What lies beneath revisited – supergene enrichment and conductivity

Overview

At the AEGC 2019 conference in Perth, the paper ‘What Lies Beneath?’ (Witherly 2019) examined several porphyry copper systems that had significant conductivity associated with the deposits. The source of this conductivity was not always well understood. The current study is an extension of this earlier work and will examine the interplay of supergene enrichment and conductivity. Unknown to much of the exploration community, high-grade supergene zones in porphyry copper systems, typically comprised primarily of chalcocite, can show a strong electrical conductivity that can be detected with techniques such as DC resistivity (part of an Induced Polarisation (IP) survey) and electromagnetics (EM). In this article, I’ll look at how the industry responded, or did not respond, to evidence of the geophysical character of the supergene in porphyry copper systems over a forty-year period.

Sillitoe’s seminal paper on the supergene (Sillitoe 2005) provides the following introduction that sets the stage.

Supergene leaching, oxidation, and chalcocite enrichment in porphyry and related Cu deposits take place in the weathering environment to depths of several hundred meters. The fundamental chemical principles of supergene processes were 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 better search technology.

Introduction

The 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 chalcocite’s 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 field experiments that did not draw upon any specific knowledge of earlier laboratory studies as mentioned here.

This in effect means that during the heyday of modern porphyry copper exploration (defined as roughly 1950-1980), much of the industry seemed unaware and hence unable to exploit a significant petrophysical attribute of supergene chalcocite. The major strategic advantage that conductive chalcocite offers is that this response can be detected remotely from an airborne platform, thereby allowing for rapid coverage of large areas.

The possible exception to this could be the very early work Newmont carried out in the late 1940s-early 1950s in their development of the induced polarisation or IP (then called overvoltage). As part of this programme they carried out studies on several porphyry systems in Peru, namely Quellaveco and Cuajone (Baldwin 1959). While the porphyry systems which they studied showed supergene/chalcocite development (see Figure 1), scant attention seemed to be paid to the resistivity of the porphyry system, the major focus being on the IP (also termed chargeability) response.

Figure 1. Overvoltage profile over north end of Quellaveco ore body, Peru (Baldwin 1959).

In a major review of the state of mining geophysical interpretation in 1966 (Hansen et al. 1966), the anomalous conductivity of chalcocite is noted. It is also noted that it is a mineral with a high content of copper, but nowhere is it mentioned that it’s petrophysical character offered an exploration opportunity.

The author had a serendipitous encounter with chalcocite in 1982 when assessing a line of IP data (acquired with the Zonge Complex Resistivity system) that covered the newly found La Escondida porphyry system. This line of data (Witherly 2014, Figure 2) showed an anomalous conductivity feature that at the time was unexplained, but ultimately attributed to conductive ground water. While several strategies were proposed to attempt to map this feature using either ground and airborne EM, the perceived geopolitical risk at the time was such that the project was largely shuttered, and the exploration team dispersed.

Figure 2. Escondida DC resistivity data; top-observed, bottom-2D smooth layer inversion (Witherly 2014).

Ten years later, there was a surge of activity that highlighted the conductive response of chalcocite. The first field work known to the author that showed chalcocite could be conductive in a field setting was a series of downhole geophysical logs acquired in 1989-90 by the USGS at the Santa Cruz deposit in Arizona (Figure 3, Nelson 1991). This was done as part of a study to extract copper from a copper oxide deposit using in situ leaching. A number of chalcocite zones were encountered that showed strong conductivity.

Figure 3. Electrical logs; resistivity, Self Potential (SP) and IP plus copper analysis (Nelson 1991).

However, the chalcocite zones were relatively thin and quite deep; situated at a depth of 1450 and 1700 feet (442 m - 523 m), suggesting that they were likely formed by hydrothermal processes and were not supergene in origin.

Soon after the Arizona study, significant programmes in Chile and Iran took place that highlighted the importance of conductive chalcocite.

In the early 1990s, efforts by Rio Tinto Exploration (RTE) in South America are discussed by Barrett (pers. comm. 2023); “I think it was probably Quantec who started running EM surveys in Chile, first 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 et al. 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 bring EM equipment to Chile and set up trials over several known supergene deposits.

