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Friday, August 26, 2011

Accretionary Wedge #37: Sexy Geology: Bancroft, Ontario

For the first time in this blog, I've decided to join a host of other geobloggers in the geoblog carnival, which is aptly named The Accretionary Wedge.  This time around, Lockwood at Outside the Interzone has called for posts around the topic "Sexy Geology", described as geology that makes you heart race and your pupils dilate.

In thinking about what to contribute many places came to mind, but in the end so many of them are found near a magical place called Bancroft, Ontario.  Bancroft is a small town that is world renowned for its mineral specimens.  It is the "Mineral Capital of Canada", due to the great allure of the area to rockhounds and mineral collectors, who are drawn there not only for the vast number of minerals that can be found but also the quality of specimens in size and shape.  Of the ~4000 known minerals, at least 1600 are known in the Bancroft area, some sources have said the number is even higher.

There are way too many sites around Bancroft to really give a full account; I'll highlight just a few of my very favorites:

1) Egan Chutes Provincial Park at the York River:
This locality includes several stops on both sides of the York River-
The Goulding-Keane Nepheline Pegamtite Quarry (N 45° 04.201’, W 077° 43.947’) is an abandoned small quarry with easily seen large crystals of magmatic nepheline, biotite, calcite, sodalite, and zircon.  The zircons are large enough to be seen in hand specimen!  This quarry is the source of the large dump of crushed rock in the center of town (N 45° 03.432’, W 077° 51.379’), where rockhounds can pick through these rocks and take what they like.
Continuing down the trail from the G-K quarry, you'll reach the main chute, where the river pinches down at a terrific waterfall (N 45° 04.473’, W 077° 44.081’). The nepheline gneisses here also contain diopside, scapolite, hornblende, plagioclase, and at one point in time contained sapphire (but the larger samples have all been removed).
Egan Chute Falls
Heading back, across the York River from the G-K quarry mentioned above lies the York River Skarn (N 45° 04.210, W 077° 43.914’). The unaltered marble can be seen in a roadcut (N 45° 04.066’, W 077° 43.874’) just before the trail to get to the skarn, which is good to visit before seeing the skarn to truly appreciate the changes that took place in these rocks as the nepheline pegmatite intruded (and it has some spectacular sigma clasts). At the skarn, about 50 minerals have been described at this single outcrop, including garnet, diopside, wollastonite, vesuvianite, forsterite, clinohumite, spinel, clintonite (a green, Ca-mica), among many others.

2) MacDonald Mine:
This is an old uranium mine in a granite pegmatite  (N 45° 09.928', W 077° 49.189'), dominated by GIANT K-feldspar & quartz crystals. Some of the K-spar crystals are ~10' long, and some quartz grains are the size of beach balls. Much of the quartz is dark & smokey, due to the presence of the uranium ore mineral, uranian pyrochlore ("ellsworthite"). The pyrochlore grains themselves are typically round & ~4 inches in diameter. And they are HOT. They will send your Geiger counter into awesome mode. Also present are andradite (black Fe+3-rich garnet), sphene, and zircon. As an American, it is really surprising to be able to freely enter a place like this; in the U.S. this mine would be surrounded by 20' tall fences, barbed wire, and very large guards with very large dogs and guns; the size of the KEEP OUT signs would reach epic proportions. But here in Bancroft, you can just park at N 45° 09.846’, W 077° 49.160’ and hike in.




3) Orange Calcite Roadcut: 
Along Monck Road, a cut at the top of a small hill (N 45° 00.169’, W 078° 00.360’) displays some fascinating rocks with orange calcite and large grains of white tremolite.  Several grains of a highly radioactive mineral I have yet to identify are present here, which are easily found with a Geiger counter as they set it on fire.
orange calcite and large white fibers of tremolite


4) Green Mantle Farm Eco-Tour:
If you have the time, and really, you should make it, there is no substitute for taking a tour with Mark Bramham through his property and adjacent Crown land to see some truly fantastic mineral specimens in the wooded areas surrounding his home (N 45° 00.663', W 078° 14.903').  No sample collecting is allowed, but you won't regret going.  Let me just say "fluororichterite stream".  Richterite is an amphibole like tremolite, but substitute 2 Na+ for Ca+2, and the richterite here contains significant fluorine for the OH- group.  There are very few places in the world where fluororichterite is known, so your choices for viewing it are limited.
Large dark green crystals of fluororichterite
You'll also see gorgeous GIANT euhedral hornblende, orthoclase, green apatite, and museum quality red clinohumite.  The tour lasts 2.5-3 hours, and their playful dogs add to the charm of this wooded walk.  Although he's sitting on a mineralogical gold mine, Mark's passion for conservation is inspiring.  Besides the material found on his own property, He has used the mining law to stake a claim on the adjacent Crown land so that he could protect the minerals and keep them in their natural place for future generations to see.
Euhedral hornblende crystals

Euhedral orthoclase crystals

So there you have it, my take on Sexy Geology, with a virtual visit to Bancroft, Ontario.  And this is but a small slice of what is available in this area!