A dog doesn't try to give advice, or judge you; they just love you for who you are. It's nice to have someone who will just sit and listen to you. So sayeth Charlie Brown.
A
Blog doesn’t try to judge you; it just lets you write what spews out from the
inner recesses of your brain. It’s nice
to have a medium where a small audience may just sit and read to their heart’s
content, or maybe just skip over it. Their choice; however, the writer has
experienced an enjoyable day. Life is good. So sayeth Mike.
So,
I finished finding out how to warm my toes on a cold winter night. It is now
time to continue my brief descriptions of new (to my collection) of Yooperland Minerals--grunerite was added June 24. That is, minerals from the fantastic rocks of
the northern peninsula of Michigan. For
those who have not read some of my previous postings (see August 2, 2019 for
copper arsenides) about Yooperland, it is the UP (Upper Peninsula) of Michigan
that is connected to the south part of the State by the Mackinac Bridge. So, the Yoopers live north of Big Mac while
the Trolls live under (south) the bridge.
This introduction to Michigan terminology was explained to me by my good
Yooper friend Duncan. The Yoopers also love
Nordic winter sports. I mean what else is there to do when it snows for 10
months each year (other than ice fish)? Duncan loves the skinny
skis and is a highly competitive cross-country skier and always finishes well
in the top Nordic ski event in North America, the 50k American Birkebeiner. In
fact, he is classified as an Uberlegger with 41 Birkies (and counting) under
his belt.
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Yooperland (the UP) is connected to Troll Land (lower Michigan) by the Big Mac (the Mackinac Bridge). Public Domain sketch.
Counties in Yooperland. The Keweenaw Peninsula protrudes into Lake Superior. Public Domain photo.
Ontonagon County is in the most northwestern section of the UP, one of the largest counties in the state, and one of the least populated counties. I have camped several times in the county due to the scenery along the coast of Lake Superior and the ruggedness of the Porcupine Mountains, AKA the Porkies.
Most of the county is heavily forested and often covered with Pleistocene glacial drift. However, the bedrock is exposed at places, especially along the coast and in river valleys, and is Middle Proterozoic (Late Precambrian) in age and associated with the Mid-Continent Rift System (see Posting October 31, 2013). As in other parts of northern Michigan, especially the nearby Keweenaw Peninsula, these Rift rocks produce copper and silver along with a large variety of other non-ore minerals.
The MRS is centered in Lake Superior with two well-defined arms and one sort of trending west. Map Public Domain (I think).
Rockland
is a sparsely populated township in Ontonagon County that is well known for its
fall colors, orchards and cider mills, and pumpkin patches. It is also the home
of the National Mine, a past producer of primary copper and secondary silver.
According to the USGS MRDS (ID:W031505) the National was an underground mine
with five shafts producing from a 2–8-foot tabular fissure of amygdaloidal
basalt (Portage Lake Volcanics ~1095 Ma). Production started in ~1948, lasted
for half a century, and yielded ~5,268 million tons of copper.
Among
the gangue minerals from the National, barite seems the most collectible. My specimen of barite, from an unknown
collector, is a mass of transparent to translucent tabular crystals.
Barite crystals collected National Mine, Rockland Township, Ontonagon County, Michigan. Width of crystal mass above ~5 mm, specimen below 2.3 cm..
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Near
the National is the Mass Mine, an underground (three shafts), very small producer
of copper from ~1849 to 1888. Mining was
sporadic during these years and in 1899 the Mine, along with Old Mass, Ridge,
Ogima, Hazard and Merrimac Mines were consolidated into the Mass Consolidated
Mining Company. Evergreen Bluff Mining
Company was added in 1911. The Group
went belly-up in 1919 after producing ~ 25,000 tons of copper.
My specimen from the Mass Mine is a mass (pun intended) of small crystals of clear quartz and green epidote mixed with dark hacky copper (including some small poorly formed crystals). There appear to be other minerals present, probably pumpellyite (yellowish acicular crystals). A second specimen is a badly tarnished, convoluted, rather ugly piece of copper.
The
copper deposits of the UP have a complex geological history that might be best
understood by consulting the book by E.W. Heinrich, with an update by George
Robinson. entitled Mineralogy of Michigan (2004 Michigan
Technological University, A. E. Seaman Mineral Museum): All copper deposits
are in rocks associated with the middle Proterozoic (late Precambrian) Mid-Continent
Rift System. The major copper producer
is the Portage Lake Volcanics, a thick sequence (up to 9400 feet) of basalt,
andesite, and felsite flows with interflow red rhyolite pebble conglomerates. Heinrich
and Robison (2004) best described the ore formation: “although hosted by
volcanic and sedimentary rocks, the copper deposits themselves are of
hydrothermal origin…[The minerals] formed in permeable parts of the basaltic
lava flows and conglomerates by open-space filling and replacement.
Hydrothermal solutions (derived from infiltrated sea water?) …leached copper
and other elements from the basalts, concentrated them, and deposited copper
and other minerals as they cooled during their ascent.” A complex history indeed.
