Mapping the thermal history of South Australia: Insights from apatite `double dating`

Transcription

1 Mapping the thermal history of South Australia: Insights from apatite `double dating` Stijn Glorie, Alan Collins, James Hall Anthony Reid, Rian Dutch, Mark Pawley SA Thermochronometry Hub ARC LE Apatite studies to `map` preserved thermal history Case studies: northern Gawler, eastern Musgraves, Olympic Domain Apatite U-Pb map of the northern Gawler Craton 1

2 Image by A. Gleadow SA Thermochron Hub Simultaneous acquisition of 2 apatite `dates`: Apatite Fission Track + Apatite U-Pb Automated Fission Track Analysis LA-ICP-MS application Basement samples Apatite separate 238 U 238 U lf l Fission tracks 206 Pb, 207 Pb Image by A. Gleadow What do these `dates` mean? 2

3 Temperature SA Thermochron Hub Apatite Fission Track - Fission tracks `anneal` between 120 & 60 C - Date timing of cooling through this T window - Simple or complex Fission track density Fission track lengths Thermal history model Image by A. Gleadow Image by A. Gleadow time Apatite U-Pb - Pb diffuses above ~ C - Date timing of cooling through this T - Similar T as for mica Ar-Ar clocks Exhumation/ preservation level of crust AFT map Exploration Tool Kohn et al

4 Apatite double dating as exploration tool McInnes et al Example for porphyry Cu systems: `hockey stick` tt history R1 (handle of the stick): Magmatic-hydrothermal cooling Hypogene ore formation (pluton + Cu-shell) Apatite U-Pb R2 (blade of the stick): Exhumation cooling Exhumation and preservation + supergene remobilisation AFT Two dates reveal two parts of evolution ore system Ap U-Pb comparable temperatures to mica Ar-Ar, but much faster & cheaper AFT used to be rather slow: >6 months separates to data. Now: <1 month 4

5 AFT `mapping` for exploration `Could my ore deposit be preserved here? and be at minable depth?` AFT predicts (relative) crustal exposure level Deep enough exhumed? Or perhaps too deep? ~800 Ma ~500 Ma `Has this structure been reactivated and perhaps remobilised ores?` AFT can detect thermal footprint of hydrothermal fluid alteration e.g. short-lived (10k yr) at 200 C can be detected over 1km from conduit ~600 Ma ~400 Ma ~200 Ma ~400 Ma ~600 Ma Hydrothermal event ~200 Ma 5

6 AFT in South Australia Previous low-t studies: - Large AFT dataset available (e.g. Gleadow et al. 2002) - Mostly focussed on the Australian margins - Inner Australia is largely unstudied - Basement in these regions is mostly buried New potentials: - Growing availability of drill-core samples - => Potential to develop exhumation map of covered regions such as much of Inner Australia Gleadow et al Kohn et al

7 AFT in South Australia AFT map to date... Shows first-order anomalies Case study 1: Thermal anomalies in northern Gawler Case study 2: Differential exhumation eastern Musgraves Case study 3: Thermal anomalies in the Olympic Domain mine site Kohn et al

8 Northern Gawler AFT Age P1: 609 +/- 15 Ma P2: 299 +/- 3 Ma Precision 126 U (ppm) AFT Age AFT Age Challenger mine Thermal events differentially preserved with respect to the Karari Fault 116 Precision Precision 8

9 Temperature Eastern Musgraves t Thermal events differentially preserved w.r.t. structural architecture 9

10 Temperature ( C) Olympic Domain AFT Age AFT Age AFT Age RED 2 ASD 1 Blanche 1 P1: 395 +/- 32 Ma P2: 192 +/- 8 Ma P1: 389 +/- 20 Ma P2: 148 +/- 6 Ma P1: /- 59 Ma P2: 425 +/- 25 Ma Precision Precision Precision Silurian early Devonian ages Observed throughout O.D. Late Mesoproterozoic ages Only preserved at margins Limited exhumation Mesozoic thermal `corridor` Linked with mine locations Deeper exhumed? General Blanche 1 thermal reset? Age (Ma) Age (Ma)

11 Apatite U-Pb for mineral exploration `When did the mineralisation occur?` Example: Ernest Henry IOCG deposit, Cloncurry Mineral liberation analysis (MLA) shows coarse hydrothermal apatite, brecciated & veined by chalcopyrite/pyrite Apatite growth = associated with early stage ore formation Age = unknown but can be determined by U-Pb Apatites from nearby deposit ~1.18 Ga significantly younger than previously perceived (UoA Hons thesis 2016 Maughan) UoA Hons thesis 2016 O`Brien 11

12 Apatite U-Pb for mineral exploration `Can apatite vector to exploration targets?` Phosphorus (apatite) ppm >2000 ppm Cross section of E1 deposit (IOCG) deposit, Cloncurry P (apatite) recognised as a 20-50m alteration halo Used to vector towards other exploration targets Ap age = unknown 12

13 Ap U-Pb map We haven`t specifically targeted ore systems yet with this method Apatite U-Pb map for the Gawler Shows promising similarities with existing mica Ar-Ar age data 28 samples (~1000 apatites) All dated this year 13

14 Conclusions o Apatite `double dating` has promising applications for mineral exploration o It`s fast, cheap, can be done straight from thin section o Our preliminary data indicates the ability to identify different crustal exhumation/preservation levels for several parts of SA o Thermal anomalies were observed in the Olympic Domain o Preliminary Apatite U-Pb map indicates similarities with conventional (but rather slow and expensive) Ar-Ar data o Ability to date mineralisation and to use it as exploration vector (?) needs to be further investigated 14