b'What lies beneath revisitedFeatureFigure 2.Escondida DC resistivity data; top-observed, bottom-2D smooth layer inversion (Witherly 2014).Escondida porphyry system. This line of data (Witherly 2014,to extract copper from a copper oxide deposit using in situ Figure 2) showed an anomalous conductivity feature that at theleaching. A number of chalcocite zones were encountered that time was unexplained, but ultimately attributed to conductiveshowed strong conductivity.ground water. While several strategies were proposed to attempt to map this feature using either ground and airborne EM, theHowever, the chalcocite zones were relatively thin and quite perceived geopolitical risk at the time was such that the projectdeep; situated at a depth of 1450 and 1700 feet (442 m - 523 m), was largely shuttered, and the exploration team dispersed. suggesting that they were likely formed by hydrothermal processes and were not supergene in origin.Ten years later, there was a surge of activity that highlighted theSoon after the Arizona study, signi\x1dcant programmes in conductive response of chalcocite. The \x1drst \x1deld work knownChile and Iran took place that highlighted the importance of to the author that showed chalcocite could be conductiveconductive chalcocite.in a \x1deld setting was a series of downhole geophysical logs acquired in 1989-90 by the USGS at the Santa Cruz deposit inIn the early 1990s, e\x1eorts by Rio Tinto Exploration (RTE) in South Arizona (Figure 3, Nelson 1991). This was done as part of a studyAmerica are discussed by Barrett (pers. comm. 2023); I think it was probably Quantec who started running EM surveys in Chile, \x1drst with an old SIROTEM Mk2 (?); if my memory serves me correctly RTE had Hugh Rutter (Australian geophysical consultant-KEW) come out to help getting us going. Quantec then brought in a EM-37. In late 1992, RTE bought a Zonge GDP-16 and GGT 10 system where yours truly cut his teeth using to run TEM surveys in Ecuador before bringing it to Chile. Here we ran a few TEM surveys for a short time but then, switched to a variant of the Kennecott-style recce IP, although often acquiring TEM data as well.In Chile the La Escondida deposit owned by BHP and several partners was being developed. Other companies were sending teams to Chile to explore for additional deposits. Geophysical contractors, especially from North America, were also arriving to support these programmes.The Collahuasi district of Chile is host to three separate major deposits, namely Ujina, Rosario and Quebrada Blanca. Quantec Geoscience, a Canadian geophysical company, successfully carried out an IP survey over the Ujina prospect and follow-up showed that a major deposit had been located (Dick etal. 1993). The supervising geophysicist noted that the supergene zone associated with the deposit was quite conductive (Figure 4) and as an experiment, he organised to Figure 3.Electrical logs; resistivity, Self Potential (SP) and IP plus copperbring EM equipment to Chile and set up trials over several analysis (Nelson 1991). known supergene deposits.45 PREVIEW DECEMBER 2023'