Micro-gravity surveys for gold exploration at Fosterville

Example Research Project supported by the ASEG Research Foundation

Eric Gozlan: Micro-gravity surveys for gold exploration at Fosterville

Host Institution: Monash University

Supervisor: Jim Cull

Contact: eric@mail.earth.monash.edu.au

Project Summary

The Fosterville Goldfield, situated 22 km east-north-east from the city of Bendigo in Central Victoria, forms part of the Central Victorian Goldfields. The geology of the area is dominated by the Fosterville Fault, which provides a prominent mineralised trace striking in a north-south direction extending over several kilometres. Previous geological interpretations have indicated an extension of the Fosterville Fault under cover at the northern end of the mine site. However, no corresponding anomalies have been detected in the geophysical data.

Apart from airborne magnetics and regional gravity data several other methods (IP, TEM, and ground magnetics) have been used in an attempt to locate high-grade prospects in close proximity to the fault. These surveys have been designed, both to delineate the Fosterville Fault structure under cover, and to detect any anomalous concentrations of sulphide mineralisation. However, the results have been inconsistent and there are currently no viable guidelines using conventional methods for routine gold prospecting at Fosterville.

With some notable exceptions (e.g. Olympic Dam) gravity surveys are seldom considered to be a primary tool for mineral exploration. Gravity data are normally slow to accumulate and the sensitivity is often sufficient only for locating major structures. However, productivity can be greatly improved using the new generation of digital gravity meters and fine structure can be reasonably resolved at the microgravity scale. Consequently, high definition microgravity surveys were conducted at Fosterville in an attempt to define any regional trends associated with faulting and to locate any isolated anomalies associated with dense sulphide ore bodies.

A Scintrex CG3M gravity meter was used to obtain data with an average station spacing of 25 m along 22 traverse lines (separated by approximately 45 m) running east-west across the inferred Fosterville fault-line. Additional in-fill readings were obtained in the region of the fault-line giving a typical station interval of 10 m in the critical region. At the completion of the 2-week survey period a total of 564 data points were obtained; these separate segments were processed and adjusted for static offsets to ensure minimum closure errors prior to gridding using SURFER and ERMAPPER software.

A regional trend to the gravity dataset was observed consistent with an independent structural report suggesting a broad structural northern plunge of basement rocks under cover. Field procedures ensured the precision of the gravity survey and enabled in the identification of a north-south trending gravity residual lineament after regional removal. Two regions of subtle anomalous gravity (5µm/s2) distinguished the gravity lineament. These features have been interpreted as excess mass due to the underlying sulphide mineralisation associated with the Fosterville Fault.

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