b'Pyritethe firestoneFeatureIn the tapped coil, variable capacitor, primitive radio receiver, theMaxwells equation. For dispersed conductive spheres, one cats whisker crystal detector could be galena, but pyrite was betterapproximate version of this equation is (Shuey 1975):with regard to: easy placement of the cats whisker, withstanding mix= [(1+2p)/(1-p)]mambient conditions, continuity of sensitivity over long periods, and tonal purity in reception. However, apparently, only certain limitedwhere p is the volume percent of porphyritic metallic,mis types of pyrite were suitable, so galena was more frequently used.the conductivity of the continuous, insulating matrix, andpThe exploration relevance or significance of asymmetric currentthe conductivity of the dispersed metallic is assumed to be conduction, or diode effect, in natural sulphide types, would seemmany orders of magnitude greater thanm(so it does not to warrant some study. appear in the equation). Form= 10-3 S/m, p = 10%, the overall conductivity of the mix is 0.0013 S/m (> 752 ohm m res.) barely Pyriteconductivity above the matrix, as one would expect. But in EM testing of such material, a response is obtained from small eddy current Samples loops confined to individual particles and not from eddy currents circulating around the entire core. This gives a pseudo-There is a considerable spread of conductivity values reportedconductivity dependent on particle conductivity, concentration, for pyrite, 1 to 100 000 S/m (Shuey 1975). To contributediameter and the core diameter (Yang and Emerson 1997). Here to understanding better pyrites electrical characteristics,the measured conductivity is:opportunity was taken to carry out conductivity measurements a= pp(d/D)2on the suite of 36 pyritics listed in Table 3 in the following ten categories: large single crystal (#14), massive medium towherepis the conductive particle conductivity, d the coarse grained low porosity aggregate (#510, 36-nodular),particle diameter, and D the core diameter. For 10% crystal high porosity aggregate (#1115), high alteration (#9), massivepyrite,p = 4000 S/m (say), d = 1mm, D = 25.4mm core, very finely crystalline with minor silicates and other sulphidesthena= 0.62 S/mquite a different result, but of no use in (#1619), massive with minor pyrrhotite and other sulphidesestablishing the actual conductivity of the core. However, (#2023), minor pyrite-networked veinlet (#25, 26), fine grainedthe equation is useful in gauging the conductivity of the banded (#2733), fine grained banded with minor chalcopyriteparticles ifa , p, d, and D are known. [This effect is quite (#24, 34, 35). The pyrite in all the samples is networked i.e. therepronounced in the case of native copper platelets or crystals, is electrical continuity through the core or across the core inset in a resistive matrix, owing to coppers extremely high the case of banded samples drilled normal to foliation to giveconductivity, 60 x 106 S/m.] Any inductively measured maximum flux coupling in the EM energisation producing eddysample suspected of isolated metallic particle behaviour currents around the core. Disseminated pyritics with grains orshould first be tested with a galvanic two electrode ohm clusters of grains electrically isolated from one another were notprobe to establish sulphide electrical continuity over the included in the sample suite for reasons given below. entire core, or in bands in the core (i.e. is the sulphide networked?). If the sulphide particles are completely isolated Measurements electrically then all that the test core induction coil EM Laboratory mesoscale measurements were carried out onresponse tells the geophysicist is that there are sulphides or cored, or shaped, air-dried samples for electrical conductivityother conductors disseminated in the core.and magnetic susceptibility. Induction coils were used andAnother pitfall is worthy of mention. Core testing by the EM energised to 1 MHz for induced electromagnetic conductivityconductivity method for conductivities upwards of a few 100s and 400 Hz for magnetic susceptibility. Changes in themS/m, is convenient and fast. However, on no account should resistance (R) and inductance (L), when cores were inserted,a core be held by the fingers in any induction coil as the coil were measured by an RCL metre. Following Yang and Emersonwill couple to the fingers through the core. The fingers of a (1997) conductivity was determined from R, and susceptibilityhuman hand have a conductivity of a few S/m, so a resistive from L. Mass properties were measured, following Emersoncore will show a quite spurious conductivity. Although this does (1990), so that the conductivity data could be viewed in thenot matter too much for a very conductive core e.g. massive perspective of density. Although the writer has carried outpyrrhotite, it is good practice, if a core has to be held in a coil, many galvanic measurements on pyritics, EM conductivityto use plastic tongs. In using a short coil to scan long lengths of was the preferred technique here. The EM measurementbenched core, hands-off measurement is essential.(Figure 10) is not responsive to insulating minerals, it just sees conductors and induces eddy currents in them; also it is quickerResultsto do. Lab EM favours conductive features normal to the coreThe mass property, magnetic susceptibility, and EM conductivity axis; galvanics, parallel to the core axis. The differences, whichdata are given in Table 3. Ward (1966) defines a massive do exist for banded pyritics, are related to texture and willsulphide as being at least 50% by volume sulphides and having not be dealt with here where only maximum conductivitiesa minimum density of 3.8 g/cc. However, for this 36 sample are presented and plotted. Auxiliary four electrode DCdata set it is deemed preferable to classify the 25 samples with galvanic measurements were made to check some of the EMair dried bulk densities exceeding 4.2g/cc as massive, the five measurements, and two electrode galvanic microprobing wassamples (#2831, 35) with densities in the 3.74.0 g/cc range as also undertaken in investigating alteration films and pockets. semi massive, and the remaining six samples (#2527, 3234) Disseminated pyrite is not included in the test suite. Such pyrite,in the 2.73.3 g/cc range as low density pyrite rock. Half the dispersed and disconnected in a resistive matrix, is not suitablesamples tested have inferred grain densities in excess of 4.6 g/cc for measurement by EM induction. The conductivity () ofattesting to their heavily pyritic nature. The presence of minor such a mix would be quite low. It is best addressed by galvanicamounts of sulphate alteration, silicates, and sulphides such as methods and modelled by a mixing law such as a modifiedsphalerite, will result in densities below the nominal 5.0 g/cc DECEMBER 2019 PREVIEW 60'