Airborne EM in Exploring for Epithermal Gold Deposits: Three Examples from the Great Basin and Western Cordillera

摘要

Airborne magnetics and electromagnetics are fast and cost effective tools for epithermal gold exploration. Continuing improvements in the sensitivity of helicopter time-domain electromagnetic systems has seen them move from simply deep direct-detection tools for highly conductive base-metal targets into broader use as geologic mappers of lithology, structure, and alteration, based on resistivity contrasts. Three examples of AEM applied to different types of epithermal gold deposits in the Great Basin and Western Cordillera are examined. A ZTEM (Z-Tipper Axis Electromagnetic) helicopter AFMAG EM and magnetic survey was conducted over the Gold Springs low-sulfidation epithermal gold project, in southeastern Nevada. Previously explored using ground CSAMT surveys, synthetic forward modeling showed that ZTEM could also defined buried, subvertically-dipping resistivity highs associated with epithermal centers more quickly and more economically. AEM and magnetic results were consistent with silica alteration and magnetite depletion over the known deposit areas. A helicopter-borne VTEM (Versatile time-domain electromagnetic) EM, magnetic and radiometric survey was carried out the Cerro Quema high-sulfidation epithermal gold deposits in Panama. Geophysical signatures that are characteristic of high-sulfidation epithermal targets were observed in the EM and magnetic data over the known deposits. These included high and low resistivities from the silica and clay-altered Au-mineralized zones, as well as airborne induced polarization effects. ZTEM helicopter electromagnetics and aeromagnetics were flown for epithermal gold and gold-skarn targets in the Guerrero Gold Belt of Mexico. Survey results showed contrasting response from the two deposit types on the property: the Ana Paula intrusive-hosted, low-sulfidation, epithermal-style gold deposit featured low resistivity and low magnetism locally within a larger magnetic high and resistivity high related to the mafic intrusion; whereas the San Luis oxidized skarn-style deposit hosts weak magnetism and high resistivity from the altered, intermediate-composition intrusion.