Exploration and Discovery of the Chidliak Kimberlite Province, Baffin ...

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Jun 22, 2017 - Canada's Newest Diamond District ... The southern half of Baffin Island, in the Canadian Arctic, ...... k = 1870 9 10-5 SI and Q = 9.9 (Table 4).
Exploration and Discovery of the Chidliak Kimberlite Province, Baffin Island, Nunavut: Canada’s Newest Diamond District Jennifer Pell, Herman Gru¨tter, Sonya Neilson, Grant Lockhart, Shawn Dempsey, and Hugo Grenon

Abstract

Sixty-two kimberlites discovered on Hall Peninsula, southern Baffin Island, Nunavut between 2008 and September 2011 comprise Chidliak, the most recently discovered kimberlite province in Canada. These discoveries were the result of traditional kimberlite exploration techniques used in glaciated terrains, including kimberlite indicator mineral (KIM) sampling and analysis, ground and airborne magnetic and electromagnetic geophysical surveys, and ground prospecting and drilling. Capacitively coupled resistivity ground surveys and comprehensive KIM classification and interpretation techniques also played a significant role. Both sheet-like and larger pipe-like bodies have been discovered. The sheet-like bodies are mainly steeply dipping hypabyssal kimberlites, which may contain basement xenoliths. The majority of the pipe-like bodies have pyroclastic or apparently coherent kimberlite infill containing sedimentary xenoliths derived from now-eroded Paleozoic strata in addition to basement xenoliths. The presence of these sedimentary xenoliths, along with other textural features, suggests that at least some of the apparently coherent kimberlites are not intrusive but are extrusive, either effusive or clastogenic in origin. Many of the kimberlites manifest as strong remanently magnetized 0.5–2.0 ha bodies that are expressed as magnetic high and low anomalies with associated weak and shallow conductivity responses. Bodies dominated by pyroclastic kimberlite infill can have neutral to weak magnetic signatures, appear to be more conductive, and can be much larger (up to 4–5 ha) than those with dominantly coherent kimberlite infill. Perovskite U–Pb dating of 25 of the kimberlites indicates magmatism spanned a period of approximately 18 million years, from 156 to 138 Ma (Late Jurassic to Early Cretaceous). Keywords

Exploration



Indicator mineral sampling



Geophysics



Kimberlite geology



Age dating

Introduction

J. Pell (&)  S. Neilson  S. Dempsey  H. Grenon Peregrine Diamonds Limited, Vancouver, BC, Canada e-mail: [email protected] H. Grütter BHP Billiton Canada Inc., Vancouver, Canada G. Lockhart Petra Geophysical Consulting Inc., Kelowna, Canada

The southern half of Baffin Island, in the Canadian Arctic, was targeted for greenfields diamond exploration by BHP Billiton and Peregrine Diamonds Ltd. in 2005. The area was selected for exploration because the regional geology was known at reconnaissance-level and the area was thought to be relatively underexplored. Further, 1.85–2.92 Ga aged zircons had been recovered from basement ortho- and paragneisses in the region (Scott 1999), suggesting that at least

D. G. Pearson et al. (eds.), Proceedings of 10th International Kimberlite Conference, Volume 2, Special Issue of the Journal of the Geological Society of India, DOI: 10.1007/978-81-322-1173-0_14, Ó Springer India 2013

209

210

a portion of the region might be underlain by Archean-aged cratonic basement. As a result of the 2005 sampling campaign, kimberlite indicator minerals (KIMs) were recovered from 5 of the 166 reconnaissance glacial till samples on the Hall Peninsula (Fig. 1). Follow-up KIM sampling in 2006 better defined the anomalous KIM distributions. The initial land tenure on the Chidliak project was secured in 2007 and additional glacial till samples were collected at this time. In 2008, an airborne geophysical survey was completed over part of the project area and the first three kimberlites were discovered by surface prospecting. In 2009, exploration rights were secured on the Qilaq project adjacent to the Chidliak project (Fig. 1). Between 2009 and 2011, continued till sampling and the implementation of more sophisticated KIM classification and interpretation techniques, as well as further airborne surveying, ground geophysical surveys, ground prospecting/ mapping and drilling led to further discoveries (Table 1). The Chidliak project area now hosts 59 kimberlites and 3 kimberlites are present in the Qilaq project area. These occurrences span an area of approximately 70 km in a north–south direction and 40 km east–west, referred to herein as the Chidliak kimberlite province (Chidliak). Forty-six of the 62 kimberlites have been tested for

J. Pell et al.

diamonds using caustic fusion analyses and all but four are diamondiferous. Commercial-sized (+0.85 mm) diamonds were recovered from the initial samples of 18 of the kimberlites in the Chidliak kimberlite province (Pell 2008, 2009, 2010; Pell and Farrow 2012).

Geological Setting The geology of the Hall Peninsula is poorly understood as it has been mapped only at a reconnaissance scale (Blackadar 1967). Based on this early work and a reconnaissance geochronology study (Scott 1996), the peninsula has been divided into three major crustal entities (Scott 1996, 1999; St-Onge et al. 2006), which, from west to east, are the Cumberland Batholith, a central belt of Paleoproterozoic metasediments and an eastern gneissic terrain now termed the Hall Peninsula block (Whalen et al. 2010). The Cumberland Batholith comprises mainly granulite facies intracrustal (I-type) granitoids that are *1.865–1.845 Ga in age (Whalen et al. 2010). The central Paleoproterozoic supracrustal belt is a metamorphosed continental margin shelf succession that has been correlated with the Lake Harbour Group strata on the MetaIncognita Peninsula (St-Onge et al.

