Goswami, Naina1,2, Forster, Marnie1,2, Reid, Anthony3
1 Research School of Earth Sciences, Australian National University, Canberra ACT 2601, 2 MinEx CRC, Canberra ACT 2601, 3 Geological Survey of South Australia, Adelaide SA 5001
40Ar/39Ar ultra-high-vacuum (UHV) step-heating experiments can routinely provide key data using small samples taken from drill core, in this case revealing the timing of the evolution of alteration zones, in the Anabama Granite. 40Ar/39Ar geochronology was conducted in conjunction with 39Ar diffusion experiments on K-feldspar, white-mica and biotite from different depths from seven core samples, and provided evidence for alteration in what appeared to be unaltered host rocks. This is because potassium feldspar can grow and regrow during periods of alteration and metasomatism, with the timing of these events constrained precisely by the ages of the high retentivity-core domains. White-mica and biotite behave differently to K-feldspar due to their hydrous nature, crystal structure and chemical composition, but nevertheless can still preserve important age information and retentivity data that helps constrains the evolution of temperature with time, as extracted by conjoint inversion of the data from 39Ar UHV diffusion experiments and 40Ar/39Ar geochronology.
The Anabama Granite is hosted within weakly metamorphosed metasediments in the Adelaide fold belt. The proposed early Ordovician age of ~468Ma is based on a Rb/Sr isochron, implying emplacement at the climax of the Delamerian Orogeny. The granite shows progressive stages of alteration grading from fresh granitic rocks to greisen envelopes hosted in variety of lithologies (granite, granodiorite and adamellite). The region is host to variety of alteration styles introducing secondary minerals such as quartz-sericite-pyrite (phyllic alteration), epidote-chlorite-albite (propylitic alteration), fleshy-pink K-feldspar (potassic alteration), kaolinite-illite (argillic alteration) and precious metals such as Cu, Mo in various sulfide-bearing phases (sulfide-mineralisation) which are indicative of existence of different environments and range of temperature conditions during these process(s). A Cu-Mo prospect has been indentified, but its geological history is yet to be completely deciphered.
Our results show younger ages of ~375-371 Ma preserved within the retentive cores of K-feldspar domains that otherwise show slow cooling from a minimum age of ~380Ma with growth of highly retentive new K-feldspar at ~360Ma. The co-existing white-micas when analysed, reflect an older age of ~457Ma, thus in agreement with previously determined K-Ar ages for muscovite in greisen from alteration zones, at ~455Ma. However, since microstructural analysis demonstrates the presence of K-feldspar overgrowing previously crystallised white-mica, we suggest the presence of an as yet unrecognised alteration event. This later-stage potassic alteration did not exceed temperatures required (or was not of sufficient duration) to reset argon-systematics in mica. These ages can be correlated with ages of thermal perturbations in the Lachlan Fold Belt (LFB) to the east, in which case Anabama Granite represents a western extent of the metamorphic and deformation imprint.
This work has been supported by the Mineral Exploration Cooperative Research Centre (MinEx CRC) whose activities are funded by the Australian Government’s Cooperative Research Centre Program.
Naina Goswami is a second year PhD candidate at The Australian National University (ANU). She finished her Masters (advanced) from ANU in 2018 and her B.Sc (Hons) Chemistry from University of Delhi 2016. She specialises in 40Ar/39Ar geochronology and geochemistry. Currently she is undertaking her PhD at ANU working on a MinEx-CRC project based in South Australia.