Endless prairies with herds of bison and buffalo roaming free—that's a typical landscape of the Wichita Mountains in southwestern Oklahoma. It's the best destination in the state for hiking, mountain biking, rappelling, rock climbing, camping, and fishing. Granite rocks from Wichita Mountains are ov

We enjoyed the Iron County Fair in Parowan UT as well as the vast views of Sycamore Canyon and nostalgia of Route 66 in Williams, AZ.

Looking for a chance to escape into nature? You will love taking on one of the many scenic hiking trails in Arizona like the ones along the Mogollon Rim.

Laramide Orogeny

The Rocky Mountains, commonly known as the Rockies, are a major mountain range in western North America. The Rocky Mountains stretch more than 3,000 miles (4,800 km) from the northernmost part of British Columbia, in western Canada, to New Mexico, in the Southwestern United States. Within the North American Cordillera, the Rockies are somewhat distinct from the Pacific Coast Ranges and the Cascade Range and Sierra Nevada, which all lie further to the west. The Rocky Mountains were initially formed from 80 million to 55 million years ago during the Laramide orogeny, in which a number of plates began to slide underneath the North American plate.

Every geologist mapping or describing folds in the field probably has the same question in mind: how did these structures actually form? As geologists we tend to look for a simple history or mechanism that can explain our observations reasonably well. Folding is no exception, and there are different approaches and process-related terms. One approach is to consider the way force or stress acts on a layered rock, which leads to the three-fold classification and terminology. Other terms are related to how the layer(s) react to force and stress, for instance whether layers fold by layer-parallel shearing, orthogonal flexure or some other mechanism that is controlled by rock rheology. Still other classes of folding, such as kinking and chevron folding, are related to fold geometry. For this reason, several different fold mechanisms are defined, and many of them overlap in definition. This is why terms such as buckling, kink folding and bending can be confusing when discussed in terms of mechanisms such as flexural slip and simple shear. In summary, we are dealing with differences in orientation of stress axes relative to the layering, kinematics, and mechanical and rheological properties, and thus mechanisms that emphasize different aspects of folding. The most important distinction between the ways folds form probably lies in whether the layering responds actively or passively to the imposed strain field. Now considering active folding (buckling), where the competence or viscosity contrast between the folding layer and its host rock is important. We will then look at passive folding, where layers are simply passive markers with no rheological influence, and then consider bending, where forces are applied across the layering. The following sections will then discuss models known as flexural folding mechanisms (flexural slip, flexural shear and orthogonal flexure), which can contribute to both active folding and bending. Finally, we will discuss kinking and the formation of chevron folds. Active folding or buckling (Class 1B folds) Active folding or buckling is a fold process that can initiate when a layer is shortened parallel to the layering, as shown schematically. Folds appear to have formed in response to layer-parallel shortening. A contrast in viscosity is required for buckling to occur, with the folding layer being more competent than the host rock (matrix). The result of buckling is rounded folds, typically parallel and with more or less sinusoidal shape. Two folded layers of different thickness. The upper and thinner one shows a smaller dominant wavelength than the lower one. Buckling occurs when a competent layer in a less competent matrix is shortened parallel to the length of the layer. If an isotropic rock layer has perfectly planar and parallel boundaries and is perfectly parallel with a constantly oriented s1 or ISA1, then it will shorten without folding even though there is a significant viscosity contrast between the layer and the host rock. However, if there are small irregularities on the layer interfaces, then these irregularities can grow to form buckle folds with a size and shape that depend on the thickness of the folded layer and its viscosity contrast with its surroundings. Buckling or active folding implies that there is layer parallel shortening and a viscosity contrast involved, and also irregularities on which folds can nucleate. Buckling of single, competent layers in a less competent matrix is relatively easy to study in the laboratory and has also been explored numerically. Single-layer folds formed by buckling have the following characteristics: Alternating Class 1B and 3 folds are commonly seen in folded layers. CompetentlayersexhibitClass1Bgeometry. The fold wavelength–thickness ratio (L/h) is constant for each folded layer if the material is mechanically homogeneous and if they were deformed under the same physical conditions. Such folds are often called periodic folds. If the layer thickness varies, then the wavelength is changed accordingly.   The effect of the folding disappears rapidly (about the distance corresponding to one wavelength) away from the folded layer.    The folds in the competent layer approximate Class 1B folds (constant layer thickness). If there are two or more folded competent layers then the incompetent layers in between are folded into Class 1A and Class 3 folds. Cusp (pointed) hinges point to the more competent layers.   Strain distribution in the hinge zone of a folded limestone layer in shale. Outer-arc stretching is separated from inner-arc shortening by a neutral surface. The outer part of the competent layer is stretched while the inner part is shortened. The two parts are typically separated by a neutral surface. Note that layer-parallel shortening, which always takes place prior to folding, can reduce or eliminate the outer extensional zone.   The normal to the axial surface or axial cleavage indicates the direction of maximum shortening (Z). Experiments and theory show that homogeneous shortening (T) occurs initially, together with the growth of irregularities into very gentle and long-amplitude fold structures. When the most accentuated folds achieve opening angles around 160–150 , the role of layer-parallel shortening decays. From that point on the folds grow without any significant increase in layer thickness.  Buckling has been modeled under the assumption of linear or Newtonian viscosity. It is likely that most rocks show non-linear rheological behavior during plastic deformation, which has consequences for the buckling process. A power-law rheology is then assumed, where the exponent n > 1. The higher the n-exponent, the quicker the fold growth and the less the layer parallel shortening T. Many natural folds show low T-values, and, together with low L/h ratios (L/h < 10), this indicates a non-linear rheology. However, the differences between the results from viscous and power-law rheology models are not great. Buckle folds are most easily recognized as single competent layers, but can also occur where several competent layers occur in parallel. Ld/h is significantly less for multilayer than for single layer buckling. Where two thin layers are close they will behave more like a single layer whose thickness is the sum of the two thin layers, as seen from the experimental results. Folding of multi-layered rocks. Far-apart layers act as individual layers (left). The closer they get, the more they behave as a single layer with thickness larger than that of the thickest of the individual layers. Where we have alternating thick and thin layers, the thin layers will start to develop folds first. At some point the thick layers will start to fold (with longer wavelength) and take control over the further development. The result is relatively large folds controlled by thick layers together with small, second-order folds formed earlier in the process. Illustration of how folding initiates in thin layers. Once the thicker layer starts to fold, the smaller folds in the thin layer become parasitic and asymmetric