New-age thermobarometry techniques The association of indicator - PDF document

Lecture IV: Application of modern indicator techniques to data from Greenland and Ontario Herman Grütter UBC Lectures - Mantle Indicators in modern use 30 October 2013 New-age thermobarometry techniques • The association of indicator minerals with diamond or graphite implies pressure (P) and temperature (T) information must be obtained from indicator grains recovered during exploration • The relevant “new-age” P-T approaches for garnet and clinopyroxene are: – T-Ni & P-Cr for G10 or G9 garnets (Ryan et al, 1996) – T-Mn for G10 or G9 garnets (Grütter et al, 1999; Creighton 2009) – P and T for Cr-diopside (Nimis & Taylor, 2000) – P 38 for G10 or G9 garnets (Grütter et al, 2006) • P and T are used to constrain the geotherm for the exploration project and hence the extent of the “diamond window” • Applications discussed: West Greenland & Northern Ontario Slide 2 1

Pyroxene thermobarometry: Kaapvaal (Grp 1’s) 20 20 P NG85 & T NT00 P NT00 & T NT00 Grütter (2009) Lithos 112S 1167–1178 30 30 40 40 P (kbar) P (kbar) 50 50 50 50 GIBEON 60 60 GIBEON 45 45 N LESOTHO N LESOTHO KIMBERLEY MINES KIMBERLEY MINES 35 40 35 40 70 70 Temp (° C) Temp (° C) 600 800 1000 1200 1400 600 800 1000 1200 1400 Xenolith P-T: Opx + Cpx + Gt Cpx single-grain P-T • Kaapvaal Grp1 kimberlites show little variation from “classic” 40 mW/m 2 geotherm • Cpx single-grain P-T results similar to xenoliths, but with larger scatter Slide 3 Pyroxene thermobarometry: Canada 20 20 P NG85 & T NT00 P NT00 & T NT00 P NG85 & T NT00 P NT00 & T NT00 Grütter (2009) Lithos 112S 1167–1178 30 30 40 40 P (kbar) P (kbar) 50 50 50 50 60 60 SOMERSET ISLAND SOMERSET ISLAND SOMERSET ISLAND 45 45 KIRKLAND LAKE KIRKLAND LAKE KIRKLAND LAKE SLAVE LdG & N SLAVE LdG & N N & C SLAVE 35 40 35 40 70 70 600 800 Temp (° 1000 C) 1200 1400 600 800 Temp (° 1000 C) 1200 1400 Xenolith P-T: Opx + Cpx + Gt Cpx single-grain P-T • Canadian geotherms are separated: cold, normal and hot (37, 40 and 43 mW/m2) • Separated P-T arrays also obtained using Cpx, with slightly larger scatter Slide 4 2

Cpx geotherm – Safartoq (Kangerlussuaq), West Greenland 24 Data from Jensen et al (2004) GEUS Report 2004/117 CPX: P NT00 & T NT00 Grütter & Tuer (2009) Lithos 112S 124–129 32 40 P (kbar) 48 SOMERSET ISLAND 56 KIRKLAND LAKE C & N SLAVE GREENLAND 64 Temp (° C) 600 800 1000 1200 1400 Cpx from till samples - Safartoq geotherm is cold (~ 36 mW/m 2 ) - Inside diamond field at T > 850 to 900° C - Whole mantle section sampled by kimberlite sources Slide 5 Merging cpx P-T with garnet compositions 16 24 Data from Jensen et al (2004) GEUS Report 2004/117 CPX: P NT00 & T NT00 G10 G9 Grütter & Tuer (2009) Lithos 112S 124–129 14 32 12 10 40 Cr2O3 (wt%) 8 P (kbar) 48 6 4 SOMERSET ISLAND 56 KIRKLAND LAKE 2 C & N SLAVE TILL GREENLAND 0 64 0 2 4 6 8 10 12 0 3.38 6 CA_INT Temp (° C) 600 800 1000 1200 1400 CaO (wt%) CPX: P-T space GAR: Compositional space (Cr-Ca) No compositional information No P-T information Slide 6 3

Apply T-Mn to Cr-pyrope garnet to obtain mantle section 500 500 G10 G9 G10 G9 T-Mn from Grutter et al (1999) 7IKC Proc V2: 307 600 600 T<900° C GSF 700 700 19% 29% 800 800 GAR T-Mn GAR T-Mn 900 900 SHALLOW DSF 1000 1000 9% 22% 1100 1100 DEEP DSF 1200 1200 4% 15% 1300 1300 <1% 2% 1400 1400 0 1 2 3 4 5 6 0 1 2 3 4 5 6 CA_INT (wt%) CA_INT (wt%) 32% 68% GAR T-X section n = 5921 Slide 7 Safartoq mantle tenor; ≥ 4 garnets / sample Many GSF grains (yellow) Variable deep mantle tenor Source-specific ♦ potential in six distinct areas Isolated samples of interest 60 km Slide 9 4

Safartoq mantle tenor; ≤ 3 garnets / sample Many GSF grains (yellow) Low-count and high-count anomalies correspond Subtle low-count anomalies 60 km Slide 10 Garnet Lake focus area Deep mantle tenor has best ♦ potential No mixing in elevated areas of felsenmeer Mixing into valleys Itisooq Garnet Lake Slide 11 5

