Electromagnetic modeling of porphyry systems from the grain-scale to the deposit-scale using the Generalized Effective Medium Theory of Induced Polarization

A new conductivity model, the Generalized Effective Medium Theory of Induced Polarization (GEMTIP) is tested with complex resistivity data and detailed mineralogy of porphyry system rock samples. The induced polarization (IP) effects are important phenomena for EM exploration. GEMTIP represents an expansion of the rock properties used for electromagnetic modeling of bulk apparent resistivity. The new model includes, mineral type, mineral size, mineral conductivity and other petrographic information. Rocks containing disseminated sulfides from porphyry systems are chosen as a good analog to the testing of the spherical grain analytic solution of GEMTIP. GEMTIP predicts the same trend in peak IP response as function of grain size for both chalcopyrite and pyrite containing synthetic rocks as a previous study, study. Inversion routines are developed and tested using synthetic data to recover the two empirical variables from recorded complex resistivity data. The two empirical variables are surface polarizability (a) and the decay coefficient (C). For three porphyry system rock samples, detailed geologic analysis using optical mineralogy, and X-ray tomography is conducted to determine GEMTIP model inputs. Sulfides in the rock samples exhibited a range of forms including near perfect cubes, stacked cubes, rounded, and complex amorphous forms. The sizes of sulfides varied from less than 0.01 mm to over 2 mm in radius. Measured surface area and surface area to volume ratios for each sample do not match the computed values assuming uniform spherical grains. Complex resistivity values are calculated from recorded EM data from 0.0156 Hz to 9216 Hz. Using the observed mineralogical data the GEMTIP model was able to fit the recorded complex resistivity data for the three samples with a the inclusion of an empirical factor to account for the difference in measured and computed spherical surface area reenforcing the role of surface area in the IP effect. Successful GEMTIP modeling of the rock samples provided insight into controlling factors of the IP effect. Forward geophysical modeling of copper porphyry systems is accomplished using geologic inputs from rock-scale to deposit-scale. For deposit scale modeling an Integral Equation method Electromagnetic forward modeling code IBCEM3DIP, developed by the Consortium for Electromagnetic Modeling and Inversion (CEMI) is used. A new interface to allow modeling of geometrically complex geologic systems was developed for the IBCEM3DIP code. The GEMTIP conductivity model was incorporated into IBCEM3DIP. Both the rock type and associated electric properties and mineralogical properties (approximate) are used for synthetic data creation. Using the new interface and developed Simplified Porphry Model as a template the effect of deposit-scale changes in sulfide distribution are tested on synthetic IP data. Although differences in the apparent resistivity data are subtle, changes in sulfide distribution strongly influence the apparent phase data. This highlights the importance of IP data and its use for mineral disrimination. With advances in the understanding of the IP effect through GEMTIP, forward modeling and inversion, detection and discrimination capability will improve for porphyry systems and other geologic targets, leading to greater efficiency in mineral exploration.