Tuesday, July 2, 2013

I LOVE THE BLACK HILLS



The Black Hills of South Dakota (although some extend into Wyoming) have enough “geology” exposed to last most travelers a lifetime!  The Hills, as they are affectingly known within the State, remain one of my favorite localities since I “discovered them” way back in 1965.  As a student at the University of South Dakota I was on a field trip designed to collect mammals; however, I spent much time looking at the rocks, picking up minerals, and trying to determine when and how I could return.  Since those youthful days I have returned and collected and hiked and fished and traveled the Hills many times. Virtually any road you choose to travel through the Hills offers potential collecting sites at many road cuts (assuming you are not in a park where collecting is prohibited).  In addition, the Hills contain untold numbers of mines ranging from small glory holes to the massive Homestake.  Some of these old mines are claimed, some are open, and many are closed to the public.  Rockhounds should do their homework before heading to the Hills and try to determine potential collecting sites.  Information also may be secured from local rock shops (I found the shops in Custer and Hermosa to be very helpful), the U. S. Forest Service (USFS) office (especially Custer), and the local rock and mineral club in Rapid City (www.wdgms.org).  In addition, Tom Loomis at Dakota Matrix Minerals (www.dakotamatrix.com) sells several books and CD’s describing the mines and minerals of the Hills (and has great mineral photos).  At any rate, check on land ownership and never enter a shaft without appropriate equipment and supervision.
 

Diagram of the Black Hills, courtesy of A.N. Strahler.

The Hills are a wonderful example of a Laramide (refers to the Late Cretaceous-early Tertiary uplift of the Rocky Mountains) anticline with Precambrian rocks in the center and a nice contingent of Paleozoic and Mesozoic rocks tilting up and encircling the central core. Unlike the Laramide ranges in Colorado (for example the Front Range) and other western states, the Hills are compact with few large scale faults to “mess up” the dome.  The central peaks, with Harney Peak at 7244 feet, are the tallest in the lower 48 east of Fisher Peak (near Trinidad, Colorado).  In addition to great exposures of rocks of many ages, the Hills have produced tremendous values of metallic ores and industrial minerals.  Certainly the best known of the metals is gold (perhaps because of the ubiquitous Black Hills Gold Jewelry), first discovered by members of the George Custer army expedition near what is now Custer City in 1874.  This discovery, of course, created numerous conflicts with the Native American population and many individuals on both sides of the argument lost their lives. 

As the alluvial gold began to disappear, miners started to prospect for the source of the nuggets and the dust, and numerous hard rock mines were opened.  Probably the most famous mine in the Hills was the Homestake Mine near Lead in the northern section.  The Homestake, at the time of its closure as an active mine in 2002, was the deepest and largest mine in the Western Hemisphere.  I once visited the mine on a tour (in the days before law suits and federal regulations) and went down a few thousand feet; however, at closure it was over 8000 feet deep.  During its 125 year existence the Homestake produced something like 40 million oz. of gold and close to 10 million oz. of silver.  Most of this production came from ore with an assay of less than 1 oz. per ton!  Today the mine, the underground section, is known as the National Science Foundation Deep Underground Science and Engineering Laboratory and is used to study “dark matter” and neutrinos.

Visitors may easily see outcrops associated with the Homestake at the “open cut” in the town of Lead (Highways US 85; US 14a).  The site is a fascinating place to visit and is easily accessible in the “middle” of Lead.  A small museum next to the cut offers information.  And, for history buffs, Lead is next door to Deadwood (remember Nuttal and Mann’s Saloon and a “Dead Man’s Hand”) where modern-day miners are usually successful in removing gold from the visitors (in the numerous casinos). 
 

Tertiary rhyolite dikes cutting Precambrian rocks at the Homestake “open cut” in Lead.  Notice that the dikes flatten out near the surface where they met the Cambrian Deadwood Formation.

Gold is still located in the Black Hills and in 2010 two prospectors found a 5.7 oz. (troy) nugget.   My panning find last trip consisted of three flour flakes of the mineral from a stream near Custer!  However, be aware that most gold panning in the Hills in done at “fee” sites and some/many purchased buckets are “salted” for the tourist trade.

