Two compositionally distinct groups of discrete nodule megacrysts (more than 1 cm in diameter) occur in kimberlite of the Colorado-Wyoming State Line and Iron Mountain districts.The megacrysts are characterized by Cr-rich and Cr-poor assemblages and include garnet, clinopyroxene, orthopyroxene, olivine (rare), ilmenite and clinopyroxene-ilmenite intergrowths.The latter three types are essentially restricted to the Cr-poor group.Most Cr-rich nodules have relatively high Mg/Mg + Fe and CaO and are quite similar chemically to comparable minerals from peridotite nodules (Fig. 1).Some of the Cr-rich megacrysts could be peridotite disaggregation products; however, their coarse grain size is atypical of the peridotite fabric.The Cr-poor megacrysts differ from the Cr-rich discrete nodules in their relatively more subcalcic nature, higher Ti02, and higher, more variable contents of iron, along with lower Cr203 (Fig. 2).Many megacrysts of both groups contain mineral in¬ clusions of one or more members of that group.The megacryst assemblages are characterized by distinctive color variations between Cr-rich and Cr-poor members.Most Cr-rich garnet megacrysts (more than 6 percent Cr203) are lav¬ ender to deep purple whereas Cr-poor garnets (less than 4 percent Cr203) are typically reddish bro\m.Cr-rich vs. Cr-poor clinopyroxene (more than 1 per¬ cent vs. less than 1 percent Cr203) are emerald green and bluish gray to gray green respectively.Cr-rich orthopyroxene megacrysts (more than .4percent Cr203) are bright to pale green; Cr-poor varieties (less than .3percent Cr203) are gray green to gray brown.Cr-poor megacrysts are quite similar to discrete nodules described by Nixon and Boyd (1973) and Boyd and Nixon (1975) from Lesotho and the Monastery Mine, South Africa (Fig. 1).However, the Colorado-Wyoming samples are more enriched in Cr203, have slightly higher Mg/Mg + Fe, and no high temperature (greater than 1300°C) orthopyroxene megacrysts have been found.Cr-rich megacrysts are not reported from the Lesotho and Monastery kimberlites.Megacrysts from the Iron Mountain, Wyoming kimberlite district are almost exclusively of the Cr-poor assemblage, and show a prominent enrichment in iron relative to State Line district nodules.Chemical trends in ilmenite are especially pronounced.In addition to significantly higher average iron con¬ tents, a MgO vs. Cr203 plot (Fig. 3) shows a parabolic relationship similar to that reported by Haggarty (1975) for African ilmenites; most of the Iron Mountain samples fall on the bottom and left side (low MgO side) of the par¬ abola.Chemical variations also occur in ilmenite from kimberlite within the State Line district.Megacrysts from the Schaffer, Ferris and Moen pipes are more enriched in iron than those from the Sloan and Nix pipes, but they do not show the degree of enrichment or extreme variation of iron as in the Iron Mount¬ ain ilmenite megacrysts.Similar chemical trends occur in garnet and clino¬ pyroxene megacrysts.Clinopyroxene-ilmenite intergrowths are found primarily at Iron Mountain and contain the most magnesian ilmenite and Fe-rich clino¬ pyroxene in that district.However, Mg concentrations in intergrowth ilmenites are appreciably lower than those in most State Line ilmenite megacrysts (Fig. 3).Rare earth element concentrations determined for representative Cr-rich and Cr-poor diopside and garnet megacrysts suggest small but distinct

The Mössbauer milliprobe allows the determination of Fe3+/ΣFe in samples as small as 50 μm. For the first time this technique is applied to a suite of diamonds of eclogitic paragenesis, where three garnet and five clinopyroxene inclusions in diamonds from George Creek, Colorado have been analysed. For garnet Fe3+/ΣFe ranges from 0–7%, while values for clinopyroxene range from 8–14%. These results are consistent with the low oxygen fugacity conditions implied by the presence of the inclusions in diamond.

