b'Discovery of the Havieron Au-Cu deposit, WAFeaturemodel response (dotted red) was systematically too strong to the south and too weak to the north of Body 3 by about 0.1 to 0.2 milligals.The south side of the magnetic anomaly has a steeper gradient than the north side but the anomaly does not exhibit a low to the south as expected for induced polarization in the southern magnetic hemisphere. The lower geological credibility of the dipping model (and to a lesser extent the gravity discrepancy which could have easily been resolved using a very deep body that altered the gravity gradient) led to the inclusion of a remanent vector (Inc = -7.0, Dec = 180 and Q = 2.2). Further adjustments led to the accurate simulation of the entire magnetic anomaly using a simpler, more believable model in the nature of a vertical cylinder, and for which there was no systematic discrepancy in the gravity. The changes led to the model presented above (Figure 6.1 through Figure 7.4) which was offered as an interpretation of the data. Figure 9.2.Three numerical experiments illuminate the effects of density, depth extent and remanence.Introducing the remanent vector required shifting the magneticremanence, the apparent location of the source would be too body southward some 200 m so that the same body, whenfar south (and would exhibit a low to the south that would given a density contrast of 0.11 gm/cc, also simulates theneed to be simulated in some way other than the vertical gravity data. The discovery hole was designed to intersect rockscylinder).represented by the now coincident source of the magnetic and gravity anomalies.IOCG and gabbro potentialAs part of the post-discovery analysis, 25 remanence measurements from three early HAD holes became available.After some initial testing, Body 3, with its inferred 1.0 percent Table 1 is a statistical summary of the results. The medians of themagnetite and 3.7 percent hematite + sulphides, was revised measurements are consistent with the remanence vector usedas a dense, non-magnetic, 40 m thick layer with an intrinsic in the model. density contrast of 0.7 gm/cc (app%ht + s = 25) resting on a deeper 900 m thick magnetic unit having a density contrast Density effect, Depth extent, Effect of remanence of 0.024 gm/cc (as expected for felsic rock with 1 percent magnetite within non-magnetic felsic country rock). This Figure 9.2 shows results from three numerical experiments. Incomprises the IOCG scenario and its response (not shown) the first, the density and susceptibility contrasts of Body 3 weresimulates, and is therefore permitted by, the data. However, set to zero to yield a model response (red dotted and lowermostthe inferred hematite + sulphide component is an order of blue dotted profiles) showing that this body contributes 90 nTmagnitude less than what occurs with known IOCG deposits, to the magnetic anomaly and less than half a milligal to theand with no justification for interpreting prodigious amounts local gravity anomaly. of iron, this mineralisation type was removed from the In the second experiment the 900 m depth extent of Body 3prognosis.was alternately increased by 200 m and decreased by 200 m,In a similar experiment, when the volume of Body 3 was to yield the two (dotted blue) magnetic model responsesassigned a density contrast of 0.37 gm/cc, which is the value closest to the magnetic data profile (solid blue). One responseexpected for mafic rock with one percent magnetite, the is too sharp, the other is too broad. The differences are minormagnetic data was simulated as before but the gravity response but systematic, and are sufficient to suggest a confidencewas too strong by about 0.7 mGal (not shown). Gabbro, interval for the depth extent parameter; however, because theeven though minor mafic units occur locally, was deemed bottoms of the model bodies are flat, unlike what is expectedimprobable as a source for the anomaly.geologically, the conclusion would refer to the overall form of the deposit itself.Magnetite vs pyrrhotiteIn the third experiment in this group the remanence vector was removed and the magnetic susceptibility was revisedIn the original description of the phase/scatter diagram upwards from 0.01 to 0.031 SI notionally to reassign formethod (Hanneson, 2003), the question of pyrrhotite rather induction, magnetite otherwise bound up in remanence. Thethan magnetite as the dominant mineral in the magnetic calculated induced anomaly is the dotted blue response withcategory was raised, and, an easy but unsatisfactory suggestion the 200 m northward shift relative to the data peak. Withoutwas to lump it in with the magnetite and merely tolerate any inaccuracy for want of an alternate solution.Table 1.Statistical summary of remanence data On learning from post-discovery drilling that pyrrhotite is the dominant magnetic mineral, we reset the initial phase/scatter Parameter MinMax Mean StdDev Median diagram parameters for the magnetic component to 1.25 SI and Inclination -83. 73. -9.44 48.8 -7.0 4.65 gm/cc suggested for pyrrhotite by Telford etal. (1980, p121, 28). The physical properties of Body 3 were not altered, and so Declination 77. 349. 202. 71.3 182.the responses still simulated the data; however, the component K-Ratio (Q) 0.30 11.6 3.31 3.1 1.9 percentages were translated differently by the MagGravJ AUGUST 2022 PREVIEW 46'