Geophysical inversion in an integrated mineral exploration program: examples fromthe San Nicolas deposit

Nigel Phillips∗ and Douglas W. Oldenburg, UBC-Geophysical Inversion Facility, University of BritishColumbia, Vancouver, Canada

Summary

The ability to produce three-dimensional physical prop-erty models of the subsurface from surface geophysicaldata, coupled with an increasing need to explore forminerals in concealed terranes, results in geophysicalinversions providing more significant information tothe exploration team. We examine the role that geo-physical inversion can play in an integrated mineralexploration program, and the impact it can have on theresults. As an example, geophysical data from the SanNicolas copper-zinc massive sulfide deposit in Mexico areinverted.

Density and magnetic susceptibility distribution models,inverted from regional gravity and magnetic data respec-tively, define large-scale structures that reflect the tec-tonic setting of the region. Several distinct anomaliesthat exhibit high density and magnetic susceptibility val-ues are identified. A correlation method that determinesvolumes of high density and magnetic susceptibility iso-lates five anomalies, two of which are associated with min-eralization.

At a more detailed scale, the deposit is well defined bygravity, magnetic, CSAMT, and IP methods individually.Each of these data sets has been inverted to generate a3D physical property model. A drill-hole that is targetedon the intersection of these favorable physical propertydistributions would have intersected the heart of the de-posit. This demonstrates the advantages of using thesemethods in concert.

Lastly core physical property measurements are used toimprove inversion results. The inclusion of data from asingle drill-hole is shown to significantly enhance the de-tailed magnetic susceptibility distribution and producesmodels that correlate better with known mineralization.

Introduction

Mineral exploration programs commonly employ a stagedapproach when assessing large amounts of land or data.The aim of this approach is to systematically discernthe most prospective targets from a large amount ofland without allowing any economic mineralization to beoverlooked. These targets can then be evaluated with alocal exploration program and drill-testing. Figure 1 is asimplified flowchart that shows the role geophysics playsat any stage of a mineral exploration program. Severaliterations of this process might occur as a programadvances and homes in on an economic resource.

Geophysical Exploration Flowchart

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Fig. 1: Exploration flowchart with the geophysical componentsthat are addressed in this study being highlighted

San Nicolas, owned by Teck Cominco Limited and West-ern Copper Holdings Ltd., is an unmined, volcanic-hosted, massive sulphide deposit containing ore-gradecopper and zinc with associated gold and silver. It is lo-cated in Zacatecas State, Mexico. The deposit is hosted inmarine volcanic and sedimentary rocks with mineraliza-tion predominately found stratigraphically after the for-mation of rhyolitic lava domes and before the depositionof mafic extrusives above. Throughout the region, therock assemblages are intruded by granitic plutons, andare unconformably overlain by felsic volcanic flows andtuffs (Johnson et al., 2000). A section of rocks has beenuplifted to form a horst, within which the San Nicolasdeposit is found.

The massive sulphide ore at San Nicolas is primarily madeup of pyrite, chalcopyrite, and sphalerite, with some mag-netite and possibly pyrrhotite. From this mineralogy,the deposit is expected to have the following physical at-tributes: high density; high magnetic susceptibility (dueto the presence of magnetite and pyrrhotite); high con-ductivity (this variable property often depends on theconnectivity of the conducting paths); and high charge-ability due to the presence of metallic minerals.

Inversion in mineral exploration

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Fig. 2: Plan view of survey locations at San Nicolas.

Data

Data were collected at San Nicolas (figure 2) duringthe last five years, and provided to U.B.C. by TeckCorporation. For the regional-scale modeling we invertgravity and airborne magnetic data. Local-scale model-ing involves the inversion of gravity, ground magnetic,CSAMT, and IP data. For detailed modeling of thedeposit, we include magnetic susceptibility measurementsthat were made on core from one drill-hole.

Inversion

In a typical geophysical inverse problem we are suppliedwith data dobs, some information about their errors, andan ability to carry out forward modelling that relatesm, the physical property of interest, to the data. Ourgoal is to find the distribution of the physical propertythat produced the data. However, the inverse problemis nonunique and it is common to find a particularsolution that is consistent with our a priori informationabout the model m and also adequately reproducesthe data. The a priori information usually consists ofgenerating a solution which is close to a reference modeland is also smooth in the three spatial directions. Theinversion algorithms used here all adopt that philosophy.The result is a model that hopefully exhibits the grossfeatures of the earth and is geologically interpretable.

Regional inversion

Regional-scale inversion of gravity and magnetic datais performed in order to generate areas for detailedfollow-up exploration. The survey areas for these datasets are shown in the upper portion of Figure 2. Theinversion algorithms are those developed by Li and Old-enburg(1996, 1998). Regional inversion of gravity dataproduces a model that contains 707625 density-contrastcells of size 250m x 250m x 100m. The distribution ofvalues within this model shows: a horst, within whichthe deposit is located; denser mafic rocks that havebeen intruded along regional faults; both large, deepand smaller, shallow intrusive bodies; and a coarse rep-resentation of the massive sulfide deposit. An isosurfaceplot of the density is shown in Figure 3a. Inversion ofairborne magnetic data generates an equally large modelthat enables intrusive bodies, an outcropping magneticrhyolite, and the deposit to be identified. An isosurfaceplot of susceptibility is shown in Figure 3b.