Just
to the east of the mines in Ontonagon County is Houghton County, a territory that
also extends about halfway up the Keweenaw Peninsula. Like Ontonagon County, Houghton is home to
trees and glacial drift and makes up a significant portion of Copper Country
where, at one time, copper was king.
One
of the Houghton mines was the LaSalle
Mine where copper was extracted from the Kearsage Amygdaloid, one of the
vesicular basalt flows of the Portage Lake Volcanics. In fact, the Kearsage was probably the
richest (in copper) of the many basalt layers in Copper Country. The LaSalle
operated from 1910-1920 and produced ~3.5 metric tons of copper.
Perhaps
the most interesting thing about the LaSalle is the abundance and variety of
gangue minerals, and as MinDat noted, the “site is especially popular with
microcollectors.” My specimen from the LaSalle is a small piece of vesicular
basalt with beautiful microcrystals of pistachio colored epidote. I also have a
large, flattened nugget of tarnished copper. However, the epidote is a much
nicer specimen to mount.
About a half mile from the LaSalle Mine is the more famous underground Laurium Mine that produced copper from the Kearsarge Lode (1907-1920). I found it most interesting that the mine had a shaft reaching 1,660 feet. MinDat noted that the “Laurium provides perhaps the best collecting in the Keweenaw, especially for the micro-mineral collector.”
One of the microminerals I have from the Laurium is saponite, a clay mineral of the Smectite Group and one that is related to the better-known clay mineral, montmorillonite. As with many clay minerals, saponite is difficult for me to identify since it seems similar to other fibrous microminerals forming in amygdaloidal cavities in Keweenawan basalt. Mostly I depend on previous descriptive analyses of these microminerals and the photographs in MinDat. For example. At first glance I thought the specimen shown in the photograph below was pumpellyite, a micro cavity filler in the basalt. However, I ran across a MinDat photograph of saponite that appears to be almost identical to my specimen so saponite it is.
I also note the “be careful” statement of Deer and others (1966): “optical methods alone are in general not reliable for identification of smectites [saponite].” That means a an ole paleontologist like me needs to understand:
Saponite:
Ca0.25(Mg,Fe)3((Si,Al)4O10(OH)2-nH2O)
Pumpellyite:
Ca2MgAl2[Si2O5OH][SiO4](OH)2(OH)
Saponite
usually occurs in the basalts as spherules of acicular or bladed crystals that
resemble puffballs, or spherules that appear solid botryoids; however, there
are other shapes. Some are snow white to
colorless while others are gray or yellow but mostly some shade of green. The
specimen I have is a broken spherule showing the mass of radiating narrow
bladed crystals.
Broken spherules showing radiating crystals of saponite. Width FOV ~7 mm.
What’s
a poor ole softrocker to do? Especially since there are specimens of
pumpellyite rich in aluminum or ferrous iron or ferric iron or manganese---just
change the cations. Heinrich and Robinson (2004) stated, “it is impossible to
say which species of pumpellyite is present without complete chemical and
structural data.” So, to answer my question—make friends with a mineralogist
proficient in using electronic gizmos like an XRD!
It seems most Keweenaw specimens of pumpellyites are Mg- dominant and are groups of radial prismatic crystals usually white to brown to green, or masses of scattered needle-like crystals thrown together in a jackstraw arrangement. It also is tough, at times, to distinguish pumpellyite from epidote or chlorite group minerals. Whatever, they are very tiny crystals found in broken vesicular cavities.
Vesicular basalt filled with mass of jackstraw green crystals of pumpellyite? along with white "puffballs" of pumpellyite.The width of the "puffballs", above and below are ~1mm or less.
Unfortunately,
I do not have a specimen of chlorastrolite, a variety of pumpellyite described
by Heinrich and Robinson (2004) as “ beach pebbles showing a finely radiating
or stellate pattern of slender green crystals.
The pebbles are derived from vesicle fillings in the amygdaloidal
basalts of Copper Country.” In 1972 chlorastrolite (AKA Island Royale
greenstone) was designated as the official state gemstone.
Microcline of the variety adularia (KAlSi3O8) is a common potassium feldspar (K Spar) in Copper Country occurring in low temperature veins and other hydrothermal deposits (Heinrich and Robison, 2004). It is often a prominent gangue mineral and the bright red to pink crystals are easy to spot and observe in specimens. I have several specimens, all from mines in Keweenaw County near the top of the Keweenaw Peninsula, or its neighbor to the southwest, Ontonagon County.
Microcline var. adularia, Seneca Mine. width FOV ~7 mm. Note green unknown mineral bordering adularia.
Epidote and adularia in a vug from Seneca Mine, Keweenaw County. Width FOV ~6mm.
