New Suspected Kimberlite Northern Colorado

Stephanie Gallegos1 and Uwe Kackstaetter2, Ph.D. Undergraduate Research

Introduction Methodology Results and Conclusions

Abstract

Kimberlite pipes are small in diameter, carrot shaped, geologically elusive, ultra

mafic igneous structures which are penetrating the crust all the way from the mantle.

They often occur in swarms, such as in the Colorado – Wyoming district, are primary

source rocks for diamonds, and are very difficult to detect. A small Kimberlite is

believed to exist in a road cut on County Road 45E and Highway 287, just North of

Virginia Dale, Colorado, in a minor fault line. While first surveys confirmed the

presence of chlorite, a common decompositional mineral of ultra-mafic lithologies,

additional data such as PLM (Polarized Light Microscopy) investigation of thin

sections, XRD (X-ray Diffraction), chemical testing through the ICP (inductively

coupled plasma) and XRF (X-ray Fluorescence) analysis, as well as heavy mineral

identification, strongly supports the initial hypothesis. Additionally, preliminary

scans, with the most up to date research technology (TIMA mineral analyzer), has

unambiguously shown the presence of pyrope garnets, clinoclore, diopside and

magnetite; strong indicator minerals of kimberlitic rocks.

Identification of Kimberlites

In order to positively identify this minor Kimberlite Pipe the following analytical

procedures were employed:

1. Thin section analysis: Selected samples were ground to standard section

thickness of ~30 microns and investigated under the PLM (Polarizing Light

Microscope) to identify mineralogy and diagenetic textures.

2. Heavy mineral separation: A sample is disintegrated and subjected to

bromoform, thus separating minerals with a density greater than 3.0g/cm3 from

those of lesser density. Products were analyzed under the PLM and standard

binolcular microscope for mineral identification.

3. XRD analysis: Samples were pulverized and studied using an x-ray diffraction

unit (XRD). Positive identification of certain minerals even in mineralogical

mixtures were attained.

4. ICP-MS chemical analysis: Selected samples were digested using hot aqua regia

and borate fusion methods and then analyzed with the ICP-MS (Inductively

Coupled Plasma – Mass Spectrometer) to attain the geochemistry of the rock.

CIPW norm calculations and similar approaches aided in comparing kimberlite

rock samples to known kimberlitic geochemistry.

Kimberlite Indicator Minerals (KIMs)

Kimberlite hosts a suite of heavy minerals, which have unique geochemical and

physical characteristics. Cr-pyrope garnet, eclogitic pyrope-almandine garnet, Mg-

ilmenite, Cr-diopside, Cr-spinel, Mg-olivine, and enstatite are the most common

KIMs (Fig. 3). These minerals are used for kimberlite reconnaissance studies because

of their uniqueness in diamond bearing ultramafics. However, Mg-olivine may also

occur in other ultramafic lithologies.

• Garnet: Mantle-derived garnets are considered to be the most important

kimberlite indicators. Chemical elements present in mantle-derived garnets are Cr,

Ca, Mg, Fe, Ti and Na.

• Clinopyroxene: Chrome-bearing, green to bright green clinopyroxenes are easily

identifiable in heavy mineral concentrates and are therefore considered effective

KIMs.

• Ilmenite: This most widely used kimberlite indicator mineral is a common

member of the megacryst suite. Major ilmenite oxides are TiO2, MgO, CrO2,

MnO2 and Fe2O3 and are used to distinguish kimberlitic ilmenites.

Results

• PLM on thin sections and heavy minerals show garnets (Fig 5), olivine,

phlogopite and possible diopside all indicating kimberlitic material.

• XRD Analysis: The suspected kimberlite is a light- to dark-green or gray-green

rock that is decomposed at the surface. The kimberlites contain considerable

amounts of clay material which is indicative of a weathering kimberlite

• ICP-MS: Using the CIPW Norm calculations, and plotting Niobium vs. Cerium,

the suspected kimberlite is approaching a type II kimberlite.

• TESCAN TIMA: This new and emerging technology is relatively young.

Kimberlite samples have never before been analyzed with this method. Promising

results verify high amounts of chlorite-mg, chlorite, as well as quartz, zircon, and

diopside. TIMA resolves geochemistry at 1µm resolution and identifies minerals

through compositional algorithms.

Conclusion

• Analytical results indicate indeed a newly discovered State-line kimberlite pipe,

alas deeply weathered, as evidenced by:

• Common decompostional mineral: Mg-chlorite [XRD]

• KIMs: Pyrope garnets, Diopside, Phlogopite, Olivine [PLM]

• Possible new techniques for kimberlite identification: Tescan™ mineral analyzer

[TESCAN, a.s. Libušina třída 21 623 00, Brno - Czech Republic]

• This new kimberlite to be kept accessible for educational purposes.

