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DHEM at Las Cruces, Spain - successes and failures Gavin R. Selfe* Geofocus Johannesburg, South Africa gavin@geofocus.co.za

INTRODUCTION

The Iberian Pyrite Belt (IPB), located in the southwest of the Iberian Peninsula, Spain, is one of the largest volcanogenic massive sulphide (VMS) provinces in the world. Within this belt lies the Las Cruces deposit, one of the richest copper deposits in the world. Las Cruces is 100% owned by First Quantum Minerals Ltd and has a proven and probable reserve of 4.3 MT at 4.54% copper (FQML, 2017). This is being mined as an open pit. The surface geology of the Las Cruces deposit consists of a Tertiary-aged highly conductive, horizontal marl layer, beneath which lies a thin layer of sandstone. The primary massive sulphide (PMS) lies beneath this sequence, within a moderately conductive volcano-sedimentary complex. The primary massive sulphides are in turn underlain by a stockwork vein system. The mineralisation is characterised by an overlying iron-oxide gossan and a secondary enrichment zone, as shown in Figure 1. Drilling is continuously being carried out at Las Cruces during mining in order to further delineate the resource at depth. A number of these holes have been logged with downhole electromagnetics (DHEM) in order to confirm the presence or otherwise of new sulphide extensions. The geophysical complexity of this environment in terms of the highly variable lithological conductivities creates a number of challenges for EM modelling.

In addition, there is the recent discovery of a small satellite body some 100 m x 150 m in size, approximately 100 m to the northwest of the primary orebody. This body has become the target of current exploration holes and DHEM. Modelling of this body’s EM responses in amongst the responses of the primary massive sulphides as well as the background geology presents the greatest challenge of all.

Figure 1. Sketch of the geology of the Las Cruces deposit (Miguelez et al., 2011). Previous to this, DHEM was done at Las Cruces in three holes by Terratec in 2014. In addition, two holes were logged by GRM Services in 2017. In February 2018 another two holes were logged by GRM Services, and at the same time GRM collected a number of petrophysical logs such as conductivity, magnetic susceptibility and density. Thereafter, detailed loop planning was undertaken before a new round of drilling on the small satellite body was started. Two of these new holes were DHEM logged in September 2018 by IGT. This paper details the complex modelling of the results from all of the latter four holes, and the successes and failures thereof.

METHOD AND RESULTS Holes CR726 and CR729 were surveyed in February 2018 by the Finnish contractor GRM Services using a DigiAltantis fluxgate probe and a Zavet transmitter, with a single loop on the pit floor. The loop current ranged from 40-43 A. Readings were taken downhole every 5-10 m. The base frequency used was

SUMMARY Downhole electromagnetic (DHEM) logging was undertaken within four holes drilled at the Las Cruces VMS body in Spain. The geology of the deposit is geophysically complex inasmuch as the background and overlying geology is conductive. A number of loop positions were pre-modelled in order to optimise coupling with the sulphides. The objective in all cases was to locate additional off-hole massive sulphides, belonging to both the main sulphide body and a smaller satellite body. The DHEM was undertaken with a variety of loop positions and at a number of base frequencies. Modelling was undertaken using Maxwell EM modelling software. In three of the holes, by using a number of thick plates and various conductive layers and half-spaces, the DHEM results could be explained by the known sulphides. In the case of a fourth hole, off-hole anomalies were located at depth. A new borehole was recommended based on these data. This borehole was later drilled and an extension to the satellite body intersected. Key words: Las Cruces, DHEM, downhole, electromagnetics, Spain, VMS, copper

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0.25 Hz. Holes CR750 and CR753 were then surveyed in September 2018 by the Spanish contractor International Geophysical Technology (IGT). These holes were logged with a Geonics BH43-3D coil sensor probe and a Protem TEM-57 Mk2 transmitter at three different base frequencies, being 0.25 Hz, 2.5 Hz and 25 Hz. In addition, they were logged using two different loop locations, one within the pit (below the highly conductive marls) and the other on the north side of the pit, draped over the northern pit wall for improved hanging-wall coupling. Readings were again taken every 5-10m. Loop current was 20 A.