Fig. 1 Simplified geological map of southern Baffin Island showing the major tectonostratigraphic assemblages and bounding crustal structures (after St-Onge et al. 2006 and Whalen et al. 2010). The Chidliak project area is marked in blue and Qilaq in yellow

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Table 1 Exploration statistics for the Chidliak project Year/activity

2005

2006

Till samples collected (#)

166

232

Probe confirmed KIM-positive till samples (#) Analyzed KIMs from till (#)

2007 872

2008 221

5

31

275

102

44

460

3,811

1,798

Airborne geophysics (line-km) Ground geophysics (line-km) Anomalies ground-checked (#) Kimberlite discoveries (#)

2009

2010

2011

Total

1861

1402

1039

5,793

126

172

400

1,111

2,679

3,593

9,587

21,972



20,442

15,178

47,320

1,096

1,884

2,113

5,250

12

63

113

99

287

3

13

36

10

62

3,952

7,672

8,865

20,489

1,445

1,713

3,158

11,700 157

Core drilling (m) RC drilling (3.625-inch, m) Analyzed KIMs from kimberlites (#)

1,635

5,451

8,487

17,447

33,020

Microdiamond sample processed (kg)

899

3,404

7,472

7,771

19,546

49.7

61.3

32.5

143.5

Mini-bulk sample processed (t)

2006). The Hall Peninsula block comprises Archean orthogneissic and supracrustal rocks of *2.92–2.80 Ga age and possibly, younger clastic rocks that have been tectonically reworked to some degree (Scott 1999). All of the kimberlites discovered at Chidliak are hosted by rocks of the Hall Peninsula block (Fig. 1). It has been speculated that the Hall Peninsula block may be: (i) part of the Nain/North Atlantic craton (Scott 1996; Scott et al. 2002; St-Onge et al. 2002); (ii) reworked Archean gneisses correlative with those of the Trans Hudson Orogen in Canada and the Nagssugtoqidian Orogen of west Greenland (St. Onge et al. 2007); or, (iii) one of several microcontinents that were accreted during a two-phase, three-way collision of the Superior, Rae, and North Atlantic cratons that occurred between 1.865 and 1.79 Ga (Snyder 2010; Whalen et al. 2010). Additional geological mapping and geochronology studies are required to resolve the tectonic affinity of the Hall Peninsula block and the underlying diamond-bearing lithospheric mantle. Despite uncertainties in the geological history of the region, the area was targeted for diamond exploration in part because zircon-dating evidence (Scott 1999) recorded the presence of Archean strata. The Quaternary geology of the peninsula is complex and poorly understood. The area was inundated by the Laurentide Ice Sheet during the last glacial maximum (LGM; approximately 18,000–9,000 years B.P; Dyke 2004; Dyke et al. 2003), and remnants of this ice sheet persist at Chidliak to the present day, at approximately 700 m above sea level. Regional ice flow directions were believed to be dominated by the Hall Ice Divide, with the primary ice flow direction interpreted to be from northwest to southeast parallel to the ice divide and then emanating to the north and south away from it (Dyke and Prest 1987). Fieldwork and KIM distributions at Chidliak bear evidence for at least three main ice flow regimes (Johnson and Ross 2010). There is evidence of ice flow related to the Hall Ice Divide;

however, glacial dispersion is dominated by a later north and northeastward ice flow, which is particularly evident in the northern half of the project area. Southeasterly ice flow associated with deglaciation postdated the north and northeastward ice flow but importantly, had only minor effects on KIM dispersion.

Exploration History The discovery of diamond-bearing kimberlites at Chidliak is a classic example of the application of traditional diamond exploration techniques in a glaciated terrain. Reconnaissance-scale sampling of glacial sediments led to delineation of an area containing KIMs and follow-up sampling better defined localized KIM anomalies. Mineral composition data from electron-microprobe analysis and abrasion studies of KIMs further prioritized the potential location of outcropping or subcropping kimberlite. Studies of the glacial history and indicator dispersion trains allowed for the design of focused airborne geophysical surveys. Kimberlite-type anomalies, typically discrete, isolated magnetic high, magnetic high dipole, or magnetic low anomalies were identified from the geophysical surveys and ground prospecting resulted in the discovery of three kimberlites (CH-1, CH-2, and CH-3) in 2008. Microdiamond analyses using caustic fusion methods of samples of these three kimberlites proved they were diamondiferous, containing between 0.75 and 1.3 stones per kg. In addition, commercial-sized stones (+0.85 mm) were recovered from samples of two of the bodies, CH-1 and CH-2. These results set the stage for the next phase of work, the evaluation of the economic potential of this new diamond district, which involved additional till sampling, ground and airborne geophysics, and core and reverse circulation (RC) drilling and sampling of kimberlites to quantitatively recover KIMs

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and micro- and macrodiamonds. Table 1 summarizes exploration activities and kimberlite discovery statistics from the first sampling campaign in 2005 through the 2011 field season.

Kimberlite Indicator Mineral Sampling, Analysis, and Interpretation After the initial five anomalous till samples were recovered from Chidliak in 2005, follow-up KIM sampling was conducted in 2006. Additional sampling in 2007 and 2008 was designed to increase sample density. A total of 1,491 KIM samples had been collected by this time, 413 of which contained KIMs (Table 1). The compositions of 6,113 the recovered KIMs as determined by electron-microprobe analysis showed evidence that the kimberlites sampled mantle peridotite from within the diamond stability field. Data for over 2,000 peridotitic pyrope garnets showed that approximately 10 % of the grains had a high Cr2O3/low CaO harzburgitic (G10) geochemical signature (Dawson and Stephens 1975; Gurney 1984), indicating a good diamond potential for the source kimberlites (Fig. 2a). Diamond inclusion-type chromites and eclogitic garnets (Fig. 2b) were also recovered. Single-grain clinopyroxene thermobarometry (after Nimis and Taylor 2000) constrained a model-conductive geotherm colder than obtained for the diamondiferous kimberlites of the central Slave craton, with entry into the diamond stability field at approximately T [ 850 °C (Fig. 2c). The pressure–temperature data obtained suggest a thermal disturbance over the approximate pressure range 62–68 kbar, at the base of the lithosphere. The abrasion characteristics of many of the KIMs suggested minimal glacial transport and a proximal kimberlite source(s). Initial studies of the picroilmenite mineral compositions revealed project-scale broadly variable signatures, suggesting that multiple kimberlite sources were present at Chidliak. As sample density within the project area increased (Table 1), it became evident that in order to resolve individual mineral dispersion trains, rate their prospectivity, and link them to potentially unique kimberlite sources, it was necessary to evaluate the data more thoroughly. Picroilmenite proved to be of limited use, as it is absent from many indicator-positive samples. Starting in 2009, in order to leverage the information available from G9 (lherzolitic) and other garnet types, the Chidliak project implemented KIM classification and interpretation techniques similar to those described in Grütter et al. (2004) and Grütter and Tuer (2009), with substantial emphasis on Mn-thermometry (T-Mn) of pyrope garnets. A project-specific T-Mn calibration (modified after Grütter et al. 1999) was tailored to reflect the specific analytical protocol and Mn-standardization of

