- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
NARROW
GeoRef Subject
-
all geography including DSDP/ODP Sites and Legs
-
Atlantic Ocean
-
Mid-Atlantic Ridge (1)
-
North Atlantic (1)
-
-
Canada
-
Eastern Canada
-
Maritime Provinces
-
New Brunswick
-
Gloucester County New Brunswick
-
Bathurst New Brunswick (2)
-
-
-
-
Quebec
-
Abitibi County Quebec
-
Val d'Or Quebec (3)
-
-
Montreal and Jesus Islands County Quebec
-
Montreal Quebec (1)
-
-
Oka Complex (1)
-
-
-
Mackenzie Mountains (1)
-
Western Canada
-
Northwest Territories
-
Great Slave Lake (1)
-
-
Yukon Territory (1)
-
-
-
North America
-
Canadian Shield
-
Slave Province (1)
-
Superior Province
-
Abitibi Belt (3)
-
-
-
-
South America
-
Chile (1)
-
Peru (1)
-
-
United States
-
Colorado Plateau (3)
-
Four Corners (1)
-
Utah
-
San Juan County Utah (1)
-
-
-
-
commodities
-
metal ores
-
copper ores (1)
-
gold ores (3)
-
-
mineral deposits, genesis (2)
-
-
elements, isotopes
-
metals
-
chromium (1)
-
-
-
geochronology methods
-
Pb/Th (1)
-
U/Pb (2)
-
-
geologic age
-
Cenozoic
-
Tertiary
-
Neogene (1)
-
Paleogene
-
Oligocene (1)
-
-
-
-
Mesozoic
-
Cretaceous
-
Upper Cretaceous (1)
-
-
-
Paleozoic
-
Ordovician
-
Tetagouche Group (1)
-
-
-
Precambrian
-
Archean
-
Neoarchean (1)
-
Yellowknife Group (1)
-
-
upper Precambrian
-
Proterozoic (1)
-
-
-
-
igneous rocks
-
igneous rocks
-
kimberlite (1)
-
plutonic rocks
-
alnoite (1)
-
lamprophyres (1)
-
-
volcanic rocks (1)
-
-
-
metamorphic rocks
-
metamorphic rocks
-
eclogite (3)
-
metaigneous rocks
-
serpentinite (1)
-
-
metasomatic rocks
-
serpentinite (1)
-
-
-
-
minerals
-
oxides
-
boehmite (1)
-
chromite (1)
-
goethite (1)
-
hydroxides (1)
-
-
phosphates
-
monazite (1)
-
-
silicates
-
chain silicates
-
jade (1)
-
pyroxene group
-
clinopyroxene
-
omphacite (1)
-
-
-
-
framework silicates
-
silica minerals
-
coesite (1)
-
-
-
orthosilicates
-
nesosilicates
-
garnet group (1)
-
zircon group
-
zircon (2)
-
-
-
sorosilicates
-
lawsonite (1)
-
-
-
sheet silicates
-
mica group
-
phengite (1)
-
-
-
-
sulfides (2)
-
-
Primary terms
-
absolute age (3)
-
Atlantic Ocean
-
Mid-Atlantic Ridge (1)
-
North Atlantic (1)
-
-
Canada
-
Eastern Canada
-
Maritime Provinces
-
New Brunswick
-
Gloucester County New Brunswick
-
Bathurst New Brunswick (2)
-
-
-
-
Quebec
-
Abitibi County Quebec
-
Val d'Or Quebec (3)
-
-
Montreal and Jesus Islands County Quebec
-
Montreal Quebec (1)
-
-
Oka Complex (1)
-
-
-
Mackenzie Mountains (1)
-
Western Canada
-
Northwest Territories
-
Great Slave Lake (1)
-
-
Yukon Territory (1)
-
-
-
Cenozoic
-
Tertiary
-
Neogene (1)
-
Paleogene
-
Oligocene (1)
-
-
-
-
crust (2)
-
Deep Sea Drilling Project
-
Leg 37
-
DSDP Site 334 (1)
-
-
-
deformation (1)
-
earthquakes (1)
-
economic geology (2)
-
faults (1)
-
geochemistry (4)
-
igneous rocks
-
kimberlite (1)
-
plutonic rocks
-
alnoite (1)
-
lamprophyres (1)
-
-
volcanic rocks (1)
-
-
inclusions (4)
-
intrusions (7)
-
mantle (1)
-
Mesozoic
-
Cretaceous
-
Upper Cretaceous (1)
-
-
-
metal ores
-
copper ores (1)
-
gold ores (3)
-
-
metals
-
chromium (1)
-
-
metamorphic rocks
-
eclogite (3)
-
metaigneous rocks
-
serpentinite (1)
-
-
metasomatic rocks
-
serpentinite (1)
-
-
-
metamorphism (3)
-
metasomatism (2)
-
mineral deposits, genesis (2)
-
North America
-
Canadian Shield
-
Slave Province (1)
-
Superior Province
-
Abitibi Belt (3)
-
-
-
-
Paleozoic
-
Ordovician
-
Tetagouche Group (1)
-
-
-
petrology (3)
-
phase equilibria (1)
-
plate tectonics (6)
-
Precambrian
-
Archean
-
Neoarchean (1)
-
Yellowknife Group (1)
-
-
upper Precambrian
-
Proterozoic (1)
-
-
-
sea-floor spreading (1)
-
sedimentary rocks
-
clastic rocks (1)
-
-
seismology (1)
-
South America
-
Chile (1)
-
Peru (1)
-
-
stratigraphy (2)
-
structural analysis (1)
-
tectonics (1)
-
tectonophysics (2)
-
United States
-
Colorado Plateau (3)
-
Four Corners (1)
-
Utah
-
San Juan County Utah (1)
-
-
-
-
sedimentary rocks
-
sedimentary rocks
-
clastic rocks (1)
-
-
Front Matter
The Discovery of the Argyle Pipe, Western Australia: The World’s First Lamproite-Hosted Diamond Mine