References

Arndt, N. T., et al. "What olivine, the neglected mineral, tells us about kimberlite petrogenesis." eEarth Discussions 1.1 (2006): 37-50. Boyd, F. R., and Henry O. A. Meyer. Kimberlites, Diatremes, and Diamonds: Their Geology, Petrology, and Geochemistry. Washington: American Geophysical Union, 1979. Print. Colorado Geological Survey, ROCKTALK. "What Are Diamonds?" What Are Diamonds? 2.3 (1999): 1-12. Print. Coopersmith, H. G. "Technical Report on the Northern Colorado Diamond Project." Technical Report Northern Colorado Diamond Project (2009): 1-71. Web. Nov. 2012.Eggler, and Braddock. "Geologic Map of the Cherokee Park Quadrangle, Larimer County, Colorado and Albany County, Wyoming." Map. Print. Hausel, W. Dan. "Searching for Placer Diamons." Www.wsgs.uwyo.edu. Wyoming State Geological Survey. Web. <http://www.wsgs.uwyo.edu/docs/PlacerDiamondsPamphlet.pdf>. Hochleitner, Rupert. Minerals: Identifying, Learning About, and Collecting the Most Beautiful Minerals and Crystals. [Hauppauge, NY]: Barron's, 1994. Print. "Kansas Geological Survey GeoRecord Vol 6.1." Welcome to the Kansas Geological Survey. Web. 10 Apr. 2012. <http://www.kgs.ku.edu/Publications/GeoRecord/2000/vol6.1/Page1.html>. Diagram of Kimberlite pipe was borrowed. Ray, Jyotisankar, Gautam Sen, and Biswajit Ghosh. "Ch. 16 - Kimberlites, Supercontinents, and Deep Earth Dynamics." Topics in Igneous Petrology: A Tribute to Professor Mihir K. Bose. Dordrecht: Springer, 2011. Print. "TESCAN :: TIMA." TIMA. N.p., n.d. Web. <http://www.tescan.com/en/products/tima>. Special thanks to Veronika Králová, Ph. D.

Department of Earth and

Atmospheric Sciences

Figure 1: Location of study area.

Area of Research: Virginia Dale, Colorado

This suspected small Kimberlite pipe exists in a road cut on County Road 45E and

Highway 287, just North of Virginia Dale, Colorado. The unusual mineralogy, leading

to the proposed rare ultra-mafic lithology, was discovered by students during a

mineralogy / petrology field trip held by Dr. Uwe Kackstaetter. The area was last

surveyed in 1988 by William A. Braddock and David H. Eggler, making no mention

of such a structure. There findings indicate that the area is mainly composed of Silver

Plume Granite and “Inner Cap Rock phase”. There are several known Kimberlite

pipes in the vicinity, the closest (approx. ½mi) being the diamondiferous Moen

Kimberlite. The host lithologies in the area are very different, consisting of granites

and quartz monzonite of the Virginia Dale ring dike complex. It is believed that this

small previously undiscribed kimberlite belongs to the Moen swarm and intruded the

acidic igneous systems of the area.

Figure 2: Picture s of suspected kimberlite samples. Right shows a cut segment of the kimberlite / felsic igneous contact zone.

Figure 5: Thin sections showing unambiguous garnets under crossed polars.

Figure 6: XRD Whole

Pattern Fitting and Reitveld

Refinement (Special thanks

to Adam Boehlke from

USGS for interpretation)

Figure 7:The graph indicates that the

suspected kimberlite is approaching

type II kimberlites, analogous to the

neighboring Moen kimberlite.

Figure 8: Resulting TIMA phase diagrams of a polished kimberlite sample .

Figure 4: (Left to right) Heavy mineral separation in progress, heavies under binocs, (far right) grain mount

of heavy minerals showing residual olivine.

Figure 9: Stephanie and Dr. Kackstaetter at the area of interest and bordering vicinities.

Acknowledgements

Special thanks to Adam Boehlke from the USGS, Veronika Králová, Ph. D. from TESCAN, All faculty, staff, and students from the EAS department, and Dr. Uwe Kackstaetter.

5. TESCAN Integrated Mineral Analyzer (TIMA): TIMA, a

fully automated, analytical scanning electron microscope /

EDS system (Fig 3). A polished sample section of ~30

microns was thoroughly analyzed using this new, state of the

art TIMA system, an approach never attempted before in

surveying kimberlitic materials.

Figure 3: TIMA

1Stephanie Gallegos is a undergraduate research geoscientist at Metropolitan State University of Denver, Department of Earth & Atmospheric Sciences, sgalle30@msudenver.edu, 303-859-2876. 2Dr. Kackstaetter is the project supervising geology faculty at Metropolitan State University of Denver, kackstae@msudenver.edu, 303-556-3070