J. Pell et al.

the single commercial microprobe service provider used since the first KIM recoveries at the Chidliak project. Chrome-pyropes were categorized as graphite-facies (T-Mn \ 900 °C), shallow diamond-facies (900 °C \ T-Mn \ 1100 °C) and deep diamond-facies (T-Mn [ 1100 °C), or as high titanium (G1 or G11 garnets with TiO2 [ 0.6 wt%). The categorization provided a simple, four-fold representation of peridotite-affinity garnet abundance relative to graphite-diamond in any given sample (i.e., the mantle tenor of Grütter and Tuer 2009). The mantle tenor observed in Chidliak sediment samples highlighted that distinct mineral trains often occurred at high angles to interpreted regional ice flow direction, which led to a reinterpretation of the local ice flow patterns (see Neilson et al. 2012). Mantle tenor fingerprints were also able to discriminate individual, source-specific KIM trains in somewhat confusing areas characterized by high KIM garnet recoveries. The distribution and relative abundance of diamond-facies peridotitic (G10D), eclogitic (G3D), and websteritic-eclogitic (G4D) garnets (Grütter et al. 2004) were additionally used to prioritize high-potential prospecting areas and geophysical targets. The discovery of 13 new kimberlites during the 2009 field season, and assessment of their KIM and diamond content through heavy mineral abundance and composition studies and caustic fusion analyses, respectively, stimulated further evolution of the interpreted relationships between KIM garnet compositions and the diamond content of the kimberlites at Chidliak. It was recognized that the absolute abundance per unit weight kimberlite of diamond-facies eclogitic-websteritic garnets constitutes an important gauge of diamond mineralization (Table 2). Fortunately, the garnet classification scheme implemented readily separates the important G3D (diamond-facies eclogite) and G4D (diamond-facies websterite-eclogite) garnet categories from compositionally similar G1 (megacryst) garnets, unlike the Angolan case discussed in Rogers and Grütter (2009). It was also recognized that a low tenor of diamond-facies Cr-pyrope garnets and a correspondingly high tenor of graphite-facies Cr-pyrope garnets should not be interpreted as indicating poor diamond potential at Chidliak (Table 2). This outcome strongly contrasts with examples from the Daldyn-Alakit, Archangelsk (e.g. Malkovets et al. 2007), and Sarfartoq (Grütter and Tuer 2009) kimberlite provinces, in which kimberlite diamond content has been correlated with the tenor of diamond-facies Cr-pyrope garnets. At Chidliak, any kimberlite with significant total garnet content per unit weight is assessed as potentially having significant diamond content, especially if eclogitic or websteritic garnets are present (Table 2). The corollary for exploration at Chidliak is that any source of high garnet counts in sediment samples is considered worthy of pursuit, regardless of garnet compositions (Neilson et al. 2012).

Exploration and Discovery of the Chidliak Kimberlite Province, Baffin Island, Nunavut

(a)

213

(b)

Cr-poor megacrysts

Diamond eclogites

(c) 20

SOMERSET ISLAND KIRKLAND LAKE SLAVE C & N

30

CHIDLIAK

P NT00 (kbar)

50

60

45

35

70 600

800

TNT00( °C)

40

1200

1400

Fig. 2 Indicator mineral signatures of Chidliak till samples; a Cr2O3CaO compositions of garnets recovered from Chidliak sediment samples between 2005 and 2008, with fields from Grütter et al. (2004). b TiO2-Na2O plot of chrome-poor garnets recovered from Chidliak sediment samples between 2005 and 2008, with fields from Schulze

(1999). Unlabeled field near the origin is Group II eclogite field. c Nimis and Taylor (2000) single-grain thermobarometry results for Chidliak clinopyroxene, in comparison to other Canadian localities (after Grütter 2009). Very deep mantle sampling is indicated, on a partly inflected (interpreted) geotherm

During the 2009 exploration program, it was recognized that picroilmenite chemical fingerprinting techniques (e.g., Lee 1993) were only of limited help in resolving the relationships between grains derived from known kimberlites and those found in till (Neilson et al. 2012) as many of

the kimberlites at Chidliak, including some of the significantly diamondiferous bodies, were found to be picroilmenite-free. At this time, it was also recognized that integration of garnet Mn-thermometry results served to fingerprint specific kimberlite sources and their glacial

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Table 2 Mantle tenor of garnets (in %) and diamond content of selected kimberlites Kimberlite name CH-6 B CH-7 N CH-3

Deep diamond-facies Pgar 2.0

Shallow diamondfacies Pgar

Graphite-facies Pgar

High TiO2 Pgar

Diamond-facies Egar

Diamond content (ct/t)

12.9

82.4

2.7

69.9a

2.8dms

a

30.2

23.5

42.7

3.6

90.1

3.6

15.3

74.5

6.6

none

1.04dms \0.1cf

dms

2.8 —diamond content determined by mini-bulk sample processed by dense media separation 0.1cf —diamond content estimated from exploration sample processed by caustic fusion Pgar peridotitic garnet; Egar eclogitic and websteritic garnet a 100 9 (G3D ? G4D)/(G3 ? G4 ? G3D ? G4D)

dispersion trains. This information helped to constrain ice flow models and to resolve mineral trains derived from known kimberlites and those from discrete sources that remain to be discovered (Neilson et al. 2012). Understanding the relationships between grains derived from samples of the known kimberlites and those found in glacial tills has been of paramount importance in determining the local ice flow regime, which includes a surprising array of flow directions. Travel distances for KIMs across the property vary. Distinct source signatures can be recognized over distances of approximately 3 km to over 20 km in one instance; however, in areas of interpreted stagnant or oscillating ice movement, transport distances can be as short as 1 km or less. At Chidliak, KIM transport is often more affected by topography than by larger scale interpreted glacial flow, implying that later localized Alpine-type glaciations had a greater effect on KIM dispersion than earlier, regional ice flow (Neilson et al. 2012).

