Magnetic characteristics of Broken Hill type deposits and their host provinces

In technical geological publications it has been demonstrated that Broken Hill Type deposits and their host provinces are characterised by similarities in their geological settings and ore deposit mineralogy at both local and regional scales. The purpose of this thesis was to examine magnetic characteristics of the globally most significant BHT deposits, and their host provinces, in order to develop a magnetic exploration model for BHT deposits. This involved detailed aeromagnetic interpretation of available datasets with reference to mapped geology. Four significant BHT provinces were studied: Broken Hill Block - Australia Eastern Succession, Mt Isa Block - Australia Bergslagen District - Sweden Aggenys -Gamsberg District - South Africa The study showed that host provinces to BHT deposits share common magnetic characteristics which reflect common geological characteristics. However, significant differences in the aeromagnetic character occur which are related to differences in volume of BHT host stratigraphy, basement stratigraphy and structural style of the host province. BHT deposits naturally divide into two classes, based on the presence or absence of magnetic minerals within the ore. This may result from differing ore-forming environments and/or fluid chemistry. Deposits which contain magnetic minerals within the ore are commonly associated with magnetic anomalies which are amongst the strongest within the host meiasedimentary /metavolcanic sequence. Deposits which do not contain magnetic minerals within the ore itself are associated with a locally anomalous signature when the immediate environs of the ore deposit are considered. The most economically signifcant deposit, Broken Hill (Australia), is of this type, although both types include large tonnage deposits. Thermodynamic modelling was undertaken to test whether the variations of magnetic deposit signatures could be explained by variations in the environment of exhalation. The roles of boiling, cooling and seawater mixing (oxidised and reduced) as precipitation mechanisms were examined. The modelling showed that a single fluid can give rise to a spectrum of iron oxide distribution, depending on whether the dominant precipitation mechanism was boiling, cooling, mixing with oxidised sea water or mixing with reduced sea water. Precursors to metamorphosed BHT ore and gangue minerals were produced by several of these thermodynamic models. It is concluded that the interpreter, when defining the likely spectrum of magnetic responses of a BHT deposit, must consider the likely range of environments of deposition consistent with known geology.