We studied diamonds and barren and diamondiferous eclogite xenoliths from the Jericho kimberlite (Northern Slave craton).
The majority of the diamonds are non-resorbed octahedral crystals, with moderately aggregated N (IaB < 50%, N < 300 ppm) and δ13C = −5 to −41‰. The diamonds belong to “eclogitic” (90% of the studied samples), “websteritic” (7%) and “peridotitic” (3%)
assemblages. The Jericho diamonds differ from the majority of “eclogitic” diamonds worldwide in magnesian compositions of
associated minerals and extremely light C isotopic compositions (δ13C = −24 to −41‰). We propose that metasomatism triggered by H2O fluids may have been involved in the diamond formation. Multiple episodes of the metasomatism and associated melt extraction
of various ages are evident in Jericho eclogite xenoliths where primary garnet and clinopyroxene have been recrystallized
to more magnesian minerals with higher contents of some incompatible trace elements and to hydrous secondary phases. The model
is supported by the general similarity of mineral compositions in diamondiferous eclogites to those in diamond inclusions
and to secondary magnesian garnet and clinopyroxene in recrystallized barren eclogites. The ultimate products of the metasomatism
could be “websteritic” diamond assemblages sourced from magnesian eclogites.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
High-pressure and high-temperature experiments using a laser-heated diamond anvil cell (LHDAC) and synchrotron X-ray diffraction have revealed a phase transition in MgAl2O4. CaTi2O4-type MgAl2O4 was found to be stable at pressures between 45 and at least 117 GPa. The transition pressure of CaTi2O4-type phase in MgAl2O4 is much lower than that in the natural N-type mid-oceanic ridge basalt composition. The Birch–Murnaghan equation of state for CaTi2O4-type MgAl2O4 was determined from the experimental unit cell parameters with K0=219(±6) GPa, K0′=4(constrained value), and V0=238.9(±9) Å3. The observed compressibility was in agreement with the theoretical compressibility calculated in a previous study. ε-MgAl2O4 was observed at pressures between 40 and 45 GPa, which has not been reported in natural rock compositions. The gradient (dP/dT slope) of the transition from the ε-type to CaTi2O4-type MgAl2O4 had a positive value. These results should resolve the dispute regarding the stable high-pressure phase of MgAl2O4, which has been reported in earlier studies using both the multi-anvil press and the diamond anvil cell. 相似文献
The EPR-study showed that natural purple diamonds from kimberlites of Eastern Siberia (Russia) contain well known P1, P2 (in
some samples), W7, and N2 centers. The EPR spectra of these centers were typical of plastically deformed diamond single crystals.
Besides, several intense additional spectra of di-nitrogen centers were observed in purple diamonds. The angular dependence
analysis of these spectra showed that they can be attributed to known M2 centers. Comparison of principal axis directions
observed for sites of the M2 center in purple diamond crystals with theoretically predicted directions in the twin crystal
revealed that these centers are allocated exclusively to the twinned lamellae. Unusual phenomenon of the ordered distribution
of paramagnetic centers in natural purple diamonds confirmed that the plastic deformation in natural diamonds can be induced
not only by the slip of dislocations but also by the mechanical twinning. 相似文献
Over 10,000 discontinuity orientations from various sources at the EKATI Diamond Mine in northern Canada were collected and amalgamated into a common database. A new technique in which structural domains were determined in the area of three closely spaced active pits by quantitatively comparing stereonets from different regions was developed. The frequencies of discontinuity poles plotting in orientation windows on stereonets were determined and compared using a correlation coefficient to quantify the degree of similarity between different stereonets. Structural boundaries between regions were established wherever their associated stereonets display low correlation coefficients. The technique was able to determine boundaries in the vertical direction as well as radially in a number of pie-shaped zones extending around the kimberlite pipes. 相似文献
About half the diamonds studied from the Cenozoic placer deposits along the Namibian coast belong to the peridotitic suite. The peridotitic mantle source is heterogeneous ranging from lherzolitic to strongly Ca depleted (down to 0.24 wt.% CaO in garnet) and shows large variations in Cr/Al ratio, illustrated by very low to very high Cr2O3 contents in garnet (2.6–17.3 wt.%). The Cr-rich end of this range includes exceptionally high Cr2O3 contents in Mg-chromite (70.7 wt.%) and clinopyroxene (3.6 wt.%). Garnet-olivine thermometry appears to indicate two groups, one that equilibrated at temperatures between 1200 and 1220°C and a second between 960 and 1100°C. Combined estimates of pressure and temperature based on garnet-orthopyroxene pairs indicate a large variance in geothermal gradients, corresponding to 38–42 mW/m2 surface heat flow.
