Diamond Indicator Minerals Research Articles

Occurrence of dravitic tourmaline in a diamond-bearing breccia: a possible lamproite deposit in the Alto Paranaíba Igneous Province

The mineral chemistry of diamond indicator minerals is used to trace mineralizations in primary rocks such as lamproites/kimberlites and in secondary deposits. Subcalcic garnets and magnesian ilmenites are both minerals associated with the diamond potential described in many deposits in the Alto Paranaíba Igneous Province where several kimberlitic pipes and alluvial deposits occur. Our study was conducted on the Romaria diamond deposit of (Brazil), where garnet, ilmenite and tourmaline grains were recovered from a diamon-bearing breccia were analyzed under electron microprobe. The garnet composition is compatible with G10, G9 and G5 types. Ilmenite grains are Mg- to Mn-rich. Nearly 80% of the tourmaline grains are dravite-type. This association of dravite tourmaline with diamond in rocks of lamproitic affinity may be common, as evidenced in the Argyle, Ellendale, Prairie Creek, Sask, Presidente Olegário and Ymi-1 pipes. We therefore suggest a possible lamproite origin for the studied diamond deposit.

Composition of the Diamond Indicator Minerals on the Mitchell Chart—Criteria of CLIPPIR Diamonds in Kimberlites and Conditions of their Mantle Crystallization

Composition of the Diamond Indicator Minerals on the Mitchell Chart—Criteria of CLIPPIR Diamonds in Kimberlites and Conditions of their Mantle Crystallization

New Findings of Diamonds and Diamond Indicator Minerals in the Middle Timan and Prospects of the Search for Their Original Sources

In the poorly studied southeastern part of the Chetlas uplift, ring structures typical for magmatic pipe-type bodies (diatremes) have been established. The main part of the ring structures is located in the areas of the distribution of the Upper Devonian sediments. This fact indicates the lower age threshold of the formation of the objects forming them as Late Devonian, i.e., similar to the age of kimberlites of the Arkhangelsk diamond-bearing province. To assess the prospects of one of the most numerous groups of the ring structures, heavy-mineral concentrate sampling of the modern stream sediments was conducted. While studying the heavy-mineral concentrates, chromium-bearing pyrope—a kimberlite indicator mineral—was revealed, and the first diamond grain was found. About 20% of pyropes are characterized by a high preservation of relict endogenous surfaces. The diamond has the appearance of a flattened aggregate with the distinct octahedron faces modified by surfaces of joint growth with other mineral grains. Findings of diamonds and signs of the formation of the diamond indicator minerals in the stream sediments of the studied area make it possible to detect their nearby original sources.

Evidence for a 200 km thick diamond-bearing root beneath the Central Mackenzie Valley, Northwest Territories, Canada? Diamond indicator mineral geochemistry from the Horn Plateau and Trout Lake regions

The Central Mackenzie Valley (CMV) area of Northwest Territories is underlain by Precambrian basement belonging to the North American Craton. The potential of this area to host kimberlitic diamond deposits is relatively high judging from the seismologically-defined lithospheric thickness, age of basement rocks (2.2–1.7 Ga) and presence of kimberlite indicator minerals (KIMs) in Quaternary sediments. This study presents data for a large collection of KIMs recovered from stream sediments and till samples from two study areas in the CMV, the Horn Plateau and Trout Lake. In the processed samples, peridotitic garnets dominate the KIM grain count for both regions (> 25% each) while eclogitic garnet is almost absent in both regions (< 1% each). KIM chemistry for the Horn Plateau indicates significant diamond potential, with a strong similarity to KIM systematics from the Central and Western Slave Craton. The most significant issue to resolve in assessing the local diamond potential is the degree to which KIM chemistry reflects local and/or distal kimberlite bodies. Radiogenic isotope analysis of detrital kimberlite-related CMV ilmenite and rutile grains requires at least two broad age groups for eroded source kimberlites. Statistical analysis of the data suggests that it is probable that some of these KIMs were derived from primary and/or secondary sources within the CMV area, while others may have been transported to the area from the east-northeast by Pleistocene glacial and/or glaciofluvial systems. At this stage, KIM chemistry does not allow the exact location of the kimberlitic source(s) to be constrained.