Geophysics Starting in 2008, systematic high-definition helicopterborne magnetic and frequency-domain electromagnetic (EM) surveying has been undertaken over KIM-defined priority areas at Chidliak. To date, Fugro DIGHEM V and RESOLVE EM data has been obtained for an area of 4245 km2. Priority airborne geophysical anomalies were typically screened using the KIM data, detailed ground magnetic and EM surveys and ground prospecting, and only then nominated for drill testing. Three ground geophysical survey techniques have been used: ground magnetic surveys (‘‘walk-mag’’ or ‘‘sledmag’’), horizontal loop EM surveys, and more recently, capacitively coupled resistivity (OhmMapper) surveys. The OhmMapper technique was introduced in 2011 to replace the outdated horizontal loop EM technique, and it has been successful in both determining kimberlite body geometry and differentiating lake-bottom sediments from features attributed to basement geology.

Kimberlite Discovery The first body (CH-1) was discovered in July 2008 when a kimberlite outcrop was found by ground prospecting an airborne geomagnetic anomaly in an area of high-interest KIMs. An initial 289 kg sample collected from CH-1 showed the body contained diamonds with a coarse size distribution, including two stones larger than a 1.7 mm square mesh sieve (Peregrine Diamonds Ltd. 2008; Pell 2008). The CH-2 and CH-3 kimberlites were also discovered in 2008 by prospecting. In 2009, 13 kimberlites were discovered, 6 by prospecting and 7 by drilling. Further 36 kimberlites were discovered in 2010:34 at Chidliak and 2 at Qilaq. Eleven of these were discovered by core drilling, 8 by reverse circulation drilling and 17 by prospecting. In 2011, an additional nine kimberlites were discovered at Chidliak and one at Qilaq, six by core drilling and four by reverse circulation drilling. As detailed in Table 1, a total of 62 kimberlites are now confirmed at Chidliak, and currently available information indicates additional kimberlites remain to be discovered in the province.

Kimberlite Geology Surface Expression, Body Morphology, and Infill The Chidliak kimberlites have varied surface expressions. Some are covered by a thin veneer of till with little or no surface expression and others are associated with vegetation anomalies within till plains (Fig. 3a). Twenty-four of the 62 kimberlites discovered to date have kimberlite outcrop, subcrop, or overlying areas with abundant kimberlite float, which in some cases, occur within sub-circular, steep-sided topographic depressions (Fig. 3b). Only seven of the kimberlites discovered to date occur under lakes. The surface areas of the known kimberlites range from less than one to over five hectares.

Exploration and Discovery of the Chidliak Kimberlite Province, Baffin Island, Nunavut

Fig. 3 Topographic expression of selected Chidliak kimberlites. a 100 by 150 m subtle depression and vegetation anomaly associated with the CH-5 kimberlite in a flat till plain; b 150 by 200 m

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topographic depression associated with the CH-28 kimberlite; note the controlling linear structure marked by lighter colored broken rock debris

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The kimberlites occur as both sheet-like and larger pipelike bodies. They contain olivine macrocrysts and phenocrysts as well as variable abundances of mantle-derived xenocrysts (Cr-pyrope and eclogitic-websteritic garnets, chrome diopside and spinel, with sporadic picroilmenite as determined from heavy mineral concentrates). Some bodies contain a variety of fairly fresh mantle xenoliths including garnet lherzolite, garnet harzburgite, websterite, and eclogite ranging up to 35 cm in size. Rare kimberlites, including CH-1, CH-17, CH-28, and Q-2 contain conspicuous 1–10 cm apple-green megacryst clinopyroxene that commonly is accompanied by mono-crystals of megacryst garnet and ilmenite. Primary groundmass minerals (or their pseudomorphs) include spinel, perovskite, monticellite, phlogopite, apatite, carbonate, and serpentine. These petrographic features indicate that the rocks are kimberlites sensu stricto (Woolley et al. 1996) and that they can be classified as Group I (Smith 1983; Skinner 1986) or archetypal (Mitchell 1986) kimberlites. The observed country rocks hosting the kimberlite bodies are basement ortho- and paragneisses. The sheet-like bodies are mainly steeply dipping and consist of typical hypabyssal kimberlite (HK), which may contain basement xenoliths. The pipe-like bodies are different in that all or parts of them also contain sedimentary xenoliths in addition to the basement xenoliths; however, the overall combined crustal xenolith abundance is generally low, commonly less than 10 modal percent. The sedimentary xenoliths are dominated by carbonate rocks derived from a now-eroded Paleozoic stratigraphy. The closest preserved Paleozoic strata on south Baffin Island occur over 150 km to the southwest (Fig. 1). The pipe-like bodies can also contain magmaclasts, which are typically mixed with variably fragmented melt-free olivine crystals (Scott Smith et al. 2013); together these are interpreted as juvenile pyroclasts (melt-bearing and melt-free) formed during volcanic eruptions. The varied morphology and infill of the Chidliak pipe-like bodies can be illustrated by the three bodies CH-31, CH-7, and CH-6 discussed below. The CH-31 pipe is one of the larger bodies found to date, with an estimated surface area of four hectares and an irregular outline. Both basement and carbonate xenoliths are present in the pipe infill but their distribution is internally variable with respect to their relative proportions as well as their overall abundance and size (Fig. 4a). Carbonate xenoliths include single blocks over 12 m in core length and can locally comprise [15 % of the pipe infill, while basement xenoliths rarely exceed 5 % (Fig. 4). Crustal xenoliths are more common at CH-31 than in most of the other pipes at Chidliak. Juvenile pyroclasts are present throughout the body (Fig. 4b). The melt-bearing pyroclasts (Scott Smith et al. 2013) have somewhat variable morphology and mineralogy, possibly indicating formation from separate

