INDUCED POLARIZATION AND RESISTIVITY MODELS IN 2D

The Poderosa mine is located in the Pataz Batolith, Northern Perú has been a difficult geophysical target in the past. Our experience in the area dates from the mid 80s and we have had very favorable results in the area mapping silificied structures within the batolith. These structures, together with their alteration halo, have a lower average Resistivity than their intrusive host rock which is the reason why the present as low resistivity targets with spurious chargeability anomalies, mainly due to the small concentrations of sulphides across these veins.
 
 
3D IP & RESISTIVITY

During the last few years, great developments in 3D modelling of Resistivity and IP surveys have been developed. As it turns out, 3D models are significantly more precise than their two-dimensional counterparts (see example above). The main requirement for a quality 3D model is that individual lines are spaced no more than 2 to 3 times the minimum electrode spacing. The example shown below was taken on a mine operation in Central Perú, where line spacing was 100m and electrode spacing 50m. The impressive graphics software that generated this image was Fracsis.
 
 

CUAJONE NORTHWEST 1993

In July 1993, Southern Perú Copper Corporation decided to perform a geophysical survey over a 400m x 400m grid of stations, previously prepared to control geochemical sampling between the open pit and the Moquegua-Puno highway some 3 km NW. From our experience in previous IP surveys on porphyry-type deposits, we recommended not to exceed 200m between IP sounding stations, covering an area of some 16 km² with 392 stations with no less than 200 to 300 meters of depth reach, depending on the post-mineral cover thickness. The survey was completed over approximately 13 km² with 326 Induced Polarization soundings, using the Wenner array, between October and December 1993.
   

True Chargeability in mV/V. The colored area gives an idea of how fieldwork was performed because stations were eliminated beyond areas where Chargeabilites dropped to levels below 10 mV/V (cyan & blue). That's how fieldwork progress was optimized, and no sounding stations were executed far beyond the anomalous zone, except on particular cases where we were required to define farther prospective boundaries. Level 3310 of the Cuajone open pit is indicated, as well as the extension of mineralization with more than 0.2% Cu at the same level. Two distinctive areas with Chargeability anomaly concentrations may be observed: the largest and most extensive appears as a NW extension of the present pit, while the smaller and weaker anomaly is centered around the crossing point of two creeks, to the Northwest. The two anomalous centers are 1 km apart.

    
    

IP Ratio. We prepared this Chargeability/Resistivity map to put emphasis on high M anomalies corresponding to areas with low R (altered rocks). We obtained two prospective targets, one just NW of the current open pit and the other 1km further Northwest. The first zone coincides with the area with new reserves have been found; these, along with additional ore found deeper in the pit, exceed 1 billion tons. The second zone has proven to be low-grade ore.

(Note that this ratio is equivalent to the very wrongfully named "Metal Factor")