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Colorado – Wyoming State Line Diamonds

by David A. Brown

         The diamond-bearing kimberlites of the Colorado-Wyoming state line province, offer valuable insight into the physical and chemical conditions in the mantle and the evolution of the craton.  The occurrence of diamonds is almost invariably associated with proximity to Archean cratonal regions (Heaman et al., 2003).   Due to their relative impermeability and non-reactive nature, diamonds are very effective at preserving the chemistry of their formation conditions.  Mineral inclusions in diamonds are shielded from alteration, and chemical zoneation of the host grains is also observable.  Significant technological advances have been made recently that make possible very detailed spot analysis of these inclusions and chemical zones.  Two especially valuable techniques being Secondary Ion Probe Mass Spectrometry (SIMS) (described on the Methods page), and the electron microprobe.


            The State Line district contains over 100 kimberlitic intrusions, and has produced over 130,000 diamonds including one gemstone of 28.3 carats.  In addition to the many diamondiferous kimberlites of the State Line area, there is a non diamondiferous diatreme near Estes Park and another on the slopes of Green Mountain outside of Boulder.  Also noteable is the Iron Mountain district, which is approximately 40 miles to the north of the State Line district.  All of these occurances are south of the Cheyenne Belt in the Proterozoic Yavapai province, (Lester et al., 2003).  There are numerous diatremes further north in Montana and Canada.



          The figure at right, (from Schulze et al., 2008), shows the locations of some of the noteable kimberlites of the State Line region.  The insert shows the relative position of the Archean Wyoming Craton with the Proterozoic Mojave and Yavapai provinces.






















                   The figure at left shows a well formed diamond in an eclogitic xenolith.
            This sample is from a South African locality. 
            Image courtesy of Joseph Smyth.

Petrology and Mineralogy of Diamond-Bearing Diatremes

Diamonds occur in small potassic ultra-basic intrusive formations known as kimberlite pipes (named after the classic locality in Kimberley, South Africa; Heaman et al., 2003).  A kimberlite pipe is a type of diatreme, which is often described as a narrow carrot shaped body of brecciated material believed to be the result of a gas-driven explosive event.  The alkaline magma associated with diatremes typically has low silica content, high magnesium and potassium, and abundant volatiles such as CO2 and H2O (Philpotts and Ague, 2009).  The formations themselves are quite unusual in that the brecciated country rock that fills the pipe is so thoroughly mixed that lithic fragments originating from strata well over 1000 meters above the present exposure may be found juxtaposed with material from lower down.  It is noteable that the fossiliferous Silurian xenoliths found in some of the State Line pipes represent the only Silurian sedimentary rocks found in Colorado or Wyoming (Lester et al., 2001).  Very little material is ejected from these eruptions and lava flows are not produced, suggesting that these events are solely driven by exsolved gases.  There is also an absence of contact metamorphism in the vicinity of the intrusion.  The presence of high pressure mineral phases such as diamond in kimberlites constrains their depth of formation to be at least 150 km, and some have even argued for an origin at the core-mantle boundary (Heaman et al., 2003; Philpotts and Ague, 2009).  Some diamonds have been found to contain inclusions of majoritic garnet, the stability field of which is in the mantle transition zone.


            There is significant variation in the mineralogy of kimberlites but they typically contain mafic minerals such as olivine, clinopyroxene, phlogopite mica, garnet, and ilmenite, as well as a matrix of calcite, perovskite, phlogopite, and spinel.  Accessory minerals which may be present include apatite, zircon, monticellite, rutile, serpentine, and various sulfides (Heaman et al., 2003; Page et al., 2007).  Both eclogitic and peridotitic xenoliths are brought to the surface in kimberlites, and similarly the mineral inclusions in diamonds are broadly classified as being either eclogitic or peridotitic.  The peridotitic (P-suite) inclusions mainly consist of olivine, orthopyroxene and Ca-poor/Cr-rich garnet, whereas the eclogitic (E-suite), inclusions are dominated by Cr-poor garnet and clinopyroxene and may also include rutile, kyanite, coesite and sanadine (Kirkley et al., 1991).  The peridotitic suite inclusions can be further subdivided into harzburgitic, lherzolitic, and wehrlitic groups (Schulze et al., 2008).  Peridotitic garnets are typically Cr-pyrope, and can be classified as; harzburgitic, lherzolitic, or wehrlitic based on the degree of calcium and chromium saturation.  E-suite garnets are aluminous, have variable FeO, MgO, and CaO contents, and commonly have Na2O > 0.07% by weight.  Eclogitic pyroxenes, such as omphacite, are also more sodic (GrÜtter et al., 2004).


          The figure above is a crossed-polars image of an eclogitic xenolith from a south African kimberlite.  The isotropic phase is garnet, upper right is OPX, lower right is CPX, and to the left is an "inverted maajorite" grain which has exsolved into pyroxenes.  Image courtesy of Joseph Smyth.





          


 The figure at left is a crossed-polars image of a typical peridotitic (lherzolite) xenolith.  This sample is also South African, the olivine grains here are quite pristine whereas it is common to observe significant serpentinization.  Image courtesy of Joseph Smyth.














          The table below modified from (Schulze et al., 2009) shows chemical analysis of some peridotitic and eclogitic diamond inclusions from the Kelsey Lake kimberlite.
         


 
          Diamonds are relatively impermeable; hence their inclusions are shielded from alteration and can preserve information concerning the conditions at the time of formation (Schulze et al., 2008).  It is important to note that the P-suite inclusions in diamonds (which are more common than E-type) do not have the same composition as the same minerals found in peridotitic xenoliths of the same kimberlite.   Also, whereas diamondiferous peridotitic xenoliths are rare, diamonds are commonly found in eclogitic xenoliths.  Various mechanisms have been proposed to explain this apparent dichotomy in the paragenesis of these minerals.  Some have suggested the diamonds originate from a greater depth than the xenoliths, others have argued that the P-suite inclusions in the diamonds could have been derived magmatically from the peridotite nodules in the presence of volatiles (Harte et al., 1980).  Isotopic zoneation is also observable in many diamonds, and can give insight into the crystal’s residence time in the mantle and temporal changes in fluid compositions and the fugacity of gases (Craven et al., 2009).

          The figure to the right (by Craven et al., 2009) is a cathodoluminescence (CL) image of a polished diamond section showing distinct zoneation.  This type of zoneation correlates strongly with nitrogen content.