The primary source of alluvial diamonds in the Guyana Shield of Venezuela, Guyana, and Brazil has been the subject of controversy from the time of their initial discovery in the late 1800s. Based on available evidence it is suggested that at least two source regions are currently present for diamonds in the Guyana Shield. One source, albeit secondary, is the thick (>2,000 m) sequence of Proterozoic clastic sediments, referred to as the Roraima Group. However, the primary source of the diamonds and the source of the clastic material constituting the Roraima Group is unknown. A West African source has been suggested by some authors but supporting evidence is lacking. A second source of diamonds in the Guyana Shield is responsible for the diamond-bearing alluvial deposits in the Guaniamo district of western Venezuela. For over a decade the kimberlite indicator minerals pyrope and garnet and Mg ilmenite have been known to occur in these alluvials and recently the primary Proterozoic kimberlite-type source rock(s) have been discovered. However, the characteristics of Guaniamo diamonds as a whole are quite different from Roraima diamonds and a common source is unlikely. It is highly probable that other Proterozoic kimberlite (or lamproite) primary sources remain to be discovered in the Guyana Shield area of the Amazonian craton.

Mineral inclusions have been recovered from 40 small (<1.5 mm) diamonds from the Sloan 1 and 2 kimberlite pipes in northern Colorado. Primary inclusions (i.e., proto- or syngenetic) are olivine, clinopy-roxene, sanidine, rutile, and possibly native Fe and some phlogopite. Phases interpreted as epigenetic are acmite, richterite, most phlogopite, perovskite, Mn-ilmenite, Cr-spinel, magnetite, calcite, serpentine and possibly Mg-ilmenite. Acmite, richterite, and perovskite have not previously been reported in diamond. The epigenetic minerals commonly occur as inclusion assemblages such as phlogopite + richterite, phlogopite + perovskite + ilmenite + magnetite + serpentine, and phlogopite + clinopyroxene + spinel + magnetite + calcite. The majority of the primary inclusions have an eclogitic suite affinity, which is uncommon but similar to diamonds in the Premier, Orapa, and Mbuji Mayi kimberlites of southern and central Africa and from a few sites in Russia and Western Australia. Secondary inclusions reflect extensive alteration by, and crystallization of, kimberlite fluids and at least some post-intrusive ground water induced alteration. The abundance of epigenetic assemblage inclusions is related to the cracked, fractured and broken nature of large numbers of the Colorado diamonds. Some epigenetic inclusions may be products of a metasomatic event in the upper mantle in which MARID-type minerals may have been introduced into diamonds along cracks generated by primary inclusions. Most cracks and fractures probably formed during a shallow, early or pre-emptive event.

Results of small-scale fiuidization, spouting, and channeling experiments suggest that similar processes may have been involved in the formation of many breccia pipe features. Various cylindrical, parallel plate, and boxlike model containers were filled with mixtures of particulate materials ranging from clay-sized (powdered kaolinitc) to 1-cm gravel and each container was attached at its base to a large-volume compressed air source. Features generated during the course of the various experimental runs are analogous to the majority of features observed in breccia pipes.Complex branching fracture systems developed in models containing abundant cohesive material appear to be similar to incipient crackle breccias. Steep-walled, funnel- to cone- shaped conduits formed readily, and in some models, inward-dipping fractures (ring fractures), marginal shear folds and/or downwarps, and/or high-angle normal faults developed in adjacent material. Thorough mixing of heterogeneous particulate materials from throughout the stratigraphic column commonly was achieved and blocks of more cohesive materials were readily rounded and comminuted. Convective particle motion (spouting) was most effective in the mixing process and was accompanied by downwarping and fragmentation of adjacent strata, downward migration of wall-rock blocks along conduit margins, and generation of annular zonation of conduit-filling material; in some systems, annular-size classification resulted in fine layering parallel to conduit walls. Crude, centroclinally, nested saucer bedding developed in some experiments where air-fall ejecta were generated and these beds commonly were transported essentially intact downward within the pipe during a bubbling fiuidization event. A gas-streaming or channeling process, in which finer material was elutriated or cleared out of coarser material, led to the formation of channelways or chimneys which in natural systems would provide excellent avenues for introduction of later magma or hydrothermal solutions. In model systems enriched in strongly cohesive clay-sized particles, accretionary pelletal spherules commonly were generated.Fluidization and related processes have been defined to describe highly controlled industrial systems. Problems of scale and strength of materials are significant when comparing experimental with geologic systems, but the striking similarity of many features observed in both experimental and natural breccia pipe systems suggests that a similar genetic process is reasonable. Although many pipes clearly are not formed by fiuidization and related processes, these processes may be important in the development of, or some developmental stage of, some breccia pipes.