J. Pell et al.

eruptions of contrasting batches of magma. In most areas, the contrasting melt-bearing pyroclasts are mixed with occasional broken melt-bearing pyroclasts and more common broken melt-free olivine crystals. The rocks have clast supported, poorly sorted textures and in parts are closely packed. Although the kimberlites are inhomogeneous, welldeveloped layering or sharp, internal contacts are not common but have occasionally been observed (Fig. 4c). These features, together, suggest that the final deposit resulted from pyroclastic processes and that there may have been some pyroclastic recycling of original unlithified pyroclastic kimberlite (PK). No coherent kimberlite (CK) is observed within the CH-31 pipe but external HK sheets and stringers were encountered in drillcore within 30 meters of the margins of the pipe. The surface area of CH-7 is estimated to be one hectare and the kimberlite body comprises at least two distinct lobes, both with apparent elliptical outlines. The smaller northern lobe contains massive macrocrystic CK containing rare basement gneiss xenoliths and lacking supracrustal xenoliths. Since the CK resembles HK texturally, it may be intrusive; however, there is no unequivocal evidence to confirm this. The southern lobe is an asymmetrical, steeply plunging pipe in which different types of kimberlite occur as four main distinct units. All four units comprise variable amounts (rarely exceeding 5 % each) of both carbonate and basement xenoliths, which are generally less than 20 cm in size. Larger basement and carbonate xenolithic blocks, up to 4.9 and 1.5 m in core length, respectively, have been encountered. Three of the units are interpreted as PKs; each is somewhat internally variably with respect to olivine content, packing, and grain size and has inhomogeneously distributed xenoliths (Fig. 5a) but does not display obvious bedding and each contains a distinct juvenile pyroclast population (Fig. 5b, c). There is no unequivocal evidence for resedimentation of material or pyroclast mixing between units, suggesting that formation and deposition was by primary pyroclastic processes and that each unit was consolidated prior to the formation of the next. The fourth unit is an apparent CK that has a carbonate-rich crystalline groundmass and contains carbonate xenoliths (Fig. 6a). Unlike in the PK units, some of the carbonate xenoliths show distinct zonal alteration and irregular outlines with impinging olivine grains, features typical of high deposit temperatures (Fig. 6d). The presence of numerous broken garnet and olivine grains (Fig. 6b), inhomogeneous distribution and close packing of mainly melt-free olivine macrocrysts and the presence of diffuse magmaclasts that resemble relict melt-bearing pyroclasts and occur near the top of this unit (Fig. 6c), suggest that the apparent CK is an extrusive pipe infill unit (Scott Smith 2011a). More work is required to determine if it is formed by agglutination or if it is, at least partly, effusive in origin. The contacts between

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217

Fig. 4 Pyroclastic kimberlite from CH-31. a NQ core (47 mm diameter) showing inhomogeneously distributed basement and white, light grey, and dark grey carbonate xenoliths; b NQ core with well developed melt-bearing pyroclasts, often cored by coarse olivine

macrocrysts; c NQ core with and example of a rarely observed clear contact between a relatively fine-grained, carbonate xenolith-poor, and a carbonate xenolith-richer unit

the different units in CH-7 are sharp and steeply dipping and, based on three dimensional models constructed from drilling to date, appear to define a series of asymmetric nested crater infills, comprising one unit of apparently coherent and at least three units of pyroclastic kimberlite. The CH-6 kimberlite is a steep-sided, slightly southwestplunging, and kidney-shaped to elliptical body with a surface area estimated at one hectare. The pipe infill can be sub-divided into two broad textural types, distinguished megascopically by the presence or paucity of carbonate xenoliths. Carbonate xenolith-bearing kimberlite forms the main pipe infill. It contains variable amounts (generally less than 5 %) of carbonate xenoliths, the majority of which are less than 15 cm in size (Fig. 7a). Rarely, large carbonate xenolith blocks and carbonate xenolith-dominated zones up to 13 m in core length are present. The kimberlite is texturally inhomogeneous and comprises a high proportion of

variably clast supported, commonly broken olivine, broken mainly fresh, garnet macrocrysts (Fig. 7b, c), and diffuse magmaclasts similar to those in the apparent CK in the southern lobe of CH-7 (Fig. 6c), set in a patchy, finegrained, and carbonate-rich groundmass. Intercalated with the carbonate xenolith-bearing kimberlite are thin zones (generally 15 m or less in thickness) of carbonate xenolithpoor, locally magmaclastic kimberlite. The majority of the carbonate xenolith-bearing and intercalated volumetrically minor, carbonate-xenolith-poor textural varieties do not resemble HK and have features that are more characteristic of extrusive, clastogenic CK (Scott Smith 2011b), somewhat similar to those from the Victor Northwest kimberlite pipe in Ontario (van Straaten et al. 2011). More work is required to determine if these rocks are effusive in origin, if they formed by agglutination or by some combination of the two processes. In the uppermost parts of the pipe, carbonate

218

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Fig. 5 Pyroclastic units at CH-7. a NQ core (47 mm diameter) from the middle PK unit showing inhomogeneously distributed carbonate rock xenoliths; b Close up of polished slab from upper PK unit comprised of clast supported melt-free serpentinized olivine and ovoid

melt-bearing pyroclasts, the largest containing a basement xenolith; c Close up of polished slab from middle PK unit comprised of clast supported pale colored suirregular melt-pyroclasts and melt-free olivine crystals set in a pale carbonate ± serpentine interclast matrix

xenolith-bearing kimberlite displays features approaching those of PK, including better developed magmaclasts that are interpreted as being probable melt-bearing pyroclasts (Fig. 7d, e). Deeper in the pipe, a unit of homogeneous carbonate xenolith-free kimberlite has a low basement xenolith content (Fig. 8a), few or no magmaclasts, little to

no broken olivine and, in contrast to other parts of the pipe infill, the garnets are almost entirely pseudomorphed by kelyphite-like material (Fig. 8b). The olivine distribution and crystalline groundmass characteristics in this unit resemble that of typical HK (Fig. 8b, c) but there is no evidence of intrusive emplacement. Sharp contacts are rare

