Partitioning and Distal-to-Proximal Trends of Trace Elements Among Phyllosilicates in Altered Komatiites from the Kerr-Addison Gold Deposit, Ontario, Canada

In situ techniques for measuring trace elements in rock-forming minerals, e.g., laser ablation-inductively coupled plasma-mass spectrometry, have become key tools for understanding the development of ore systems. However, knowledge gaps exist in our understanding of the impact of crystal structure on trace-element substitution in phyllosilicates. This is coupled with the paucity of corresponding data on phyllosilicates in orogenic gold environments, despite these minerals being intimately associated with mineralization. Given these knowledge gaps, a focused study on the altered komatiitic rocks of the 513 t Au Archean Kerr-Addison orogenic gold deposit (Virginiatown, Ontario, Canada) was undertaken. This contribution seeks to (1) document and explore the crystal-chemical controls behind trace-element distributions in phyllosilicates (talc, clinochlore, and muscovite), with a focus on Goldschmidt’s rules, as well as to provide the first known dataset for trace-element incorporation in talc, and (2) apply trace-element incorporation in phyllosilicates to understand the processes involved in the formation of these minerals in relation to ultramafic-hosted orogenic gold deposits. Results show the following: (1)The partitioning behaviors of elements in phyllosilicates can be described based on geochemical groupings: compatible R2+ cations (Mg, Mn2+, Fe2+, Co2+, Ni, and Zn; clinochlore ∼ talc ≫ muscovite); R3+ cations (Al, V3+, Cr3+, and Ga3+; muscovite > clinochlore ≫ talc); large-ion lithophile elements (Na, K, Rb, Sr, Cs, Ba, Tl+, and Pb2+; muscovite ≫ clinochlore > talc); Li (clinochlore ≫ muscovite ≫ talc); high-field-strength elements (Ti4+, Zr4+, Nb3+/5+, Sn4+, Sb3+/5+, and W4+; muscovite ≫ clinochlore > talc); the light elements Be and B (muscovite > clinochlore ∼ talc); along with the chalcophile semi-metals Ge and As (clinochlore > talc > muscovite).(2)Despite having generally similar partitioning behaviors, the R3+ cations show varying distal-to-proximal behaviors in clinochlore (Al increases; V3+ increases from the talc-chlorite to carbonate-chlorite facies, then decreases moving into the carbonate-muscovite facies; and Cr3+ and Ga3+ show ambiguous distal-to-proximal trends) and muscovite (Al increases; V3+ decreases; and Cr3+ and Ga3+ show ambiguous distal-to-proximal trends). This is attributed to V preferentially partitioning into muscovite, as well as competing isovalent R3+ substitutions.(3)Clinochlore shows significant enrichments in Li (55 to 400 ppm), interpreted to be related to fluid input, and the variable mobility of Li between the carbonate-chlorite and carbonate-muscovite facies suggests that the alteration assemblages may have been produced by multiple fluids of different chemistries. Clinochlore also shows depletions in Ti (160 to 69 ppm) and Co (100 to 1.0 ppm), associated with the development of Ti- and Co-bearing minerals (rutile and the cobaltite–gersdorffite series, respectively).

These results imply that the incorporation of trace elements into phyllosilicates can be elegantly explained based on Goldschmidt’s rules, with differences among ionic radii of the elements being considered as having a greater tolerance on their incorporation as compared to differences in valence. Secondary trends in trace-element distributions can be considered with respect to element partitioning in coexisting minerals and fluids. Finally, the lack of spatial variability with respect to muscovite chemistry at Kerr-Addison suggests that muscovite chemistry may be a useful tool for global-scale comparisons of the evolution of ultramafic-hosted gold deposits over time and space.