GEOLOGICAL SCENES FROM THE BLACK HILLS

I recently returned from a fall camping and collecting trip spread out from the high country of Colorado to the Black Hills of South Dakota.  It is such a great time to wander around in the “great outdoors.”  Luckily I returned home to Colorado Springs right before the Great October Blizzard dumped four feet of snow on what was my camping site in Spearfish.

The Pierre Shale (Cretaceous) crops out east of the Black Hills on the plains.  In some localities the concretions in the Pierre produce beautiful cephalopod fossils and crystals of barite and calcite.  This particular concretion gave up a few small bivalves and a barite crystal.

 

These are the famous Fairburn Agate Beds east of the Black Hills along French Creek.  The gravel beds are lag gravels weathered from the Chadron and\or Chamberlain Pass formations (White River Group) while the red colored material is an old (Eocene) tropical soil zone(s).

Cathedral Spires is located along the famous Needles Highway.  The spires developed along vertical joints (see photo below) with weathering in the Precambrian Harney Peak Granite (~1.72 Ga).

No one could visit the Hills without viewing the carved faces of Presidents Washington, Jefferson, Roosevelt and Lincoln on Mt. Rushmore.  The bedrock is the Precambrian Harney Peak Granite, part of a large domed mass of intrusions.

Sylvan Lake at 6,145 feet is usually considered the most beautiful lake in South Dakota.  Again, the Harney Peak Granite..

American bison, buffalo in the vernacular, roam the prairies of Custer State Park in a semi-wild state..

Did you ever wonder where and how South Dakota rivers started.  Now you know the answer!  This ole bull gave us several visits in the Custer State Park Campground at Legion Lake.

Golden Park along French Creek just east of Custer city where men of the George Custer expedition first located gold in August 1874.

Cross sections of several large black tourmaline (schorl) crystals exposed in a pegmatite southwest of Custer city.

FAIRBURN AGATE: WYOMING

I have written a couple of other blogs about Fairburn Agates in South Dakota and one about Teepee Canyon Agates.  Since I have long been interested in these agates, I report on another specimen.  But readers should be aware that I really lack any expertise either in their discovery, formation, or distribution.  I saw my first Fairburn back in the mid-1960’s and decided “they were pretty neat” and my interest has remained.  The initial Fairburn that I collected still sticks out in my mind---I had found a Rapid City (I suppose) AM radio station that played “rock & roll” that I could receive out on the plains east of the Hills.  I had cranked up the radio and decided to look at some gravel exposures to see what was there---Tommy James’ My Baby Does The Hanky Panky was playing when I looked down and there it was.  Now, my mind has a penchant for being able to remember trivial factoids from decades ago but is unable to recall where I left the car keys.  But, life is good.

At any rate, South Dakota Fairburn Agates are “plains’ agates” mostly to the south and east of the Black Hills and mostly associated with terrace gravels along streams and/or conglomerate beds and lag deposits on top of the Eocene-Oligocene White River Group (Chadron Formation or perhaps the underlying Eocene Chamberlain Pass Formation).  However, Fairburns or Fairburn-type agates have been located in the adjacent states of Wyoming and Nebraska.  Buena Vista Gem Works (2013) noted that in Nebraska the area of collecting is north of US 20 from Chadron to the Wyoming state line, generally following outcrops of the Chadron.  Major collecting localities in Wyoming include Lance Creek and Hat Creek (southwest and reasonably close to the Hills) and Glendo and Guernsey (further to the south).  Gemworks (2013) also reported “stray finds” of Fairburn-type agates from the Yellowstone River in Montana, near New Raymer, Colorado, (northeast Colorado) and along the Yampa River in northwest Colorado.  The Chadron crops out near New Raymer but is absent from the Yellowstone and Yampa River localities. Pabian and Cook (1976) reported Fairburn-type agates at localities along the east-flowing Platte River in Nebraska.

Most agate hunters describe Fairburns as fortification agates with a holly leaf banding. They have sort some of red to orange banding due to iron oxides with the black banding ascribed to manganese.  The “type area” for Fairburns is near (east) of the small village of Fairburn along French Creek (see blog postings on 6-12-12and 8-19-12).  It seems well established that the agates out on the South Dakota plains originated in the Minnelusa Formation of  Pennsylvanian-Permian age and were transported away from outcrops in the Black Hills by Cenozoic streams.  Perhaps the best-know site for observing in situ Fairburn agates is at Teepee Canyon west of Custer (see blog posting 8-18-12).