Abstract Argyle diamond mine discovery is a tale of scientific exploration, belief, and persistence over a nine-year period from project formulation in 1971 to pipe discovery in 1979. Initial inspiration came from a suggestion that leucite lamproites that crop out in the Kimberley region of Western Australia were derived by differentiation from a kimberlite-like peridotite magma. A 200,000-km 2 -wide program of reconnaissance stream sampling was initiated in the north of Western Australia and discovered kimberlite indicator minerals and diamonds in three parts of the Kimberley basin and adjacent King Leopold and Halls Creek orogens. An innovative laboratory was constructed for indicator mineral recovery, and the use of chromite as an indicator was pioneered and proved a key mineral. Follow-up of indicator/diamond anomalies led to the world’s first recognition of olivine lamproites. The program led to the discovery of the smaller-scale Ellendale lamproite diamond deposit en route to finding the tier 1, high-grade Argyle pipe, the world’s first economic olivine lamproite-hosted diamond deposit, which has produced >750 million carats since 1983. Prior to these discoveries, traditional targets for diamond pipe exploration were areas of ancient cratonic basement. The Argyle and Ellendale deposits lie within the Halls Creek and King Leopold orogens, respectively, over thrust upon ancient cratonic lithosphere. The discoveries thus introduced a new target rock, olivine lamproite, for diamond exploration in nontraditional orogen environments.
Evaluation of the AK1 Deposit at Argyle Diamond Mine
Abstract The 36-year evaluation history of the AK1 lamproite of the Argyle diamond mine (1980–2016) captures an important chapter in the development of diamond deposit evaluation. The resource potential was recognized early in the evaluation process, despite indications of a fine diamond size-frequency distribution and lower-quality stones. High diamond density and the size-frequency distribution characteristics allowed development of sampling methods. These included large-diameter core sampling and analysis of 20-kg samples of NQ/split HQ core for microdiamonds, which were significantly cheaper than traditional methods and unique to the diamond industry at the time. These methods permitted increasingly deep evaluation programs that augmented resources, underpinned the underground block cave feasibility study and, for the first time, revealed four coalescing vents of the AK1 orebody to –735 m reduced level. The majority of the deposit has high to moderate grades (2–5 ct/t), with extremely high grades in the uppercentral parts (>10 ct/t) and low grades in the north (0–1 ct/t). Despite some anomalous areas, at the scale of mining the AK1 deposit is striking in its size-frequency distribution homogeneity. Diamond size-quality distribution has also remained consistent for many years, being lower in the south and higher in the central-north with respect to the main central part of the deposit. The largest diamond deposit in the world (by contained diamond carats) and the largest producer, AK1 has sustained annual production averaging >25 million carats per annum (more than twice its nearest competitor). Size, homogeneity, and high grades (average 3.5 ct/t over 36 years) have been crucial to its longevity and economic success.
The Geology of the Argyle (AK1) Diamond Deposit, Western Australia
Abstract The Argyle (AK1) pipe in the Kimberley region of Western Australia is the world’s largest source of natural diamonds, and it has produced more than 835 million carats since mining began in 1983. The ~1.2 Ga olivine lamproite pipe lies within the Paleoproterozoic Halls Creek orogen at the southeastern margin of the Kimberley craton, and it was emplaced during a period of extension associated with large-scale, NNE-trending strike-slip faulting of the orogen. AK1 is composed of four coalesced, steep-sided diatremes aligned along an NNE-trending fault and tapering to narrow feeder zones at depth. The body is infilled by volcaniclastic olivine lamproite and cut by late olivine lamproite dikes. Two distinct groups of volcaniclastic rocks are present. The dominant type comprises mainly quartz-rich lapilli tuffs and coarse ash tuffs formed by numerous phreatomagmatic eruptions, when olivine lamproite magma was erupted through water-rich sands and silts of the Carr Boyd Group. In contrast, late-stage olivine lamproite lapilli tuffs, devoid of accidental quartz grains, fill the center of the northern diatreme. Following emplacement, the body was tilted 30° to the north and extended north-south by the sinistral north-northwest–south-southeast Gap fault system and east-northeast–west-southwest by the dextral Razor Ridge fault. The Razor Ridge fault offsets the most southerly diatreme (southern tail) from the main part of AK1. Reversal of structural offsets affecting AK1 reveals a geometry which closely resembles that of other ultramafic diatremes. Variations in diamond grade and quality within AK1 indicate that at least two distinct magmas with unique diamond contents have been emplaced at Argyle.
