EVALUATION OF INDUCED POLARIZATION SURVEYS TO DETECT SULFIDE MINERALIZATION AT CONSTRUCTION SITES

Induced polarization (IP) is an electrical geophysical method developed by the mining industry to prospect for disseminated sulfide deposits. A small number of published case studies have shown that IP can be used to detect pyritic rock in proposed highway corridors, thus avoiding acid rock drainage (ARD) problems. Some recent PennDOT highway construction projects in Central Pennsylvania have exposed pyritic rocks, providing an opportunity to evaluate IP in a variety of geologic settings.

IP surveys were conducted over two Devonian Black Shale formations (Mandata and Marcellus) exposed in a deep road cut along Route 544, near Lewistown. These black carbonaceous shales, ranging from 1.12 to 11.57% carbon (~ half as hydrocarbons) and 0.83 to 4.83% sulfur, are a source of ARD. The chargeabilities measured over these rocks ranged from 0 to 9 mV/V, barely distinguishable from background levels. The low chargeabilities of these shales are probably due to the very fine grain size of the pyrite.

In contrast, chargeabilities measured over the epigenetic sulfide veins at Skytop, 10 km west of State College, were as high as 40 mV/V. The exact chargeability value measured depended on both the electrode configuration used and the nature and concentration of cross-strike vein sets. These range from fine veinlet networks to thick veins (>1cm) spaced 50 cm to 1 m apart, to a fault zone, 60 cm thick, of massive (up 67 %) pyrite. An IP anomaly of 20 mV/V was detected beneath several meters of oxidized cap rock along I-80 in Curtin Gap.

After establishing optimum electrode configurations over mapped veins at Skytop IP surveys were used to map concealed sulfide mineralization in areas where the surface had been disturbed by, or covered during construction.