Monday, November 15, 2010

Iron Springs District, Utah

Iron Springs District.

West of Cedar City is the Iron Springs District.  This area includes Iron Mountain, Granite Mountain, Three Peaks, and many other iron deposits that have been mined over the decades.  The district is flanked on three sides by desert and is at the eastern edge of the Basin & Range province.

The iron came to be by events recorded in the rock.  The (watered-down) sequence of events would be as follows…Limestone was deposited (Carboniferous) and erosion later deposited minor layers of shale, conglomerate, and more limestone.  Then came the laccolith intrusion.  A laccolith is a small magma body that works its way toward the surface in an effort to erupt like a volcano but never erupts out the top.  This laccolith was very rich in andesite (feldspar).  This laccolith caused the rock layers to tilt as it pushed its way through.  It also caused metamorphism in the rock around the laccolith.

Once the laccolith had pushed its way through the rock, then ore-depositing solutions could fill the cracks and fissures made by the laccolith.  This was a secondary ore deposit which caused further metamorphism in the surrounding rocks.

Now comes the erosion.  Surface  weathering has removed, by some estimates, over 1,000 feet of sediment to expose the iron ore bodies we can see today.

As the ore body cooled, it shrank.  This allowed even more hot ore-bearing solutions from inside the earth to creep up and mix with the laccolith and the host rock that was metamorphed from the intrusion.  It is at this contact area that there was room for nice crystals to develop.  The shrinking of the laccolith has been calculated to be 200 to 500 feet.  That is a lot of room for collectable minerals.

In this area we find many quartz crystals including amethyst, martite crystals (magnetite pseuodomorphed to hematite) siderite, feldspars, biotite, pyrite, garnet, and apatite.  This is the zone which mineral collectors like to be able to get to.  Martite crystals can reach well over 2 inches.  They are often associated with pale-apple-green apatites from fine needles to 2 inches tall and 0.5 inches in diameter.  Siderite crystals are usually irredescent red/yellow/orange and can reach 0.5 inches.  Any of these minerals, or a combination of them on one piece make for an attractive addition to any collection.

Most of the mines in the area are closed and posted.  The may pit at Iron Mountain is used as the landfill for Cedar City and a couple of the mines have been worked off and on.  If you are attempting to collect here, beware you are not trespassing first.

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