Kimberlite Dikes Research Articles

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.

Contact zones and hydrothermal systems as analogues to repository conditions

Radioactive waste isolation efforts in the U.S.A. are currently focused on examining basalt, tuff, salt and crystalline rock as candidate rock types to encompass waste repositories. As analogues to near-field conditions, the distributions of radio- and trace elements have been examined across contacts between these rocks and dikes and stocks that have intruded them. The intensive study of the Stripa quartz monzonite in Sweden has also offered the opportunity to observe the distribution of U and its daughters in groundwater and its relationship to U associated with fracture-filling and alteration minerals. Investigations of intrusive contact zones to date have included: (1) a Tertiary stock into Precambrian gneiss; (2) a stock into ash flow tuff; (3) a rhyodacite dike into Columbia River basalt, Washington, and (4) a kimberlite dike into salt. With respect to temperature and pressure, these contact zones may be considered “worst-case scenario” analogues. Results indicate that there has been no appreciable migration of radioelements from the more radioactive intrusives into the less radioactive country rocks, either in response to the intrusions or in the fracture-controlled hydrologic systems that developed following emplacement. In many cases, the radioelements are locked up in accessory minerals, suggesting that artificial analogues to these would make ideal waste forms. Emphasis should now shift to examination of active hydrothermal systems, studying the distribution of key elements in water, fractures, and alteration minerals under pressure and temperature conditions most similar to those expected in the near-field environment of a repository.

Groundmass oxide minerals in the Koidu kimberlite dikes, Sierra Leone, West Africa

Petrology and oxide mineral chemistry are presented on 5 kimberlite dikes that are classified into three groups: (1) one dike is highly carbonated and highly oxidized (> MH) and is characterised by chlorite+Mn-titanomagnetite+rutile+hematite (after chlorite)+maghemite (after titanomagnetite), with ilmenite and perovskite being absent; (2) three dikes are typified by atoll-textured spinels +phlogopite+euhedral Mn-picroilmenite, of intermediate oxidation state (WM-FMQ) with coexisting deuterically serpentinized olivine+Ni-Fe alloys and magnetite; (3) the remaining dike records an early crystallization event under very low (≤ WM) redox conditions that precipitated anionic-deficient spinels and a Mg-Ti-Cr-wustite-type phase, followed by late stage more oxidizing (=FMQ) Mnpicroilmenite. Spinels are complexly zoned and crystallization trends among the dikes are diverse, underscoring the fact that no single compositional trend, or evolutionary sequence is typical of kimberlites. Ilmenites are euhedral, and criteria for groundmass crystallization are established. Extraordinarily high MnO (max 17 wt%) contents and high geikielite (62 mole%) concentrations expand the ilmenite field typically assigned to that of kimberlites. Zirconium, Nb and Cr are present in concentrations of 0.5–3 wt% (as oxides) in ilmenite. These highly incompatible elements, along with Mn, are concentrated in late stage melt fractions. The high pyrophanite contents, which are more typical of silicic alkali suites, are accompanied by phlogopite in the Koidu dikes. Objective evaluations of kimberlite-carbonatite relations, as outlined in the literature, cannot be made based on the oxide mineral group. Much of the compositional data for oxides in kimberlites are on mantle-derived xenolith suites and are not from oxides derived from the crystallization of kimberlitic melts. Assessments of the fO2's of kimberlites have considerable potential in evaluating diamond survival through redox reactions. Manganese-rich (+Nb, Zr, Cr) ilmenites are typical of many kimberlites and should be considered in the suite of index minerals employed in prospecting.

Rapidly crystallized garnet pyroxenite xenoliths possibly related to discrete nodules

Abundant small xenoliths in the Mzongwana kimberlite dike, Transkei, southern Africa, are predominantly pyroxenites composed of ilmenite, pyrope, orthopyroxene, clinopyroxene, rutile, and phlogopite; two of the xenoliths contain small amounts of Ti-rich amphibole near kaersutite in composition. A majority of the pyroxenites have polygonal granoblastic textures, but many have fasciculate, acicular and skeletal growths. The latter are believed to be the product of rapid crystallization because of similarities to textures of lunar and terrestrial volcanic rocks and quenched experimental charges. Segregations of garnet or ilmenite and pyroxene are common, and these are believed to have originated by crystallization from supersaturated magma. Pyroxenes in the rocks that appear to have crystallized most rapidly are richer in Al and Ti and the garnets are richer in Ti than comparable phases in the granoblastic rocks.

