Summary of results. 40Ar/39Ar step-heating hornblende dating, Johnnies Reward prospect, Strangways Range, Aileron Province

Abstract

This Record details 40Ar/39Ar step-heating analysis of hornblende sampled from the polymetallic Cu-Pb (Zn-Ag-Au) Johnnies Reward prospect, ALICE SPRINGS, in Aileron Province, central Australia. A sample of hornblende (AS15MVM034) was collected from an outcropping quartz vein associated with pegmatite intrusions that cross-cut the Cadney Metamorphics and mineralisation at the prospect. Hornblende was dated to determine the timing of vein formation and any potential overprinting episodes of fluid-flow and/or metamorphism. If the quartz vein is pegmatite-related, any age yielded may date pegmatite emplacement. Because the sampled quartz vein cross-cuts the mineralised host assemblage, dating of the hornblende will also constrain a minimum age for mineralisation at the prospect.

Hornblende from sample AS15MVM034 produced a discordant age spectrum of anomalously old apparent ages of ca 3.8 -1.6 Ga for the low-temperature steps (steps 1-3), decreasing to ca 900-800 Ma (steps 4-5), and then to ~650 -600 Ma for the remaining 55% of the gas release profile (steps 6-12). Inverse isochron ages for the same sample (making no assumption about initial 40Ar/36Ar sample values) produced concordant data for the mid- to high-temperature (steps 6 to 12), and revealed the presence of excess argon in the sample. This indicates that age spectrum ages (which are calculated assuming an atmospheric initial 40Ar/36Ar ratio) are verestimated. The inverse isochron age generated through steps 6 to 12 is 428 ± 31 Ma (2s; MSWD = 1.6, probability of fit = 0.16). This age is consistent with the timing of the ca 450-300 Ma Alice Springs Orogeny and provides a minimum age constraint for mineralisation at Johnnies Reward.