b'Discovery of the Havieron Au-Cu deposit, WAFeatureRather than creating separate models for the magnetic and gravity data, however, a joint interpretation of the two data sets was made possible through recognition that magnetite, the main and often sole contributor to the local magnetic response, is dense (5 gm/cc), and, in sufficient concentrations, can contribute to the local gravity field. For example, MagGravJ makes it possible to calculate the gravity response of a magnetic phase like magnetite or pyrrhotite separately from the gravity response of dense non-magnetic material like hematite and/or sulphides. Furthermore, MagGravJ uses the physical properties of model bodies to estimate percentages of a dense non-magnetic mineral category like a hematite-sulphide mixture (because they have similar properties) and dense magnetic minerals like magnetite or pyrrhotite (subject to assumptions about the lithology). Terms like App%hts and App%mag are used to report these estimates and are more meaningful to geologists than density and susceptibility, once they become familiar.These are important considerations in the search for iron-oxide- Figure 6.1.Model depth slice; d = 420 m with drill collars and surface trace copper-gold [IOCG] deposits which, in early days, was invokedfor non-vertical holes.as a potential deposit model for the Havieron anomaly. Known IOCGs tend to occur under significant cover, and can exhibit ovoid magnetic and gravity anomalies that appear coincident in plan but normally derive from shallower, dense, non-magnetic, oxidised rocks, that transition to more reduced magnetic rocks at greater depth.Modelling resultsTo apply the method, the magnetic model response is calculated at all points where there is magnetic data, and likewise for the gravity. Changes for subsequent iterations are influenced by plan views of the point-by-point differences and by discrepancies in profiles in numerous cross-sections. The model can be displayed by selecting the horizontal depth slices and vertical cross-sections that best illustrate the model.While the shallow bodies, discussed above, that simulate the supposed cover variations are not shown in plan, depth slices at the unconformity depth of 420 m (known from the historicFigure 6.2.Model depth slice; d = 1320 m.drilling) and at 1320 m are presented in Figures 6.1 and 6.2, respectively. The dark green Body 3 was deemed to be the main drill target; it is 420 m deep, has a depth extent of 900 m, and, upon applying the MagGravJ method, has the density expected for felsic rock averaging a percent magnetite and 3.7 percent of a dense non-magnetic component like a hematite + sulphide mixture. In addition, Body 3 has a shallow, south-directed remanence vector with a Koenigsberger ratio of 2.2. Some large regional bodies extend outside the study area and have been clipped for the presentation.The 1320 m deep yellow-green Body 1 seen in Figure 6.2 has the same magnetic properties as Body 3. It was introduced to simulate a northward bulge in the low amplitude contours of the anomaly and is interpreted to represent deep rocks similar to those represented by Body 3; however, it was not assigned the dense non-magnetic component since its depth makes the gravity data too insensitive to require it.Figure 7.1.Model cross-section and profiles along line P1.Figures 6.1 and 6.2 show straight lines annotated P1 to P4 that indicate the locations of model cross-sections shown in Figureswas not recorded in the archive. Consequently, coloured 7.1 to 7.4 which were designed for proximity to the historiccircles indicate relative values of down-hole susceptibilities in drill holes and show drill collars and down hole points thatlogarithmic intervals from blue (low) to red (high).are within 50 m of the section. A modest amount of magnetic susceptibility readings were available for holes HAC9101 andIn model displays, the bodies are given colours that are HAC0301; values ranged from 0 to 83 but the unitary systemconsistent with the background colour at their plot points on AUGUST 2022 PREVIEW 44'