DENVER GEM SHOW: COPPER 2012

The colorful copper minerals of Arizona.

Each fall the Greater Denver Area Gem and Mineral Council (eight local clubs) sponsors the Denver Gem and Mineral Show with event proceeds distributed as monetary grants for the advancement of the arts and sciences within the field of the earth sciences. There are a number of competitive and non-competitive cases as well as tens of dealers housed in the Denver Merchandise Mart Expo Hall (exit 215 off I-25 north of downtown).  The 2013 show is schedule for September 13-15 with a theme of “Tourmaline”.

In addition to this “main show”, there are nine other gem, mineral, fossil and jewelry shows scattered around the city---most on the north side.  Something like 800 vendors will be at these ancillary events. 

 

I attended the 45^th Annual Show in September 2012 and found the exhibited specimens quite beautiful.  The Show theme in 2012 was “Copper and Copper Minerals” and varieties of copper-bearing minerals, as well as large hunks of native copper, were displayed in a spectacular manner.  As usual, the colorful blue azurite and green malachite glowed from inside their cases.  I spent a large amount of time sort of staring at the displays wondering why I could never find such specimens!  I also made the rounds of several dealers and was able to visit with one of my heroes, Bob Jones, the Senior Editor of Rock and Gem Magazine.

 

New Mexico has produced numerous copper minerals.

Copper (Cu) occurs either as native copper, or is combined with other elements to form a “copper mineral”.  The former is perhaps best known from the Keweenaw Peninsula of Upper Michigan (the Yooper  Region, check out Da Yoopers web site at www.dayoopers.com; known otherwise as the UP). There is evidence that Pre-Columbian Native Americans in the area mined (shallow pits) and gathered the loose copper chunks. 

 

Native copper chunk from the Keweenaw Peninsula.

Archaeological evidence points to the mineral’s use as weapons (projectile points), tools, cookware, and personal ornaments.  It is interesting to note that Pleistocene glaciers moved pieces of this native copper (known as drift copper) to large areas of the Midwest and native peoples gathered and use the metal as described above.

The  UP copper is found in rocks associated with the Midcontinent Rift System (MRS), one of the more interesting geological structures created in the Proterozoic (Precambrian ~1.1 Ga).  The MRS (splitting apart) was probably composed of three arms, the result of what geologists term a “triple junction” with the center (the junction of the three arms) positioned approximately at the location of modern Lake Superior.  To fix this image in your mind, just imagine the top crust of a pie and how triple cracks develop during baking (but magnify it by zillions!).  One arm extended southeast through Lower Michigan while a second arm trended west along the Minnesota-Ontario border.  The best known arm, and the longest/largest, extended southwest from the junction for ~ 1200 miles into eastern Kansas.  As the rift opened, intrusive rocks such gabbro formed (Duluth Complex) while extrusive basalt flowed from surficial vents (the rocks are lumped together into the Keweenawan Supergroup).  In addition, erosion of the adjacent highlands dumped (streams and fans) clastic sediments (now sedimentary rocks) into the trough. The extensional rift is similar to the current East African Rift System.

Somewhere in the Proterozoic (after ~15-20 million years of rifting) the continent splitting “stopped” and the rift started to close (a failed rift in geological terms).  Perhaps the compression stopping the rifting was the result of orogenic activity (mountain building) on what we now know as the east coast of North America.  The trench/rift then acquired a variety of sediments and ultimately became buried beneath the Paleozoic rocks of the craton (stable part of interior North America).  Today the MRS rocks generally are buried from Kansas to near (just north) of Minneapolis/St. Paul, Minnesota, but are especially well-exposed around Lake Superior and the Upper Peninsula of Michigan.  However, geologists are aware of the well-defined subsurface rift rocks in the south since geophysical magnetic surveys produce a gravity high (dense, iron-magnesium basalt surrounded by low density sedimentary rocks), and in place deep wells (looking for oil) have bored into MRS rocks.

Midcontinent Rift System.  Map courtesy of Winona State University.

Scientists at Michigan State University have described the (www.geo.msu.edu/geogmich/copper.html ) formation of UP copper as follows: most of the native copper occurs at the top of the MRS basalt in a unit known as the Portage Lake Volcanics/Lavas.  However, this series actually contains over 200 individual lava flows (now basalts and some rhyolite), and 20 discreet conglomerate beds, that collectively have produced over 11 billion pounds of copper.  Over one billion pounds of copper have been extracted from copper sulfides (mostly chalcocite, CuS) in the overlying Nonesuck Shale.  The original source of the copper was from secondary deep seated hydrothermal solutions percolating toward the surface with native copper crystallizing in the open vugs and pore spaces. 

Whereas the native copper produced in the UP is almost 100% copper (or mixed with other metals such as silver), the copper content of porphyry copper deposits is much less, often less than 1%!  However, it is these types of deposits that currently are producing most of the world’s copper from giant open pit mines observable at famous localities such as Bingham, Utah, and the Lavender Pit at Bisbee, Arizona (now closed but viewable).  Here, hydrothermal fluids associated with a cooling magma plume deposited copper, and other metals, in both the igneous rocks and the surrounding country rocks such as limestones.  At such low percentages of metal, the mining companies have developed extremely efficient methods of extraction and smelting.  Native copper is sometimes found at these pits and their dumps; however, the nuggets are quite small (compared to the UP.

 

Exquisite crystals of green dioptase.

Chalcoalumnite on azurite.

Besides the copper exhibits at the Show, several cases displayed a wide variety of other beautiful minerals.  I was especially impressed with: a specimen of dioptase crystals [CuSiO₂(OH)₂] from Namibia (Denver Museum of Nature and Science); chalcoalumite [CuAl₄(SO₄)(OH)₁₂-3H2O] from Bisbee.  (University of Arizona Mineral Museum); aquamarine [Be₃Al₂Si₆O₁₈] and quartz [SiO2] from Pakistan; and of course Gold (Au) from California (both from Mineralogical Association of Dallas—I think).

Large crystals of aquamarine and quartz.

I really enjoy the Denver shows and look forward to seeing some magnificent tourmaline specimens this coming September. Somewhere a hunk of gold like this is just waiting for me to stumble upon it (the only way that I could find such a treasure)!

Native gold.