The trace-element composition of peridotitic garnet inclusions (determined by SIMS) also indicates large diversity. Two principal groups, corresponding to different styles of metasomatic source enrichment, are recognized. The first group ranges from extremely LREEN-depleted patterns, through trough-shaped REEN to sinusoidal patterns with the position of the first peak gradually moving from the LREEN to the MREEN. This series of REE patterns is interpreted to reflect a range of metasomatic agents with decreasing LREE/HREE. Only in the case of the two garnets with REEN peaking at Sm–Eu is this process connected with enrichment in Zr, without significant introduction of Y and Ti. The metasomatism responsible is interpreted as reflecting percolation of CHO-fluids through harzburgite under sub-solidus conditions. A second group of garnets shows an increase from LREEN–MREEN and almost flat (lherzolitic garnet) to moderately declining MREEN–HREEN at super-chondritic levels. This second style of metasomatism is caused by an agent carrying HFSE and showing only moderate enrichment in LREE over HREE, which points towards silicate melts. 相似文献
Major element and Re–Os isotope analysis of single sulfide inclusions in diamonds from the 240 Ma Jwaneng kimberlite has revealed the presence of at least two generations of eclogitic diamonds at this locality, one Proterozoic (ca. 1.5 Ga) and the other late Archean (ca. 2.9 Ga). The former generation is considered to be the same as that of eclogitic garnet and clinopyroxene inclusion bearing diamonds from Jwaneng with a Sm–Nd isochron age of 1.54 Ga. The latter is coeval with the 2.89 Ga subduction-related generation of eclogitic sulfide inclusion bearing diamonds from Kimberley formed during amalgamation of the western and eastern Kaapvaal craton near the Colesberg magnetic lineament.
The Kimberley, Jwaneng, and Premier kimberlites are key localities for characterizing the relationship between episodic diamond genesis and Kaapvaal craton evolution. Kimberley has 3.2 Ga harzburgitic diamonds associated with creation of the western Kaapvaal cratonic nucleus, and 2.9 Ga eclogitic diamonds resulting from its accretion to the eastern Kaapvaal. Jwaneng has two main eclogitic diamond generations (2.9 and 1.5 Ga) reflecting both stabilization and subsequent modification of the craton. Premier has 1.9 Ga lherzolitic diamonds that postdate Bushveld–Molopo magmatism (but whose precursors have Archean Sm–Nd model ages), as well as 1.2 Ga eclogitic diamonds. Thus, Jwaneng provides the overlap between the dominantly Archean vs. Proterozoic diamond formation evident in the Kimberley and Premier diamond suites, respectively. In addition, the 1.5 Ga Jwaneng eclogitic diamond generation is represented by both sulfide and silicate inclusions, allowing for characterization of secular trends in diamond type and composition. Results for Jwaneng and Kimberley eclogitic sulfides indicate that Ni- and Os-rich end members are more common in Archean diamonds compared to Proterozoic diamonds. Similarly, published data for Kimberley and Premier peridotitic silicates show that Ca-rich (lherzolitic) end members are more likely to be found in Proterozoic diamonds than Archean diamonds. Thus, the available diamond distribution, composition, and age data support a multistage process to create, stabilize, and modify Archean craton keels on a billion-year time scale and global basis. 相似文献
The present paper provides C- and N-stable isotope characteristics, N-contents and N-aggregation states for alluvial diamonds of known paragenesis from placers along the Namibian coast. The sample set includes diamonds with typical peridotitic and eclogitic inclusions and the recently reported “undetermined” suite of Leost et al. [Contrib. Mineral. Petrol. 