Abstract The richly diamondiferous ~1180 Ma Argyle AK1 lamproite pipe at the margin of the Kimberley craton of Western Australia is underlain by a depleted Archean lithospheric root composed mostly of garnet-poor lherzolite. Peridotite xenoliths (some diamondiferous) define a cratonic paleogeotherm with a ~200-km-thick lithosphere, comparable with estimates from present-day seismic S-wave tomography. The Argyle lamproite is highly enriched in incompatible elements and formed by very small degrees of partial melting under reduced H 2 O- and HF-rich conditions of depleted lithospheric mantle that had undergone long-term (>2 Ga) geochemical enrichment. Multiple metasomatic and thermal events including episodic formation of diamond have impacted on the lithosphere of the Kimberley craton, both predating and postdating Paleoproterozoic reworking of its margins and amalgamation within the larger North Australian craton. At Argyle, the inventory of older (Archean?) peridotitic diamonds in the craton root was augmented by Proterozoic (1.58 Ga) eclogitic diamonds with distinctive light carbon isotope compositions to generate Argyle’s rich diamond grades. The Argyle lamproite and other brief episodes of kimberlite, lamprophyre, and lamproite magmatism (some diamondiferous) on the Kimberley craton all bear their own geochemical and isotopic signature, reflecting variable relative contributions from asthenospheric and enriched lithospheric mantle sources. Mantle melting and eruption of small volumes of these diverse magmas may have been triggered by small short-lived thermal perturbations from the asthenosphere and/or tectonic events elsewhere on the Australian continent, triggered by global plate reconfiguration.
Argyle Diamonds: How Subduction Along the Kimberley Craton Edge Generated the World’s Biggest Diamond Deposit
Abstract Based on the mineral inclusion content, diamonds from the Argyle mine, Western Australia, derive primarily (~90%) from eclogitic sources with a minor peridotitic contribution from both harzburgitic and lherzolitic lithologies. The eclogitic inclusions cover a large compositional range and, in part, show unusually high concentrations of mantle-incompatible elements (P, Ti, Na, and K). Coherent trends in major elements (e.g., of Ti or Na versus Mg-number) suggest that the eclogitic diamond source was created by a single process, namely igneous fractionation. Calculated bulk-rock chondrite-normalized rare earth element (REE N ) patterns match a section of oceanic crust reaching from lavas and sheeted dikes to upper gabbros. Positive Eu anomalies for garnet and clinopyroxene, with calculated bulk-rock REE N patterns similar to upper (nonlayered) gabbros, are strong evidence for plagioclase accumulation, which is characteristic for the gabbroic portions of oceanic crust. Linking previously published oxygen isotope analyses of eclogitic garnet inclusions with their major element composition reveals a correlation between δ 18 O (mean of 7.2‰) and Na content, consistent with coupled 18 O and Na enrichment during low-temperature alteration of oceanic crust. The carbon isotope composition of Argyle eclogitic diamonds forms a normal distribution around a δ 13 C value of −11‰, indicative of mixing and homogenization of mantle-and crustal (organic matter)-derived carbon prior to diamond precipitation. Previously published noble gas data on Argyle diamonds support this two-component model. Inclusion-and nitrogen-in-diamond–based thermometry suggests an unusually hot origin of the eclogitic diamond suite, indicative of derivation from the lowermost 25 km (about 180–205 km depth) of the local lithospheric mantle. This is consistent with emplacement of an oceanic protolith during subduction along the Kimberley craton margin, likely during the Halls Creek orogeny (about 1.85 Ga). For Argyle eclogitic diamonds, the relationship between the rate of platelet degradation and mantle residence temperature indicates that both temperature and strain play an important role in this process. Therefore, ubiquitous platelet degradation and plastic deformation of Argyle diamonds are consistent with derivation from a high-temperature environment (softening the diamond lattice) close to the lithosphere-asthenosphere boundary (inducing strain). In combination, the Argyle data set represents a uniquely strong case for a subduction origin of an eclogitic diamond source, followed by mixing of mantle and crustal components during diamond formation. Some lherzolitic inclusions show a similarity to the eclogitic suite in incompatible element enrichments (elevated P, Na, and K). The presence of a mildly majoritic lherzolitic garnet further supports a link to eclogitic diamond formation, as very similar majoritic components were found in two eclogitic garnet inclusions. The carbon isotope composition of peridotitic diamonds shows a mode between −5 and −4‰ and a tail extending toward the eclogitic mode (−11‰). This suggests the presence of multiple generations of peridotitic diamonds, with indications for an origin linked to the eclogitic suite being restricted to diamonds of lherzolitic paragenesis.
The Unique Nature of Argyle Fancy Diamonds: Internal Structure, Paragenesis, and Reasons for Color
Abstract The Argyle lamproite mine is world famous for fancy colored diamonds, ranging from cognac and champagne shades of brown through Cape yellows to rare pink to intense red and even rarer blue stones. The majority of diamonds are type IaAB, displaying platelet degradation and having plastic and brittle deformation, mainly after diamond formation, indicative of high pressure/temperature conditions. The deformation produced optical defects, causing different Argyle diamond colors. White and brown Argyle diamonds belong to both eclogitic and peridotitic parageneses, but yellow- and pink-colored ones studied here are eclogitic. Nitrogen contents, N aggregation states, and formation temperatures of yellow diamonds are correlated with their internal structure. The strongly deformed and internally brecciated yellow diamonds have low to moderate contents of highly aggregated N and high temperatures of formation. The undeformed yellow diamonds are richer in moderately aggregated N, and their formation temperatures are lower. The intensity of N3, H3, and H4 bands of the photoluminescence spectra of these diamonds is higher in the more deformed crystals. The pink eclogitic diamonds contain low to moderate N, are highly aggregated, and have high temperatures of formation, but differ from each other in the deformation level recorded by their internal structures. All identified photoluminescence peaks in these pink diamonds are higher in intensity in the strongly deformed crystals. Geothermometry, based on N contents, aggregation state, age, and temperature relationships of the diamonds, shows that most eclogitic diamonds resided at 1,250° to 1,300°C near the base of the lithospheric mantle, slightly deeper than the peridotitic diamonds. One eclogitic pink diamond contains a two-phase garnetomphacite inclusion reequilibrated from precursor majorite with composition indicative of formation pressure of 9.5 to 10 GPa, equivalent to ~300-km depth, the deepest identified for Argyle diamond formation to date. Internal structures of eclogitic diamonds reflect evidence of their formation under stress in a subduction zone setting.