Map showing location of Hartville Uplift in eastern Wyoming.  The topographic expression of the Uplift is limited to the notation “Hartville Uplift” while a subsurface structural high (arch) extends northeast to the Black Hills.  Map from Dickinson and others, 1988.

I find it interesting that Fairburn-type agates found at the Glendo and Guernsey locations in southeastern Wyoming most likely came from rocks similar to the Minnelusa Formation (Hartville Formation) exposed around the perimeter of the Hartville Uplift (Sutherland, 1990).  This uplift is a Laramide structure, not unlike the Black Hills with which it connects on the north, but much more topographically and structurally subdued.  It separates the Powder River Basin (northwest) from the Denver Basin.  The Uplift does have a core of Precambrian rocks with a surrounding ring of Paleozoic and Mesozoic rocks; most of the uplift is now covered by Cenozoic rocks.  I have written about the Hartville Uplift in a previous blog (12-22-12) and noted: “The Precambrian exposures are of interest to Coloradans since the rocks contain large deposits of iron, both banded iron formations and specular hematite.  Iron was first produced from the Sunrise mine, and later the Chicago, Central, and Good Fortune mines, near the towns of Hartville and Sunrise in the late 1800’s.  These mines then shipped this hematite ore to the Colorado Fuel and Iron Corporation open-hearth furnaces in Pueblo, Colorado. (Sims and Day, 1999). At the time when mining ceased at the Sunrise mine in 1980, the Hartville district had produced about 45 million tons of iron ore (Hausel, 1989).”

It appears, then, that the Hartville Formation (Hartville Uplift) is stratigraphically equivalent to the Minnelusa Formation (Black Hills); both produce similar fortification agates. I also find it interesting that Pabian and Cook (1976) believed the "fortification agates [Fairburn-type] found in Nebraska originated chiefly in sedimentary rocks…of Pennsylvanian age in the Hartville Uplift and the Front range of Wyoming.  Some may have originated in the Black Hills but this appears to be a minor source."  It appears that Pabian and Cook (1976) expressed this belief because of statements in a 1955 paper by Shultz and Stout noting that “no physiographic evidence exists to show a major north-south drainage from the Black Hills to Nebraska.”  I don’t know enough about recent stratigraphic studies on the White River Group to speculate on the accuracy of this statement.  So, perhaps Nebraska Fairburn-type agates came from the Black Hills and/or from the Hartville or Front Range Uplifts?  Perhaps some reader knows the answer to this question?

Fairburn agate collected near Douglas, Wyoming.  Length ~7 cm.

My Wyoming Fairburn was collected near Douglas, Wyoming, from lag gravels associated with rocks of the White River Formation.  That is I presume a Fairburn, or is it a Fairburn-type?  Did it originate in the Black Hills or from the Hartville Uplift?

As I remember my first Fairburn (the real thing) the music floats through my mind::

I saw her walking on down the line
You know I saw her for the very first time
A pretty little girl standing all alone
Hey, pretty baby, can I take you home…

My baby does the hanky panky

REFERENCES CITED

Buena Vista Gem Works: www.buenavistagemworks.com/agates/fairburn.htm

Dickinson, W.R., M.A Klute, M.J. Hayes, S.U. Janecke, E.R.Lundin, M.A. McKittrick and M.D. Olivares, 1988, Paleographic and Paleotectonic Setting of Laramide Sedimentary Basins in the Central Rocky Mountain Regions: Geological Society of America Bulletin, v. 100. 

Hausel, W.D., 1989, The Geology of Wyoming’s Precious Metal Lode and Placer Deposits: Geological Survey of Wyoming Bulletin 68.

Pabian, R.K. and Allan Cook,1976, Minerals and Gemstones of Nebraska: A Handbook for Students and Collectors: Nebraska geological Survey Educational Circular no. 2.

Sims, P. K. and W. C. Day (compliers), 1999, Geologic Map of Precambrian Rocks of the Hartville Uplift, Southeastern Wyoming with a section on Mineral Deposits in the Hartville Uplift by Terry Klein: U. S. Geological Survey  Map I-2661.

Sutherland, W.M., 1990, Gemstones, Lapidary Materials, and Geologic Collectables of Wyoming: Geological Survey of Wyoming Open File Report 90-9.

