Effects of hydrothermal alteration on the magnetic properties of rocks : case studies for stratabound and porphyry copper deposits of chile

In nature, the great mayority of metallic ore deposits are of hydrothermal origin, hence they present a mayor or minor degree of associated hydrothermal alteration. Alteration is metasomatic and affects the mineralogical and chemical composition of the host rock, be it by replacement of host rock minerals or by crystallization of hydrothermal minerals. Many types of hydrothermal alteration associations exist for many types of metallic ore deposits, depending upon factors such as composition and thermodynamic characteristics of hydrothermal fluids, lithology and permeability of host rock, temperature and depth of emplacement, among others. These processes of hydrothermal alteration, understood in terms of partial to total metasomatism, affect in a mayor to minor degree the "ferromagnetic" (sensus latus) minerals and/or generate "ferromagnetic" minerals, in such case that the magnetic properties of the rock are affected. This concept is dealt in exploration by use of magnetic surveys at a practical level: the effects of hydrothermal alteration, being of local scale, must present contrast of magnetic properties (susceptibility and remanent magnetization) with respect to the hosting country rock. This being true, does not take into consideration the magnetic properties of rocks at a mineral scale, observeable constrast of magnetic surveys much dependant of scale and detail of data (ie. aeromagnetic survey vs. ground survey). A study of the particular magnetic properties associated with different types of alteration was done for two stratabound copper deposits (Carolina de Michilla and Mantos Blancos, northern Chile) and for the giant porphyry copper deposits Chuquicamata (northern Chile) and El Teniente (central Chile). The objetives were to determine the effects of hydrothermal alteration on "ferromagnetic" minerals. Preliminary results of these effects and relationships are presented, with a brief discussion on the implications of magnetic property contrast due to locallized changes of magnetic susceptibility and remanence, in terms of exploration magnetic surveys and scale at which contrast can actually be detected.