As one approaches the Black Hills, especially from the east, there are two signature landforms that stand out: 1) the Precambrian rocks of the high peaks; and 2) the outer hogback standing about 400 feet higher than the surrounding plains.  The latter is composed of an erosion-resistant Cretaceous sandstone unit termed the Lakota Formation capping the hogback with the Fall River Sandstone forming the front dip slope (away from the hills, remember the Black Hills are a large anticline or dome ).  Inside of the hogback is a feature known to many travelers—the Red Valley or the Racetrack.  The structure is a strike valley in the Permian-Triassic Spearfish Formation.  That is, the redbeds of the Spearfish have eroded away between the outer (dipping away) Cretaceous hogback and the inner, massive Paleozoic Minnekahta Limestone (also dipping away). The Racetrack was an “easy” place to build railroads as well as auto roads.  The Spearfish Formation represents the “drying up” and receding great Paleozoic seaway.  The waters became quite saline and redbeds and gypsum were left behind.  Today the massive gypsum (alabaster) is quarried at several localities, and a few selenite crystals are sometimes located.  It is easy to collect alabaster, especially in exposures along I-90 between Rapid City and the Wyoming state line (where the highway travels through the Racetrack).

 

The Red Valley near the South Dakota-Wyoming state line along I-90.  Notice the eroding redbeds in the foreground along the creek while the more resistant white beds of gypsum form a dip slope in the grassy background.


The Central Black Hills, the high peaks, have several different Precambrian units exposed with dates on the major terranes as follows (Gosselin and others, 1988; Hark, 2009): the Little Elk and Bear Mountain Terranes ~2.5 Ga (Archean) and are part of the Wyoming Craton; the Little Elk also has some radiometric dates of ~1.85 Ga (Proterozoic) associated with the Trans-Hudson orogenic event; the Boxelder Creek Metaconglomerate with a date of ~2.5 Ga (Archean) has detrital zircon dates of up to ~3.37 Ga (Archean; indicating the zircons came from older rocks); the Bogus Jim intrusive rocks with a date of around 2.0 Ga and representing a maximum age for the Homestake gold; and the Harney Peak Granite is ~1.7 Ga (Proterozoic; famous for the presidential “faces” at Mount Rushmore).  
For rockhounds exploring the Hills, the Harney Peak Granite is a paradise since in excess of 20,000 pegmatites have been identified in, and surrounding, the intrusion and these pegmatitic units contain at least 175 mineral species (Gries, 1996)!  I have collected in the Hills, off and on, since my graduate school days in the mid 1960’s.  My collection is not large, but I find the specimens fascinating (I am easy to please) and will describe a few.


One of the most spectacular mines in the Black Hills is the Etta Mine near Keystone, now in private hands and probably off limits to collectors.  The Etta, originally a mica mine in a pegmatite, has produced monster crystals of spodumene, a lithium aluminum silicate. Hess (1939) noted that huge crystals of spodumene are mixed at every possible angle like toothpicks in a translucent gel (quartz).  In 1904, a crystal 42 feet long and 3 feet by 6 feet in cross section was found...The crystal weighed about 65 tons.  How would you like to find space for that crystal in your collection? 
 
 

Giant spodumene crystals in the wall of the Etta Mine near Keystone, SD.  Note miner for scale.  Photo taken in 1904 and courtesy of W.T Schaller and the U. S. Geological Survey archives.

It should also be noted that spodumene is the source of three gemstones—kunzite, hiddenite, and colorless/clear (sometimes called triphane although the name is not in common usage).  Kunzite is pink to lilac in color due to small amounts of manganese.  Hiddenite, perhaps best known from the mines in North Carolina, is the emerald green variety with the color coming from chromium.  Triphane, the colorless to pale yellow variety, receives any color from iron.  Roberts and Rapp (1965) reported all three gems from pegmatites in the Hills.  I have kunzite, very pale pink, from the Tin Mountain Mine and a clear variety from the Helen Beryl Mine.  The specimens are all small cleavage pieces and were collected in the 1960’s.  However, even the non-gemmy variety of spodumene often has a nice greenish sheen along the cleavage faces.
 

Fragment of spodumene showing greenish sheen.  Fragment of pink kunzite, ~3.4 cm, placed on specimen for scale.

