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.
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Diagram of the Black Hills, courtesy of A.N.
Strahler.
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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).
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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.
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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).
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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).
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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?
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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.
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Fragment of spodumene
showing greenish sheen. Fragment of pink
kunzite, ~3.4 cm, placed on specimen for scale.
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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.
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Roadcut pegmatite
west of Custer displaying large specimens of schorl tourmaline, the largest is
~ 30 cm.
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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.
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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).
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Micaceous schist,
nickel for scale. Collected near the
garnet locality where the rock unit contains numerous small garnets.
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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”).
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Lavender lepidolite, a lithium mica. Width about 5 cm.
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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.
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Muscovite with included crystal of tourmaline
(elbaite?).
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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”.
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Greenish columns of actinolite with
bronze-colored flakes of phlogophite in a white dolomitic marble. Width of specimen ~5.5 cm.
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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):
Dakota Matrix Minerals: http://www.dakotamatrix.com/Black_Hills_pegmatites.asp
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
Arrojadite; A Rare Phosphate---April 2, 2013