Update on Exploration for Diamonds in Colorado-Wyoming Kimberlite Province: ABSTRACT

The discovery of diamonds in kimberlite diatremes in 1975 led to a joint effort by Colorado State University and the Wyoming Geological Survey to explore for additional kimberlite occurrences within Colorado and Wyoming. Presently, more than 90 separate kimberlite localities are known in the Colorado-Wyoming State Line district and the Iron Mountain district of Wyoming. Additionally, an isolated kimberlite pipe is present west of Boulder, and there is a kimberlite dike in the Estes Park area of Colorado, extending the known kimberlite occurrences in a roughly north-south trend over approximately 120 mi (192 km). Diamonds have been recovered only from diatremes in the State Line district, except for an isolated occurrence of placer diamonds recently identified in stream-se iment concentrates from the Medicine Bow Mountains. Exploration continues with the examination of the Front Range by available remote-sensing imagery. Target areas given highest priority are those showing apparent relations and similarities to known kimberlite districts. Drainages in these areas are systematically sampled for heavy mineral indicators (i.e., pyrope garnet, magnesium ilmenite, chrome diopside), and the heavy mineral trains are traced upslope to potential kimberlite sites. Detailed ground surveys are conducted over several miles around all new discoveries, with special emphasis placed on associated linear trends (faults, dikes, joints, etc). Limited soil and alluvial geochemical sampling has been used with variable success. Several geophysical methods have been used, but electrical resistivity and magnetics appear to be the most useful. Electrical resistivity methods show that weathered kimberlite is highly conductive (80 to 250 ohm-ft) compared to the enclosing Precambrian granitic host rocks (300 to 7,400 ohm-ft) and that magnetics are variable, showing only small dipolar anomalies (±30 to 150 ^ggr). End_of_Article - Last_Page 830------------

Shortite in Kimberlite from the Upper Canada Gold Mine, Ontario

Shortite, Na₂Ca₂(CO₃)₃, occurs in the groundmass of micaceous kimberlite dikes from the Upper Canada Gold Mine, Ontario. X-ray, optical, and chemical properties of shortite from the Upper Canada Mine are virtually identical to those of shortite from the Green River Formation, Wyoming. The grains poikilitically enclose phlogopite, magnetite, apatite, and perovskite and are closely associated with calcite, olivine, and aggregates of serpentine in the groundmass of the kimberlite. The groundmass assemblage is interpreted to have precipitated from kimberlite magma at low temperature.

A crustal-upper mantle model for the Colorado plateau based on observations of crystalline rock fragments in a kimberlite dike

A crustal-upper mantle model for the Colorado plateau based on observations of crystalline rock fragments in a kimberlite dike

The Kimberlites of Åvike Bay, on the Bothnian Coast of Sweden

Abstract The finds of kimberlite dikes and veins which tend to strike towards a centre, and the finds of angular breccia boulders in the same area suggest a new centre of volcanic activity within the Avike Bay region, situated approximately 15 kilometres east of the wellknown Alno district in Sweden. The volcanism of the two areas are probably related to each other. However, the rocks of Avike seem to be more kimberlitic in character and appear to be younger than those of the Alno region.

Diamantiferous diatremes; reply

In the discussion by A. P. Lebedev and A. S. Trofimov (p. 784-786), it is pointed out that Czechoslovakian diatremes are basaltic or basalt cementing detrital kimberlite and therefore not comparable to those of the Siberian platform. Discrepancies between geologic age and K-Ar ages of kimberlites are not real but are due to introduced argon. Diamonds form at temperatures above 200 degrees C, as indicated by experimental data and fluidal structures in host rock. Diamond-bearing kimberlite dikes without explosion or brecciation features suggest that shallow explosions are not essential. In the author9s reply it is held that although differences between Czechoslovakian and Siberian kimberlites noted by Lebedev and Trofimov are real, xenoliths are similar, particularly in breccias. K-Ar dates for phlogopite cited in the original article have been supported by Pb-U dates and indicate emplacement in a relatively cold state. Diamond-bearing kimberlite dikes do display brecciation features. The upward increase in diamond content in an intrusive system suggests growth of diamonds during rather than prior to intrusion. Kyanite is another high-pressure mineral that has apparently formed by overpressures at high crustal levels. For reference to the paper under discussion, see this Bibliography Vol. 29, Davidson 1.

Ett fynd av frisk olivin i kimberlit från Kalix skärgård i Norrbotten

Abstract The kimberlite dikes in the Kalix-Lulea archipelago, carlier described by Geijer and W. Larsson, have their content of olivine replaced by serpentine etc. A kimberlite boulder from this area that was handed over for examination in 1948, contained however fresh olivine in abundance. The Li-bearing mica cryophyllite was noticed in the groundmass of this kimberlite.

On the Association of Faulting with Dike Intrusion

A structural study of some of the kimberlite dikes near the head of Cayuga Lake, New York, has shown a close relationship between the intrusion of the dikes and the stress at fault planes. In one case, the pattern of a group of dikes crossed by a fault suggests that tension, resulting from fault stresses, tended to open joint fractures and thus facilitate the entrance of dike stringers. Similar tension patterns have been obtained experimentally. In another case, fault stresses caused a constriction in a dike. Above and below the constriction the magma removed the country rock for the entire width of the dike by mechanical stoping. Other structural features tended to show that diastrophic forces were active when the dikes were intruded.