Ilmenite as a recorder of kimberlite history from mantle to surface: examples from Indian kimberlites

Five compositional-textural types of ilmenite can be distinguished in nine kimberlites from the Eastern Dharwar craton of southern India. These ilmenite generations record different processes in kimberlite history, from mantle to surface. A first generation of Mg-rich ilmenite (type 1) was produced by metasomatic processes in the mantle before the emplacement of the kimberlite. It is found as xenolithic polycrystalline ilmenite aggregates as well as megacrysts and macrocrysts. All of these ilmenite forms may disaggregate within the kimberlite. Due to the interaction with low-viscosity kimberlitic magma replacement of pre-existing type 1 ilmenite by a succeeding generation of geikielite (type 2) along grain boundaries and cracks occurs. Another generation of Mg-rich ilmenite maybe produced by exsolution processes (type 3 ilmenite). Although the identity of the host mineral is unclear due to extensive alteration and possibility includes enstatite. Type 4 Mn-rich ilmenite is produced before the crystallization of groundmass perovskite and ulvospinel. It usually mantles ilmenite and other Ti-rich minerals. Type 5 Mn-rich ilmenite is produced after the crystallization of the groundmass minerals and replaces them. The contents of Cr and Nb in type 2, 4 and 5 ilmenites are highly dependent on the composition of the replaced minerals, they may not be a good argument in exploration. The highest Mg contents are recorded in metasomatic ilmenite that is produced during kimberlite emplacement, and cannot be associated with diamond formation. The higher Mn contents are linked to magmatic processes and also late processes clearly produced after the crystallization of the kimberlite groundmass, and therefore ilmenite with high Mn contents cannot be considered as a reliable diamond indicator mineral (DIM) and kimberlite indicator mineral (KIM).

Use and misuse of Mg- and Mn-rich ilmenite in diamond exploration: A petrographic and trace element approach

Magnesian ilmenite is a common kimberlite indicator mineral, although its use in diamond exploration is still controversial. Complex crystallisation and replacement processes have been invoked to explain the wide compositional and textural ranges of ilmenite found in kimberlites. This work aims to shed light on these processes, as well as their implications for diamond exploration. Petrographic studies were combined for the first time with both major- and trace-element analyses to characterise the ilmenite populations found in xenoliths and xenocrysts in two Angolan kimberlites (Congo-Kasai craton).A multi-stage model describes the evolution of ilmenite in these pipes involving: i) crystallisation of ferric and Mg-rich ilmenite either as metasomatic phases or as megacrysts, both in crustal and in metasomatised mantle domains; ii) kimberlite entrainment and xenolith disaggregation producing at least two populations of ilmenite nodules differing in composition; iii) interaction of both types with the kimberlitic magma during eruption, leading to widespread replacement by Mg-rich ilmenite along grain boundaries and fractures. This process produced similar major-element compositions in ilmenites regardless of their primary (i.e., pre-kimberlitic) origin, although the original enrichment in HFSE (Zr, Hf, Ta, Nb) observed in Fe3+-rich xenocrysts is preserved. Finally (iv) formation of secondary Mn-ilmenite by interaction with a fluid of carbonatitic affinity or by infiltration of a late hydrothermal fluid, followed in some cases by subsolidus alteration in an oxidising environment.The complexities of ilmenite genesis may lead to misinterpretation of the diamond potential of a kimberlite during the exploration stage if textural and trace-element information is disregarded. Secondary Mg-enrichment of ilmenite xenocrysts is common and is unrelated to reducing conditions that could favour diamond formation/preservation in the mantle. Similarly, Mn-rich ilmenite should be disregarded as a diamond indicator mineral, unless textural studies can prove its primary origin.

Manganilmenite in the Magnetite Ore Body from Pokphur Area of Nagaland, North East India and the Possibility of Microdiamonds in the Ophiolites of Indo-Myanmar Ranges

Manganilmenite is found to be associated with the magnetite ore body of Pokphur area in the Nagaland ophiolites, North East India. There is perhaps no earlier description of the mineral from the Indian subcontinent. It occurs as an accessory mineral with magnetite and Fe-chlorite (chamosite). Electron probe micro-analytical data reveal that the mineral contains 5.6-8.5 wt% MnO and traces of MgO, ZnO and Cr2O3, while the TiO2 content remains within narrow limits of 50-53 wt%. The calculated pyrophanite end-member varies from 13% to 18%. Although the magnetite body of Pokphur has been reasonably proved to be a hydrothermally altered product of basic and ultrabasic igneous rocks, and most of the minerals in the magnetite body are supergene in nature, different end-member compositions of mangan ilmenite indicate that it has originally crystallized with the basic suite of rocks and has survived the alteration process with only marginal effects. Since manganilmenite has been considered as a diamond indicator mineral and ophiolites are a newly documented host of microdiamonds elsewhere in the world, the presence of manganilmenite in the Pokphur magnetite hints towards occurrence of microdiamonds in the ophiolite suite of rocks of the Indo-Myanmar ranges.

