GROUND PENETRATING RADAR IMAGING OF MIAROLITIC CAVITIES: A GEOPHYSICAL APPROACH TO PEGMATITE EXPLORATION AND MINING

Discovery of a major gem tourmaline pocket at the Cryo-Genie Pegmatite Mine, San Diego County, in September, 2001, prompted the author to suggest to the mine owner that Ground Penetrating Radar (GPR) might be a useful tool to assist in the exploration efforts. Previous investigations at other San Diego County gem pegmatite mines demonstrated the effectiveness of GPR for delineating pegmatite orientation and for finding gem-bearing miarolitic cavities (pockets). However, not all gem pegmatites are amenable to the use of GPR. High clay contents in the overburden and wet clay seams within a dyke limit the depth of penetration and reduce the GPR signal quality. Poor understanding of the mineralization sequence and internal structure of a pegmatite hamper data interpretation, even if the data quality is good. Therefore, these studies were undertaken to improve our interpretive models. So far, we have been able to discern small (< 5cm), medium (5 cm – 2 m), and large (> 2 m) air, clay, or zeolite filled gem-bearing miarolitic cavities GPR data were collected with GSSI SIR type instruments in a continuous recording mode. Broadband antennas, with central frequencies between 100 and 1,000 MHz, were selected for use to provide reasonably detailed images of the near surface region. On the surface the antenna is pointed down, but underground the antenna can be pointed in any direction and is usually positioned sub-parallel to the vertical walls. This allows pockets and fracture zones to be imaged in or near the direction of mining. Geologically, the GPR model of a pegmatite consists of a tabular sheeted dyke using the classic Jahns or Foord cross-sections. The interpreted GPR scans match very well the underlying geology and mine workings, based on direct measurements and excavations. Open drifts, declines, and geologic features are clearly visible on the recorded scans. As mining progresses, and we are able to measure the exact distances of the working face to the anomalies, we should be able to improve the accuracy of our interpretation and complete a detailed geologic mapping of the complex features of the pegmatites based on both geology and GPR. In our opinion, GPR is proving to be a useful geophysical prospecting tool in the pegmatite districts of southern California and has broad applications to worldwide miarolitic pegmatite districts.