Teledetective analysis refers to the coordinated interpretation of remote sensing, geomorphological, and geophysical data to define the surface and subsurface character of kimberlite regions that occur in extremely different environments. Study areas in Colorado-Wyoming, Tanzania, and Yakutia were chosen on the basis of their diversity: e.g. elevation (2400m, 1200m, 300m, respectively), climate (temperate, equatorial, subarctic), country rock (granite, granite, limestone), and vegetative cover (montane forest, grassland, taiga). The analysis of similarly scaled Landsat images, drainage networks, and geophysical maps permits the composite character of a given kimberlite province to be considered and compared to other kimberlite provinces, particularly in terms of textural patterns. The teledetective technique indicates that all three kimberlite study areas are characterized by some type of cross-cutting textural pattern where the regional trend is interrupted or intersected by another trend. In noting the association of such transverse textures with other kimberlite regions, previous investigators have frequently cited the intersection of deep-seated zones of structural weakness, e.g. intersecting fault systems, as providing conduits for the ascent of kimberlite magma. The same argument can be made for the regions studied here. The present study demonstrates that transverse textural patterns are a common characteristic of widely separated kimberlite provinces, and may therefore have value as a textural signature in delineating favorable target areas during the reconnaissance phase of kimberlite exploration programs.

Belt is more than 30 mi (50 km) to the south, and the intervening area contains only a few small precious-metal mining districts.The Northgate fluorspar district 10 mi (15 km) northwest of the study area has been a large producer.Thus, attention is focused on the possible occurrence of fluorspar in the area, although none has yet been found.Uranium minerals in a fault zone in Proterozoic rocks 4 mi (6.5 km) east of the study area suggest the possibility of similar deposits in the study area, but the traces discovered thus far are too small to permit an inference of uranium resources.No mining has taken place in the study area, and little evidence of prospecting or claim staking were found.One mining claim southeast of Montgomery Pass was staked in an area of fractured, sheared, and faulted granitic rocks and Permian-Triassic red beds.The rocks are locally silicified, but no ore minerals were found, and chemical analysis of samples did not disclose the presence of significant amounts of metals.Several small prospect pits in a gneiss xenolith west of Glendevey and north of Mclntyre Creek contain minor amounts of rutile and topaz; however, excavation activity at this site was probably triggered by the local presence of lazulite, which may have been mistaken for a blue copper mineral.A few prospects in the northeastern part of the study area contain minor amounts of copper and silver but no suggestion of mineable deposits.The geochemical survey disclosed only a few weak anomalies of copper, silver, arsenic, tungsten, uranium, thorium, and molybdenum.Anomalous amounts of copper and silver were obtained from rock samples taken from small veins west of and in the prospect pits in the northeast part of the study area.The highest silver value is 1.5 ppm.Arsenic was found in sheared, silicified, argillized, and pyrite-bearing granitic rocks at small prospect pits in the southern part of the area in amounts that range from 500-2,000 ppm.Tungsten was detected in a few stream-sediment and pannedconcentrate samples in areas that suggest weak tungsten mineralization along some of the major faults.Uranium was found in anomalous amounts in a few samples, the highest of which (>120 ppm) was in a very small carbonate vein.Thorium that is commonly detected in stream-sediment samples in amounts ranging from about 50-150 ppm is probably present in allanite and other trace minerals from the granitic rocks.Molybdenum is present in low amounts (mostly 5-10 ppm) in many stream-sediment samples and in a few samples of fractured and altered granitic rocks along faults.The geochemical survey suggests the possibility of tungsten and uranium resources in deposits along major faults although no specific exploration targets are indicated.