Metaconglomerate preserves evidence for kimberlite, diamondiferous root and medium grade terrane of a pre-2.7 Ga Southern Superior protocraton

We studied heavy minerals extracted from a diamondiferous metaconglomerate that formed 2697–2701 Ma in a successor basin within the Michipicoten Greenstone Belt (MGB) of the Wawa–Abitibi Terrane (Southern Superior Craton). The conglomerate is metamorphosed in the greenschist facies and contains mainly locally derived igneous mafic to felsic detritus, but also very minor components of medium grade metamorphic minerals, diamonds and paragenetic diamond indicator minerals. Comparison of the size distribution, resorbtion and N aggregation of diamonds in nearby Wawa lamprophyres and the metaconglomerate diamonds confirms that the latter were not derived from the proximal lamprophyric source. The heavy minerals in the metaconglomerate include diopside, olivine, corundum, chromite, almandine, pyrope with kelyphitic rims, picroilmenite, amphibole and anorthite. Low abundances of the heavy minerals (several grains per 4–70 tons of the metaconglomerate) are, in part, explained by their complete or partial replacement by the greenschist mineral assemblage. Detrital almandine and amphibole are inferred to originate in amphibolite facies rocks. Cr-diopside, olivine, chromite and anorthite were sourced from mafic–ultramafic anorthosite- and chromitite-bearing layered complexes mapped in the MGB. The presence of pyrope with more than 6 wt.% Cr 2O 3 suggests derivation from a cratonic root. Picroilmenite has compositions typical of kimberlite and unlike that of ultramafic lamprophyres and other unconventional diamondiferous volcanics. The Wawa metaconglomerate, therefore, should be considered analogous to the Witwatersrand successor basin conglomerate in recording indirect evidence for Archean kimberlites. The tight localization of the diamondiferous conglomerate in time and space was controlled by a quick (~ 3 Ma) erosion of the source kimberlite body. The location of the kimberlite-bearing > 2.7 Ga Superior protocraton was inferred from the provenance of the metaconglomerate detrital material. The clasts could have originated as close as the northern Wawa–Abitibi Terrane or as distant as the Opatica terrane. The pre-2.7 Ga diamonondiferous cratonic root below the Southern Superior was removed in the Neoarchean–Proterozoic. The existence of Archean kimberlites and deep diamondiferous roots below smaller pre-2.7 Ga protocratons emphasizes the similarity of Neoarchean and Phanerozoic mantle processes.

Use of airborne hyperspectral and soil geochemical surveys in diamond exploration at Terowie, southern Flinders Ranges, South Australia

Two new kimberlite intrusions in the Pine Creek area, east of Terowie, South Australia were located by soil sampling across anomalies identified from airborne hyperspectral data. The area includes the state’s largest kimberlite diatreme and has been widely prospected in the past using diamond indicator minerals, airborne and ground magnetics, and follow-up drilling. Thin soil cover on kimberlite contains Fe-Mg smectite and talc, which provide the target spectral anomaly. While not unique to kimberlite, the mineral combination produces a relatively small number of constrained anomalies in this area of dominantly clastic sedimentary rocks. Soil geochemical traverses were used to test spectral anomalies, with elevated levels of Ni, Cr and Nb confirming the presence of ultrabasic intrusions. The newly identified kimberlites are associated with subtle highs in the magnetic data that were not recognised as priority targets in previous investigations. In this region of the Flinders Ranges, spectral methods are capable of locating weathered kimberlite in residual and erosional terrains. The combination of detailed magnetic and spectral data offers an approach to prioritising anomalies, from either data set, that will help to locate kimberlite at or near the surface, and accessible to shallow sampling to test their diamond content.