due to flexural flow. Several mechanisms can be involved during buckling. The simple stones can collectively be termed flexural folding and are separated into orthogonal flexure, flexural slip and flexural flow. In addition there is always the possibility of having volume change, particularly in the hinge zone. Buckled multi-layers. Note how the largest folds affect the entire layer package. Passive folding (Class 2 folds)  Passive folding is typical for rocks where passive flow occurs, i.e. where the layering exerts no mechanical influence on the folding. In these cases the layering only serves as a visual expression of strain with no mechanical or competence contrast to neighbouring layers. Such layers are called passive layers. Perfectly passive folds produced by simple shear are Class 2 (similar) folds, and passive folds that are associated with simple shear, or atleast a significant component of simple shear, are called shear folds. Formation of Class 2 folds by (a) simple shearing and (b) pure shearing of a gently curved layer. No viscosity contrast is involved, meaning that the folds can be regarded as passive. Passive folds generated by simple shearing are perfectly similar folds. Passive folds of perfect Class 2 geometry can easily be generated by differentially shearing a card deck. Drawing lines perpendicular to the cards prior to shearing helps visualize the fold. However, the formation of passive folds is not restricted to simple shear. Passive folds can form in response to any kind of ductile strain, for instance sub-simple shear, transpression and even coaxial strain. Hence simple shear is only one of an infinite spectrum of kinematic models that can produce passive folds. Passive folding produces harmonic folds where the layering plays no mechanical role and therefore no influence on the fold shape.  Examples of passive folding are found where passive layers enter shear zones or otherwise are affected by heterogeneous strain. Drag folds along faults are examples typical for the brittle regime, although many layered sequences contain beds of quite different mechanical properties so that slip occurs between layers. Passive folds are frequently found in mylonite zones, particularly in mono-mineralic rocks such as quartzite, marble and salt. Passive harmonic folding of quartzite in a Caledonian mylonite zone. The similar geometry of this Z-fold and its setting in a Caledonian shear zone indicate that it is a shear fold. Bending  Bending occurs when forces act across layers at a high angle, unlike buckle folds where the main force acts parallel to a layer. This is also the case for passive folding, and the two are closely related. However, bending is generally thought of as something that is more directly forced upon the layers by geometries and kinematics of the bounding rock units. Several aspects of bending have been studied in great detail by engineers because of its importance in the field of construction engineering, such as in horizontal beams supported by vertical pillars. Bending occurs when forces act across layers, and may involve more than one mechanism. Classic geologic results of bending are the forced folds created in sedimentary layers blanketing faulted rigid basement blocks. The displacement is forced upon the sediments by fault movement on a pre-existing basement fault, and the sediments are soft enough to respond by monoclinal folding until at some critical point they rupture and the fault starts propagating up-section. Such structures are particularly well exposed in the Colorado Plateau–Rocky Mountains area, where numerous Laramide-related uplifts have created such structures. Bending as such is a boundary condition- or external load-related model, not a strain model, particularly not when a free surface is involved such as during forced folding mentioned above. In other words, there are many ways that folding and strain can accumulate internally in a fold during bending. Examples of bending in various settings and scales: (a) between boudins; (b) above thrust ramps; (c) above reactivated faults; and (d) above shallow intrusions or salt diapirs. An obvious response to bending is deformation by simple shear, in which case we are back to passive folding. The simple shear passive folding model may work if we have a wide fault zone underneath the fold or if the fold is very narrow. In most cases the fold widens upward, telling us that we have to modify the simple shear model. In this case trishear comes in handy. Trishear distributes shear in a triangular zone ahead of a propagating fold, and seems to work very well for several mapped examples. Still, trishear cannot explain all features seen in many forced folds. Field studies show evidence of bedding parallel slip or shear. This is manifested by striations on weak bedding-parallel surfaces or by bedding-parallel deformation bands. Also the related flexural mechanism described below as orthogonal flexure can result from bending loads.  There are many other examples of bending. One is fault-bend folds, for instance where thrust sheets are passively bent as they move over a ramp structure. Such folds are commonly modelled as kink folds, again related to flexural slip. They may also be modelled by means of simple shear, which is commonly done for fault-bend folds formed above non-planar (e.g. listric) faults.  Differential compaction, where a sedimentary sequence compacts more in one area than in another due to different degrees of compaction of the underlying layers, is also a type of bending. This is common across the crests of major fault blocks in post rift-sequences in sedimentary basins, but can also occur along salt diapirs and shallow intrusions. Folds formed by differential compaction are gentle.  Forceful intrusion of magma or salt can also bend roof layers. Again the strain accumulation mechanism may vary, with flexural slip being a common constituent. In the plastic regime, bending is less common because of the high ductility of all or most parts of the deforming rocks. However, bending is frequently associated with rigid boudins. Passive folding of layers between boudins. Flexural slip and flexural flow (Class 1B)  (a) Flexural slip, showing opposite sense of slip on each limb, decreasing towards the hinge zone. (b) Flexural flow, where fold limbs are being sheared. Ideally, layer thickness is preserved in both models. Flexural slip implies slip along layer interfaces or very thin layers during folding. It is one of three kinematic models of folding (the others being flexural flow and orthogonal flexure) that maintains bed thickness and thus produces Class 1B or parallel folds. Simple flexural slip experiments can be performed simply by folding double sandwiches with jelly. The sandwich maintains its thickness even though slip occurs between the pieces of bread, until the fold becomes too tight. It is a prerequisite for flexural slip that the deforming medium is layered or has a strong mechanical anisotropy.  In nature, the anisotropy could be mica-rich thin layers in a quartzite or mylonite, or thin shale layers between thicker sandstone or limestone beds in sedimentary rocks. Flexural slip can occur in the middle crust where plastic deformation mechanisms would be involved, but is perhaps more common where sedimentary strata are folded in the upper crustal brittle regime. In the latter case, bedding surfaces act like faults, and slicken lines will sometimes develop on slipping surfaces.  