Copper infused agates from the Wolverine #2 Mine in Keweenaw County, Michigan

Wayne Sukow (USA)

Found in situ at the Wolverine #2 Mine in Houghton County, MI is a rare and unique Lake Superior agate. Some of those recovered are infused with varying amounts of massive copper, ranging from flecks visible to the unaided eye to solid stretches of native copper. Some of the most spectacular have several chalcedony fortification bands replaced by native copper. A few of these rare agates also contain metallic silver.

After completing my series of articles on agate formation (see table below), it seems to me that the occurrence of native copper in Lake Superior agates is a post-agate-formation feature that deserves its own comment. Specifically, we are concerned with the massive copper inclusions in small agates from the Wolverine #2 Mine, the Seneca Mine and the St Louis Mine in Houghton County, Michigan. The native copper masses or tendrils are a rich, penny-pink colour.

The size and distribution range from copper flecks, a fraction of a millimetre across with a random distribution, to the replacement of an entire fortification band with copper, to a massive chunk of copper having no structure. All of these are within the confines of a rather small agate – 2.5cm is the largest dimension found. Other unusual features include:

Most, if not all, the iron oxide appears to have been leached from them, which is consistent with an oxidation-reduction process where copper ions are reduced, becoming copper atoms, while iron compounds are solubilised.
Some of these copper infused agates also have calcite and prehnite structures.

I have one of these agates, which has an elongated cubic copper crystal that has been replaced by cupric oxide to give a beautiful transparent red crystal. This is in marked contrast with many other agates found in situ in the basalt mountains of Michigan’s ‘Copper Country’ and in the basalt under the waters of Lake Superior around the Upper Peninsula. Many, if not most, of these agates are characteristically opaque, pastel red, pink and/or brown, suggesting that they are loaded with iron compounds.

In comparison, some of the copper infused agates being mined in the Wolverine #2 dumps have chalcedony bands, which range between transparent to an opaque white or pink and rarely deep-red (the colour of chalcotrichite). In particular, note the following:

The pink and deep red colouring of some bands is due to the disseminated native copper and deep-red chalcotrichite or cupric oxide.
In a few instances, there are tiny blebs (irregular bulges) of native silver associated with the copper.
The native copper in these agates is not composed of copper crystals as are the microscopic native copper aggregates in Lake Superior agates from the glacial tills of Wisconsin and Minnesota.

While these agates not been studied in detail, I suggest they result from corrosive, super-hot copper solutions that flood the agate bearing strata. I also suggest that the solutions are alkaline, perhaps containing copper sulphate, copper carbonate and other copper minerals. Their super-hot nature requires them to originate below the agate-bearing strata and near a magma source.

These solutions erode the agate’s rind and some of the chalcedony. Some of these agates contain not only the massive copper, but also calcite and prehnite, sometimes as bands. Erosion of the chalcedony or other quartz granules is followed by the deposition of copper. Perhaps that deposition is through an oxidation-reduction process involving a very hot copper sulphate solution. Finally, I have to emphasise that the Wolverine #2 Mine copper-agates show a post-agate formation feature and that’s an excellent place to end this discussion of Lake Superior Agate formation. Again, the data is the photographs…

While a few of the Wolverine #2 Mine copper infused agates have small blebs of silver embedded in the copper, an agate from a beach on the north coast of the Keweenaw Peninsula has a rich deep red and brown fortification pattern on one side and a massive sphere of crystallised native silver. What’s left, gold?

At this time, I am not going to offer a theory for the sequence of events that leads to the massive copper in these agates, other than to say that it’s a post-agate-formation phenomenon and involves very hot alkaline solution, which is needed to etch away the chalcedony and euhedral quartz. It can be suggested that the solution may be highly reducing as the iron aggregates are solubilised and the copper ions reduce to their elemental form thereby replacing the iron. However, for those who have read all the articles in this series, hints have been dropped. Of course, since chemical reactions are speeded up by elevated temperatures it’s reasonable to conclude that there were also elevated temperatures, when the copper was replaced by quartz.

To bring this discussion of Wolverine #2 Mine copper replacement agates to a close, note that they appear to be devoid of all red iron oxide colouring, whether in the form of amorphous aggregate or minute flecks. A recent agate theorist will have to reconsider his statement that all Lake Superior agates are coloured by red iron oxide.

I look forward to the next surprise that these unique agates have in store for us. Or should we call them “post-agate” agates? Regardless of what we call them, they are causing us to reconsider our theories of agate formation.

Postscript

Finding and reading the articles by David Lee, Juan Manuel García-Ruiz, Marc Wilson and others on agate band-building mechanisms and processes, band structure, and self-assembly of carbonate minerals and then going to Google to find other original articles was a success. Thanks to the information provided by Google software, particularly the “referred to by ‘n’ others” feature, it was immediately obvious that agate collectors have more reading and studying to do. The extent of the information and number of active researchers was surprising, if not overwhelming.

