) . Copper mineralization in this deposit is mostly present within stockwork veining. related with repetitive fracturing along an active fault system, and disseminated (Ossandon et al., 2001). Later superimposed supergene alteration generated an important zone of secondary enrichment mineralization (chalcosite-covellite) and extensive surface zones of leaching. The Fortuna