Abstract The Bunder diamond project comprises a cluster of diamondiferous volcanic pipes and dikes intruding Paleoproterozoic and Mesoproterozoic intracratonic sedimentary rocks covering the Archean Bundelkhand craton, underlain by depleted peridotitic mantle lithosphere. The discovery resulted from an 18-month regional exploration program targeting potentially well preserved diamond orebodies within Neoproterozoic to Paleoproterozoic intracratonic sedimentary rocks along strike from the Majhgawan diamond mine, or possibly more eroded bodies within the exposed Bundelkhand craton to the north. High-chromium magnesiochromites and, in particular, rare G9/G10 garnets from stream sediment samples proved to be effective regional diamond indicator minerals in terms of their dispersion, recovery, and analysis. Follow-up stream sediment surveys for indicator minerals, ground magnetic surveys, and soil geochemical surveys plus visual outcrop prospecting proved to be fit for the purpose and successful in locating diamondiferous source rocks. Key factors for the Bunder discovery were the funding and creation of a dedicated team with a strong commodity focus that was able to secure exploration licenses ahead of the competition, negotiate local exploration access, and then apply local knowledge to tried-and-true exploration and sampling techniques. The ability to prioritize, process, observe, and analyze stream sediment indicator mineral samples in a timely manner at in-house laboratories was crucial.
The Bunder Diamond Project, India: Geology, Geochemistry, and Age of the Saptarshi Lamproite Pipes
Abstract The Bunder diamond project comprises a cluster of seven known diamondiferous volcanic pipes and dikes known as the Saptarshi field. The largest of these is Atri, which comprises two adjacent coalesced volcanic pipes, Atri North and Atri South. This paper reports data that have been compiled into a new three-dimensional geologic model and, together with new geochemical and geochronological information, provides further insight on the internal geology, emplacement history, classification, and age of the Atri pipes. The range of texturally diverse geologic units within the Atri pipes suggests a complex emplacement history, with variations in eruption energy and source magmas identified. The steep-sided pipes were infilled by multiple phases of primary pyroclastic material as well as variably coherent material now locally preserved along the pipe margins. Atri North postdates Atri South and displays a marked change in both the locus and style of volcanic activity. Comparison between the Saptarshi intrusions and the Majhgawan and Hinota diamondiferous pipes (the only other known diamondiferous deposits on the craton) reveals similarities in the marginal cratonic setting, petrogenesis, and age of emplacement. The classification of the Atri pipes within the traditional kimberlite-orangeite-lamproite scheme is not possible due to conflicting discrimination evidence. The magmatic mineral assemblage of the Atri pipes (olivine, phlogopite, apatite, spinel, rutile/anatase, and ilmenite) is not diagnostic. The expanded dataset of phlogopite mineral chemistry has both lamproite and orangeite affinities, while the Sr and Nd systematics of the pyroclasts ( 87 Sr/ 86 Sr 0.7038–0.7048, ε Nd +1.6 to –1.8) are more consistent with archetypal kimberlites. Many of these characteristics are similar to those of the nearby Majhgawan and Hinota pipes. Consequently, these pipes are best classified as members of the alternative metasomatized lithospheric mantle magma group. Rb-Sr dating of phlogopite indicates a pipe emplacement age of 1079 (± 6) Ma, similar to published phlogopite ages from Majhgawan (1067–1084 Ma, recalculated).
A Study of Garnet and Chromian Spinel Xenocrysts from the Atri South Ultramafic Intrusion, Bundelkhand Craton, India
Abstract The Bundelkhand craton is one of five Archean cratons that make up the Indian shield, four of which have yielded diamond discoveries. The Bunder diamond project consists of seven ultramafic intrusions, with the Mesoproterozoic Atri pipe being the largest and most prospective in terms of its diamond content. Despite the Majhgawan diamond mine also being located on the Bundelkhand craton, little information exists regarding the section of subcontinental lithospheric mantle that these deep-seated intrusions have sampled. This paper reports major and trace element data for xenocrystic chromian spinels and garnets, allowing some new interpretations of the mantle section and geotherm to be made. All of the xenocrysts recovered are of peridotitic paragenesis, with no evidence of eclogitic material being observed in concentrate. Calculated equilibration temperatures for garnet and chromian spinel indicate sampling at a range of depths. The chromian spinel can be divided into three groups based on major and minor element characteristics, with each group being derived from different horizons within the subcontinental lithospheric mantle. The garnet data can be divided into five groups based on their Ca, Cr, and rare earth element (REE) contents. The REE profiles of the groups span from a near-primitive mantle signature to extremely sinusoidal, which can be accounted for by varying amounts of initial melt depletion and/or metasomatic reenrichment. Equilibration temperatures for garnet overlap with chromian spinel at temperatures from ~1,100° to 1,250°C, near the base of the sampling profile. Using calculated minimum garnet equilibration pressures, the data suggest a geotherm relating to a model heat flow of ~40 mW/m 2 , which is similar to that previously determined using xenoliths from numerous intrusions in the eastern Dharwar craton to the south. Despite the similarity in their geotherms, previously reported geophysical data have suggested differences in the thickness and composition of the subcontinental lithospheric mantle between the two cratons. While modification or destruction of the Dharwar cratonic root after the breakup of Gondwana has been documented, seismic evidence suggests the same event may not have affected the Bundelkhand craton where the root appears to be preserved. If the subcontinental lithospheric mantle beneath the Bundelkhand craton escaped significant heating and metasomatism in the Mesozoic, then Cretaceous-aged kimberlite or lamproite intrusions may have greater potential to be diamond bearing, whereas intrusions of this age in the Dharwar or Bastar cratons do not.
