Episodic alteration within a gold-bearing Archean shear zone revealed by in situ biotite Rb–Sr dating

Abstract

Many economic gold deposits formed in the Neoarchean under greenschist facies conditions in association with ductile–brittle shear zones that were conduits for sulfur-bearing hydrothermal fluids. Although the large-scale architecture and formation history of Neoarchean structurally controlled gold deposits has been the subject of many studies, the timing, rates, and durations of mineralization-related alteration remain challenging to deconvolve owing to a lack of event-specific geochronometers and the potential multitude of overprinting processes. In an effort to better understand the alteration history along the mineralized Zuleika Shear Zone in the Eastern Goldfields Superterrane of the Yilgarn Craton in Western Australia, we present the results of biotite Rb–Sr from 13 drill core samples (seven with apatite Sr isotopes) from the Kundana gold camp and the Carbine area. The Kundana district is a lode gold deposit that represents an ideal case study area to test the longevity of mineralization-associated alteration systems. We use in situ laser ablation collision cell triple quadrupole mass spectrometry on biotite from a range of characteristic alteration assemblages with well-defined relative growth ages. Biotite Rb–Sr ages constrain the timing of four different alteration stages between ∼2700 and 2550 Ma. The correlation between relative alteration chronology and Rb–Sr age and the lack of any relationship between grain size and age supports an interpretation of Rb–Sr ratios reflecting biotite growth during hydrothermal alteration with subsequent rapid cooling. Initial 87Sr/86Sr ratios of biotite and apatite are similar and indicate compositions that are more radiogenic than chondritic values and generally slightly more radiogenic than modelled Neoarchean Bulk Silicate Earth values, consistent with a mixture of mantle and crustal fluid sources. Ti-in-biotite thermometry yields temperatures between 540 and 615 °C, peaking towards the main gold mineralization phase. Although it is not possible to link individual Rb–Sr ages to specific faulting events, the absolute age dating for the various alteration assemblages indicates a prolonged, multiphase alteration system lasting ∼150 Ma. Long-term permeability networks with episodic short-lived fluid pulses that outlast terrane- or camp-scale deformation may not only apply to Neoarchean lode gold deposits, but to any tectonically driven zone of weakness in the continental crust.