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The significance, however, for gold exploration in a previously unrecognised hydrothermal mineralising system with possible links to extrusion of the Gawler Range Volcanics was clearly established.
A subvolcanic epithermal mineralisation model was developed by Aberfoyle geologists as a focus for exploration on northern Eyre Peninsula and the southern margin of the Gawler Range Volcanics.
These three sites have different thermal Ar retentivity characteristics, such that the fluid-inclusion-held Ar is released during step heating at lower temperatures, the OH site-bound Ar is released at moderate temperatures and finally the domains within the sulfate crystal itself are the most retentive, outgassing at furnace or laser temperatures Ar dating of alunite from Phanerozoic (Cenozoic) ore deposits and alteration systems have demonstrated that the ages obtained from alunite in this way are geologically significant and that alunite represents a very useful mineral for dating acidic alteration episodes (e.g. This increase in age suggests that either the alunite underwent slow cooling post-crystallisation, or that it underwent a thermal event that partially modified the age profile within the crystal.Other mineralisation has been linked to this event, including epithermal-style precious metal, lead–zinc, tin and copper occurrences along the southern margin of the Gawler Range Volcanics, on northern Eyre Peninsula. The result confirms the preservation of a Mesoproterozoic epithermal mineral system and adds support for exploration models that anticipate remnants of extensive, interrelated and varied epithermal mineralisation associated with the southern extent of the Gawler Range Volcanics.The timing and relationship of these apparently disparate mineral occurrences, however, remain poorly constrained. During mineral exploration by Aberfoyle Resources in 1991, siliceous breccia in low hills south of Nankivel Dam and 4.5 km WNW of Peterlumbo Hill, northern Eyre Peninsula, was recognised as evidence of shallow crustal hydrothermal activity, most probably related to igneous activity during eruption of the Gawler Range Volcanics (Coutts et al. The observation raised the potential for epithermal-style gold mineralisation.1585 Ma, based on the dating of alunite within the advanced argillic alteration assemblage. These age spectra suggest that the alunite has not remained completely closed to Ar diffusion since initial crystallisation.
However, the alunite samples do not provide flat age spectra (Fig. Rather, in three-step heating experiments on three alunite grains from two different samples, the age spectra obtained show a progressive increase in age from minimum ages of c. In general, alunite has been shown to retain Ar over geological time where the crystals have remained unaffected by secondary alteration processes or thermal events exceeding temperatures of ~200–220 °C (Landis, Snee and Juliani 2005). However, in Cenozoic examples, alunite Ar age spectra typically yield relatively simple age profiles consistent with minimal post-crystallisation disturbance.The age of crystallisation was interpreted from incremental heating plateau ages, defined as a sequence of two or more steps corresponding to a least 50% of the total Ar data (Table 1) implies that these rocks cooled to temperatures below ~350 °C shortly after the Kimban Orogeny (1730–1690 Ma), as revealed by the muscovite cooling ages of c.