The Tin Mountain Mine is located about seven miles west, on US 16, of Custer, then north on a Forest Road 287 (.25 mi) and east on FDR 265 (.5 mi).  The mine may or may not be available for collecting the dumps; check with the local USFS office (it was posted in summer 2012).  The mine commercially produced several lithium minerals, plus the “rare cesium mineral pollucite” (a zeolite), from a complex pegmatite (Gries, 1996).
Columbite-Tantalite is often found in lithium-and phosphate-rich pegmatites and associated with such minerals as spodumene, beryl and lepidolite. I have a small twinned specimen of "columbite-tantalite" (probably the latter) collected many years (decades) ago from somewhere near Custer, South Dakota.  At times in my life, especially when younger, my note taking and locality information was not the best; I thought my memory would last forever!  (see tantalite Blog posting 10/11/12).  The various pegmatites of the Black Hills in South Dakota have produced many tons of columbite-tantalite.  Roberts and Rapp (1965)  stated the Black Hills have received world-wide recognition for the many excellent specimens of columbite-tantalite collected from pegmatites in the area since first reported in 1884...In addition to specimens, over 65 tons of columbite-tantalite have been produced since 1918 as a by-product of mining other minerals.  I am unaware of current mining for columbite-tantalite in the Black Hills.
 

Roadcut pegmatite west of Custer displaying large specimens of schorl tourmaline, the largest is ~ 30 cm.

In road cuts immediately west of Custer are some really nice tourmaline-mica pegmatites.  The host pegmatites are speckled with black (iron-rich) prismatic crystals of schorl tourmaline and large books of muscovite and biotite.  In fact, I believe the pegmatites of the Black Hills may be the “easiest” place to collect fine crystals of these minerals.

Many of the pegmatites also have fantastic "books of mica", both biotite and muscovite, and these are easily collected at roadcuts and mine dumps.



Books of biotite ~4 cm. in length with quartz and feldspar (top).  A specimen almost completely composed of muscovite books, quarter for scale (lower).  Collected a few miles west of Custer at an abandoned quarry.


Microcline is one of the more common minerals in the Black Hills; however, very few of the specimens are greenish-blue amazonite such as collected in the local Pikes Peak pegmatites.  I do have a very nice crystal collected in 1966 from, I believe, the Hugo Mine (very near the Etta Mine near Keystone).  During and immediately after World War II the Black Hills were the second leading producer of feldspar (North Carolina was first).  And, at one time, the Black Hills produced about one-third of the mica in the United States (Loomis, no date). 
Between miles six and seven west of Custer on US 16 are fantastic exposures of a wavy, shimmering micaceous schist along the west side of the road.  At the seven mile mark and west along FDR 287, the schist produces “gemmy, transparent, ruby-red modified dodecahedral crystals of almandite [almandine garnets]” (Roberts and Rapp, 1965).  These garnets are the iron-rich end member of a solid-solution series with pyrope garnets having magnesium substituting for the iron.  They may be collected in the schist host rock or loose in the associated wreathing product (sediment). 

Micaceous schist, nickel for scale.  Collected near the garnet locality where the rock unit contains numerous small garnets.

The Teepee Canyon agate locality, in the Paleozoic Minnelusa Formation, is located west of Custer along U.S. 16 about two miles west of Jewel Cave National Monument, or perhaps 14 miles from Custer.  (see the 8/18/12 Blog posting).

The Bob Ingersoll Mine near Keystone is “a mine with more varieties [minerals] than Heinz has pickles” (Johnson, 1989).   I once saw miners “highgrading” nice beryl crystals, most likely for the beryllium since the mineral is a beryllium aluminum silicate or perhaps for their aesthetic value since they were beautiful six-sided crystals.  They actually gave me a small hexagonal crystal that has small patches that are close to green (probably colored by chromium) gemmy.  In the early 1900’s, a large beryl crystal was exposed at the Ingersoll, a nearly perfect hexagon 46 inches across the face. In 1933, another crystal was found that measured nine feet high and over eight feet wide and produced 24 tons of ore (Loomis, no date).  The Ingersoll also was an important producer of lepidolite, another lithium mineral; potassium aluminum lithium silicate and a “mica”). 
 

Lavender lepidolite, a lithium mica.  Width about 5 cm.

Another interesting specimen from the Bob Ingersoll, collected decades ago, is a large piece of muscovite with a crystal of tourmaline enclosed.  This crystal does not appear 3-dimensional, but almost flattened.  It is tough to tell the variety of tourmaline (without taking apart the muscovite); however, I believe it is elbaite.  At any rate, a sort of strange specimen.
As previously stated, almost all outcrops of pegmatites in the Black Hills have nice crystals of feldspar and mica.  Pacer Minerals at Custer, immediately across from the Flintstones Campground, currently mines potassium feldspar that is used in ceramics and tile.  I was able to take some close looks at their large bounders brought in for processing, both for feldspar and muscovite and they were impressive.
 