Figure 4. Ujina deposit geology-upper image; TEM section-lower image (Nickson 1993).

This work was carried out in 1993 and the results circulated by Quantec to interested parties (Nickson 1993). One such group was BHP who have been investing considerable effort at upgrading airborne EM technology along with Aberfoyle LLC and Geoterrex Ltd (Smith et al., 2003). Based on the Quantec work, and other trials carried out by BHP near the Escondida Mine, BHP then undertook a major programme of airborne EM looking for the supergene part of undiscovered porphyry systems. This work was summarised in Harrison (2002).

The scope of the exploration programme allowed for a more careful examination of actual conditions within the ore body. Figure 5 shows a downhole resistivity log from Escondida Norte. Harrison (2002) also provided the following comments about what was observed:

Figure 5. Graph showing strong correlation between downhole conductivity and total sulphide content for ZERD62 at Escondida Norte.

Figure 5 is a physical property log from Escondida Norte showing the strong correlation between conductivity and total sulphide content (Pyrite, Chalcopyrite, Chalcocite and Covellite). Chalcopyrite tends to be more disseminated and not as well connected in hypogene ore and is generally not seen as a source of conductivity. Pyrite is the most abundant mineral, and although it does not have the lowest resistivity, it makes up for it in connectivity. In addition, chalcocite can form as surficial covering on incompletely oxidised pyrite crystals, increasing the connectivity and lowering the resistivity for non-economic occurrences of chalcocite. Metallic chalcocite is well connected and very conductive

Almost concurrent with this work, Aerodat Ltd., a Canadian airborne survey group, was commissioned by the Iran National Copper Company to carry out an extensive airborne EM, magnetic and radiometric survey (37 000 line-km) in the Kerman District of Iran.

The Kerman District was known to host a number of porphyry copper deposits, including the Tier 1 Sar Cheshmeh deposit. This work was documented in 1994 (Pitcher et al. 1994) and in their report, it was noted that a number of chalcocite-rich supergene zones were showing up as good conductors with the EM component of the Aerodat survey. This was not expected going into the survey, and unlike the Chilean experience where a theory was being tested, in the Iranian survey a serendipitous discovery led to mapping high grade copper zones with airborne EM. An example of these results is shown in Figure 6.

Figure 6. Darrehzar deposit-Kerman District, Iran (Pitcher et al. 1994).

In the late 1990s, BHP was exploring for porphyry deposits in western Pakistan. As part of their exploration programme, ground TEM was carried out over a number of prospects (Schloderer 2003). One deposit, designated initially as H-4 (later called Tanjeel) had a well-developed supergene system that was clearly defined with the TEM survey (Figure 7). Also, in the mid-1990s, TEM was being carried out on a VMS deposit at Las Cruces located in southern Spain, which showed a well-developed supergene zone; Figure 8 (McIntosh et al., 1999).

Figure 7. H-4 (Tanjeel) deposit: TEM channel 15 + drilling-top image; geological section with supergene zone-bottom image (Schloderer 2003).

Figure 8. TEM line over Las Cruzes (Spain) deposit-supergene zone (McIntosh et al. 1999). The figure show Zonge Engineering 1D STEMINV inversion results.

While chalcocite is believed to be primarily responsible for the incidents of conductive responses discussed above, the supergene process can also generate native copper, which is believed to have produced a strong conductivity response at the Ernest Henry deposit; Webb and Rowston (1995) and M. Webb pers. comm. (2023).

In some instances, a supergene development can exist concurrently with massive sulphide veins that can also be quite conductive. The Rosario deposit in the Collahuasi district in Chile, which sits adjacent to the Ujina deposit discussed earlier, shows supergene development. However, an extensive suite of massive sulphide veins dominates the conductivity response (Dick et al. 1994).

The change in sulphide habit from disseminated to massive and its effect on conductivity, was looked at by Nelson and Van Voorhis (1983). Their findings are summarised in Figure 9. At Rosario, this attribute has been exploited on several campaigns. The outcomes of a Crone DEEPEM ground TEM survey from the late 1980s (Wilt 1991), is shown in Figure 10. And in 2004-5 (A. Watts pers. comm. 2021), deep penetrating TEM was able to image the Rosario vein system to depths over 500 m (Xstrata 2006, Figure 11).

