Please use this identifier to cite or link to this item: https://geoscience.nt.gov.au/gemis/ntgsjspui/handle/1/86862
Title: Summary of results. 40Ar/39Ar step-heating hornblende dating, Johnnies Reward prospect, Strangways Range, Aileron Province
Report ID: NTGS Record 2017-013
Year: 2018
Author: McGloin, MV
Matchan, E
Phillips, D
Geological Province: Aileron Province
Arunta Region
Map250: Alice Springs SF5314
Mine or Prospect: Johnnies Reward
Subject: Geochronology
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.
Publisher: Northern Territory Geological Survey
ISSN: 1443-1149
ISBN: 9780724573240 (PDF)
Document Type: NTGS Record
Access Constraint: Creative Commons Attribution 4.0 International (CC BY 4.0). Please observe and retain any copyright or related notices that accompany this material and give attribution to: Northern Territory of Australia (Northern Territory Geological Survey).
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