Diamonds from the Atri South Pipe, Bunder Lamproite Field, India, and Implications for the Nature of the Underlying Mantle
Abstract The diamonds sourced from the Mesoproterozoic Atri pipe are white to brown in color and often plastically deformed, with two morphological populations present, octahedra dominant in the finer sizes, and resorbed dodecahedra in the coarser sizes. The color and resorbed shapes have some resemblance to properties of diamonds from the Majhgawan lamproite 80 km to the east-northeast and of the brown, plastically deformed diamonds from the Mesoproterozoic Argyle lamproite pipe in Australia. The diamond infrared spectra indicate low to moderate nitrogen content and IaA to IaAB aggregation and show occasional spikes related to hydrogen and carbonate presence, which suggests diamond formation through cooling of hydrous fluids that contain both CH 4 and CO 2 . Platelet peaks are commonly prominent and show regular correlation with the IaB component, unlike the irregular diamonds from Argyle. The diamond inclusions are dominantly peridotitic olivine and Cr-rich magnesiochromite formed from depleted lithospheric mantle at ~150-km depth at an estimated 1,140° to 1,200°C, corresponding to a typical cratonic geotherm equivalent to 40-mW/m 2 model surface heat flow. The predominance of harzburgitic-type inclusions and the lack of eclogitic-type indicators in the heavy mineral concentrates suggest that both the diamonds and mantle section sampled by the Atri intrusion are mainly or entirely peridotitic in nature, unlike those of the Argyle lamproite, despite their similar craton-marginal tectonic positions.
Exploration History and Discovery of the Diavik Diamond Deposits, Northwest Territories, Canada
Abstract The 1991 discovery of the diamondiferous “Point Lake” kimberlite in the Lac de Gras region of the Archean Slave craton ignited one of the greatest staking rushes in history. Rio Tinto initiated a large land consolidation, predominantly up-ice and to the south of the BHP-DiaMet Point Lake discovery, including a 600,000-ha property owned by Aber Resources Ltd. Exploration commenced on this property in 1992 with airborne geophysics and kimberlite indicator mineral sampling. Prior to drilling, targeting decisions utilized ground geophysical surveys that resulted in a high (>90%) kimberlite discovery success rate. Early discoveries were made by drilling strongly magnetic (but reversely polarized) lows. Subsequent discovery of diamondiferous kimberlites characterized by conductive anomalies with subtle to no magnetic responses changed the exploration paradigm. The East Island area in Lac de Gras contained a preponderance of high-interest indicator minerals; drill testing of several conductive anomalies as imaged by EM techniques resulted in the discovery of kimberlites that would eventually comprise the Diavik diamond mine orebodies. Microdiamond results from more than 60 kimberlites on the property indicated that A154N, A154S, A21, and A418 were the most likely pipes to contain economic grades. Subsequent large-diameter drilling confirmed that A154S and A154N were indeed high grade, with respective sample grades of 4.59 and 2.32 carats per tonne. Initial delineation drilling and favorable valuation of 240 carats from A154S prompted the authorization of a $60 million feasibility study by the end of 1995. The rapid discovery of Diavik was a result of the quick decision to acquire significant land in and surrounding Lac de Gras, the utilization of state-of-the-art geophysical techniques, the sound implementation of kimberlite indicator mineral exploration in glaciated terrains, and the use of microdiamond size-frequency curves to rank the kimberlites and assess the macrodiamond potential.
Abstract The 23-year evaluation history of the Diavik kimberlites (1994–2017) commenced with elevated microdiamond stone counts from small samples of drill core. These stone counts equated with high sample grades, grades that were higher than anyone had expected (2–5 carats per tonne [ct/t]), and caused many experienced explorers to change their opinions on the prospectivity of small kimberlites (1–2 ha in surface area). The diamond size-frequency distributions were coarser trending, and the diamond quality was “mixed,” resulting in >1-mm diamond prices of about US$100 per ct, slightly better than the world average. This grade and price combination gave ore values ranging from US$200 to US$500 per tonne and, with four kimberlites, the Diavik joint venture partners estimated that there would be sufficient ore to underpin a 20-year mine life at a treatment rate of 2.2 million tonnes per annum. Primary diamond deposits are usually sampled on 50- or 100-m grids. Grade drilling at Diavik was conducted on 25-m centers, allowing for 12- × 6-in holes in each kimberlite. The kimberlites are located beneath Lac de Gras and had to be bulk sampled underground to generate diamond parcels (approximately 12,000 and 8,000 ct) for sorting and valuation. Three of the kimberlites transitioned from open-pit to underground mines (two sublevel retreats and one cut and fill). The fourth kimberlite narrows at depth and is unlikely to be mined from underground. Grade reconciliation has presented challenges; each kimberlite comprises three to four geologic units and plant feed can comprise ore from three kimberlites concurrently. Careful ore management and tracking has overcome these difficulties, meaning that grade, size-frequency, and size-quality reconciliations have been meaningful and, for the most part, have varied less than ±5% on an annual basis. The Diavik mine is neither the largest nor the smallest deposit in the world. However, by the end of its life it will have produced about 7 million carats of diamonds per annum for 22 years at an average grade of 3.4 ct/t. It represents an exceptional diamond resource for kimberlites ~100 m in diameter, considering that it is also located in one of the coldest and remotest locations on Earth.