Maximum slip occurs at the inflection points and dies out toward the hinge line, where it is zero. The sense of slip is opposite on each limb, and the slip is consistent relative to the hinges, where sense of slip changes. Relative slip on the convex side of a flexural slip fold is always toward the fold hinge, whereas on the concave side slip is opposite. Slicken lines on folded weak layers and constant bed thickness reveal flexural slip. In cases where strain is more evenly distributed in the limbs in the form of shear strain, as is more commonly the case in the plastic regime, flexural slip turns into the closely related mechanism called flexural shear or flexural flow. Flexural flow experiments are conveniently done by bending a soft paperback book or a deck of cards (remember to draw circles for strain markers). During this process slip occurs between individual paper sheets. If we put strain markers on our paperback, we would see that strain is zero in the hinge zone and increasing down the limbs. This is so because the shear strain is directly related to the orientation (rotation) of the layers: the higher the rotation, the higher the shear strain. For originally horizontal layers folded into an upright fold, shear strain is directly related to dip (g ¼ tan (layer dip)), and the sense of shear is opposite on each side of the axial trace. This results in a characteristic strain distribution in the fold. For example, the neutral surface separating extension from contraction, typical for many buckle folds, is not found in pure flexural-flow folds. Flexural flow produces identical strain in the inner and outer part of a fold, but strain increases away from the hinge. Note that evidence for a combination of orthogonal flexuring and some flexural flow or slip is commonly found in buckle folds, in which case a neutral surface may well exist. Pure flexural folds have no neutral surface, and strain increases away from the hinge zone. Pure flexural folds are perfect Class 1B folds. We can estimate the amount of layer-parallel shortening for such folds by measuring the length of any one of the folded layers. This layer has maintained its original length because it was the shear plane throughout the folding history. Constant layer length and thickness are assumptions that simplify restoration of cross-sections. Orthogonal flexure (also Class 1B)  Orthogonal flexure, also called tangential longitudinal strain, is a deformation type with its own specific conditions: All lines originally orthogonal to the layering remain so throughout the deformation history. The result is stretching of the outer part and shortening of the inner part of the folded layer. The long axis of the strain ellipse is therefore orthogonal to bedding in the inner part of the layer and parallel to bedding in the outer part. Layer-parallel shortening resulting in orthogonal flexure and flexural flow. Note what happens to the originally orthogonal lines. Strain ellipses are indicated. Orthogonal flexure and flexural flow have in common that they produce parallel (Class 1B) folds. But the two models produce quite different strain patterns: The neutral (no strain) surface separating the outer-extended and inner-contracted part of the folded layer does not exist in flexural flow, where strain is identical across the fold along dip isogons. During the folding history, the neutral surface moves inward toward the core of the fold, which can result in contraction structures overprinted by extension structures. Orthogonal flexure produces parallel folds with a neutral surface. Pure orthogonal flexure is only possible for open folds. When folds get tighter, the conditions for orthogonal flexure become harder and harder to maintain, and flexural slip or flow will gradually take over. Evidence for orthogonal flexure is typically found in stiff, competent layers that resist ductile deformation. Some have simplified the definition of orthogonal flexure to a mechanism resulting in outer-arc contraction and inner-arc extension. By getting rid of the requirement of orthogonality, the model becomes more general and embraces many more natural examples. Kinking and chevron folding  (a) Conjugate kink bands in mylonitized anorthosite gabbro, Bergen, Norway. (b) Kink folds related to Laramide thrusting in north Wyoming (Dead Indian Summit) (c) Kink-like folds in oceanic sediments in Oman. Kink bands are common in well-laminated and anisotropic rocks rich in phyllosilicate minerals, and some field occurrences. Kink bands are centimetre to decimetre wide zones or bands with sharp boundaries across which the foliation is abruptly rotated. Wider zones are sometimes referred to as kink folds. Kink bands and kink folds are characterized by their strong asymmetry and their Class 2 fold geometry. They are closely related to chevron folds, which also are Class 2 folds, but differ in terms of symmetry. Both are relatively low-temperature (low metamorphic grade) deformation structures where there is a significant mechanical anisotropy represented by lamination or repeated competent–incompetent layers, and both imply layer shortening. Classic kink bands have very angular hinges and lack even the narrow hinge zone found in the outer arc of chevron folds. There is another important difference between the two. While chevron folds initiate with their axial surface perpendicular to the shortening direction, kink bands form oblique to this direction, typically in conjugate pairs. (a) The orientation of s1 can be determined from the orientation of conjugate sets of kink bands. (b) Continued kink band growth can produce chevron folds. When conjugate sets of low-strain kink bands are observed, s1 or ISA1 is commonly assumed to bisect the sets. Going from strain to stress is not straightforward, but the smaller the strain the better the correlation. When a single set of kink bands occurs, we know that s1 is oblique to the band, but its precise orientation is unknown because kink bands may rotate during progressive deformation. Kink folds generated by bending do not directly reveal the orientation of stress. Such kink folds have orientations that are controlled by the local geometries of ramps or fault bends. Hence, in such cases the bisecting axis between two kink zones does not in general represent s1 or ISA1. Experiments have shown that conjugate sets can nicely merge to form chevron folds if strain is high enough (around 50%). However, 50% shortening is not commonly achieved by kinking in naturally deformed rocks, so this way of forming chevron folds may not be the most common one after all. Classic chevron folds with beds on the centimetre scale are more likely to form by flexural slip of multi-layered rocks during layer-parallel shortening. The typical setting is where competent beds are separated by thin incompetent layers, for instance quartzite or chert separated by shale or phyllite. Flexural slip then occurs between the competent layers, which are strained only in the thin hinge zones. Just like buckle folds, the hinges have to stretch in the outer arc and shorten in their inner parts. Furthermore, geometric problems in the hinge zone require flow of the incompetent rock into the hinge, or alternatively inward hinge collapse of the competent bed. Hinge collapse is particularly common in relatively thick competent layers that occur between thinner ones. Another way of resolving hinge compatibility problems is by reverse faulting. Credits: Haakon Fossen (Structural Geology)

INTRODUCTION Granite xenolith (foreign rock) exposed in the mine rib (wall) of the Sloan 2 diamondiferous kimberlite in Larimer County, Colorado. Bernie Free poses (photo by the author). Revised, 3/9/2022. On Sunday, May 19th, 1996, the 1996 Industrial Minerals Forum scheduled a field excursion to the Sloan Ranch kimberlites in the southern part of the Colorado-Wyoming State Line district, to learn about two of the six diamondiferous kimberlite intrusives within the Sloan cluster in the southern part of the State Line district. These two intrusives, referred to as the Sloan 1 and 2 pipes, are two of the more than 40 known diamond-bearing kimberlites in the district (McCallum, 1991). In addition to these kimberlites, several large clusters of cryptovolcanic structures in the region have rounded boulders and cobbles and carbonate-rich soil that were identified by DiamonEx Ltd in 2007 and 2008.  