Muscovite with included crystal of tourmaline (elbaite?).

The Hills have nice exposures of a wide variety of “micas”.  Besides the previously mentioned “lavender” lepidolite (potassium, aluminum, lithium silicate mica), “brown-black” biotite (potassium, magnesium, iron silicate mica) and “light-clear” muscovite (potassium, aluminum silicate mica) the Black Hills also have outcrops of phlogophite (potassium, magnesium silicate mica).  This latter mica is often a nice bronze color and in the Hills is associated with metamorphic rocks, especially marble.  I have a specimen of actinolite, phlogophite, and marble (evidently dolomitic marble as effervescence is ‘slow”) that I know was collected from near Custer in the 1960’s.  The description provided by Roberts and Rapp (1965) seem to confirm the specimen came from “about 11 miles northwest of Custer”.
 
 

Greenish columns of actinolite with bronze-colored flakes of phlogophite in a white dolomitic marble.  Width of specimen ~5.5 cm. 

The State Mineral of South Dakota is rose quartz, and the Black Hills have produced “millions of tons”.  In fact, collectors from “around the world” head to the Hills when looking for specimens.  The Scott Rose Quartz Mine, southeast of Custer, has produced more rose quartz than any other mine in the world (Roberts and Rapp (1965).

In summary, the Black Hills are a paradise for mineral collectors and Farrar (2002) noted they contain approximately 390 different minerals.  Many of these come from the hundreds of mapped pegmatites, with their accompanying mines and dumps, that are scattered throughout.  Great road cuts are abundant and produce a wide variety of really nice specimens.  The Hills have a network of roads that will take the rockhound into the inner depths.  But, collectors should be aware that many/most of the old mines are on private property and/or have active claims.  My advice is to visit the local rocks shops and the Custer office of the USFS, and always ask permission if exposures are on private lands.
 
I have additional specimens from the Hills and other postings will be forthcoming.

REFERENCES CITED AND OTHERS
A short article like this cannot begin to list all of the collectable minerals or localities, so I would suggest the following references prior to visiting (or just for a great learning experience): 


Farrar, B., 2002,  Quoted in Loomis, unknown date, as Per. Com.: Loomis, T. A., unknown date, Black Hills Pegmatites: Matrix, v. 10, no.3, www.dakotamatrix.com.

Gosselin, D. C., J. J. Papike, R. E. Zartman, Z. E. Peterman, J. C. Laul, 1988,  Archean Rocks of the Black Hills, South Dakota: Reworked Basement from the Southern Extension of the Trans-Hudson Orogen: Geological Society of America Bulletin, v. 100, no. 8. 

Gries, J. P, 1996, Roadside Geology of South Dakota: Montana Press Publishing Company, Missoula.

Hark, J. S., 2009, Zircon, Monazite, and Xenotine as Provenance Indicators in Selected Precambrian Crystalline Rocks, Black Hills Uplift, South Dakota: MS Thesis, Kent State University, College of Arts and Sciences, Department of Geology.

Johnson, A. I., 1989, Western Mining in the Twentieth Century, Oral History Series: University of California, Berkeley, California. 

Roberts, W. L. and G. Rapp Jr., 1965, Mineralogy of the Black Hills: South Dakota School of Mines Bulletin 18.

South Dakota School of Mines & Technology, Museum of Geology: http://museum.sdsmt.edu/home/

Zeitner, J. C., 1998, Midwest Gem, Fossil and Mineral Trails: Prairie States: Baldwin Park, CA. Gem Guides Book Company.

Also see Blog postings:       
          Fairburn Agates—June 17 and August 19, 2012
          Bear Butte Laccolith—June 10, 2012
          Crow Peak Laccolith—August 14, 2012
               Arrojadite; A Rare Phosphate---April 2, 2013 

3 comments:

  1. Thanks for the post. I appreciate the work you put into it to help those of us with an interest, but who have a lot less knowledge of geology

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    1. Thanks for your comments. I just returned from a trip to South Dakota and was able to spend a couple of days in the Black Hills. I came back with some interesting specimens, especially from a couple of those Tertiary laccoliths. Will describe those in later posts.

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  2. Thank you for posting all this knowledge! I moved to SD a year ago October and my family and I enjoy rockhounding. I've been searching all over the internet to see what we can find and your blog was the first one to pop up. I have it bookmarked and have came back to your blog many, many times in the past couple of months.

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