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Understanding physical property : mineralogy relationships in the context of geologic processes in the ultramafic rock-hosted mineral deposit environment : aiding interpretation of geophysical data Sterritt, Victoria Athena

Abstract

Inversion of potential field geophysical data to generate physical property models is becoming increasingly common in the exploration industry. This study aimed to develop relationships between physical properties and mineralogy in ultramafic rock-hosted mineral deposits, based on an analysis of the crater facies-dominated Anuri kimberlite (Canada) and the intrusive magmatic sulfide deposit at Kabanga (Tanzania). Physical property distributions for rock types and minerals that contribute to density and magnetic susceptibility were characterized in both deposits. Magnetic susceptibility is directly related to magnetite abundance, which is an igneous phase and is produced by serpentinization of ultramafic rocks of both deposits. Magnetite in kimberlite also occurs in crustal xenoliths, which dilute the diamond content. Consequently, susceptibility in the Anuri kimberlite is inversely related to diamond grade. In the Kabanga magmatic sulfide deposit, susceptibility is unrelated to ore content, but does indicate the degree of serpentinization. Density is a function of dense minerals concentrated with ore during primary sorting and settling processes in both deposits. As such, density is directly correlated with ore in both volcaniclastic kimberlite breccia of the Anuri kimberlite and ultramafic rocks at Kabanga. However, serpentinization decreases the density of ultramafic rocks significantly, masking any density anomalies associated with sulfide minerals. Modeling demonstrates that a pervasively serpentinized rock with up to 50% sulfide minerals can have a density equal to that of a barren ultramafic rock. A combination of susceptibility and density can be used to identify high-grade rocks in both deposits. In the Anuri kimberlite, rocks with high diamond contents have susceptibilities less than 10 x 10⁻³ SI and densities of 2.42 - 2.51 g/cm³. In magmatic sulfide deposits, susceptibility and density can be used to accurately calculate ore mineral abundances. Relationships developed between physical properties and mineralogy for these deposits can not only be applied to other crater facies-dominated kimberlites and both intrusive and extrusive magmatic sulfide deposits, but also to other ultramafic rock-hosted mineral deposits with comparable geologic processes. Consequently, both magnetic and gravity surveys can be interpreted in combination to give a powerful remote tool in predicting grade.

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