The kimberlites are visible on GoogleEarth at 40o50'38.74"N; 105o27'20.97"W, and more than a hundred cryptovolcanic structures are found in this region suggesting a potential for many undiscovered kimberlites - but only one anomaly was ever drilled! Since every kimberlite in this district has yielded diamonds, the district should be explored in more detail! To the north, the author also identified 200 similar anomalies that may include  more kimberlites in the Twin Mountains, Lost Lakes, Vadawuoo and other areas. Total diamond production from the State Line district, includes 140,000 diamonds. The diamonds ranged from microdiamonds to a 28.3 carat gemstone (Hausel, 1998; Erlich and Hausel, 2002). Cumulative production from the Sloan 1 and 2 kimberlites totaled 39,616 diamonds (about 1/4 of the total production in the district). The Sloan diamonds ranged from microdiamonds to a 5.51 carat gemstone (Hausel, 1995). These diamonds were all during bulk sampling in the 1990s. Bulk samples recovered from the Sloan property ranged from 0.01 to 0.60 carats/tonne (Hausel, 1998). Exploration of the property began with Superior Minerals Company. Acknowledgements and Access Sloan complex from the air. At top of photo is a diamond-shaped tailings pond with the mill or diamond ore next to the pond. Along the bottom right of photo are several test pits dug into the Sloan 1 diamondiferous kimberlite. Perpendicular to this pipe is a linear depression - the Sloan 2 buried kimberlite (photo by the author).  Permission was granted for a visit to the property by the 1996 Industrial Minerals Conference to visit the kimberlites by Frank Yaussi, Prairie Divide Corporation, and Cominco American Incorporated. The kimberlites lie in the S/2 section 10, T10N, R72W in Larimer County, Colorado, at an elevation of about 7,250 feet. The pipes can be reached from U.S. Highway 287 which runs between Laramie and Fort Collins. From Laramie, the pipes are accessible by driving 40-miles south along Highway 287 to the Larimer County Road 80C (Cherokee Park Road) turnoff. From that point, travel west 8 miles to the Larimer County Road 82E junction. Turn left on County Road 82E and continue west another 4.5 miles. The pipes lie south of the county road adjacent to a large, blue, metal building which at one time housed Superior Minerals' and later Dia Met's diamond mill. Equipment from the mill was rumored to have been flown to Canada at the end of the 20th Century, for testing the Ekati kimberlites. To access the property, permission is needed, although one can view the kimberlites from the County Road. This field trip summary is an overview of several publications listed in the references cited section at the end of the paper and summarizes the activities of the International Field Trip conference in 1996. An additional field trip to the pipes included the 2003 International Kimberlite Conference (Coopersmith, Mitchell, and Hausel, 2003). HISTORY Diamonds were discovered in the Colorado-Wyoming State Line district in 1975. The lack of historical discoveries in this region is curious especially when one considers the district hosts several deeply eroded diamondiferous kimberlite intrusives. And in all probability, some drainages in the district contain considerable placer diamonds due to significant portions of the upper one to two thousand vertical columns of the diamond pipes in this region being eroded over the past 60 million years. The lack of historical placer discoveries may be due to two reasons. (1) The "1872 Great Diamond Hoax" was located just 180 miles due west of the State Line district (Hausel and Stahl, 1995). The "hoax" was of such magnitude that few people would have believed diamonds could be found anywhere in Colorado or Wyoming for decades to come. (2) Much of the State Line district is underlain by relatively unmineralized granite, thus very little gold prospecting has ever occurred in the district. The general lack of placer gold activities probably contributed to the lack of placer diamond discoveries. Aerial view of nearby Sloan 5 diamondiferous kimberlite showing typical anomalous characteristics of kimberlite (cryptovolcanic) pipes. Note the open, circular park (depression) surrounded by trees. Also note the prominent lineament on the right side of the depression formed by stand of trees that mark a deep fracture. When I took this photo, Cominco American was testing for diamonds and you should be able to see the shallow pits that expose the gray soil (blue ground) (photo by the author). Kimberlite was first identified in this district in 1964, by Dr. M.E. McCallum from samples collected from a small terrazzo quarry along the western edge of the Sloan 1 diatreme. Diamonds were later discovered in 1975 during routine thin-section preparation of a serpentinized garnet peridotite nodule collected from the Schaffer 3 kimberlite in Wyoming. During preparation of the section, Florian Nowacki and Ross Jensen of the U.S. Geological Survey noted deep scratches cut into a carborundum grinding plate. Upon examination of the nodule, a small white crystal was isolated and confirmed as diamond by x-ray diffraction analysis by T. Botinelly and B.F. Leonard of the U.S. Geological Survey. Other small diamonds were subsequently recovered from this nodule (McCallum and Mabarak, 1976). However, Frank Yaussai (personal communication) reported finding several diamonds in Rabbit Creek while prospecting for gold. This was also supported by a prospector from North Carolina who panned in the same area as Frank Yaussai following the release of a 2014 book by the author. The prospector reported panning out several diamonds including one of approximately 5-carats in weight with verification from the Geology Department at the University of North Carolina. Frank Yaussai indicated samples of kimberlite mined from the Sloan quarry scratched a carborundum wheel owned by the Cheyenne Tile and Terrazzo Company during polishing of the material for use in the Cheyenne airport. It was also indicated that Dr. Arnold Waters, Jr., chief geologist for DeBeers, applied for a lease on the Sloan property following the processing of a small bulk sample recovered from the Sloan 1 kimberlite sent to South Africa for processing. These events took place prior to the 1975 announcement of diamonds being found in Wyoming, but the results were not published. Commercial interest in the Colorado-Wyoming State Line district was later initiated in 1979 after the State of Wyoming, in cooperation with Rocky Mountain Energy Company (Union Pacific), issued a call for exploration proposals for the Wyoming portion of the State Line district. Cominco American Incorporated submitted the successful bid and explored the Schaffer and Aultman kimberlites in Wyoming. The results of bulk sample tests by Cominco American were discouraging as only subeconomic grades (0.5 to 1.0 carats/100 tonnes) were detected. The largest diamond recovered in these tests was a 0.86 carat gemstone.  Above - aerial photo over group of cryptovolcanic structures showing structural alignment. Interest in the Sloan 1 and 2 kimberlites was initiated by Superior Minerals in the following year in 1980. This was followed by a Superior-Lac joint venture in 1983, and in 1984, the property was quit-claimed back to the owners. This latter event coincided with a scheduled advanced royalty payment, and the purchase of Superior Minerals by Mobil Oil Company. Mobil eventually disposed of all of Superior's mineral interests while Lac Minerals was entering into litigation in Canada concerning its control of the Hemlo Gold mine (Oliver, 1990). In 1994, Royalstar Ltd. drove a 700-foot adit into the Sloan 2 kimberlite to test for economic mineralization. In 2006, DiamonEx Ltd of Brisbane Australia, leased the property with the plan of sampling the property and recovering a large parcel of diamonds for valuation. At about the same time (2006-2008), exploration by DiamonEx resulted in discovery of a few hundred cryptovolcanic (kimberlite-like) anomalies and kimberlitic indicator mineral anomalies in the Colorado and Wyoming region. One of several anomalies (depression with considerable amount of calcium carbonate) on shoreline. These occur in a granitic and gneissic basement terrain (photo by the author).  