Figure 2. Maxwell model result for mid-late times, borehole CR729. The downhole data were modelled using EMIT’s Maxwell EM modelling software. The modelling result for hole CR729 is shown in Figure 2. Two 35-40 m thick plates were placed within the closest PMS (primary massive sulphides, shown in yellow in Figure 2) in order to represent the nearby orebody. By adjusting the dip, depth, thickness, conductivity and rotation of these plates it could be shown that the PMS could fully explain the mid-late time EM response within this hole. This can be

seen by the red versus black curve match at the bottom of Figure 2. In order to explain the earlier time response, an additional plate had to be added to the model to represent the sulphides intersected in a small satellite body during drilling. The downhole conductivity and magnetic susceptibility logs proved very useful for identifying minor sulphide intersections and for measured conductivity values which could be used in the modelling. In Figure 3 below, the thick magenta plate represents the in-hole massive sulphides. The curve match at the bottom shows a reasonable, although by no means perfect, fit. It is clear that between the PMS and the intersected in-hole sulphides, the DHEM can be largely explained. However, reality is far more complicated than this and additional off-hole sulphides cannot be ruled out in the upper part of the hole.

Figure 3. Maxwell model result for early times, borehole CR729. Once a reasonable fit had been obtained, the CSIRO conductive half-space algorithm (Leroi) was applied, and the plates adjusted in order to see if the fit still held true. A background resistivity of 200 ohm.m was used, as indicated by

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petrophysical logging. A number of small adjustments to the plate conductivities and geometry showed that the model held true. Adding a conductive (50 ohm.m) marl layer to the top of the half-space made little difference as both the loop and the borehole collar lie beneath it. A similar exercise was undertaken for hole CR726 which is a much simpler scenario as no in-hole sulphides were intersected. Again, thick plate models within the PMS could explain the DHEM logs quite adequately and the logs showed no off-hole anomalies. A period of forward loop modelling preceded the next round of DHEM logging. The objective was to calculate whether an angled loop directly overhead of the satellite body, draped over the pit wall within the conductive marl, would deliver better results than an offset loop at the bottom of the pit, below the conductive marl. The forward modelling showed that an overhead loop delivered strong coupling and could be just as effective as an offset loop within the pit. Ultimately, both loop positions were logged. Holes CR750 and CR753 were drilled to better delineate the satellite orebody. Of the three frequencies logged, the 0.25 Hz data were very noisy and were not used. In general, the 25 Hz data were the best and seemed to be more than adequate for the late time response from the massive sulphides. Hole CR750 showed a number of in-hole anomalies which, upon modelling, could all be explained by sulphide intersections. However, hole CR753 proved interesting as a number of new anomalies were present at the base of the hole. The clearest anomalies were obtained at 25 Hz from the overhead (northern) loop draped over the pit wall. The model results are shown in Figure 4 below. In Figure 4 above, a moderately conductive half-space was used with the conductive marls at surface. The green body represents the satellite sulphide body as currently understood. The thin sub-horizontal plates shown below it in red and blue represent new, off-hole sulphides. Although the model response is complex and by no means perfect, there is a clear indication, particularly in the V-component and the deepest anomaly in the U-component, that proximal off-hole sulphides go part of the way towards explaining the DHEM response. For this reason, it was recommended that new boreholes be drilled to the west, or south-west, of CR753. This was subsequently done and a new hole, CR758 some 32 m to the west, intersected massive sulphides with up to 8% Cu over 44 m from 116-160 m.

CONCLUSIONS Detailed DHEM modelling at Las Cruces has shown that the results can be very complex, with complications in the form of the moderately conductive background and the conductive surface geology, and EM contributions from the overlying primary orebody as well as newly intersected in-hole sulphides. Interpreting new sulphide bodies in this environment is not without its hazards. However, in one case where proximal off-hole sulphides were modelled, positive drilling results were obtained nearby which reinforces that DHEM has its merits. Attention to detail in an environment such as Las Cruces is critical, and forward modelling can add great value.

Figure 4. Maxwell model result for late times, borehole CR753.

ACKNOWLEDGEMENTS

I would like to acknowledge Chris Wijns of First Quantum Minerals Limited who was instrumental in the initiation and design of these surveys, as well as all of the geological staff at Las Cruces for their ready responses to data requests.

REFERENCES

FQML, 2017, https://www.first-quantum.com/Our-Business/operating-mines/Las-Cruces/Reserves-and-Resources/default.aspx Miguelez, N., Arroyo, F., Velasco, F. and Videira, J., (2017), Geology and copper isotope geochemistry of the Las Cruces deposit (SW Spain). Revista de la Sociedade Espanola de Mineralogia, 15, 131-132.