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| THE "CUT" AT THE CALUMET IRON MINE. THE LARGE BLOCKS COMING DOWN THE HILL ARE PIECES OF THE LATE PALEOZOIC LIMESTONE. MANY DISPLAY SURFACE AREAS COVERED WITH CRYSTALLINE EPIDOTE AND DIOPSIDE--ACTINOLITE. NOTE PERSON FOR SCALE (DIGGING FOR QUARTZ CRYSTALS) |
Colorado is fortunate to
have a large number of collecting areas available for visiting by rock and
mineral clubs. One of the standard
collecting localities for club members is the Calumet Iron Mine down in Chaffee
County about 7 miles north-northeast of the city of Salida. There seems to be a gazillion articles
written about field and collecting trips to the Calumet, so I will just add
another one!
Fall weather in Colorado 2012
has been spectacular for collecting trips but less so for the skiers. At this writing in early December the highs
are in the low 60’s and several ski resorts remain closed---no snow. So, when Yam of the West called me about a
trip on Saturday, I eagerly accepted. Where to---a creek walk for petrified
wood? To northeast Colorado for barite
crystals? Or over to Salida for actinolite,
epidote and other crystals? I chose the
latter since I had never actually visited the old mine, and I am a sucker for
green minerals. So, off we (Yam of the
West, SharonRocks, and Phil the Digger) went the next day and with a couple of
coffee stops arrived mid-morning.
The Mine was discovered/located
shortly after 1880 and seemed to be a major producer until the end of the
century, generating somewhat less than 250k tons of iron ore. The ore was taken from the large veins of high
grade magnetite shot through the area.
Today there is a large open cut (actually “cuts”) on the hillside plus a
number of shafts leading underground (to where I don’t have the slightest idea
since the tunnels seem quite decrepit and my momma didn’t raise no fools). According to Modreski (2005) late Paleozoic carbonate
rocks were baked and altered by igneous rocks associated with emplacement of the
Whitehorn Granodiorite; remnants of the limestone are clearly visible. Wrucke (1974)
described the Whitehorn as including a pluton and a couple of small satellite
bodies that intrudes the Paleozoic rocks (and also some of Precambrian age) along
the east flank of the Arkansas River Valley east and northeast of Salida. The rocks generally range in composition from quartz
monzonite (less quartz than granite and equal amounts of plagioclase and orthoclase
feldspars) to granodiorite (similar to granite but with more plagioclase
feldspar than orthoclase feldspar) along an outcrop area of ~16 miles by ~5
miles. The small satellite body at Calumet
is a sill where the igneous rocks are tabular or sheet-like in shape. Wrucke
(1974) reported radiometric dates of ~70 Ma or Late Cretaceous and associated
with the Laramide Orogeny (the mountain building event associated with the
Colorado Rocky Mountains). The Turret
Mining District (includes the Calumet) also has produced small amounts of
copper, gold, silver, vermiculite, marble, and feldspar. In 2011 Real Aspen (www.realaspen.com) noted that, “earlier
this month, Canada-based Rare Earth Industries announced it is exploring
tantalum, beryllium and manganese at the formerly abandoned Turret Mines it
acquired northeast of Salida in Chaffee County, Colo”.
One of the more
interesting aspects of the Calument mine era was the construction of the
Calumet Branch of the Denver & Rio Grande Western narrow gauge railroad in
1881. The Calumet was a spur line off
the main line (Tennessee Pass) and ran from Hecla Junction in Browns Canyon
about 8 miles to the mine. The short
line brought the ore down from the mine and transferred it to a line that
ultimately moved it to the big steel mill, Colorado Fuel & iron (C&FI),
in Pueblo. The short line was especially
steep, about 7% grade, and the curves were tight. The empty ore cars returning to the mine were
“pushed” up the canyon with a locomotive in the rear. The cars coming down the canyon were “pulled”
but the train had a brakeman stationed on each car (shoes wore out
quickly). The mine closed in 1899 and
the line was wiped out by a flood in 1901 (above from Rio Grande Info, www.DRGW.net). Today, concrete remnants of the loading docks
and rail line are still in place, having survived weathering for over 100
years.