The available data suggested possibility for commercial diamond mineralization at the Sloan Ranch depending on the price per carat. Thus it was the intention of DiamonEx to determine a value per carat and ore grade and attempt to place a small gemstone operation in production to recover diamonds, Cape Rubies (pyrope garnet), Cape Emeralds (chromian diopside) and specimens if the diamond value showed the property to be economic. The concept included searching for additional commercial pipes and placers while the Sloan kimberlites were being mined. However, the economic collapse of 2008 took its toll on the DiamonEx and company operations ceased in Colorado, Wyoming and Botswana. GEOLOGICAL SETTING The Colorado-Wyoming State Line district lies within the southern Laramie Range of Wyoming and continues south into Colorado along the flank of the northern Front Range. Geographically, the Laramie and Front Ranges are a continuous mountain range. This range was uplifted during the Laramide orogeny and its core was exposed following several million years of erosion. The core of range in the vicinity of the State Line district consists of Proterozoic crystalline rock. The Proterozoic basement continues north, where in the vicinity of the Sheep Rock area, the Cheyenne Belt separates the Proterozoic basement from an Archean basement farther to the north. The Archean core consists of Archean gneisses, schists and granites of the Wyoming Province. Thus in terms of cratonic environments, an Archon lies north of the Cheyenne Belt, and a Proton consisting of a Proterozoic mobile belt, lies to the south. Precambrian exposures in the vicinity of the State Line district consist of 1.9 to 1.7 Ga metamorphic rocks of the Colorado Province that are disconcordantly intruded by 1.4 Ga granitic rocks. The metamorphic rocks are amphibolitic, quartzofeldspathic, and pelitic gneisses and schists (McCallum and others, 1977) that are part of a island arc succession which collided with the Archean craton of the Wyoming Province nearly 1.7 Ga ago. This collision boundary is marked by a major suture referred to as the Cheyenne Belt (Karlstrom and Houston, 1984). Even though the district lies 50- to 70-miles south of the Wyoming Province, Shaver (1988) suggested that the Proterozoic rocks of the Colorado Province may have thrust over the Wyoming Province and an Archean fragment may lie at depth in the district. The dominant regional structure in the district is the Virginia-Dale ring dike complex, a circular granitic pluton of nearly 9-miles in diameter. The ring-dike intrudes 1.4 Ga Sherman Granite and the older metamorphic rocks (Eggler, 1968). The complex is interpreted as a phase of the Log Cabin batholith located to the south, and represents Precambrian caldera. Map showing location of the Colorado-Wyoming State Line district. Areas of interest include the Iron Mountain  kimberlite district, the Sheep Rock and Happy Jack anomalous areas; the Estes Park, Isolation Peak, Hayden Peak,  and Green Mountain kimberlites; the Leucite Hills lamproites; and the Cedar Mountain anomalous area (The Cedar  Mountain area contains widespread 'kimberlitic' mineral indicators as well as the recent discovery of 10  diamondiferous breccia pipes and a dike complex hosting eclogite xenoliths. A 200-pound core sample from one of the  pipes recently was reported to have yielded two diamonds). The only post-Precambrian rocks found in situ in the district are arkosic sedimentary rocks of the Pennsylvanian Fountain Formation (McCallum and others, 1979). In situ Lower Paleozoic rocks do not occur in the district, although some Cambrian, Ordovician, and Silurian sedimentary xenoliths are found in some kimberlite diatremes. It was the presence of these sedimentary xenoliths that first attracted the curiosity of geologists to the diatremes in the early 1960s (see: Chronic and Ferris, 1961). The age of the kimberlites in the Colorado-Wyoming kimberlite province, which extends as far south as Boulder, Colorado, and as far north as Sybille Canyon in the central Laramie Range, were assumed to be early Devonian based on the presence of Paleozoic xenoliths in some of the intrusives. Fission track dating of sphene (Larson and Amini, 1981) and zircon (Naesser and McCallum, 1977), and Rb-Sr dating of phlogopite (Smith, 1979, 1983) has since established a late Devonian age for many of the kimberlites. The Green Mountain pipe located 50 miles south of the district near Boulder, Colorado, recently yielded paleomagnetic, K-Ar, Ar40-Ar39, and Rb-Sr ages between 600 to 700 Ma. Additionally, Ar40-Ar39 and Rb-Sr data also indicate a Proterozoic age for one of the Chicken Park kimberlites located in the district, just five miles west of the Sloan kimberlites (Alan P. Lester, written communication, 1996). These dates provide further evidence of the several episodes of kimberlite and lamproite magmatism in the Wyoming craton (Hausel, 1996) [the Wyoming craton is here defined to include both the Wyoming Province (Archon) and the Colorado Province (Proton)]. Kimberlite, lamproite, diamond, and kimberlitic indicator mineral anomaly map of the Wyoming craton. The approximate boundary of the Wyoming (Archean) Province lies within the dashed line. Diamond symbols are areas were detrital diamonds have been reported. Dots are locations of indicator mineral anomalies, and stars represent sites of kimberlite  and lamproite occurrences, districts, and related rocks. Kimberlites in the region occur as diatremes (pipes), related plugs or blows, and dikes. Intrusive breccia (tuffisitic kimberlite breccia) predominates in the diatremes, whereas massive porphyritic kimberlite (hypabyssal facies) is dominant in dikes and blows. Kimberlite is typically intensely serpentinized, carbonated, and locally silicified and/or fenitized (McCallum and Eggler, 1971; Smith and others, 1979). Diatreme facies kimberlite consists of abundant subrounded to angular rock fragments along with serpentinized macrocrysts of olivine and lesser enstatite, Cr-rich and Cr-poor pyropic garnet and diopside, picroilmenite, clinopyroxene-ilmenite intergrowths, and phlogopite in a finely crystalline matrix of serpentine, carbonate, olivine, diopside, picroilmenite, phlogopite, perovskite, magnetite, Cr-rich spinels, hematite, apatite, and zircon (Rogers, 1985; McCallum, 1991). A very nice suite of indicator minerals collected from the Sloan 1 and 2 kimberlites during exploration by DiamonEx Ltd. All of the garnets (purple to orange) and chromian diopsides (emerald green) in this parcel are gem-quality. At the bottom of the photo are five opaque, metallic, picroilmenites and chromites. This deposit had known diamond reserves outlined by previous  operators and would have made an excellent diamond mine (photo by the author). Hypabyssal facies kimberlite occurs in dikes, sills, and small plug-like bodies that most likely represent root zones of deeply eroded pipes. Kimberlite and kimberlite breccia both occur, although kimberlite breccia is common only locally. Xenolithic fragments rarely exceed a few percent (unlike the diatreme facies) and are typically moderately well-rounded upper mantle and lower crustal nodules. Aphanitic hypabyssal kimberlite devoid of macrocrysts is rare. The mineralogy of the hypabyssal kimberlite is essentially the same as for the diatreme facies. However, globular, emulsion-like segregations of serpentine and calcite are more abundant in hypabyssal types (Rogers, 1985). The State Line district has been truncated by late Tertiary to Pleistocene erosion. The kimberlites are generally poorly exposed, have negligible relief, and are deeply weathered. Geophysical surveys suggest weathering continues to depths of at least 100 feet below the surface (Hausel and others, 1979), although moderately fresh exposures are present at some sites. Most kimberlites are covered by alluvium and or colluvium, and some have been recognized by the presence of grey, weathered ("blue-ground"), kimberlitic soil that is commonly associated with lush, grassy, vegetation, animal burrows, and/or cliche. Other sites are recognized by the presence of Lower Paleozoic xenoliths as well as mantle and lower crustal xenoliths and nodules of peridotite, pyroxenite, eclogite, granulite, and/or megacrysts of pyrope, ilmenite, or diopside (McCallum and Eggler, 1979). GEOLOGY OF THE SLOAN RANCH KIMBERLITES The Sloan cluster consists of six kimberlites near the southern margin of the Colorado-Wyoming State Line district. The field trip attendees will visit only the Sloan 1 and 2. Known diamondiferous kimberlite localities of the Colorado-Wyoming State Line district (does not include recent discoveries of cryptovolcanic structures). The Sloan 1 consists of diatreme-facies kimberlite, and the Sloan 2 consists of hypabyssal- and transitional-facies kimberlite (Shaver, 1994). The Sloan 1 has an aerial extent of 500 by 1,800 feet while the Sloan 2 is 200 by 2,000 feet. These two pipes have yielded a significant number of subcalcic 'G10' harzburgitic pyropes (McCallum, 1991) (author's note: Many