The individual inversions are informative, but for massivesulfide exploration we are looking for targets that havehigh density and high susceptibility. We therefore carryout a correlation procedure to find volumetric regions thatare high in both of these properties. This produces theimage in Figure 3c. We note, that with the cutoff levelsused, there are 5 locations that are characterized by highvalues of correlation. One was immediately identified asa deep intrusive. Of the others, one is the San Nicolasdeposit, and the volume at the bottom has also been ex-plored and found to be mineralized. This illustrates thatthe joint interpretation of gravity and magnetic inversioncan be very beneficial at the regional scale.

Local inversion

Having localized likely areas on the regional scale, thenext step is to explore these areas using more detailedsurface geophysical surveys. The enhanced resolutionmeans that the inversions can be carried out withsmaller meshes and more detail about the physicalproperty characterization should be available. This stageof the work builds on the regional knowledge alreadyobtained. Along with the gravity data, which was usedfor the regional modeling, more localized data sets areconsidered from ground magnetic, CSAMT, and IPmethods. The inversion of these data is discussed inmore detail in Phillips et al. (2001). The CSAMT datawere inverted using a 1D inversion algorithm (Routhand Oldenburg, 2000) and then stitched together tomake a 3D image. The IP data were inverted using thealgorithm by Li and Oldenburg (2000). The 3D modelsof density-contrast, magnetic susceptibility, resistivity,and chargeability models all define the deposit very well.Figure 4 shows north-facing cross-sections through thephysical property sections with geology overlain. It isapparent that inversion of any of these data types wouldhave resulted in a successful drill-hole. In combination,the inversion results provide even more information,especially when the images are jointly interpreted withgeologic understanding about the deposit.

Inversion in mineral exploration

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Fig. 3: Top: regions of the density-contrast model greater than0.13 g/cm3, Middle: regions of magnetic susceptibility model that

are greater than 2.6× 10−3 S.I., Bottom: regions where both den-sity contrast and magnetic susceptibility are high.

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Fig. 4: North-facing cross-sections of local physical property in-version models with geology overlain.

Inversion in mineral exploration

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Fig. 5: Top: geologic and magnetic susceptibility log from a singledrill-hole. Bottom: north-facing cross-section of magnetic suscep-tibility model that was generated by inverting ground magneticdata while honoring drill-hole measurements.

Detailed inversion

The next stage in the exploration process is to furtherdelineate the mineralization that may have been found.Geophysical inversion can play a role at this delineationscale. If a target is drill tested, and whether or notmineralization is intersected, each drill-hole can provideadditional information that can be used to furtherrefine the inversion models. The inclusion of this data-independent information will help eliminate models thatare inconsistent with known geology.

We use magnetic susceptibility measurements that were

made on core from one drill-hole to constrain the inversionof ground magnetic data. The resulting model (Figure 5)shows a dramatic improvement in magnetic susceptibilitydistribution, and corresponds well with a known magneticregion in the deposit.

Conclusions

The effectiveness of geophysical inversion has beendemonstrated by inverting various geophysical datasets at different stages of the exploration process. Atthe reconnaissance stage, the joint interpretation ofgravity and magnetics was effective in localizing pos-sible exploration targets. Integrated interpretationsof ground-based inversions provided an unmistakeabletarget for the San Nicolas deposit and the incorporationof drill-hole information into the inversion generatedenhanced detailed knowledge about the deposit. Theseresults verify that high quality geophysical data andanalysis can greatly increase the efficiency of findingand delineating mineral resources and expedite a drillprogram.

Acknowledgments

The authors would like to thank Teck Cominco Limitedfor their cooperation in providing a comprehensive dataset and allowing this study. We also thank P. Routh,J. Chen, and R. Shekhtman for their help with manyaspects of this study.

References

Johnson, B. J., Montante-Martinez, J. A., Canela-Barboza, M., and Danielson, T. J., 2000, Geology ofthe San Nicolas deposit, Zacatecas, Mexico in VMSdeposits of Latin America, eds. Sherlock, R., and Lo-gan, M. A. V., G.A.C. Mineral Deposits Division spe-cial publication, No. 2, 71-86.

Phillips, N., Oldenburg, D. W., Chen, J., Li, Y., andRouth, P., 2001, Cost-effectiveness of geophysical in-versions in mineral exploration: Applications at SanNicolas, The Leading Edge, Vol. 20, No. 12, 1351-1360.

Routh, P. S. and Oldenburg, D. W., 1999, Inversionof controlled source audio magnetotelluric data fora horizontally layered earth, Geophysics, 64, 1689-1697.

Li, Y. and Oldenburg, D. W., 1996, 3D inversion of mag-netic data, Geophysics, 61, 394-408.

Li, Y. and Oldenburg, D. W., 1998, 3D inversion of gravitydata, Geophysics, 63, 109-119.

Li, Y. and Oldenburg, D. W., 2000, 3D inversion of in-duced polarization data, Geophysics, 65,1931-1945.