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| THE DYNAMITE SHACK AT CALUMET. |
I suppose the Calumet Mine
is best known for a couple of minerals: diopside--actinolite pseudomorphs, and
epidote. The most interesting might be
the former. Diopside is a magnesium-calcium
pyroxene, MgCaSi2O6, (see blog posting November 25, 2012)
and forms a solid solution with end member hedenbergite (iron-calcium silicate)
while augite is situated somewhere in the middle with differing amounts of added
titanium, aluminum and sodium. I
remember in my intro geology class that somehow I was always confusing augite
with hornblende (a complex amphibole) since both minerals are black in color
and both are common constituents in igneous rocks. After letting me suffer for several labs the
instructor finally explained about cleavage, ~56o and 124o
in hornblende while cleavage in augite is nearly “square” at 90o. That tidbit of information really helped!
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| DIOPSIDE--ACTINOLITE PSEUDOMORPH. |
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| LONG BLOCKY CRYSTALS OF DIOPSIDE-ACTINOLITE. WIDTH OF SPECIMEN ~3.5 CM. |
Diopside is commonly found
in mafic igneous rocks (containing high amounts of magnesium and iron such as
gabbro) but also in metamorphic rocks, especially where contact metamorphism
has heated some carbonates. Diopside
crystals commonly are columnar to prismatic and are close to augite in cleavage
angles: ~87o and 93o—sort of like elongated rectangles!
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| DIOPSIDE-ACTINOLITE. NOTE LARGE BLOCKY CRYSTAL TO RIGHT. WIDTH OF SPECIMEN ~4.5 CM. |
At the Calumet Mine, results
of the contact metamorphism included the formation of many crystals of
diopside. However, post-metamorphism the
crystals have pseudomorphed, or altered, into actinolite while keeping the same
crystal shape of diopside. Now, most
readers realize that I often just fumble around with the complexities of
mineralogy and petrology. And, this is a
case of “how did it happen and why”? I
don’t have the slightest idea about the intricacies of this alteration and
certainly could use some help from a competent mineralogist. At one time the diopside-actinolite was given
the mineral name, uralite (now discredited).
Actinolite is a complex
amphibole silicate, Ca2(MgFe)5Si8O22(OH)2,
and is the middle member of a solid
solution series between tremolite (magnesium-rich end member) and
ferro-actinolite (iron-rich end member).
From my observtions I probably cannot tell the difference between
actinolite and tremolite and I believe there is much gradation between the two
(and also most likely with the ferro-actinolite). Actinolite often occurs in long prismatic
crystals with two planes of cleavage so that a cross section of a single
crystal would resemble a diamond (it is an amphibole and resembles the cleavage
of hornblende); fracture is often “splintery”.
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| PHOTOMICROGRAPH SHOWING "SPLINTERY" SMALL CRYSTALS OF DIOPSIDE--ACTINOLITE. |
Some actinolite, the non-pseudomorph
type, may have an interesting crystal appearance. The mineral occurs as blades radiating out
from a central point. I picked up my specimen
from an old mine dump on the trail back to the vehicle.
There are a couple of other
very interesting types of actinolite out in rock land. Very fibrous actinolite is classified as a
form of asbestos. I don’t believe that
the mineral is mined for such at the present.
Second, a variety called nephrite is one of the two forms of jade (and
that mineral is confusing enough), the other being jadeite, a pyroxene. No wonder I get the amphiboles and pyroxenes
confused! The famous ‘apple-green” jade
from Wyoming is nephrite.
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| RADIATING CRYSTALS OF ACTINOLITE. WIDTH OF SPECIMEN ~6 CM. |
We also returned to the
Springs with a variety of other minerals including garnet, epidote, and
quartz. However, that will be another
story.
REFERENCES CITED
Modreski, P. J., 2005,
Colorado Mineral Collecting Localities: Rocks and Minerals, Sept.-Oct.
Wrucke, C.T., 1974, The
Whitehorn Granodiorite of the Arkansas Valley in Central Colorado. U. S. Geological Survey Bulletin 1394-H.