Multiple Origins of Alluvial Diamonds from New South Wales, Australia

In eastern Australia, diamonds occur in alluvial deposits overlying Phanerozoic basement. The origin of the diamonds is not known. No feeder pipes of kimberlite or lamproite are known, and none of the usual diamond indicator minerals occurs in association with the diamonds. Alluvial diamonds from Wellington, Bingara, Copeton, and Airly Mountain in New South Wales form two distinct groups, here termed A and B. Group A diamonds are similar to those found in kimberlites and lamproites globally and are thought to have formed in Precambrian lithospheric mantle. Group B diamonds have unusual characteristics that indicate they formed in a subduction environment. Surface features of diamonds of both groups indicate that all were brought to the surface by magmas. Group A diamonds are similar in primary crystal form, internal structure, mineral inclusion composition (mainly peridotitic), and carbon isotopes to diamonds found in kimberlitic and lamproitic hosts in Archean and Proterozoic cratons worldwide. Re/Os ages (3.4 and 2.1 Ga; Pearson et al., 1998) of sulfide inclusions in two Group A diamonds constrain the origin of these diamonds to ancient mantle sources. This, along with the nature of the surface abrasion structures and radiation damage, suggests that the Group A diamonds represent an older group of diamonds that have been in secondary collectors for a significant time. If this is so, it is feasible that the Group A diamonds may have been derived from a number of primary sources, including possible sources in Antarctica. Group B diamonds are unlike any other diamond suites worldwide in their combination of shape, surface features, strained and irregular internal structures, 13C-enrichment, and Ca-rich eclogitic mineral inclusions. The features are best explained as a product of diamond growth under varying P-T conditions in a high P/T dynamic environment such as a subducting slab. The diamonds may well have formed during arc-continent collision at the time of the development of the New England fold belt. This would explain why the major known concentration of Group B diamonds is within the New England fold belt at Copeton and Bingara, and is consistent with the Phanerozoic ages for diamond emplacement determined from mineral inclusions.

Superdeep diamonds from the Juina area, Mato Grosso State, Brazil

Alluvial diamonds from the Juina area in Mato Grosso, Brazil, have been characterized in terms of their morphology, syngenetic mineral inclusions, carbon isotopes and nitrogen contents. Morphologically, they are similar to other Brazilian diamonds, showing a strong predominance of rounded dodecahedral crystals. However, other characteristics of the Juina diamonds make them unique. The inclusion parageneses of Juina diamonds are dominated by ultra-high-pressure ("superdeep") phases that differ both from "traditional" syngenetic minerals associated with diamonds and, in detail, from most other superdeep assemblages. Ferropericlase is the dominant inclusion in the Juina diamonds. It coexists with ilmenite, Cr-Ti spinel, a phase with the major-element composition of olivine, and SiO2. CaSi-perovskite inclusions coexist with titanite (sphene), "olivine" and native Ni. MgSi-perovskite coexists with TAPP (tetragonal almandine-pyrope phase). Majoritic garnet occurs in one diamond, associated with CaTi-perovskite, Mn-ilmenite and an unidentified Si-Mg phase. Neither Cr-pyrope nor Mg-chromite was found as inclusions. The spinel inclusions are low in Cr and Mg, and high in Ti (Cr2O3 10 wt%). Most ilmenite inclusions have low MgO contents, and some have very high (up to 11.5 wt%) MnO contents. The rare "olivine" inclusions coexisting with ferropericlase have low Mg# (87–89), and higher Ca, Cr and Zn contents than typical diamond-inclusion olivines. They are interpreted as inverted from spinel-structured (Mg, Fe)2Si2O4. This suite of inclusions is consistent with derivation of most of the diamonds from depths near 670 km, and adds ilmenite and relatively low-Cr, high-Ti spinel to the known phases of the superdeep paragenesis. Diamonds from the Juina area are characterized by a narrow range of carbon isotopic composition (δ13C=–7.8 to –2.5‰), except for the one majorite-bearing diamond (δ13C=–11.4‰). There are high proportions of nitrogen-free and low-nitrogen diamonds, and the aggregated B center is predominant in nitrogen-containing diamonds. These observations have practical consequences for diamond exploration: Low-Mg olivine, low-Mg and high-Mn ilmenite, and low-Cr spinel should be included in the list of diamond indicator minerals, and the role of high-Cr, low-Ti spinel as the only spinel associated with diamond, and hence as a criterion of diamond grade in kimberlites, should be reconsidered.

