b'What lies beneath revisitedFeatureWhat lies beneath revisitedsupergene enrichment and conductivityIntroductionThe conductivity of chalcocite has been documented from at least 1928 (Harvey 1928), although Shuey (1975) is considered the most complete modern reference. The most recent reference that relates to a potential geoscience application of chalcocites conductivity is Emerson (2021). So, while the petrophysical character of chalcocite has been known and studied for almost a century, the actual exploitation of its fairly unique conductivity behaviour has only occurred quite recently, starting in the 1990s. This recent activity appears to be due to either serendipity or basic \x1deld experiments that did not draw upon any speci\x1dc knowledge of earlier laboratory studies as Ken Witherlymentioned here.Condor Consulting, Inc. ken@condorconsult.com This in e\x1eect means that during the heyday of modern porphyry copper exploration (de\x1dned as roughly 1950-1980), much of the industry seemed unaware and hence unable to exploit a Overview signi\x1dcant petrophysical attribute of supergene chalcocite. The major strategic advantage that conductive chalcocite o\x1eers is At the AEGC 2019 conference in Perth, the paper What Liesthat this response can be detected remotely from an airborne Beneath? (Witherly 2019) examined several porphyry copperplatform, thereby allowing for rapid coverage of large areas.systems that had significant conductivity associated with the deposits. The source of this conductivity was not alwaysThe possible exception to this could be the very early work well understood. The current study is an extension of thisNewmont carried out in the late 1940s-early 1950s in their earlier work and will examine the interplay of supergenedevelopment of the induced polarisation or IP (then called enrichment and conductivity. Unknown to much of theovervoltage). As part of this programme they carried out studies exploration community, high-grade supergene zones inon several porphyry systems in Peru, namely Quellaveco and porphyry copper systems, typically comprised primarily ofCuajone (Baldwin 1959). While the porphyry systems which they chalcocite, can show a strong electrical conductivity that canstudied showed supergene/chalcocite development (see Figure be detected with techniques such as DC resistivity (part of1), scant attention seemed to be paid to the resistivity of the an Induced Polarisation (IP) survey) and electromagneticsporphyry system, the major focus being on the IP (also termed (EM). In this article, Ill look at how the industry responded, orchargeability) response.did not respond, to evidence of the geophysical character ofIn a major review of the state of mining geophysical the supergene in porphyry copper systems over a forty-yearinterpretation in 1966 (Hansen etal. 1966), the anomalous period. conductivity of chalcocite is noted. It is also noted that it Sillitoes seminal paper on the supergene (Sillitoe 2005) providesis a mineral with a high content of copper, but nowhere is the following introduction that sets the stage. it mentioned that its petrophysical character o\x1eered an exploration opportunity.Supergene leaching, oxidation, and chalcocite enrichment in porphyry and related Cu deposits take place in the weatheringThe author had a serendipitous encounter with chalcocite in environment to depths of several hundred meters. The1982 when assessing a line of IP data (acquired with the Zonge fundamental chemical principles of supergene processes wereComplex Resistivity system) that covered the newly found La elucidated during the early decades of the twentieth century, mainly from studies in the western United States. The products of oxidation and enrichment continue to have a major economic impact on Cu mining in the central Andes and southwestern North America, currently accounting for 50 percent of world-mined Cu and have sustained these two premier Cu provinces for the past 100 years. Enriched grades may attain 1.5 to 2 percent Cu, commonly two or three times the hypogene tenor. Deep oxidation also transforms low-grade refractory Au mineralisation into bulk-mineable ore.While many of the shallow porphyry copper deposits that host supergene systems have been found, still-to-be-found deposits, which are presumed to be located mostly under cover, could still be considered to be viable economic targets given the enhanced copper grades and lower energy budgets required to process the supergene ores. However, the still-to-be-found deposits will be more challenging to locate and require betterFigure 1.Overvoltage pro\x1fle over north end of Quellaveco ore body, Peru search technology. (Baldwin 1959).DECEMBER 2023 PREVIEW 44'