Application of Local Earthquake Tomography to Greenfield Discovery of Subduction-Related Mining Deposits

Abstract:

Mining exploration is oriented towards the discovery of new mineral deposits (greenfield) or the expansion of previously discovered deposits (brownfield). Greenfield discovery involves large-scale exploration and usually begins blindly, with little or no evidence of the existence of an ore deposit. Historically, most greenfield discoveries were made from exposed deposits. However, exposed deposits have by now been mostly discovered, so current searches focus on buried deposits. Identification of buried deposits is more difficult, and traditional techniques employing regional geology, geochemistry and geophysics have had mixed results. Almost half of northern Chile is covered by gravel, and the remainder is exposed rock. The challenge in Chile, therefore, is to develop new tools that may be applied to discovery of covered and/or deep deposits. To address this challenge, we propose applying local earthquake tomography (LET) to identify potential exploration targets at the greenfield scale, in part because buried deposits in Chile tend to extend to great depth. LET uses natural seismicity, which is abundant in Chile, and can be recorded by inexpensive, locally manufactured seismic stations. LET analysis can determine the 3D distribution of seismic wave velocities with anomalies that are directly related to the presence of copper deposits. We have conducted seismic tomography experiments at both regional and local scales, in northern and central Chile. In all the cases we observed low Vp/Vs anomalies that are spatially correlated with deep porphyry-copper deposits. Low Vp/Vs anomalies can be generated by batholiths and hypabyssal intrusions, which correspond to porphyritic textured bodies. The relationship between subduction processes and mineralization is well established, and significant metallogenic belts worldwide are often located above subduction zones. Porphyry-type deposits are generally related to arc magmatism and partial melting related to subduction. Part of the ore-forming fluids are ultimately derived from dehydration of the subducted slab, reflecting the intimate relationship between subduction processes and mineralization.