Exploration and Discovery of the Chidliak Kimberlite Province, Baffin Island, Nunavut

219

Fig. 6 Extrusive coherent kimberlite from the southern lobe of CH-7. a HQ core (62 mm diameter) showing inhomogeneously distributed carbonate rock xenoliths some of which have been concentrically altered; b Polished slab showing broken garnet and olivine in coherent kimberlite and a wide size range of olivine crystals, some of which are \0.5 mm and may be phenocrystic; c Photomicrograph showing

an angular, embayed olivine macrocryst with an incomplete, asymmetric diffuse selvage, possibly representing a relict melt-bearing pyroclast; d Photomicrograph of an irregular, zonally altered carbonate xenolith with impinging olivine grains set in kimberlite comprised of close-packed olivines in a carbonate groundmass

in CH-6 and boundaries between the varying textural types are complicated and difficult to trace through the body. In parts, the HK-like units are intercalated with clastogenic apparent CK and carbonate xenolith-bearing apparent CK underlies the main HK-like unit, suggesting that the more HK-like units are effusive, not intrusive (Scott Smith 2011b). CH-31, CH-7, and CH-6 illustrate the range of kimberlite pipe infill at Chidliak, from pipes filled entirely with PK deposits (CH-31), to those with mainly PK and lesser extrusive apparent CK infill (CH-7 south lobe), to those dominated by extrusive apparent CK (mixed clastogenic and effusive), with lesser amounts of PK (CH-6). True HK is largely restricted to the more sheet-like bodies and associated small blows.

Geophysical Expression and Emplacement Ages of Kimberlites Perovskite U–Pb dating of 25 of the Chidliak kimberlites indicates magmatism spanned approximately 18 million years from 156 to 138 Ma (Heaman et al. 2012). Numerous paleomagnetic reversals are known during this time period (Fig. 9) and this is reflected in the magnetic signatures of the Chidliak kimberlites, many of which are strong remanently magnetized. Of the 62 kimberlites discovered at Chidliak to date, 45 appear to be normally magnetized, 15 are reversely magnetized, and two are neutral in total field magnetic survey data. Paleomagnetism can provide a first-order assessment of the age of emplacement of kimberlites as magnetic minerals

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J. Pell et al.

Fig. 7 Carbonate xenolith-bearing kimberlite from CH-6. a HQ core (62 mm diameter) with inhomogeneously distributed carbonate xenoliths; b Polished slab; and c Photomicrograph showing a closepacked to grain-supported texture and angular, broken olivine, and

fresh garnet set in a fine-grained groundmass; d Polished slab showing a distinct magmaclast cored by serpentinized olivine; e Photomicrograph of well developed magmaclasts (probably melt-bearing pyroclasts) core by serpentinized olivine, set in a serpentine-rich matrix

may become oriented toward the Earth’s magnetic pole during crystallization. Paleomagnetic dating is based on a comparison of the orientation of remanently magnetized

minerals with the position of the pole relative to a crustal plate through time, which is known as the Apparent Polar Wander Path (APWP). The known 140–80 Ma APWP for

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Fig. 8 Carbonate xenolith-free coherent kimberlite from CH-6. a HQ core (62 mm diameter) of massive black kimberlite with a low basement xenolith content and one large mantle xenolith (in the upper row, right end); b Polished slab showing a completely kelyphitized garnet (bottom left corner, reddish brown with a dark brown rim) and

olivine with an HK-like distribution in a crystalline groundmass; c Polished slab showing green serpentinized, unbroken olivine with an HK-like distribution in a crystalline groundmass adjacent to a white carbonate-serpentine vein

the North American Plate (NAM) has been intersected with paleomagnetic data for kimberlite CH-6, resulting in an estimated emplacement age of 140–130 Ma (Fig. 10 and Table 3). This is consistent with the U–Pb perovskite age of 145.4 ± 6.0 Ma for CH-6 (Heaman et al. 2012). Many of the Chidliak kimberlites have remanently magnetized vectors consistent with a 140 Ma paleopole; however, there was little relative motion between the APWP and the NAM at this time, which curtails the ability of paleomagnetic methods to constrain emplacement ages for Chidliak kimberlites with higher precision. Paleomagnetic work is ongoing at Chidliak, but it has already proven to be a useful tool for prioritizing kimberlite anomalies at the discovery stage of exploration, and for providing remnant vector inputs for three-dimensional

inversion modeling of kimberlite shapes and their possible extent to depth. In ground magnetic data, many of the Chidliak kimberlites manifest as strongly remanently magnetized 0.5–2.0 ha bodies. In many cases, the kimberlites have associated very weak and shallow conductivity responses, which are detectable in the highest frequency quadrature airborne electromagnetic (AEM) data. Exceptions to the typical geophysical responses can often be attributed to rock types at source: for instance, PK can have neutral to weak magnetic signatures, appear to be more conductive, and can have much larger EM anomalies (up to 4–5 ha) than their CK counterparts. The variety in geophysical signature is illustrated by the three bodies CH-31, CH-7, and CH-6, and is discussed below.