peridotitic garnets found in kimberlite have been designated as 'G10' or 'G9'. These were derived from the peridotite xenoliths sampled by the kimberlite during ascent from the upper mantle. The peridotites are Cr-rich ultramafic rocks with olivine (>40%) and pyroxene and minor plagioclase that are subdivided on the basis of pyroxenes. For example, lherzolites are peridotites that have both clinopyroxene and orthopyroxene. Wehrlites contain clinopyroxene but lack orthopyroxene. Harzburgites have orthopyroxene but no clinopyroxene. Diamonds are found in some garnet peridotites. Most peridotitic diamonds occur in garnet-bearing harzburgite: diamondiferous pyrope-lherzolites are uncommon. Pyrope garnets from disaggregated peridotite have been designated by Gurney (1989) as either 'G10', derived from low-Ca garnet harzburgites, and 'G9' derived from the more calcic lherzolites. This suggests that a kimberlite with abundant 'G10' pyropes should be predicted to be diamondiferous as long as the magma was reducing. Reducing conditions can be predicted on the basis of MgO:Cr2O3 ratios of picroilmenite. The ratios provide an indication of the oxygen fugacity and the favorability of diamond preservation. In addition to G10 garnets, eclogitic garnets also are important in appraising the diamond potential of a kimberlite. Eclogitic garnets with Na2O contents >0.09% are indicative of a diamondiferous source terrane). Sample of diatreme facies Sloan kimberlite with large chromian diopside  megacryst typical of many samples from these pipes. Some of the better samples in the world have been collected here (photo by the author). A significant number of peridotitic garnets from the Sloan 1 and 2 kimberlites were reported by McCallum and Waldman (1991) to be subcalcic chromian garnets with G10 compositions indicating a low to moderate grade of diamonds from a harzburgitic source. The TiO2:Na2O plots for the eclogitic garnets indicated a low potential for a eclogitic diamond source. Ilmenite Cr2O3:MgO compositions indicated that reducing conditions prevailed in the kimberlite magmas suggesting that diamond preservation was favorable (McCallum and Waldman, 1991). The emplacement of the Sloan 1 and 2 kimberlites was structurally controlled. The Sloan 1 was emplaced along the Copper King fault while the Sloan 2 was emplaced along the Prairie Divide fault in the Log Cabin granite (McCallum, 1976). Typically, kimberlite erodes faster than the adjacent granite which results a slight topographic depression. Both pipes continue under Tertiary gravels in the North Fork of Rabbit Creek to the north (McCallum, 1991). The extent of kimberlite under the surficial cover is unknown (Oliver, 1990). Reported ore grades for the Sloan 2 kimberlite. Data needed for this project is value of diamonds - but remains unknown. Ore grades are reasonable for a small diamond operation, but value must be determined for the Sloan 1 and 2. Geologists from Superior Minerals Company concluded that both Sloan pipes were emplaced during separate events and were later eroded. McCallum (1976; 1991) however, concluded that both pipes were emplaced during the same event, and interpreted the Sloan 2 to be a blind diatreme. The evidence cited by McCallum supports that the Sloan 2 is a blind diatreme, in that no Paleozoic xenoliths have been found in the pipe indicating the kimberlite did not penetrate the Paleozoic cover during the Devonian. Whereas, the Sloan 1 contains abundant Paleozoic xenoliths along its western flank. The kimberlites are ultrabasic intrusive rocks that have been extensively altered through carbonatization and serpentinization. Minor phlogopite, mantle megacrysts, and cognate and xenolithic nodules from the mantle and crust occur in the kimberlites. The Sloan 1 is dominated by tuffisitic kimberlite breccias typical of diatreme facies, whereas the Sloan 2 is more characteristic of macrocrystic kimberlite breccia typical of hypabyssal facies (McCallum, 1991). The following descriptions are based on reports by McCallum (1991) and Shaver (1994). Sloan 2 DK1 (Sloan 2 phase): As we walk up the side of the hill from the vehicles towards the Sloan 2 adit, numerous fresh samples of DK1 phase kimberlite are scattered around the mine dump. These rocks were exposed by mining operations, as kimberlite did not crop out on the surface of the Sloan 2. The pipe was discovered following heavy mineral sampling (McCallum and Mabarak, 1976). The DK1 phase is a pale-green to black, phlogopite, monticellite, calcite, serpentine, macrocrystic hypabyssal-facies kimberlite breccia, with a phlogopite-serpentine groundmass. Locally, opaques (mostly spinel), perovskite, and apatite may be abundant. Olivine macrocrysts are pervasive and range from fresh to partially altered to completely serpentinized and locally carbonatized rounded grains. Macrocrysts of picroilmenite, pyrope garnet, and chromian diopside are abundant as are mantle and lower crustal xenoliths. This phase forms the northwestern two-thirds of the dike. Geologic map of the Sloan 1 and 2 kimberlites (from McCallum, 1991). DK4 (Light-green spotted phase): Samples of the DK4 phase can be found farther up the hill near the contact with the Sloan 1. This is a pale-green, locally pelletal, lithic tuffisitic transitional phase kimberlite breccia with a serpentine-clay-carbonate matrix. Some light green spots in this phase are the result of calcite-serpentine pools. Macrocrysts of serpentinized olivine are ubiquitous and macrocrysts of picroilmenite, pyrope, chromian diopside, phlogopite, and orthopyroxene are present. Crustal and mantle xenoliths are abundant. Carbonatite xenoliths are also common in this phase, and some are phlogopite-rich sovites. Some of the nodules contain barite. This phase occurs at the southeastern end of the Sloan 2. Sloan 1 Three major diatreme facies kimberlite phases were described at the Sloan 1: DK 2 (Black border phase): This is a distinctive, grey to black, tuffisitic kimberlite breccia with a serpentine matrix scattered around some reclaimed pits along the eastern margin of the Sloan 1 pipe. Serpentinized olivine macrocrysts are abundant and locally characterized by a reddish coloration due to finely disseminated hematite after magnetite produced during the serpentinization process of the olivines. Xenocrysts of pyrope, picroilmenite, and chromian diopside are relatively common, and phlogopite and orthopyroxene occur locally. Small xenoliths of Proterozoic granite are abundant, and fragments of Paleozoic sediments are ubiquitous. Sample of Sloan kimberlite with large pyrope megacryst in matrix. Photo by the author. DK3 (eastern green phase): This phase forms much of the Sloan 1 diatreme and can be found near some old reclaimed bulk sample pits. It is a pale-green tuffisitic kimberlite breccia containing abundant secondary serpentine, talc, clay, and carbonate. Unaltered minerals are rare, and macrocrysts and microcrysts of picroilmenite, pyrope, and chromian diopside are widely dispersed. Lower Paleozoic limestone and some granitic and metamorphic xenoliths are common. DK5 (Quarry phase): Located along the western flank of the Sloan 1 diatreme at the site of the old terrazo quarry. This is a dark-green, pelletal, tuffisitic kimberlite breccia with a serpentine matrix. The groundmass includes fine-grained prisms of diopside, phlogopite, and irregular pools of calcite and minor apatite. The abundant pelletal lapilli gives this phase a pseudo-conglomeratic appearance. Serpentinized olivine macrocrysts are abundant and macrocrysts of picroilmenite, pyrope, chromian diopside, and phlogopite are present. Pelletal lapilli vary from a few inches across and are cored with serpentinized olivine, Paleozoic carbonate, or Proterozoic crystalline rock clasts. Kimberlite in the lapilli is generally relatively fresh compared to the kimberlite matrix. Paleozoic carbonate and Proterozoic granite xenoliths can be found. The Paleozoic xenoliths are abundant and vary in size from inches to many feet across. DK6 : Several dikes (< 6 feet wide) of brown, black, to green macrocrystic hypabyssal-facies kimberlite with a serpentine matrix cut the kimberlite breccias. DIAMONDS The stability of diamond is constrained by pressure, temperature, and the oxygen fugacity of the host magma. Minimum depths of genesis