Geophysical Anomalies Over Epithermal Systems

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Journal Article: Geophysical Anomalies Over Epithermal Systems

The interpretation of most physical anomalies in epithermal systems needs to be integrated with available geological and geochemical data, and incorporated into a conceptual model of the original hydrology of the system. These anomalies result from the passage of hydrothermal fluids through rocks. Pervasive clay minerals associated with argillic-propylitic alteration zones cause low-resistivity anomalies comparable in size to the original geothermal system (typically 10-30 km2). Resistivities are usually in the range 10-30 Ωm, and decrease to less than 10 Ωm in areas of advanced argillic alteration. These very low-resistivity zones may be caused by the reaction of the rock with acid, steam-heated waters. Most hydrothermal fluids, including relatively cool CO2-bearing waters, destroy magnetite and cause demagnetisation anomalies. Airborne electrical and magnetic survey methods are therefore ideal for detecting epithermal systems in large exploration areas. Gamma-ray spectroscopy also has potential because of the concentration of potassium minerals (adularia and illite) in high-temperature alteration assemblages. In contrast to system-wide anomalies which have characteristic widths of several kilometres, epithermal ore bodies have characteristic widths of hundreds of metres. The precipitation of quartz and adularia which commonly accompanies gold-silver mineralisation causes an increase in resistivity, with values locally exceeding 1000 Ωm. Although this mineral assemblage causes an increase in apparent resistivity when near-surface, its subsurface extension may be difficult to delineate by electrical methods because of current channelling in adjacent low resistivity zones. The main I.P. anomalies are more likely to be associated with argillic alteration zones, unless quartz-adularia zones have significant sulphide concentrations. Gravity high anomalies of the order of mgals may occur as a result of mineral precipitation, especially in low-density, high-porosity host rocks. In relatively dense host rocks, local fracturing, alteration and late-stage leaching may actually cause gravity low anomalies of several mgal.

R. G. Allis

Published Journal 
Journal of Geochemical Exploration, 1990



R. G. Allis. 1990. Geophysical Anomalies Over Epithermal Systems. Journal of Geochemical Exploration. (!) .