Iron and sulphur in proportions appropriate to Fe7 S8 were reacted in evacuated quartz tubes for 24 hr at 500 °C. Quenching produces predominantly hexagonal pyrrhotite, whereas further prolonged annealing at 250 °C produces monoclinic pyrrhotite. We mainly report the properties of material cooled from 500 °C at 10 °C min−1, which is predominantly monoclinic but probably contains hexagonal pyrrhotite together with pyrite, FeS2 . This material, which we refer to as ‘non‐ideal’ pyrrhotite, may be a better analogue of pyrrhotite ore than ideal synthetic monoclinic pyrrhotite. The samples were characterized by X‐ray, Mössbauer effect and thermomagnetic analyses. The slow‐cooled non‐ideal monoclinic pyrrhotite exhibits thermal stability intermediate between monoclinic and hexagonal pyrrhotites. One striking observation is that the deviation from ideal monoclinic pyrrhotite increases with decreasing particle size. Particles about 1 μm in size have a low saturation magnetization value of about 6 Am2 kg−1, indicating a lower concentration of ferrimagnetic monoclinic pyrrhotite (saturation magnetization 18 Am2 kg−1) than particles of about 6 μm and above (saturation magnetization 12 Am2 kg−1). On the other hand, magnetization process parameters—coercive force, ratio of saturation remanence to saturation magnetization, coercive force of remanence, median destructive and inductive fields, the magnetic susceptibility—follow well‐behaved power law dependences on particle size, similar to monoclinic pyrrhotite. Magnetic domain patterns in the slow‐cooled non‐ideal pyrrhotite are similar in type and style to those observed in synthetic monoclinic pyrrhotite and in natural pyrrhotite particles in rocks. Dynamic observations of domain walls in changing external alternating and direct magnetic fields are consistent with macroscopic observations of magnetization change with field. The critical size for the monodomain/multidomain transition in synthetic monoclinic pyrrhotite, at about 1 μm, is essentially the same as that previously determined for natural pyrrhotite grains in rocks. The observations suggest that the domain walls lie normal to the c‐axis of the structure, consistent with the known magnetocrystalline anisotropy symmetry of monoclinic pyrrhotite. The type and style of the magnetic microstructures of hexagonal pyrrhotite are quite different from those of monoclinic pyrrhotite. The domain patterns are on a much smaller scale, wavy, and sometimes resemble those of a hexagonal material such as magnetoplumbite or cobalt.

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Monoclinic pyrrhotite was prepared by the silica tube technique. All characteristic properties (X-ray, magnetic) point to the successful synthesis of Fe&. The material was separated into seven size fractions in the particle size range 1-30 pm, in which the particles are believed to contain only few domains. The magnetization process was studied at room temperature. The characteristics of the finest fraction (<0.8pm) are consistent with coherent rotations of monodomain moments and a triaxial symmetry of anisotropy within the basal plane. The in-plane anisotropy constant is about 3 x lo3 J m-3. In the sized fractions (1-30 pm) the variation of ratio of isothermal remanence to saturation magnetization (M,/M,) can be fitted by models of increasing complexity based on the statistically expected number of domains in particles of a particular size. The data can also be fitted by (Mrs/MS) = 0.52exp (-0.047z1'2) where is the mean particle size in pm. The form of this expression is suggestive of the relevance of Poisson statistics to the magnetization process, but the numerical parameter within the exponential is physically implausible. Unlike titanomagnetite (Fe2.4Ti0.604) particles of the same size range, the variation of coercive force (H,) cannot be modelled on the basis of independent surface and volume pinning of domain walls. There is evidence for only one wall pinning mechanism in the 1-30pm range in Fe,S,. A pleasing fit to the data is obtained by H, = 75.5 exp (-0.025t1'2) kA m-'. A similar dependence is found for H,,, the coercive force of isothermal remanence. The susceptibility in the 1-30 pm range appears to be essentially the 'intrinsic' susceptibility and, if so, again provides evidence for only one wall pinning mechanism.

Airborne magnetic and radiometric datasets are used to interpret the geology and geological structural patterns which serve as potential gold mineralization zones in the Kyerano area located at southwestern boundary of the prospective Sefwi Gold Belt and the Kumasi Basin in southwestern Ghana. The geophysical data processing approach adopted concentrated on mapping geological boundaries, geological structures and possible gold mineralization zones is link to hydrothermally altered zones. The application of the enhancement filtering algorithms such as the reduction to the pole and analytic signal to the magnetic data, as well as the ternary radiometric image aided in the mapping of the mafic metavolcanics, basin metasediments and the belt-type granitoid complexes. The first vertical derivative and tilt angle derivative filters helped to delineate fractures, folds, and the contact zones of the formations such as that of the metavolcanics-metasediments that host the main Bibiani Shear Zone. Lineament analysis of the structures using rose diagram, reveals two main tectonic episodes in the area. These are NE-SW and NNW-SSE trending regional structures which account for about 90% of the extracted structures and are associated with the D1 and D2 deformational episodes of the Birimian Formation respectively. These structures are major fracture systems and play a pivotal role in the localization of gold mineralization in the study area.