40Ar/39Ar ages from sericitic alteration in central Gawler gold prospects: timing constraints on gold mineralisation?

This report presents results of a reconnaissance-scale 40Ar/39Ar geochronological study of gold prospects in the central Gawler Craton. Prospects included in the study were: Tarcoola, Tunkillia, Barns, Weednanna and Nuckulla Hill. The aim of the study was to help constrain the age of mineralisation within individual prospects, and then to consider to what extent gold mineralisation throughout the central Gawler Craton was temporally linked. The age results are seen as a first step in testing the idea of a common origin for the prospects, and thereby testing the concept of a "Central Gawler Gold Province".
Nineteen step heating experiments have been completed for samples from gold prospects in the central Gawler Craton. The majority of these samples are sericites from alteration zones spatially associated with gold mineralisation. Sixteen of the nineteen samples yield relatively well-behaved age spectra with the majority of the gas having apparent ages within uncertainty of the age bracket 1570 Ma to 1590 Ma, i.e., contemporaneous with Gawler Range Volcanics (GRV) and Hiltaba magmatism.
In the case of the Tarcoola Goldfield, a hornblende age from a mafic dyke and geological relationships between this dyke and cross-cutting sericitic alteration tightly bracket the age of sericitic alteration at ~1580 Ma. Sericite 40Ar/39Ar ages from this deposit therefore appear to closely approximate the time of sericite crystallisation.
Interpretation of age data from the other central Gawler Gold prospects is less clear at this stage. While the majority of the new 40Ar/39Ar data are consistent with gold mineralisation at ~1580 Â± 10 Ma, the sericite 40Ar/39Ar data can also be interpreted as recording thermal resetting by Hiltaba and GRV magmatism. In the absence of independent evidence for maximum ages, the sericite 40Ar/39Ar ages should therefore be regarded only as minimum constraints on the timing of alteration. A simple first order conclusion is, therefore, that sericitic alteration occurred either contemporaneous with GRV and Hiltaba magmatism or earlier. It should be noted that even if the 40Ar/39Ar ages are interpreted as sericite alteration ages, the relationship between such alteration and gold mineralisation remains to be clearly established at most of these prospects. Addressing this question should be an important component of future studies.
It is recommended that any future geochronology should be attempted only after more detailed petrologic studies on individual prospects have been undertaken. These studies are required to establish in more detail the nature and origin of alteration, the relationship between alteration and gold mineralisation, and evaluate the possibility of multiple alteration and/or mineralising events. Detailed studies at prospect-scale may identify critical relative-timing relationships that could be exploited in focussed geochronology studies to augment the reconnaissance-level results presented here. In particular, maximum age constraints for mineralisation are required at most of the prospects to complement the minimum age constraints reported here. As demonstrated at Tarcoola, maximum age constraints in some situations may be obtained from the age of dykes that are cross-cut by alteration and mineralisation. Such "second-phase" geochronology need not necessarily utilise the 40Ar/39Ar method, but may be more appropriately achieved via U-Pb or Re/Os analyses, depending on the particular questions being addressed.

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