Abstract
The Yerington Mine is an open pit mine of 'porphyry-copper' type. Primary mineralization (quartz veins, Cu-Fe sulfides, magnetite, and silicate alteration) is closely associated with a complex sequence of quartz monzonite porphyry intrusives. This primary mineralization is disrupted by subsequent alteration and faulting. The fault block containing the deposit has been rotated almost 90 degrees to the west; plan maps represent pretilt vertical sections. Almost one mile of pretilt vertical extent is exposed in the pit.Geologic observations may be taken only upon mining advances at discrete intervals. Detailed in-situ induced-polarization response and magnetic susceptibility surveys, both taken upon the pit floor, have been used to supplement geologic mapping. These in-situ measurements reveal the distribution patterns of sulfides and magnetite and are useful in interpolation or correlation of contacts, faults, and mineralization patterns between mapped advances.In-situ induced-polarization (IP) response was measured using a Scintrex IPR-7 receiver. Observations were made on a 10 X 10-ft grid using a Wenner electrode array with 3 1/3-ft a-spacing. Susceptibility measurements were taken with a Bison model 3101-A bridge and an in-situ coil arrangement 5 ft in diameter. A composite area approximately 3000 by 500 ft, representing 15,000 data points, has been surveyed.Contoured maps of in-situ IP response and magnetic susceptibility reveal the mineralization pattern in detail. These maps correlate well with the geologically mapped distribution of porphyry intrusives and also indicate zones of sulfide remobilization and/or magnetite destruction, including fault zones.IP response and magnetic susceptibility correlate well with elemental assays, mineral abundances, ratios, and textures determined from bench mapping and pit floor samples. The spatial distribution of physical properties is extremely complex, which demonstrates the need for caution in the use of simple interpretative models. The abundance of magnetite locally present in this particular porphyry-copper environment is noteworthy.