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Fig. 9 Geomagnetic polarity timescale for Chidliak kimberlite magmatism. Normal geomagnetic polarity is indicated in black and reversed polarity in white. Chidliak kimberlite magmatism spans 18 Ma, from 156 to 138 Ma, as determined from 206Pb/238U isotopic dating of groundmass perovskite (Heaman et al. 2012). Numerous polar reversals occurred during this period, and these are recorded by fossil magnetization of the Chidliak kimberlites. Base timescale created with TSCreator Pro 5.3 (Lugowski and Ogg 2012)

The magnetic signature of CH-6 is dominated by strong reversed thermoremanent magnetism (TRM) with a weak NW resultant dipole direction (Fig. 11a). In ground magnetic survey data, the total peak (trough) ground magnetic value is 57545nT and the residual intensity value is -1235nT. At 40 m vertical depth, the average magnetic

J. Pell et al.

susceptibility (k) is 700 9 10-5 SI and Koenigsberger ratio (Q) ranges from 1.6 to 6.7, while at 85 m, k = 1870 9 10-5 SI and Q = 9.9 (Table 4). There is no coincident anomalous AEM response over CH-6. The magnetic signature of CH-7 is dominated by strong normal TRM (Fig. 11b). There are two diverse magnetic phases apparent in the ground magnetic survey data. The southern lobe is volumetrically larger and has a resultant dipole in the NNW direction, compared with the smaller and more intensely magnetized northern lobe, which displays an apparent N resultant dipole direction. The total peak ground magnetic values are 59285nT for the southern lobe and 61700nT for the northern lobe, with residual intensities of 480nT and 2890nT, respectively. At 42 m depth, the average k = 154 9 10-5 SI and Q = 7.1, while at 100 m depth k = 250 9 10-5 SI and Q = 5.5 to 10.7 (Table 4). There is no coincident AEM response. The CH-31 kimberlite manifests in ground magnetic data as a very weak magnetic depression, almost a neutral expression, due to weak reversed TRM (Fig. 11c). No obvious dipole is observed in ground magnetic survey data. The resultant total peak (trough) ground magnetic value is approximately 58820nT, with a residual intensity value of -95nT. At 30 m depth, the average k = 121 9 10-5 SI and Q 2.4, while at 110 m depth k = 1090 and Q = 1.3–2.9 (Table 4). In contrast with CH-6 and CH-7, CH-31 shows an AEM response, which was primarily responsible for the discovery (Fig. 11d). The AEM survey system employed over Chidliak and Qilaq properties was the RESOLVE frequency-domain system, which utilizes an array of five co-planar frequencies: 400 Hz, 1800 Hz, 8200 Hz, 40 kHz and 140 kHz. The highest frequency induces a moderately strong EM response over CH-31 and its detection led to the kimberlite discovery. The magnitude of the magnetization vector of CH-6 and CH-7 is dominated by TRM (Q  1); this appears to be the case for the majority of the Chidliak kimberlites, and it is often indicative of the presence of CK at source. CH-31 represents a rare example where TRM nearly cancels the induced magnetization vector. In this case, PK is the dominant pipe infill. The TRM direction measured on CH-6 and CH-10 was determined to be 303.4° declination and 71.0° inclination, Fisher’s precision parameter: 212, and confidence interval a95 = 2.9° (Enkin et al. 2009). The corresponding reversed vector is 123.4° declination and -71.0° inclination. Since TRM tends to dominate these kimberlites, a detectable NW or NNW resultant dipole is present in total field magnetic survey data in nearly every case and this observation has been used to prioritize magnetic anomalies for discovery drilling. As a consequence, a robust discovery tool has been developed on southern Baffin Island through geologic observation and paleomagnetic analysis, which relates pipe infill to its magnetic properties

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223

Fig. 10 Paleomagnetic age determination of the Chidliak paleopole position. The red square is the Chidliak paleopole, the small circles are paleopole data for unoriented cores from CH-6 to CH-10 kimberlites. The grand mean TRM vector direction was determined to be Dec: 303.4° and Inc: 71.0°, Fisher’s precision parameter: 212, and confidence interval a95 = 2.9° (Enkin et al. 2009). The corresponding reversed vector is Dec: 123.4° and Inc: -71.0. The line with 10 million year increments is the North American Apparent Polar Wander Path (NAM APWP) (Besse and Courtillot 2002)

Table 3 Paleomagnetic multi-point data for grand mean solution of CH-6 and CH-10 TRM direction Core sample details Kimberlite

Drill hole

Sample

Depth (m)

Azimuth (°)

Paleomagnetic data Dip (°)

Dec (°)

Inc (°)

a95 (°)

Small circle radius (°)

CH-6

DD02

CHIDL1a

40.5

57

-90

263.2

-66.9

2.1

23.1

CH-6

DD02

CHIDL1b

40.5

57

-90

39.1

-67.8

1.0

22.2

CH-6

DD03

CHIDL2a

120.2

22

-45

263.5

-46.7

1.9

43.3

CH-6

DD03

CHIDL2b

120.2

22

-45

247.7

-48.8

2.2

41.2

CH-6

DD05

CHIDL3a

37.4

122

-60

243.0

-41.2

1.9

48.8

CH-6

DD05

CHIDL3b

37.4

122

-60

293.1

-42.0

2.0

48.0

CH-10

DD01

CHIDL4a

64.5

-45

283

105.4

59.4

1.7

30.6

CH-10

DD01

CHIDL4b

64.5

-45

283

353.2

61.1

0.9

28.9

CH-10

DD01

CHIDL5a

105.6

-45

283

254.0

-58.9

2.3

31.1

CH-10

DD01

CHIDL5b

105.6

-45

283

190.3

-61.7

1.9

28.3

CH-10

DD02

CHIDL6a

67.8

-45

23

123.6

-41.4

2.4

48.6

CH-10

DD02

CHIDL6b

67.8

-45

23

98.0

-51.1

1.3

38.9

CH-10

DD02

CHIDL7a

78.5

-45

23

261.7

-41.8

3.1

48.2

CH-10

DD02

CHIDL7b

78.5

-45

23

24.9

-40.5

2.7

49.5

Source Enkin et al. 2009

224

J. Pell et al.

Fig. 11 a CH-6 ground magnetic data. b CH-7 ground magnetic data. c CH-31 ground magnetic data with AEM flight line path. The thick line is associated with the AEM profiles. d The top three AEM frequency profiles are used to illustrate the EM response of CH-31. CPI Co-planar In-phase; CPQ Co-planar Quadrature; RES Resistivity calculated from the In-phase and Quadrature components

and its age of emplacement; however, no correlation between pipe infill or age with diamond content has been established at this time.