I don’t have a list of zeolites (porous aluminosilicate minerals) that are found in Copper Country; however, one of the best known of the group is analcime, a hydrous sodium aluminum silicate [Na(AlSi2O6)-H2O]. Most crystals of analcime that I have observed in various rocks are usually white or colorless, medium hard (~5.0 Mohs), translucent to transparent, often well formed, trapezohedral crystals, and come from late-stage hydrothermal fluids filling vugs in basalt. Others are formed as precipitates in lake sediments. Interestingly, at several mines in the Copper Falls area in Keweenaw County the analcime crystals are bright red in color. They are also a very late stage, post-copper deposition, mineral (Heinrich and Robinson, 2004).
Native
silver seemed to occur in almost every copper mine found in Copper Country.
It was produced in large amounts but rarely recorded accurately in production reports
(due to high grading by mine workers and supervisors). In fact, some mines were
so rich in silver that originally, they were designated as silver mines. It seems likely that the silver in vesicular
basalt probably formed later than the copper (Heinrich and Robinson, 2004).
One
of the more collectable specimens from Yooperland is a copper-silver mix
(formally known as halfbreeds). On 29 September 2012 a long discussion was held
on MinDat and the best summary was presented by Paul Brandes (remember that
halfbreed is now viewed as a derogatory term and is not used in the mineral
world):
The
Keweenaw deposits were formed in a relatively low temperature, low grade
metamorphic environment where hydrothermal fluids were being generated. These
fluids, which picked out copper and silver ions from the basalts, were
channeled and deposited in their present position through permeable
conglomerates and the broken tops of basalt flows. Copper and minor silver were
the predominant metals deposited, with very rare chalcocite, digenite, and
other copper sulphides due to a lack of sulfur.
There is a quite a bit of talk about what exactly is a halfbreed. Some folks regard them as any specimen with silver and copper together. Others believe they are an amalgamation, whether by natural or mechanical means, of copper and silver in the form of a nugget or other rounded form. Still others regard glacial float copper with silver on them as halfbreeds; so which is the correct usage of the term??? In my perfect world of nomenclature, I use the term “halfbreed” strictly for the nuggets of copper and silver that have been pounded together during the stamping process to remove the host basalt rock from the metal. Natural nuggets and boulders of copper with occasional silver which have been rounded and transported by glacial activity have been given the term “float copper”, while specimens of intergrown copper and silver are simply known as “native copper with silver” or “native silver with copper”, depending on the amount present in a specimen. Unfortunately, the term halfbreed is used far too interchangeably between all three types by most folks, so there is no real conciseness and therefore no consistency in its use.
Heinrich
and Robinson (2004) also noted (after
Olson, 1986) that most of the copper-silver specimens on the market are
products of the stamp mill. That certainly
would seem to be the case with my specimen collected from “Houghton, Michigan.” Regardless
of the lack of more specific information, I am happy to have the now somewhat uncommon
specimen.
Besides
the copper-silver nuggets, silver from Copper Country is found with habits
ranging from wire silver (rare to absent) to herringbone and spike aggregates
(common) to octahedral to dodecahedral crystals. A few rare cubic crystals have been found in
a prospect pit near the Petherick vein at Copper Falls (Heinrich and Robinson,
2006).
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This
last sentence is of great interest to me since a few years ago I picked up a
micromount put together by Art Smith, mainly since it was cheap and was silver
(a collectable mineral). The label said, “Silver, near Copper Falls Mine, Keweenaw
County, Michigan.” Much to my surprise the mount consists of several nicely
formed silver cubes. This is my idea of a great purchase!
Finally, I would be remiss without mentioning a nice specimen of Lake Superior Agate. One usually thinks of the agates as being collected from the shoreline gravels and adjacent gravel pits of Lake Superior, especially the North Shore in Minnesota. However, the Lakers have a fairly widespread distribution since glacial action (Pleistocene) moved specimens into adjacent Wisconsin, Michigan, Iowa, Missouri, Nebraska, and South Dakota and there are reports of a Laker or two from Kansas. In addition, Pabian and others (2006) noted Lake Superior-type agates have been found in deposits of the Mississippi River as far south as Louisiana. Essentially any “gravel pit” in the glaciated region, or along the Mississippi River, has the possibility of producing a Laker. See Posting on August 1, 2013, describing: 1) Lakers from Wisconsin; and 2) August 19, 2013, a Laker from South Dakota.
My
specimen does not have good provenance information except that it came from Keweenaw
County. One might assume a Lake Superior
beach, but it could have been found in the glacial till. The specimen label
appears “old.”
In the recent July/August 2022 edition of Rocks and Minerals Tom Rosemeyer published News from the Keweenaw, Recent Finds in Michigan’s Copper Country. This article is the 29th in a series describing minerals of Copper Country. All articles are listed in the July/August edition.
REFERENCES CITED
Heinrich,
E.W. with G.W. Robinson, 2004, Mineralogy of Michigan: Michigan Technological
University, A. E. Seaman Mineral Museum.
Olson,
D.K., 1986, Michigan silver: Mineralogical Record vol. 17.
Pabian, R., B. Jackson, P. Tandy, J. Cromartie, 2006, Agates: Treasures of the Earth: Firefly Books, Limited.
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