of diamond-bearing kimberlites are estimated at 95 miles. Garnet lherzolite (peridotite) from the State Line district. (photo by the author). Most, if not all diamonds from kimberlite are interpreted as xenocrysts derived from the disaggregation of diamondiferous mantle xenoliths during the ascent of the kimberlite. The most important xenoliths are eclogite and garnet-bearing peridotites. Some nodules are very rich in diamond: a hand-specimen of eclogite collected from the Sloan 1 diatreme yielded a grade of 2,100,000 carats/100 tonnes (Schulze, 1992). Typical kimberlite and lamproite ore may only average 10 to 680 carats/100 tonnes. Eclogite consists of clinopyroxene (omphacite) and yellow-orange garnet (almandine-pyrope) with the possible addition of accessory rutile, kyanite, corundum, and coesite. Eclogites are assumed to be cumulates of melts of garnet peridotite or possibly derived from subducted oceanic crust. Some eclogites represent primary host rocks for diamond. This is not only supported by the presence of very rich diamondiferous eclogites, but also by the common occurrence of E-type (eclogitic) diamonds containing eclogitic mineral inclusions in kimberlite groundmass. Such diamonds may contain sodic clinopyroxene and garnet with rutile, kyanite, corundum, and coesite. A genetic relationship between eclogite and inclusion minerals is corroborated by the elevated sodium and potassium contents in garnets and clinopyroxenes, respectively. Garnet harzburgite (peridotite) from State Line district. Many such nodules are diamond-bearing. Photo by the author. Most peridotitic diamonds are formed in garnet-bearing harzburgite. Diamondiferous pyrope-lherzolites are uncommon. Diamond grades in the peridotite xenoliths are up to three orders of magnitude higher than the host kimberlites, but are still 2 to 4 orders of magnitude lower than those in the eclogite xenoliths (Helmstaedt, 1993). The majority of the diamonds in the Sloan Ranch complex are E-type diamonds. A parcel of 153 diamonds from the Sloan 1 and 2 yielded 74% E-type diamonds (Otter and Gurney, 1989). Thus, any attempts made to predict the diamond content of these kimberlites based on their G10 pyrope geochemistry would be invalid as the G10 pyropes are only indicative of the garnet harzburgites (Shaver, 1994). The diamonds range from well-perserved octahedra to highly resorbed tetrahexahedra with colors ranging from D to E to some that are very dark brown-yellow (Dummett and others, 1988). Sloan 2 diamonds. This group includes a 5.5 carat gemstone. Diamonds from Royal Star Resources. Diamonds from the Sloan pipes range from microdiamonds to a 5.51 carat gemstone (Bernie Free, personal communication, 1994). The 5.51 carat diamond was sent to a cutter and two small 'windows' were polished on its surface. Based on the 'windows' the cutter estimated the diamond would produce a clear, white, brilliant-cut stone weighing at least 2 carats (Shaver, 1994). Rare kyanite eclogite nodule from the State Line district. This nodule consists of blue kyanite, green chromian diopside and dark brown to grey garnet (photo by the author). In a recent test, Royalstar Resources recovered a 3,300 tonne sample from a 700 foot adit driven into the Sloan 2 kimberlite. The Sample yielded 9,034 diamonds larger than 1 mm with a combined weight of 342.17 carats for an average grade of 12.68 carats/100 tonnes (Shaver, 1994). Individual samples ranged from 0.72 to 0.22 carats/tonne (Oliver, 1990). The material was processed in the Dia Met mill east of the Sloan 1 kimberlite. The mill was constructed by Superior Minerals in 1982 with a rated capacity of 50 tonnes/day. Resources: The Sloan 1 pipe is estimated to contain a resource of 15,300,000 tonnes of kimberlite at an average grade of 0.06 carats tonne. The Sloan 2 is estimated to contain a resource of 8,400,000 tonnes at an average grade of 0.17 carats/tonne. A transitional phase along the contact between the Sloan 1 and 2 contains 3,800,000 tonnes at an average grade of 0.13 carats/tonne (Oliver, 1990). These resource estimates were made to a depth recoverable by open-pit mining (i.e., to a depth 400 feet). Based on the grades, the Sloan 2 kimberlite represents a potential resource of 1.4 million carats of contained diamonds, and the Sloan 2 (including the transitional phase) represents a potential resource of 1.9 million carats of contained diamonds (Oliver, 1990). Eclogite nodule from the State Line district. Some eclogites have been very diamond rich including one nodule recovered from the Sloan kimberlites that contained >20% diamond! Photo by the author. SUMMARY These Sloan Ranch kimberlites were of interest to DiamonEx because they represented a potentially economic diamond resource, although small compared to most economic kimberlites in the world. The initial intention was to acquire the Sloan Ranch kimberlites along with the Kelsey Lake kimberlites, place these into production while searching for additional resources. Exposed highwall at the Kelsey Lake diamond mine north of the Sloan Ranch, Colorado and west of Highway 287. Photo by the author. The Colorado production would have contributed to the overall proposed production from Botswanna and Colorado. However, the Kelsey Lake kimberlites could not be acquired. Discovery of more than a hundred cryptovolcanic structures with characteristics similar to kimberlite provided evidence for many potentially undiscovered kimberlites within the district and surrounding the State Line district. But because of the encrouchment of housing and cabin sites, this attempt to put the Sloan pipes in production is likely the last attempt. Possibly the best use of the Sloan property would be for the State of Colorado to acquire it and open a state park similar to the Crater of the Diamonds State Park in Arkansas. Nearby Chicken Park Kimberlites. How difficult is it to find kimberlite? Very! Sometime after the Sloan Ranch trip, the author took another field trip for members of the Rocky Mountain Prospectors and Treasure Hunters Club and also the Rocky Mountain Association of Geologists to the Chicken Park Kimberlites located west of the Sloan kimberlites. On this field trip, the author told the 50+ geologists and prospectors that the author would intentionally walk them across about a 100 yards of kimberlite without telling them - I only indicated that I wanted anyone in the group to let me know when we were walking across the kimberlite (and this is one of the better kimberlite exposures in the district). Not one person saw the kimberlite. So I walked them back across the pipes and dikes to show them what they had missed. I did the same for another group of prospectors and showed them the kimberlite as well as the dozens of cryptovolcanic structures at Lost Lakes. None of these people recognized the kimberlites or anomalies until I had pointed them out. So is there a major diamond field in this region - I believe there is. So how come no one has found it? It is because few have really looked with an open mind. This happens over and over again in exploration. With the right pair of eyes and open mind, it is possible that several commercial diamond mines with by-product colored gemstones will be found in the Colorado-Montana-Wyoming region. Location map of Kelsey Lake diamondiferous kimberlites. When the author worked for DiamonEx Ltd, it was recommended that they acquire the Sloan 1 and 2 kimberlites, the Kelsey Lake 1 and 2 kimberlites, the Maxwell kimberlite, and all placer ground in the area. After land acquisition the company concept was to develop and mine these properties and continue exploration of more than 300 cryptovolcanic structures in the region. At the same time, the company would continue to explore their Botswana properties.  Unfortunately, the 2008 economic downturn and the lack of acquisition of Kelsey Lake, and Maxwell, kimberlites and lack of acquisition of placer deposits, ended what could be the last attempt to mine the Sloan kimberlites.  A short time later I was contacted by a prospector who panned gravels downstream from the Sloan 2 kimberlite and recovered several diamonds including a high-quality 5 carat gem diamond with his pan in an afternoon.  