Abstract The Diavik diamond mine includes four diamondiferous kimberlite pipes (A154N, A154S, A418, and A21) and minor kimberlite dikes that are mined by Rio Tinto and Dominion Diamond Corporation. Pipe morphologies from A154S, A154N, and A418 show similar circular near-surface expressions, with expansion of cross-sectional area and elongation at depths below 100 m above sea level, consistent with the dominantly steeply dipping to nearly vertical structures in the area (~050°). The internal geology of the pipes is highly variable; twenty-nine distinct domains form the basis for three-dimensional geologic models. The kimberlite deposits at Diavik are highly variable and suggest that emplacement into similarly shaped pipes within a single cluster can vary significantly, resulting from a single infilling sequence (e.g., A154S), multiple events separated by periods of relative volcanic quiescence (e.g., A154N), or changes in magma flux or hydrologic conditions (e.g., A418). Diavik pipe emplacements likely result from a shared five-stage emplacement continuum involving (1) exploitation of existing structures by early kimberlite magma intrusion, (2) initial pipe excavation, (3) pipe infilling, (4) sedimentation into craters, and (5) late magmatic intrusion. Indicator minerals from each kimberlite pipe contain varying proportions of the same mantle source. Differences in the relative abundances of mantle minerals are shown among different pyroclastic domains in a single pipe, suggesting emplacement via multiple magma pulses containing unique mantle populations, while units from different pipes are, in some cases, more similar and reflect cross-fertilization among adjacent pipes. Clinopyroxene thermobarometry indicates that the Diavik garnet lherzolite xenoliths were sampled from 100- to 200-km depths and suggests that magmas emplaced at A154N successively sampled deeper mantle through time. Structural analysis indicates that faulting occurred prior to kimberlite emplacement in varying stress regimes. Faults were subsequently reactivated or exploited by early-stage ascending kimberlite magmas, impacting both pipe elongation directions and excavation depths during emplacement. Exploration assessments on the basis of shallow (<150 m) drilling alone may underestimate the volume and/or diamond potential, particularly for isolated (i.e., unclustered) bodies.
Abstract The diamondiferous mantle root beneath the Lac de Gras area in the central Slave craton (northwestern Canada) is now one of the world’s best characterized lithospheric mantle sections with regard to geochemical and thermophysical information. Its most spectacular feature is its marked stratification. An ultradepleted, highly oxidized, shallow layer to ~150-km depth consists dominantly of granoblastic harzburgite with olivine Mg# (100Mg/(Mg + Fe)) of 92 to 94. Garnet in this layer has very low TiO 2 and Zr contents (avg 0.05 wt % and 9.5 ppm, respectively), and strongly sinusoidal REE patterns. The shallow stratum, which exhibits enhanced conductivity, is separated from a less conductive, less depleted, and less oxidized, dominantly lherzolitic layer by a seismically and geochemically imaged sharp discontinuity. The deep stratum features an olivine Mg# of 91 to 92, average garnet TiO 2 of 0.26 wt % and Zr of 33.4 ppm, and includes porphyroclastic varieties. It reaches the thermal and mechanical lithosphere-asthenosphere boundary at ~220 km. The ultradepletion of the shallow subcontinental lithospheric mantle (SCLM) may require an origin by polybaric melting at excess mantle potential temperature, accompanied by shallow plate interactions at ca 3.5 Ga, while the mild depletion of the deep SCLM could be explained by ca 3.3 Ga subcretion of upwelling mantle after a short melting interval, or may alternatively have formed by accretionary processes. The formation of both strata produced peridotitic diamond populations and was followed by amalgamation of the ancient (4.0−2.8 Ga) cratonic core with juvenile (2.7 Ga) domains in the eastern Slave craton, which may have led to the incorporation of coeval lithospheric mantle portions. Ancient (Proterozoic or Archean) interaction with fractionated high field strength element (HFSE)-poor fluids is inferred from garnet with strongly sinusoidal REE patterns and peridotite minerals with radiogenic Sr and Hf, but unradiogenic Nd, and was accompanied by diamond formation. A <350 Ma metasomatic event by an evolving and increasingly fractionated kimberlite liquid is indicated by a spectrum of garnet REE patterns from “normal” light rare earth element (LREE) depleted to increasingly sinusoidal, and by relatively constant 143 Nd/ 144 Nd at variable Sm/Nd. The unfractionated melt was either destructive to diamonds or at least not conducive to diamond growth, whereas the signature of fractionated melt is identified in diamondiferous peridotite xenoliths and may have produced some fibrous overgrowth on diamonds. Short-lived accretionary processes at the western craton margin are reflected in ca 1.85 Ga eclogite xenoliths that make up <5% of the lithosphere column beneath Lac de Gras and that have trace element systematics consistent with gabbroic-or boninite-like precursors. They are concentrated just below the intralithospheric discontinuity and their mode of emplacement into substantially older mantle lithosphere remains enigmatic. Some eclogitic diamonds were likely generated during the metasomatic episodes identified in peridotite samples. However, the accretion itself produced a disproportionately high (relative to the absolute eclogite/peridotite ratio) abundance of sulfide-included and perhaps also other diamonds and eventually helped to conserve the diamondiferous mantle root beneath the central Slave craton.