Diamonds from Myanmar and Thailand: Characteristics andPossible Origins

Diamonds occur in modern alluvial deposits at several localities in Myanmar, Thailand, and Sumatra; the deposits do not contain typical diamond indicator minerals, and no obvious primary sources have been found in the surrounding terrane, which is characterized by Phanerozoic tectonic and magmatic activity. Detailed studies of diamonds from Theindaw and Momeik in Myanmar and Phuket in Thailand have been undertaken to clarify their origin. Syngenetic mineral inclusions in the diamonds are largely of the peridotitic paragenesis, with a smaller eclogitic component. Carbon and nitrogen isotope compositions are typical of kimberlitic and lamproitic diamond suites worldwide. Nitrogen aggregation states indicate a long residence and/or significant deformation at mantle temperatures; many stones show plastic deformation features. The rounded and polished surfaces of most of the diamonds reflect resorption in a corrosive magma. These features do not support an exotic or unusual origin for the diamonds, for example, by subduction-exhumation. Extensive abrasion and abundant brown radiation damage spots suggest long surface transport. Their distribution within the Sibumasu terrane, and their close association with Carbo-Permian glacial-marine sediments, suggest that these diamonds were derived from primary sources in northwestern Australia or within the terrane itself, prior to the Early Permian separation of the Sibumasu terrane from the Gondwanaland margin. The isotopic data and the dominantly peridotitic nature of syngenetic inclusions rule out the Argyle lamproite as a possible source and also distinguish the Myanmar- Thailand diamonds from morphologically similar stones from eastern Australia.

Petrology and oxygen-isotope geochemistry of the Yamba Lake kimberlite rocks, N.W.T.

The Torrie, Sputnik, and Eddie kimberlite rocks, located near Yamba Lake, central Slave province, N.W.T., are volcaniclastic, macrocrystic, heterolithic, olivine-rich tuff, and olivine-rich tuff breccia. Torrie and Sputnik kimberlite rocks contain pyroxene and garnet xenocrysts and megacrysts with major-element compositions consistent with derivation mostly from disaggregated garnet lherzolite, with subordinate contributions from eclogite, spinel lherzolite, garnet harzburgite, and websterite. The presence of primary groundmass phlogopite and compositionally evolved spinel, and the absence of mantle xenocrysts, xenoliths, and megacrystic ilmenite distinguish the Eddie kimberlite pipe from the other two kimberlite pipes. Large variations in δ18O of garnet and clinopyroxene in xenocrysts and xenoliths (+3.98 to +6.36‰), nonequilibrium intermineral isotopic fractionation, and major-element heterogeneity are interpreted as resulting from infiltration of fluids or melts produced by dehydration or melting of subducted oceanic crust into overlying peridotite. Although the timing is unconstrained for the xenocysts, the xenolith must have experienced this metasomatic interaction shortly before entrainment in the kimberlite. Variable δ18O values for magnesian ilmenite are also interpreted to result indirectly from such metasomatic activity in the mantle as well. The Torrie and Sputnik kimberlite rocks have low concentrations of diamond indicator minerals consistent with their low-diamond grades. These kimberlite rocks did not sample a significant amount of garnet harzburgite, the rock type commonly associated with high-diamond grades in other kimberlite rocks. Furthermore, metasomatism just prior to kimberlite eruption may have caused the resorption of any diamond present.

Diamonds from Wellington, NSW: insights into the origin of eastern Australian diamonds

Abstract Diamonds from alluvial deposits near Wellington, New South Wales, have been characterized on the basis of morphological features, mineral inclusions, C isotope signatures, N content and aggregation state and internal structure. The diamonds are of two types. The larger group (Group A) is indistinguishable from diamonds found worldwide from kimberlitic and lamproitic host rocks. This group is inferred to have formed in a peridotitic mantle source in Pre-Cambrian subcratonic lithosphere. The second group (Group B) is unique in its internal structures (which show evidence of growth in a stress field and non-planar facets), has unusually heavy C isotopic compositions and contains Ca-rich eclogitic inclusions. This group is inferred to have formed in a subducting slab. Diamonds of both groups have external features (corrosion structures and polish) indicating transport to the surface by lamproitic-like magmas. The diamonds show evidence of long residence at the earth's surface and significant alluvial reworking: they are not accompanied by typical diamond indicator minerals.

STRUCTURAL SETTING OF THE KIMBERLITES OF THE EAST EUROPEAN CRATON

Kimberlites and associated igneous rocks were intruded along abyssal faults during episodes of crustal extension at the beginning of major tectonoagmatic cycles, in at least five epochs from Early Archean to Late Paleozoic. Contrary to Clifford's rule, they appear not to be particularly associated with Archean shields. Some of the kimberlites are diamond-bearing, even richly so, and diamonds and diamond indicator minerals in sedimentary strata point to undiscovered occurrences.