145 (2003) 15] which resulted from infiltration of high temperature, carbonate-rich melts. δ13C-values range from −20.3‰ to −0.5‰ (n=48) for peridotitic diamonds and from −38.5‰ to −1.6‰ (n=45) for eclogitic diamonds. Diamonds belonging to the “undetermined” suite span a narrower range in δ13C from −8.5‰ to −2.7‰ (n=13). When compared with previous studies, diamonds from Namibia are characterised by unusually low proportions of N-free (i.e. Type II) peridotitic and eclogitic diamonds (3% and 2%, respectively) and an unprecedented high proportion of N-rich diamonds (15% and 73%, respectively, have N-contents >600 ppm). δ15N-values for diamonds of the peridotitic, eclogitic and “undetermined” suites range from −10‰ to +13‰ without correlations with either N-content or δ13C. The similarity in N-isotopic composition and the N-rich character of diamonds belonging to the eclogitic, peridotitic and “undetermined” suites is striking and suggests a close genetic relationship. We propose that a large part of the diamonds mined in Namibia formed during metasomatic events of similar style that introduced carbon and nitrogen into a range of different host lithologies. 相似文献
Since 1998, BHP Billiton has mined diamonds at the Ekati Diamond Mine™ near Lac de Gras in the Northwest Territories of Canada. Current operations are based on mining multiple pipes by the open-pit method, but as some pits deepen, converting to underground mining is being considered.
As a test of underground mining methods and to provide access to the lower elevations of the Panda and Koala pipes, the Koala North pipe is being developed for underground mining. Initially, the top 40 m of the pipe were mined as an open pit to provide grade information and a prepared surface for the transition to underground mining. Currently, Koala North is being developed as an open-benching, mechanized, trackless operation. Although the method was successfully used at several De Beers diamond operations in South Africa, it has never been tested in an Arctic environment.
This case study describes basic geology, mining method layout and ongoing geological and geotechnical investigation. From the beginning of underground development, geotechnical daily routines have been fully integrated within the technical services department, which supports the operation. Geotechnical, geological and structural information obtained from underground mapping and core logging is compiled, processed, reviewed and analyzed on site by the geotechnical staff. Conclusions and recommendations are implemented as part of the operations in a timely manner. This ongoing “live” process enables the operators to make the most efficient use of resources both for ground support and excavations as well as to address safety issues, which are the top priority. 相似文献
Mantle-derived garnets recovered in diamond exploration programs show compositional variations in Cr, Ca, Mg, Fe and Ti that reflect the chemical, physical and lithological environments in which they occur, occasionally together with diamond. The association of diamond with mantle garnet has progressed through a number of geochemical advances, most notably those of Dawson and Stephens (1975) and Gurney (1984), which are integrated in this work with less well known petrological advances made primarily in xenolith and experimental petrology. A simple, robust garnet classification scheme is formulated which accommodates empirical garnet–diamond relationships for peridotitic (G10, G9, G12), megacrystic (G1), Ti-metasomatised (G11), pyroxenitic (G4, G5) and eclogitic (G3) lithologies in eight distinct garnet classes. The calcium-saturation characteristics of harzburgitic (G10), lherzolitic (G9) and wehrlitic (G12) garnets are described by a Ca-intercept projection that also shows promise as a relative barometer for garnet lherzolite (Grütter and Winter, 1997). Thermobarometric aspects of garnet–diamond associations are highlighted in the scheme through the use of the minor elements Mn and Na, though analysis by anything other than an electron microprobe is not required for classification. A “D” suffix is added to the G10, G4, G5 or G3 categories to indicate a strong compositional and pressure–temperature association with diamond. The scheme remains open to improvement, particularly with regard to delineation of pyroxenitic (or websteritic) diamond associations and to advances in Ca-in-garnet and Na-in-garnet thermobarometry. 相似文献