Abstract The morphology and color of Diavik diamonds, their nitrogen concentrations and δ 13 C values, and the composition of their mineral inclusions provide insights into the formation of diamonds and the evolution of the mantle root beneath the central Slave craton. The minerals which make up inclusions reflect a largely peridotitic mantle source region (77% peridotitic, 18% eclogitic, 1% ultradeep, and 4% ferropericlase bearing) predominantly composed of garnet-harzburgite. The major element geochemistry of the inclusions indicates that the degree of primary melt depletion during formation of the central Slave cratonic mantle root was distinctly lower than for other cratons worldwide—for example, beneath Yakutia and the Kaapvaal craton. The formation of peridotitic diamonds in the Paleoarchean was likely followed by lithosphere-scale cooling by about 150°C, based on differences in equilibration temperatures for touching and nontouching inclusion pairs and nitrogen aggregation-based residence temperature estimates for their respective diamond hosts. The protolith of the eclogitic diamond substrate likely was basaltic-gabbroic oceanic crust, as shown by trace element patterns that include weak positive Eu N anomalies in one garnet and one clinopyroxene inclusion, and a normal mid-ocean ridge basalt (NMORB) signature for the calculated bulk-rock eclogite. The analysis of microinclusions in fibrous diamonds reveals the presence of high-density fluids that span a continuous compositional range between carbonatitic and saline end-members. The diamonds grew from continuously evolving and fractionating melts/fluids that moved through the mantle. Cathodoluminescence imaging of fibrous and clear monocrystalline diamonds indicates that they grew in pulses with intermittent periods of resorption.
Discovery of the Murowa Kimberlites, Zimbabwe
Abstract The Murowa diamond deposit comprises a cluster of kimberlite pipes and dikes intruded into the southern edge of the Tokwe block, the oldest part of the Zimbabwe Archean craton underlain by diamond-favorable, thick, strongly depleted, peridotitic mantle lithosphere. The Murowa discovery resulted from a four-year regional exploration program by a Rio Tinto Exploration/Rio Tinto Zimbabwe joint venture targeting both the structural continuation of the Limpopo mobile belt northeastward from Venetia and River Ranch diamond pipes as well as the adjacent Zimbabwe craton. Follow-up of high-Cr magnesiochromites, often as single grain anomalies, from processed stream sediment samples was key to the discovery of the weakly diamondiferous Sese kimberlites, 70 km east-northeast of Murowa. Trials of different exploration techniques at Sese showed soil geochemistry (Mg, Cr, Ti, Nb, Ni) and horizontal loop (MaxMin) electromagnetic surveys as the most effective follow-up tools for locating kimberlites. The occurrence of fenitized granite adjacent to Sese kimberlite was used as an additional prospecting tool and blocks of fenite associated with a chromite- and diamond-bearing stream sample led to the discovery of the first Murowa pipe (K1). Airborne and ground electromagnetic surveys and soil geochemistry were then applied to locate the other Murowa kimberlites (K2-K5), followed by pitting and drilling. Key factors for the Murowa discovery were the development of a stream sediment sampling methodology closely tied to a proprietary sample-processing laboratory; close linkage of laboratory and field staff, giving fast turnaround to prioritized samples; and a management strongly committed to the program.
Geology, Structure, and Radiometric Age Determination of the Murowa Kimberlites, Zimbabwe
Abstract The Murowa kimberlite field includes three diamondiferous kimberlite pipes (K1, K2, and K3) and multiple kimberlite dikes that have been emplaced into the Archean Chibi granite batholith north of the Limpopo belt in south-central Zimbabwe. Here we summarize the key aspects of the geology of the Murowa kimberlites from previous studies and integrate these findings with new structural data to interpret a structural model governing the locations, relative positions, and orientations of emplaced kimberlite. Key observations of drill core, thin section petrography, geochemistry, and mapping of exposed rocks at the Murowa diamond mine are summarized from previous work, and these data collectively form the basis for emplacement interpretations and threedimensional (3-D) geologic models of each body. Structural observations are used to interpret the presence of a km-scale tensile bridge hosting the Murowa kimberlites and suggest Murowa is an example of kimberlite emplacement into multiple, reactivated, preexisting near-surface structures at different orientations. We propose that the physical state of the ascending magma (% of gas, extent of phase separation) can dictate whether kimberlite is emplaced along preexisting structures or creates and intrudes new fracture networks in planes of weak horizontal stress. A reproducible age of ~526 Ma is determined for two coherent kimberlite dikes at K1, while an older Rb-Sr model age of ~543 Ma is calculated for a single dike from K2, though this result is of limited reliability due to potential disturbance of the Rb-Sr system due to phlogopite alteration. These results highlight potential problems with reported ages from kimberlite pipes.