Figure 9. Weight percent sulphides vs. resistivity (Nelson and Voorhis 1983).

Figure 10. Crone DEEPEM coverage over shallow portion of Rosario vein system (Wilt 1991).

Figure 11. 3D view of Rosario vein system at depth; Xstrata Copper 2006.

There are also a number of copper deposits (many porphyry coppers but not all) that have a supergene zone developed that hosts good grade copper but are not conductive. Two examples are shown in Figure 12 (Casino, Yukon, Witherly et al., 2018) and Figure 13 (Silver Bell, Arizona, Thoman et al., 1998).

Figure 12. Casino Deposit, Yukon. IP-DC resistivity section 11200 (Witherly et al. 2018).

Figure 13. Silver Bell Mine, Arizona. Geology section with supergene deposit shown on the top and the TEM resistivity section shown on the bottom (Thoman et al. 2000).

The literature on supergene development suggests that the cycle must be repeated a number of times in order for the grade of the supergene (and assumed purity of the chalcocite) to be increased. In Alpers and Brimhall (1988), a suggested process is provided:

Supergene leaching, erosion of leached capping and lowering of base level in adjacent drainage channels during active weathering of a porphyry-copper system tend to encourage descent of the water table, causing leaching of previously formed enrichment zones and reprecipitation of supergene copper sulphides at progressively lower levels and augmenting copper grades and enrichment blanket thickness.

It is considered likely that the Casino and Silver Bell examples formed as the consequence of one-event; a supergene zone is formed, but the enrichment process does not reach the copper grade, and hence the conductivity, that might be expected from multiple events. Multiple events seem to be associated with deposits that formed in very dry climates.

Summary

While there was a surge of examples in the 1990s showing exploration applications of EM to chalcocite mapping, the last 20 years have not generated many new examples, suggesting many explorers may never have heard of the relationship and may ‘miss’ making the connection, even if they are presented with data that shows the supergene ‘effect’ is in play.

Having field data on which to base judgments is always a good starting point, but for all the work in the 1990s there was a lack of petrophysical assessments that allowed the observed conductivity to be tied back to chalcocite per se, even though there was an extensive history of laboratory work and some field work (Nelson 1991) that showed that chalcocite is conductive. The major recent development in this field was the work of Don Emerson (Emerson 2021). Don was able to show that as density increased, so did the conductivity, a relationship expected to be observed. However, Emerson did not address the issues relating to the formation of chalcocite and supergene enrichment, as was done by Alpers and Brimhall (1988). Also, Alpers and Brimhall’s ideas on how the supergene gets upgraded appear to be more ‘suggestions’ than well-documented theories. More work needs to be done to properly understand this mechanism.

The argument could be made that this sort of study is ‘too late’ to have any real impact on exploration ‘best practice’, the assumption being that most supergene deposits have already been found. However, whilst all shallow terrains may have been explored, covered areas unsuited to conventional geological mapping and geochemical exploration techniques are still potentially prospective.

The more important conclusion is that a significant petrophysical attribute, one that could help explorers map and define economic copper resources, has largely been ignored for 100 years. While a ‘renaissance’ in exploration for porphyry copper deposits appears to have occurred in the 1990s, little study of the phenomenon of conductive supergene copper deposits appears to have happened since then. My closing question is, if the chalcocite story ‘slipped under the fence’, what other potentially important petrophysical relationships has the exploration community missed?

Acknowledgments

This research would not have been possible without the help of several colleagues: Paul West-Sells, Leo Fox, Rob Gordon, Erik Tornquist, Dick Tosdal, Mark Rebagliati, Mo Colpron, Scott Casselman, Rob Carne, Jon Woodhead, José (Pepe) Perelló, Jeremy Richards (deceased), Fred Graybeal, Randall Nickson, Mike Harrison, Jeremy Barrett, Doug Pitcher, Lee Sampson, Andy Mountford, Paul Hayston, John P. Schloderer.