b'What lies beneath revisitedFeaturemagnetic and radiometric survey (37 000 line-km) in theIn some instances, a supergene development can exist Kerman District of Iran. concurrently with massive sulphide veins that can also be quite conductive. The Rosario deposit in the Collahuasi district in The Kerman District was known to host a number of porphyryChile, which sits adjacent to the Ujina deposit discussed earlier, copper deposits, including the Tier 1 Sar Cheshmeh deposit.shows supergene development. However, an extensive suite of This work was documented in 1994 (Pitcher etal. 1994) andmassive sulphide veins dominates the conductivity response in their report, it was noted that a number of chalcocite- (Dick etal. 1994).rich supergene zones were showing up as good conductors with the EM component of the Aerodat survey. This wasThe change in sulphide habit from disseminated to massive not expected going into the survey, and unlike the Chileanand its e\x1eect on conductivity, was looked at by Nelson and experience where a theory was being tested, in the IranianVan Voorhis (1983). Their \x1dndings are summarised in Figure survey a serendipitous discovery led to mapping high grade9. At Rosario, this attribute has been exploited on several copper zones with airborne EM. An example of these results iscampaigns. The outcomes of a Crone DEEPEM ground TEM shown in Figure 6. survey from the late 1980s (Wilt 1991), is shown in Figure 10. In the late 1990s, BHP was exploring for porphyry depositsAnd in 2004-5 (A. Watts pers. comm. 2021), deep penetrating in western Pakistan. As part of their exploration programme,TEM was able to image the Rosario vein system to depths over ground TEM was carried out over a number of prospects500 m (Xstrata 2006, Figure 11).(Schloderer 2003). One deposit, designated initially as H-4There are also a number of copper deposits (many porphyry (later called Tanjeel) had a well-developed supergenesystemcoppers but not all) that have a supergene zone developed that that was clearly defined with the TEM survey (Figure7).hosts good grade copper but are not conductive. Two examples Also, in the mid-1990s, TEM was being carried out on aare shown in Figure 12 (Casino, Yukon, Witherly etal., 2018) and VMS deposit at Las Cruces located in southern Spain,Figure 13 (Silver Bell, Arizona, Thoman etal., 1998).which showed a well-developed supergene zone; Figure 8 (McIntosh etal., 1999). The literature on supergene development suggests that the cycle must be repeated a number of times in order for the grade While chalcocite is believed to be primarily responsible forof the supergene (and assumed purity of the chalcocite) to be the incidents of conductive responses discussed above, theincreased. In Alpers and Brimhall (1988), a suggested process is supergene process can also generate native copper, which isprovided:believed to have produced a strong conductivity response at the Ernest Henry deposit; Webb and Rowston (1995) and M.Supergene leaching, erosion of leached capping and lowering of Webb pers. comm. (2023). base level in adjacent drainage channels during active weathering Figure 6.Darrehzar deposit-Kerman District, Iran (Pitcher etal. 1994).47 PREVIEW DECEMBER 2023'