Constraining diamond metasomatic growth using C- and N-stable isotopes: examples from Namibia

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.     

Underground geotechnical and geological investigations at Ekati Mine–Koala North: case study

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.     

The relationship between the distribution of nitrogen impurity centres in diamond crystals and their internal structure and mechanism of growth

Fifty diamond crystals of different morphological types (octahedra, dodecahedroids, cubes and single tetrahexahedroid) with differing internal structures were examined using methods of cathodoluminescence (CL), anomalous birefringence and local infrared (IR) analysis. The main objective of the study was to examine the regularities of nitrogen impurity distribution in diamond with differing internal structures. Almost all the analyzed octahedra, as well as dodecahedroids with zonal structures and the blocky dodecahedroids, are characterized either by nearly isothermic growth conditions or by a decrease in formation temperature during the crystallization process. In contrast to zoned octahedra and dodecahedroids, dodecahedroids with zonal–sectorial and sectorial internal structures show a notably different distribution of nitrogen defects, with N_tot generally decreasing from crystal cores to marginal areas, and degree of nitrogen aggregation increasing in the same direction. From this, it would follow that in these crystals, the temperature of diamond formation of the outer crystal zones is approximately 40–50 °C higher than that of the inner zones. The same result (15 to 80 °C) was obtained for diamond crystals with cubic habit, which generally show a fibrous internal structure, reflecting normal mechanisms of growth. The anomalous distribution of nitrogen centres in diamond crystals that grew through the normal mechanism, with a high rate of growth and in an oversaturated medium, might point to non-equilibrium relationships between the concentrations of different nitrogen centres. It is likely that in crystals of this type, the rate of growth is higher than the rate of structural nitrogen aggregation. Thus, it appears that in these peculiar crystals of diamond we deal with non-equilibrium concentrations of nitrogen B centres and, consequently, with anomalous, non-actual diamond formation temperatures.     

Role of geology in diamond project development

For a mining operation to be successful, it is important to bring fundamental and applied science together. The mining engineer needs to understand the importance of geology, mineralogy and petrography, and how projects can benefit from the data collected during the exploration and pre-exploration stage. Geological scientists also need to understand the process of project development from the exploration stage through mine design and operation to mine closure. Kimberlite pipe or dyke emplacement, geology and petrology/mineralogy are three areas that illustrate how information obtained from the geological studies could directly influence the mining method selection and the project strategy and design. Kimberlite emplacement is one of the fundamental processes that rely on knowledge of the kimberlite body geology. Although the importance of the emplacement model is commonly recognized in the resource geology, mining engineers do not always appreciate its importance to the mine design. The knowledge of the orebody geometry, character of the contact zones, internal structures and distribution of inclusions could directly influence pit wall stability (thus strip ratio), underground mining method selection, dilution, treatability, and the dewatering strategy. Understanding the internal kimberlite geology mainly includes the geometry and character of individual phases, and the orientation and character of internal structures that transect the rock mass. For any mining method it is important to know “where the less and where the more competent rocks are located” to achieve stability. On the other hand, the detailed facies studies may not be important for the resource and mine design if the rock types have similar physical properties and diamond content. A good understanding of the kimberlite petrology and mineralogy could be crucial not only to the treatability (namely diamond damage and liberation), but also to the pit wall and underground excavation stability, support design, mine safety (mudrush risk assessment) and mine dewatering. There is no doubt that a better understanding of the kimberlite and country rock geology has a direct impact on the safety and economics of the mining operations. The process of mine design can start at the beginning of kimberlite discovery by incorporating the critical geological information without necessarily increasing the exploration budget. It is important to appreciate the usefulness of fundamental geological research and its impact on increased confidence in the mine design. Such studies should be viewed as worthwhile investments, not as cost items.     

川滇地区地震活动演化特征分析

结合川滇菱形块体的活动特征，应用图型动力学的损伤演化诱致突变分析方法，研究了川滇地区两条地震带中强地震的损伤演化过程，并给出各地震带演化到破坏态的所有可能的各单元初始状态的组合形式图。研究结果对指导该区地震预测实践有一定的积极作用。     

The Volutionary Characteristics of Seismic Activity in the SichuanYunnan Region,China

Based on the earthquake activity characteristics of the diamond block in the Sichuan-Yunnan region and by using the method of the meso-scope damage dynamics and damage evolution,we studied the damage evolution process for moderately strong earthquakes along two seismicbelts.The original combination patterns of all the units which illuminate the changes from stable state to destroyed state are given.All these patterns can direct the earthquake prediction practice in this region     

In situ X-ray observations of phase transitions in MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel to 40 GPa using multianvil apparatus with sintered diamond anvils