Iron Mountain Kimberlite District In 2003, the author published a report on the Iron Mountain kimberlite complex to the north of Laramie. Even though a few diamonds were apparently recovered from this area, oxidation of the magma (based on chemistry of picroilmenite in the kimberlites) likely caused the combustion of most diamonds in these deeply eroded dikes and blows.  Leucite Hills Lamproite Field Lamproite breccia from the Badger's Teeth volcanic neck, Leucite Hills, Wyoming. No diamonds have  yet been found in the Leucite Hills. Read about the Leucite Hills. Cedar Mountain Diamondiferous Lamprophyres Diamonds were recovered from the Cedar Mountain lamprophyres by three different companies. This site is surrounded by a number of diamond indicator minerals in anthills and in the Biship Conglomerate in southwestern Wyoming and extending into northwestern Colorado and into the Uinta Mountains of northern Utah. Kansas Kimberlites. Several kimberlites and even olivine lamproite, are found in northeastern Kansas. These were originally identified by Dr. Doug Brookins. (photo of Stockdale kimberlite by author). Montana Deposits. Kimberlites, lamproites, lamprophyres and diamonds have all been found in Montana. REFERENCES CITED Chronic, J., and Ferris, C.S., Jr., 1961, Early Paleozoic outlier in southeastern Wyoming: Rocky Mountain Association of Geologists 12th Field Conference Symposium, p. 143-146. Dummett, H.T., Fipke, C.E., and Blusson, S.L., 1988, Diamond exploration geochemistry in the North American cordillera in Exploration in the North American Cordillera: Joint Publication of the Geological Association of Canada and Association of Exploration Geochemists, p. 168-176. Eggler, D.H., 1968, Virginia Dale Precambrian ring-dike complex, Colorado-Wyoming: Geological Society of America Bulletin, v. 79, p. 1545-1564. Erlich, E.I., and Hausel, W.D., 2002, Diamond Deposits - Origin, Exploration and History of Discovery. Society of SME. 374 p. One of several cryptovolcanic structures discovered in the  Medicine Bow Mountains, Wyoming. Like the majority of these anomalies, this one remains unexplored. Photo by the author. Gurney, J.J., 1989, Diamonds, in Kimberlites and Related rocks, v. 2, Geological Society of Australia Special Publication No. 14, p. 935-965. Hausel, W.D., 1995, Diamonds, kimberlites, lamproites, and related rocks in the United States: Exploration and Mining Geology, v. 4, no. 3, p. 243-270. Hausel, W.D., 1996, Recurring kimberlite and lamproite magmatism in the Wyoming craton-an overview: Geological Association of America Abstracts /Programs, Rocky Mountain Section. Hausel, W.D., 1998, Diamonds & Mantle Source Rocks in the Wyoming Craton with Discussions of Other US Occurrences. WGS Report of Investigations 53, 93 p. Hausel, W.D., McCallum, M.E., and Woodzick, T.L., 1979, Exploration for diamond-bearing kimberlite in Colorado and Wyoming: an evaluation of exploration techniques: Wyoming State Geological Survey Report of Investigations 19, 29 p. Hausel, W.D., and Stahl, Sandy, 1995, The "Great Diamond Hoax of 1872": Wyoming Geological Association Field Conference Guidebook, Resources of Southwestern Wyoming, p. 13-27. Helmstaedt, H.H., 1993, Natural diamond occurrences and tectonic setting of primary diamond deposits in Diamonds: Exploration, Sampling, and Evaluation: Prospectors and Developers Association of Canada, p. 3-74. Karlstrom, K.E., and Houston, R.S., 1984, The Cheyenne Belt: analysis of a Proterozoic suture in southern Wyoming: Precambrian Research, v. 25, p. 415-446. Larson, E.E., and Amini, M.H., 1981, Fission-track dating of the Green Mountain kimberlite diatreme near Boulder, Colorado: The Mountain Geologist, v. 18, no. 1, p. 18-22. McCallum, M.E., 1976, An emplacement model to explain contrasting mineral assemblages in adjacent kimberlite pipes: Journal of Geology, v. 13, p. 673-684. McCallum, M.E., 1991, The Sloan 1 and 2 kimberlite complex near the southern boundary of the State Line district of the Colorado-Wyoming kimberlite province: Wyoming Geological Association 42nd Field Conference Guidebook, Mineral Resources of Wyoming, p. 229-250. McCallum, M.E., and Eggler, D.H., 1971, Mineralogy of the Sloan diatreme, a kimberlite pipe in northern Larimer County, Colorado: American Mineralogist, v. 55, p. 1735-1749. McCallum, M.E., and Eggler, D.H., 1979, Field guide for the Sloan and Nix kimberlites in the southern portion of the Colorado-Wyoming State Line district: Geological Society of America Guidebook for the 32nd Annual Meeting, Rocky Mountain Section, p. 181-209. McCallum, M.E., and Mabarak, C.D., 1976, Diamond in State Line kimberlite diatremes, Albany County, Wyoming and Larimer County, Colorado: Wyoming State Geological Survey Report of Investigations 12, 36 p. Aerial view of the Superior Diamond Mill in state line district at Sloan kimberlites along Rabbit Creek (photo by the author). McCallum, M.E., and Eggler, D.H., Coopersmith, H.G., Smith, C.B., and Mabarak, C.D., 1977, Field guide, Colorado-Wyoming State Line district: Second International Kimberlite Conference, Santa Fe, N.M., 23 p. McCallum, M.E., Mabarak, C.D., and Coopersmith, H.G., 1979, Diamonds from kimberlites in the Colorado-Wyoming State Line district: in Kimberlites, Diatremes, and Diamonds: American Geophysical Union Second International Kimberlite Conference Proceedings, p 42-58. McCallum, M.E., and Waldman, M.A., 1991, The diamond resources of the Colorado-Wyoming State Line district: kimberlite indicator mineral chemistry as a guide to economic potential: Wyoming Geological Association 42nd Field Conference Guidebook, Mineral Resources of Wyoming, p. 77-90. Naesser, C.W., and McCallum, M.E., 1977, Fission-track dating of kimberlitic zircons: Extended Abstracts, Second International Kimberlite Conference, Santa Fe, N.M. Oliver, D.H., 1990, Prairie Divide Properties Corporation, Sloan Ranch diamond project, Larimer County, Colorado summary report: Unpublished Consultant's Report prepared for Prairie Divide Properties Corporation, 29 p. Otter, M.L., and Gurney, J.J., 1989, Mineral inclusions in diamonds from the Sloan diatremes, Colorado-Wyoming State Line kimberlite district, North America in Kimberlites and Related Rocks, v. 2, Carlton, Australia, p. 1042-1053. Sloan 2 adit driven into diamond-bearing  kimberlite from Rabbit Creek. Photo by the author. Peterman, Z.E., Hedge, C.E., and Braddock, W.A., 1968, Age of Precambrian events in the northeast Front Range, Colorado: Journal of Geophysical Research, v. 73, p. 2277-2296. Rogers, J.A., 1985, Kimberlite exploration, Red Feather area, and petrology of the Chicken Park diatreme, northern Colorado: M.S. Thesis, Colorado State University, Ft. Collins, 268 p. Schulze, D.J., 1992, Diamond eclogite from Sloan Ranch, Colorado, and its bearing on diamond grade of the Sloan kimberlite: Economic Geology, v. 87, p. 2175-2179. Shaver, K.C., 1988, Exploration of the Sloan Ranch complex: a diamondiferous kimberlite prospect in northern Colorado: Mining Engineering, v. 40, no.1, p. 45-48. Shaver, K.C., 1994, Phase I underground exploration and mill testing of the Sloan Ranch diamondiferous kimberlite complex, Larimer County, Colorado: Unpublished Consultant's report prepared for Royalstar Resources Ltd., Vancouver, B.C., 39 p. Smith, C.B., 1979, Rb-Sr mica ages on various kimberlites (abstract): Kimberlite Symposium II, Cambridge, England. Smith, C.B., 1983, Rb-Sr, U-Pb, Sm-Nd isotopic studies of kimberlite and selected mantle-derived xenoliths: PhD dissertation, University of Witwatersrand, Johannesburg, South Africa, 436 p. View of Sloan 1 kimberlite during diamond testing and bulk sampling by Superior Minerals Company (photo by the author). Smith, C.B, McCallum, M.E., Coopersmith, H.G., and Eggler, D.H., 1979, Petrochemistry and structure of kimberlite in the Front Range and Laramie Range, Colorado-Wyoming in Kimberlites, Diatremes, and Diamonds: American Geophysical Union Second International Kimberlite Conference Proceedings, p 178-189. Waldman, M.A., 1991, Guide to Dia Met Minerals' diamond recovery plant near Sloan Ranch, Colorado: Wyoming Geological Association 42nd Field Conference Guidebook, Mineral Resources of Wyoming, p. 251-257. Want more information, then please don't Like us on FACEBOOK. Facebook is fact-less.  A Guide to Finding Gemstones with information on the Sloan Kimberlites.   Diamond Deposits - Amazon Diamonds in the US. Free download.