The geophysical response of the Las Cruces massive sulphide deposit

Abstract

Riomín Exploraciones SA, a subsidiary of Rio Tinto plc, discovered the Las Cruces volcanic hosted massive sulphide deposit in 1994. The Las Cruces deposit lies at the extreme eastern end of the Pyrite Belt in southwestern Spain. Interpretation of the regional gravity data indicated that Pyrite Belt lithologies could extend further east than previously interpreted. This possible extension of the Pyrite Belt, however, lays beneath a variable thickness of post-mineral mainly Tertiary aged sediments. Rio Tinto identified the Las Cruces gravity anomaly from a survey conducted across the Faralaes licence block in 1994. The residual Bouguer gravity anomaly is approximately 29 µm/s2 (2.9 mGal). The deposit lies at a depth of approximately 120 m beneath the base of the Tertiary cover and is hosted by typical Pyrite Belt lithologies consisting of highly altered felsic to intermediate volcanics with minor sediments. Las Cruces is unusual as it contains a substantial zone of enriched secondary copper sulphides lying beneath a gold rich gossan cap. At the end of 1998 a measured and indicated secondary copper resource of approximately 14.51 Mt grading 6.13% Cu had been outlined by drilling. Las Cruces also contains primary copper and zinc mineralisation as well as a gossan gold resource lying above the other orebodies at the base of the Tertiary. A number of geophysical techniques including electrical soundings, IP, TEM, CSAMT, DHEM and mise-a-la masse surveys have been completed across the deposit. The area is difficult for electrical surveying as it lies within 18 km of the city of Seville. There are a number of domestic and high-tension power lines crossing the area. There are also roads, fences, a factory and several houses to contend with, as well as a high-speed electric rail link in the vicinity. The IP and TEM techniques both worked well in helping to define the extent of mineralisation at Las Cruces.