 Phase transitions in MgAl₂O₄ spinel have been studied at pressures 22–38 GPa, and at temperatures up to 1600 °C, using a combination of synchrotron radiation and a multianvil apparatus with sintered diamond anvils. Spinel dissociated into a mixture of MgO plus Al₂O₃ at pressures to 25 GPa, while it transformed to the CaFe₂O₄ (calcium ferrite) structure at higher pressures via the metastably formed oxide mixture upon increasing temperature. Neither the e-phase nor the CaTi₂O₄-type MgAl₂O₄, which were reported in earlier studies using the diamond-anvil cell, were observed in the present pressure and temperature range. The zero-pressure bulk modulus of the calcium-ferrite-type MgAl₂O₄ was calculated as K=213 (3) GPa, which is significantly lower than that reported by Yutani et al. (1997), but is consistent with a more recent result by Funamori et al. (1998) and that estimated by an ab initio calculation by Catti (2001). Received: 2 April 2002 / Accepted: 29 July 2002 Acknowledgements The authors thank Y. Higo, Y. Sueda, T.␣Ueda, Y. Tanimoto, A. Fukuyama, K. Ochi, F. Kurio and T. Kawahara for help in the in situ X-ray observations at SPring-8 (No: 2000A0061-CD-np and 2000B0093-ND-np). We also thank W.␣Utsumi, J. Ando and O. Shimomura for advice and encouragement during this study, and N. Funamori and an anonymous reviwer for comments on the article. The present study is partly supported by the grant-in-aid for Scientific Research (A) of the Ministry of Education, Science, Sport and Culture of the Japanese government (no: 11694088).     

Deviatoric stress in a diamond anvil cell using synchrotron radiation with two diffraction geometries

Deviatoric stress in a diamond anvil cell with gold as a pressure and stress indicator is measured by two complementary techiques using synchrotron radiation. The first method employs a white X-ray beam using energy disperisive X-ray diffraction. The incident X-ray beam is parallel to the load axis and the diffraction pattern is recorded at a low two-theta angle. Using powder diffraction patterns of polycrystallin gold, we measured the elastic strain of two crystal planes oriented normal to the diffraction vector. Stresses nearly parallel and perpendicular to the load axis can be calculated by stress-strain tensor relatonship. The other method uses a monochromatic wiggler X-ray beam. In this case, the diamond cell is oriented so that the incident beam is perpendicular to the load axis. The diffraction pattern is recorded on an image plate area detector. Elastic strains responding to stresses perpendicular and parallel to the load axis can be measured and stresses of the same orientations can be calculated from the strain data. These measurements provide a lower bound of the actual differential stress in a diamond cell. With these techniques, we can measure stress distribution in a less deviatoric gasketted sample and determine yield strength of mantle materials at high pressures and temperatures.     

Grade-tonnage and other models for diamond kimberlite pipes

Grade-tonnage and other quantitative models help give reasonable answers to questions about diamond kimberlite pipes. Diamond kimberlite pipes are those diamondiferous kimberlite pipes that either have been worked or are expected to be worked for diamonds. These models are not applicable to kimberlite dikes and sills or to lamproite pipes. Diamond kimberlite pipes contain a median 26 million metric tons (mt); the median diamond grade is 0.25 carat/metric ton (ct/mt). Deposit-specific models suggest that the median of the average diamond size is 0.07 ct and the median percentage of diamonds that are industrial quality is 67 percent. The percentage of diamonds that are industrial quality can be predicted from deposit grade using a regression model (log[industrial diamonds (percent)]=1.9+0.2 log[grade (ct/mt)]). The largest diamond in a diamond kimberlite pipe can be predicted from deposit tonnage using a regression model (log[largest diamond (ct)]=–1.5+0.54 log[size (mt]). The median outcrop area of diamond pipes is 12 hectares (ha). Because the pipes have similar forms, the tonnage of the deposits can be predicted by the outcrop area (log[size (mt)]=6.5+1.0 log[outcrop area (ha)]). Once a kimberlite pipe is identified, the probability is approximately .005 that it can be worked for diamonds. If a newly discovered pipe is a member of a cluster that contains a known diamond kimberlite pipe, the probability that the new discovery can be mined for diamonds is 56 times that for a newly discovered kimberlite pipe in a cluster without a diamond kimberlite pipe. About 30 percent of pipes with worked residual caps at the surface will be worked at depth. Based on the number of discovered deposits and the area of stable craton rocks thought to be well explored in South Africa, about 10^–5 diamond kimberlite pipes are present per square kilometer. If this density is applicable to the South American Precambrian Shield, more than 70 undiscovered kimberlite pipes are predicted to be present.     

Raman study of radiation-damaged zircon under hydrostatic compression

Pressure-induced changes of Raman band parameters of four natural, gem-quality zircon samples with different degrees of self-irradiation damage, and synthetic ZrSiO₄ without radiation damage, have been studied under hydrostatic compression in a diamond anvil cell up to ~10 GPa. Radiation-damaged zircon shows similar up-shifts of internal SiO₄ stretching modes at elevated pressures as non-damaged ZrSiO₄. Only minor changes of band-widths were observed in all cases. This makes it possible to estimate the degree of radiation damage from the width of the ν₃(SiO₄) band of zircon inclusions in situ, almost independent from potential “fossilized pressures” or compressive strain acting on the inclusions. An application is the non-destructive analysis of gemstones such as corundum or spinel: broadened Raman bands are a reliable indicator of self-irradiation damage in zircon inclusions, whose presence allows one to exclude artificial color enhancement by high-temperature treatment of the specimen.