Characteristics and Origin of the Mantle Root Beneath the Murowa Diamond Mine: Implications for Craton and Diamond Formation
Abstract The Murowa and Sese kimberlites erupted through the southern margin of the Zimbabwean craton. These kimberlites provide a unique sample of the continental lithospheric mantle in that area through their entrained mantle xenolith and xenocryst cargo. Mantle xenoliths have only been obtained from the Murowa locality so far and thus they form the focus of this review. Ultradepleted chromite-harzburgite and chromite-dunite rock units dominate the Murowa mantle xenolith inventory. No eclogite xenoliths have been found and eclogitic garnets are extremely scarce in the minerals analyzed from heavy mineral concentrate. The very low bulk rock Al and Ca contents of the Murowa peridotites, along with their extreme Pt and Pd depletions, require extensive melt extraction—to in excess of 40% melting. At the same time, their high bulk rock Cr# (100 ☼ Cr/(Cr+Al)) indicates that melting took place at relatively low pressures (<<5 GPa). Such high bulk rock Cr#s (median = 0.60) are considerably higher than those of peridotites from the nearby Venetia mine (median = 0.33) and have only been found elsewhere in cratonic peridotites from the North Atlantic craton (median = 0.89). Similar elevated bulk Cr# values are matched by spinel peridotites derived from Phanerozoic collision zones as ophiolites. This similarity favors a model in which this part of the cratonic lithosphere was formed by the subduction of peridotite that had undergone multistage low-pressure melt extraction, likely in an Archean mantle wedge that underwent flux-melting within a subduction zone prior to lateral compression to form nascent cratonic lithospheric mantle. Preliminary quantitative fitting of mantle geotherms derived from thermobarometry data is hampered by the scarcity of suitable clinopyroxene grains and is highly dependent on assumptions made regarding crustal heat production, especially in the lower crust. Nonetheless, at ~540 Ma, the time of kimberlite emplacement, the lithosphere beneath the southern edge of the Zimbabwe craton can be constrained to be approximately 200 km deep, slightly shallower than the 220-km depth estimated for lithosphere beneath the Venetia kimberlite, intruded through the Limpopo Complex. The presence of deep, ~200-km-thick lithospheric mantle beneath Murowa and Sese in early Cambrian times agrees with the minimum pressure estimates derived from Cr-Ca relationships in concentrate garnets. This estimate is close to that derived from surface-wave seismic studies and indicates that the thickness of the craton root beneath the southern Zimbabwe craton and the Limpopo Complex has not varied significantly in the last 500 m.y. The ultradepleted nature of the Murowa peridotites, together with the scarcity of eclogite/pyroxenite components, indicates a different petrogenetic history for the lithospheric root beneath the southern Zimbabwe craton compared with the mantle roots beneath the Limpopo Complex (Venetia) and the Kaapvaal craton to the south. The very high fraction (73%; n = 150) of low-Ca, high-Cr harzburgitic “G-10” garnets in the mantle garnet concentrate population at Murowa, along with their Cr-Ca relations, is consistent with the high diamond grade (0.7 ct/metric ton). The likely metasomatic origin for G-10 garnets along with the abundance of ultradepleted chromite-bearing peridotites in the Murowa mantle xenolith suite indicates that this lithology, if present in the lithosphere in the diamond stability field, may be a critical starting component for a variety of diamond- formation events in cratonic lithosphere.
Abstract Murowa kimberlites are situated in the 3.5 Ga Tokwe block, the oldest part of the Zimbabwe craton. The diamonds are predominantly white or pale brown octahedra of high gem quality. Mineral inclusions are overwhelmingly peridotitic, dominated by chromite, sulfide, and olivine, with only minor amounts of orthopyroxene and garnet, suggestive of a very depleted lithospheric peridotite mantle host. Carbon isotope values for the diamonds range from δ 13 C –5.8 to –2.5‰ and are consistent with global peridotitic diamond signatures. Diamonds with an eclogitic paragenesis are virtually absent. Most Murowa diamonds have simple octahedral zonation, sometimes with several growth stages. The diamonds have a wide range of nitrogen concentration (10–1,600 ppm) and aggregation state (20–80%), with internal variations. Murowa diamond mantle residence temperatures of 1,090° to 1,210°C, calculated from nitrogen aggregation based on a 2.9-b.y. mantle residence, are in agreement with mineral inclusion thermobarometry of 1,050° to 1,270°C, suggesting formation on a 40-mW/m 2 model geotherm with a lithospheric keel depth of ~200 km. Variable inclusion trace element compositions record diamond formation in geochemical environments changing from depleted to strongly metasomatized peridotites. Three populations of chromite inclusions in the diamonds were identified. The main population of the highest-Cr and lowest-Ti inclusions is enriched in Nb, Rb, and V, and depleted in Zr, similar in composition to worldwide chromite diamond inclusions. Two other minor Murowa groups of chromites, less rich in Cr, depleted in Nb, but having up to 2.7 wt % of TiO 2 and 10 to 34 ppm Zr concentrations, are extremely rare as diamond inclusions worldwide but common as kimberlite indicator minerals. The variable amounts of Ni, Mn, Zn, Ca, and Ti identified in the olivine inclusions suggest diamond formation occurred during high-Ti, low-Ca metasomatism. Pyrope garnet inclusions show even stronger light-medium rare earth elements (REE) enrichment compared to worldwide records. Compositions of Murowa orthopyroxene inclusions are also variable: several are rich in light REE and middle REE and, additionally, have high concentrations of Na, Ba, Zr, and Nb, but others are depleted in these elements. The wide range of inclusion trace element compositions shows that Murowa diamond geneses were prolonged, taking place in several stages. A previously published 3.4 ± 0.2 Ga Re-Os model age of a single Murowa sulfide inclusion may correspond the formation time of the main diamond population. Previously reported 40 Ar/ 39 Ar 892 ± 21 Ma model ages of yimengite inclusions from a diamond at the nearby Sese kimberlites and the occurrences of yimengite and armalcolite inclusions in Murowa diamonds suggest the possible presence of a younger diamond-forming event. Both Murowa and Sese diamonds likely originated during interaction between asthenosphere-derived carbonate and water-rich melt and extremely depleted peridotitic mantle lithosphere.