A Study on the Application of the Gravity Method in Detecting a Concealed Granite Cupola

Abstract

The main objective of this study was to assess the potential of using the gravity method in detecting and delineating the granite cupola underlying the tin-bearing sediments near the township of Emmaville, New South Wales. Models proposed for the genesis of tin-tungsten mineralization in the nearby Taronga mining area are associated with granite cupola structure. The density contrast between the low-density granite cupola and the overlying high-density metamorphosed sediments may give rise to gravity anomalies which, if detected. may define areas of future exploration activities. One area chosen for a detailed gravity survey is located east of Emmaville where a 2 km x 2 km grid with a 50 m station spacing was established.This area was chosen because it is known for the occurrence of tin mineralization from past mining activities. The resulting Bouguer gravity map is characterized by a broad minimum of 20 g.u. over the Emmaville Volcanic Formation relative to the neighbouring Tent Hill Volcanic and Gulf Silt stone-Argillite Formations. Small-scale minima on the gravity map are associated with minor intrusive bodies of granodiorite that outcrop in the area. These small-scale anomalies were attenuated to a large extent by upward continuation of the gravity field to 200 m above the datum level, indicating that they are caused by near-surface structures. A broad minimum became the dominant feature in the upward-continuation map. One simple explanation for this gravity low is based on lateral variations of rock formations as indicated by the surface geological map. Density measurements on surface rock samples revealed slightly lower densities for the Emmaville volcanics relative to the adjacent formations. Assuming a density contrast of 100kg,M3 results in a thickness of about 500 metres for the low-density Emmaville volcanics to account for a gravity anomaly of 20 g.u. Although this interpretation is consistent with surface geology, other factors such as a granite cupola may also produce the observed anomaly. To carry out a more detailed interpretation of the gravity anomaly additional data are required, such as sufficiently deep drillhole and/or seismic data that were not available during the course of this study. In addition, a more widely spaced grid should extend well outside the 2 km x 2 km area investigated in this study. This would allow gravity anomalies possibly related to the granitic cupola of the Mole Granite to be outlined more clearly than is possible with currently available data. To assess the potential for such an extended grid survey, gravity readings were taken along a 7-km N-S traverse line. The Bouguer gravity anomaly along this line is characterized by a pronounced minimum of the order of 30 g.u. The causative body responsible for this anomaly may either be a basinal structure filled with weathered. Emmaville volcanics or a rise in the Mole Granite intruding into the Emmaville Volcanic Formation, which is in turn flanked by the Gulf Siltstone-Argillite Formation in the north and the Tent Hill Volcanic Formation at its southern boundary. The relatively small density contrast between the metamorphosed sediments of the Emmaville Volcanic Formation and the Mole Granite adds to the problem of identifying the causative body. The use of the gravity method alone for the purpose of detecting unambiguously a buried cupola in a geological environment such as that prevailing in the study area is accompanied by difficulties, the most important of which is related to the fact that near-surface lateral variations in structure cannot easily be distinguished from those caused by a granite cupola at relatively shallow depth.