Clearly, to become something of an expert in the field of these self-organising patterns and structures with an eye toward applying these new concepts to understanding agate pattern formation and plume formation in agates, will be a very time consuming effort. Therefore, I intend to initiate an informed discussion with others in the agate collecting hobby, as new and potentially very important information is and becomes available.

This may guide our thoughts along new avenues about accumulation of silica SiO₄ monomers in the amorphous silica aggregates in the agate-cavity, the importance of the agate’s husk, tiny bubbles as a triggering mechanism to start the local super-saturation process, silica ‘Mobility by Metastatability’, fortification band formation, acicular crystals and crystal casts formation, plume, and “moss formation” in agates in our never ending quest to understand nodular agate formation.

The gallery

Now sit back, relax and enjoy the pictures of beautiful Lake Superior Agates – the real reason why we collect them.

Fig. 1. The red crystal to the left of centre on the top appears to be a gemmy crystal of cuprite. It is a unique feature of this 2.2cm copper infused agate from the Wolverine #2 Mine. The black crystal in the lower centre appears to be tenorite, the other copper oxide.

Fig. 2. At 43mm this is one of the largest, yet well-formed copper agates known. The massive copper banding is striking along with the heavy copper areas at the extreme left and tenorite in the lower right.

Fig. 3. A beautiful pair of 17mm agates, that shows the integrity of the native copper in reproducing the agate banding. It is remarkable.

Fig. 4. Nicknamed “The Arrow” by its field collector, David Harris of Hayward, WI, this copper-agate from Wolverine #2 has two remarkable features – the solid copper replacement in the right-hand portion of the agate and the greenish area to the left of centre. Using a 20X microscope, it was concluded the
green was due to plumes of malachite growing inward from the agate’s husk.

Fig. 5. Striking! In addition to the outer copper ring don’t miss the green and red areas on the right half. You can see the malachite and cupric oxide coating the inner surface of the back side of the agate. Clear calcite overlays them.

Fig. 6. Collected in the early 1980s by John Perona of Dollar Bay, MI from the St Louis Mine, this 5mm copper agate was not cut. What it shows is the outer surface of the agate, just under the husk, which gives the reader a different view of these agates. There are complete fortification surfaces of massive copper.

Fig. 7. The massive native copper intrusion on the left side of this 30mm agate is complemented by deep purple fortification bands. The colouring in these bands is due to densely packed cupric oxide and chalcotrichite. Note the nondescript greyish chalcedony husk on the top and right side. This specimen is from a beach on the north coast of the Keweenaw Peninsula.

Fig. 8. Plum-red pancakes of cupric oxide alternate with cryptocrystalline quartz and embedded native copper flecks. The pancake stack rests on massive native copper and white chalcedony.

Fig. 9. Some copper agates have only a small amount of copper. This 15mm nodule is one of them. To compensate, it does have a new and unusual feature. They are the black florets on the lower left side, which take a silvery polish; perhaps they are the copper mineral chalcocite.

Fig. 10. A matched pair with sharp edged copper replacement in the fortification bands. Their length is 1.8cm.

Fig. 11. Copper and color offer striking contrasts, beginning with the unusually wide copper replacement fortification bands, then the bright green malachite, the velvety smooth banded white chalcedony and, if you look closely, a plum-red mineral (perhaps it’s the copper oxide, chalcotrichite) layered on botryoidal chalcedony. On the upper left and lower right.

Fig. 12. Here is a close up of the plum-red copper mineral layered on botryoidal chalcedony in the upper left of the previous photo.

Fig. 13. Where should you look first? This agate has a marvelous combination of velvety smooth white and pale pink chalcedony, massive native copper, copper replacement of fortification bands, a pleasing green aura complementing some of the copper replacements, and on the lower right a thin arc of the black copper oxide tenorite. It was found at a copper mine dump of the Kearsarge Lode.

Fig. 14. It’s only 1.8cm long, but it’s a gem. Don’t miss the silvery crystal tips on the copper band that’s just left of center.

Fig. 15. It’s an unusual double bubble copper agate joined by a narrow isthmus.

Fig. 16. Here is 13.0 turned over. There’s a 2.5mm sphere of silver imbedded in the chalcedony in a shallow cavity. The collector polished the surface, which appears black above but is silver coloured when viewed with the naked eye. It is locked into the chalcedony of the agate and appears to be a mass of small silver crystals. A second irregular piece of silver is below and to the left of the polished ball of silver.

Other articles in this series:
Tiny bubble theory of Lake Superior agate formation (Part 1): Tiny bubbles, the husk and silica accumulation in the cavity
Tiny bubble theory of Lake Superior agate formation (Part 2): Results from the laboratories of other scientists and inclusions in Lake Superior agates such as crystals and plumes
Tiny bubble theory of Lake Superior agate formation (Part 3): Non-silica minerals and colours in agates
Tiny bubble theory of Lake Superior agate formation (Part 4): Tiny bubbles and reflections on theories of agate formation

Postscript

The gallery

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