Summary and Conclusions The Chidliak kimberlites were discovered using a traditional exploration model focused on constraining the location of primary sources for KIMs recovered from surficial glacial sediments on Hall Peninsula, southern Baffin Island. KIM compositions highlighted the prospect of locating diamondiferous source(s) in an Archean cratonic setting prior to discovery of the first kimberlite in July 2008. Subsequent interpretation of KIM data has permitted surficial indicator mineral populations to be matched with their primary sources. Mineral trains determined to have come from known kimberlites are being used to refine the understanding of the ice flow directions on Hall Peninsula. In some areas, local ice flow directions appear to have had more local influence on kimberlite indicator mineral dispersion than regional ice flow.

The Chidliak kimberlites occur as both pipes and steeply dipping sheets that were emplaced over an 18 Ma period, between 156 and 138 Ma (Heaman et al. 2012) into Paleozoic sedimentary carbonate strata overlying basement gneisses. The Paleozoic succession is now completely eroded from the Chidliak area and the only evidence of this cover is the xenoliths preserved in the kimberlites. The pipes in the Chidliak province are infilled with a variety of textural types ranging from apparently coherent to pyroclastic kimberlite. The apparently coherent kimberlites are interpreted to have formed by processes ranging from effusive to more explosive eruptions that resulted in variably clastogenic deposits. Intrusive coherent kimberlite is largely restricted to sheets. The pipe infills and their general emplacement have similarities to those at Victor in the Attawapiskat province, Ontario but, like many other Canadian kimberlites, are different from those commonly found in southern Africa (Scott Smith 2008). The timing of kimberlite magmatism at Chidliak roughly corresponds with that of some of the younger intrusions in the Attawapiskat province (Heaman et al. 2012), which were also intruded into a Paleozoic carbonate-dominated sequence. Unlike at

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225

Table 4 Magnetic property data for CH-6, CH-7, and CH-31 kimberlites Core sample details Kimberlite

Drill hole

Sample

CH-6

DD02

CHIDL1a

Depth (m) 40.5

Azimuth (°) 57

Magnetic properties Dip (°) -90

Susceptibility (9 10-5 SI) 776.2

TRM (mA/m) 506

Koenigsberger Q (SI) 1.63

CH-6

DD02

CHIDL1b

40.5

57

-90

869.5

562

1.62

CH-6

DD03

CHIDL2a

120.2

22

-45

1893.2

7830

10.34

CH-6

DD03

CHIDL2b

120.2

22

-45

1847.7

7000

9.47

CH-6

DD05

CHIDL3a

37.4

122

-60

725.0

1520

5.24

CH-6

DD05

CHIDL3b

37.4

122

-60

416.8

1110

6.66

CH-7

DD05

25.11

59.1

322

-45.0

169.0

499

7.43

CH-7

DD05

25.12

59.1

322

-45.0

161.9

423

6.57

CH-7

DD05

25.21

59.5

322

-45.0

131.0

388

7.44

322

-45.0

1361.9

406

0.75

323.4

-44.6

267.3

730

6.86

CH-7

DD05

25.22

59.5

CH-7

DD05

25.31

142.3

CH-7

DD05

25.32

142.3

323.4

-44.6

173.5

731

10.59

CH-7

DD05

25.41

142.5

323.4

-44.6

258.4

645

6.27

CH-7

DD05

25.42

142.5

323.4

-44.6

162.8

690

10.65

CH-7

DD05

25.51

142.7

323.4

-44.6

240.7

896

9.36

CH-7

DD05

25.52

142.7

CH-31

DD01

11.11

42.3

323.4

-44.6

368.1

812

5.54

195

-43.6

130.1

101

1.95

CH-31

DD01

11.12

42.3

195

-43.6

129.2

152

2.95

CH-31

DD01

11.21

42.9

195

-43.6

109.7

102

2.33

CH-31

DD01

11.22

42.9

195

-43.6

115.9

103

2.23

CH-31

DD01

11.31

161.6

195

-43.0

846.0

965

2.87

CH-31

DD01

11.32

161.6

195

-43.0

797.3

892

2.81

CH-31

DD01

11.41

161.8

195

-43.0

98.2

84

2.15

CH-31

DD01

11.42

161.8

195

-43.0

4.4

2

1.03

CH-31

DD01

11.51

162.05

195

-43.0

1415.9

746

1.32

CH-31

DD01

11.52

162.05

195

-43.0

1304.4

707

1.36

Source CH-6 Enkin et al. 2009 and CH-7 and CH-31 Van Alstine and Butterworth 2011

Chidliak, some of the Paleozoic strata are still preserved in the Attawapiskat region and the Chidliak bodies may be deeper analogues of Victor-type pyroclastic kimberlites. Many of the Chidliak kimberlites manifest as strongly remanently magnetized bodies. Numerous paleomagnetic reversals occurred during the time period in which they were emplaced and the kimberlites can have normally magnetized, reversely magnetized, or magnetically neutral signatures. In many cases, the kimberlites have associated very weak and shallow conductivity responses, which are detectable in the highest frequency quadrature AEM data. Exploration will continue to establish the potential of this, Canada’s newest diamond district. Future evaluation of these kimberlites is planned as well as exploration for new kimberlites with economic potential, which unsourced highinterest indicator mineral trains within glacial sediments suggest should be present.

Acknowledgments The authors are grateful to Peregrine Diamonds Ltd. for permission to publish this paper and to the exploration team that worked long and hard in the field to do the work and make the discoveries presented here. We would like to thank Barbara Scott Smith for her input on the geology of the kimberlites in this newly discovered province and to Barbara Scott Smith, Brooke Clements, Mike Westerlund, Laura McLean, Ferdi Winter, Kim Webb, and Bruce Kjarsgaard for providing comments on the manuscript. Meilani Zamora-Smith is thanked for assistance in preparation of some of the illustrations.

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