REALGAR--PARAREALGAR BLACK HILLS, SD

QUARTZ WITH AN INCRUSTATION OF PARAREALGAR (ALTERED REALGAR) FROM THE BLACK HILLS OF SOUTH DAKOTA.  LENGTH OF SPECIMEN ~6.5 CM.

I recently returned home from a fall leaf peeping and collecting trip to the Colorado high country (near Leadville) and continuing to the Black Hills of western South Dakota---hence my absence from the Blog!  I have often written about the latter area as the Hills are one of my favorite places to visit-especially in the fall.  The children are in school, the campgrounds are un-crowded, and the many old mines are waiting for my return.  In addition, the Hills have numerous rock/mineral shops that require stops to visit new additions as well as the dusty old bins that time seems to forget.  It is these latter cardboard boxes that require close examination.  The bottom line is that I came home with many decent collected specimens and some of those weird and strange and rare minerals that I love to acquire for my collection.

Now realgar is not a weird or strange mineral and has been mined for centuries at various world-wide locations; however, it is a rare mineral in rocks of the Black Hills.  Realgar is a sulfide mineral, an arsenic sulfide: As₄S₄ (sometimes written as AsS).  In crystal form the mineral is usually bright ruby-red or orange-red in color and is sometimes called “ruby sulfur”.  Often transparent with a resinous luster, realgar is quit soft (1.5-2.0 Mohs) and crystals are easily damaged.  Most realgar forms in low temperature hydrothermal settings but does occur as a product in hot springs such as the Norris Geyser Basin in Yellowstone National Park, or in sublimation products via volcanic vapors.  In hydrothermal veins realgar is often found with other sulfides such as gold, silver, stibnite, lead, and especially orpiment.  The latter mineral, As₂S₃, is often confused with realgar since it usually forms in the same environments and is similar in appearance (more on this in a later blog).

Realgar is one of those minerals that is fairly unstable and over time alters to a new mineral, pararealgar (As₄S₄ or AsS)—same chemical formula and chemical composition as realgar but a different crystal form.  But since I am not a mineralogist confusion often enters my mind—both pararealgar and realgar are Monoclinic and 2/m Prismatic—so at this time I fail to understand the difference.  Regardless of my confusion, with exposure to light and over time realgar alters to this yellow to yellow-orange crust called pararealgar---ah, the wonders of modern instrumentation to determine such actions!  But hold on, if you leave pararealgar in the light for a “long time” it turns into a yellow-orange powder and sort of disintegrates into what?  Dust?  I have also noticed that not every mineralogy reference seems to accept pararealgar and simply states that realgar occurs either as the red crystals or the yellow-orange crust or powder, sometimes all on the same specimen. 

In reading an article on medieval painting techniques and natural pigments I remember the author writing about red pigment made with beautiful realgar that would fade from the canvas and turn into a yellow or yellow-orange hue.  I can assume that is true although I don’t have the slightest idea for that source---probably some magazine that I browsed through at Barnes and Noble!

So, in this small rock/mineral shop in the Hills I stumbled across a couple of dusty specimens marked “realgar, mine near Sheridan Lake?, Black Hills”.  The price was right so I picked it up knowing that, at least from my limited knowledge, that realgar was not common in the Black Hills.  I only wish that I had also picked up the second specimen.  I immediately consulted the Black Hills “mineral bible” Mineralogy of the Black Hills (Roberts and Rapp, 1965) but could not find a reference for realgar, or for orpiment, occurring in the Hills.  I continued to search my other books (and that knower of all knowledge, the Internet) and the only reference that I located was in Encyclopedia of Minerals (Roberts, Rapp and Weber, 1974):  “Realgar is found in the United States…sparingly in the Homestake gold mine, Lead, South Dakota.”

So, the problem is that Sheridan Lake is not really close to the Homestake Mine!  Maybe there is a mine near the lake that produced the specimen, or perhaps the locality information is wrong.  I may never know as I cannot locate photos of Homestake specimens for comparison with my small specimen of massive quartz (with vugs of tiny quartz crystals) with a crust of pararealgar. 

As stated previously, there are many aspects of crystallography that confuse me; however, I have hope for the future since to be conscious that you are ignorant is a great step to knowledge:)  (Benjamin Disraeli)