16 Prevalence of P 38 ≥ 51, G10D G10 G9 14 and G10 garnets in till samples 12 10 Cr2O3 (wt%) 8 6 4 60 km 2 TILL 0 0 2 4 6 8 10 12 CaO (wt%) 1568 samples 5921 garnets 32% are G10 n=1906 11% are G10D n=627 0.6% have P 38 ≥ ≥ ≥ 51 ≥ Slide 12 Summary: Sarfartoq • Cpx and garnet well preserved in cold climate • New-age P-T techniques easily applied to high quality probe data • ~ 6000 garnets in ~ 1500 samples sufficient to pinpoint diamond potential across property (1 st phase follow-up data set) • T-Mn applied to G10 and G9 garnets provides statistical leverage • Very high % G10 garnets. Most are graphite-facies on a cold geotherm (T < 900° C) => substantial shallow mantle sampling • Diamond potential related to deep mantle sampling of G10D grains • Microdiamond results reflect highly variable diamond potential Grütter & Tuer (2009) Constraints on deep mantle tenor of Sarfartoq-area kimberlites (Greenland), based on modern thermobarometry of mantle-derived xenocrysts. Lithos 112S: 124–129 Slide 13 6

Northern Ontario kimberlites 100 km Kimberlite Rb-Sr age U-Pb age Province (Ma) (Ma) Attawapiskat 156 175-180 Kyle Lake 1123 1076 Slide 14 Which do you prefer ? 16 16 Data from Sage (2000) OGS Open File Report 6019 G10 G9 G10 G9 14 14 12 12 10 10 Cr2O3 (wt%) Cr2O3 (wt%) 8 8 6 6 4 4 Attawapiskat Kyle Lake 2 2 0 0 0 2 4 6 8 10 12 0 2 4 6 8 10 12 CaO (wt%) CaO (wt%) Attawapiskat province n = 3664 Kyle Lake 1 n = 250 Slide 15 7

Cpx P-T: Attawapiskat, Ontario 24 CPX: P NT00 & T NT00 Data from Sage (2000) OGS Open File Report 6019 32 40 Cpx from Victor, Charlie, P (kbar) Gulf & X-Ray 48 Geotherm is “normal” and enters DSF at T ~ 1000° C, like Kaapvaal craton SOMERSET ISLAND 56 KIRKLAND LAKE C & N SLAVE ATTAWAPISKAT 64 Temp (° C) 600 800 1000 1200 1400 Slide 16 Attawapiskat garnet Cr-Ca & T-Mn 16 500 G10 G9 G10 G9 Data from Sage (2000) OGS Open File Report 6019 600 14 700 12 800 10 Cr2O3 (wt%) GAR T-Mn 900 8 1000 6 1100 4 1200 Attawapiskat 2 1300 0 1400 0 2 4 6 8 10 12 0 1 2 3 4 5 6 CaO (wt%) CA_INT (wt%) Normal geotherm, but very limited diamond-facies mantle sampling profile Diamond potential decoupled from G10 grains ? - check low-Cr garnets Slide 17 8

Attawapiskat low-Cr garnets 16 1.6 G10 G9 Data from Sage (2000) OGS Open File Report 6019 Attawapiskat 14 12 1.2 10 GT TiO2 (wt%) Cr2O3 (wt%) 8 0.8 6 0.4 4 Attawapiskat 2 0 0 0 0.05 0.1 0.15 0.2 0.25 0 2 4 6 8 10 12 CaO (wt%) GT Na2O (wt%) Diamond potential related to low-Cr eclogitic / websteritic grains - see AT-56 interpretation (Armstrong et al. 8IKC Proc., 2004) Slide 18 Attawapiskat summary Background image courtesy of De Beers Canada (with permission) • Jurassic kimberlites cutting carbonate host rocks • “Normal” cratonic geotherm • “Normal” diamond window (1000 - 1250° C) • Low % G10 garnets and most are graphite-facies • Significant content of G3 (ECL) & G4 (WEB) garnets, with high Na 2 O content – probable source of high-quality, coarse diamonds Victor rough diamonds Slide 19 9

Cpx P-T: Kyle Lake, Ontario 24 CPX: P NT00 & T NT00 Data from Sage (2000) OGS Open File Report 6019 32 40 Cpx from core samples P (kbar) Elevated geotherm enters 48 DSF at T ~ 1220° C What is the impact of an elevated geotherm ? SOMERSET ISLAND 56 KIRKLAND LAKE C & N SLAVE KYLE LAKE 64 Temp (° C) 600 800 1000 1200 1400 Slide 20 Kyle Lake garnet Cr-Ca & T-Mn Data from Sage (2000) OGS Open File Report 6019 16 500 G10 G9 G10 G9 600 14 700 12 800 10 Cr2O3 (wt%) GAR T-Mn 900 8 1000 6 1100 4 1200 Kyle Lake 2 1300 0 1400 0 2 4 6 8 10 12 0 1 2 3 4 5 6 CaO (wt%) CA_INT (wt%) Deep mantle sampling profile for G10 and G9 grains Elevated geotherm restricts diamond window Deep eclogite could make a big difference – no low-Cr garnet data Slide 21 10

Summary: Kyle Lake • Mid-Proterozoic kimberlites underlying Ordovician carbonates • Very high % G10 garnets; all would fall inside diamond window on a normal geotherm • Cpx data show an elevated geotherm and define a restricted, high-T diamond window (1220 - 1350° C) • Content of low-Cr G3, G4 & G5 garnets unknown • Microdiamonds are present • Macrodiamond content being tested (Metalex Ventures / KWG) Slide 22 Conclusions New-age clinopyroxene and garnet thermobarometry techniques • Permit prioritization of indicator source(s) prior to discovery. Integrate the data streams to simplify the message • Bring into sharp focus the mantle sampling profile and thus the likely presence / absence of diamond • Help to constrain G10-related diamond potential. Hence highlights the relative importance of low-Cr (eclogitic and possibly websteritic) sources for diamond Slide 23 11

Questions ? 12