Electromagnetic modeling of porphyry systems from the grain-scale to the deposit-scale using the Generalized Effective Medium Theory of Induced Polarization
Creators: name: Emond, Abraham M
Publisher: name: University of Utah
Year: 2012
Subjects: Electric prospecting
GEMTIP
Simplified Porphry Model

description: 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.