Monday, October 24, 2011

Tremolite-Actinolite Skarn

A few days ago — after recovering from an average long work stint by relaxing, walking, visiting, helping to lay flooring, and running errands while at the lake, then returning to our little house via Costco and a family meal or two in the big city we sometimes drive through — I wrote a note in an irregularly kept journal indicating that I had been dreaming about diopside skarn and actinolite schist. I understand dreaming about the former: skarns have been invading my territory at work recently, and diopside skarn is one of my favorite types of skarn, introduced to me way back when on the west side of the Inyo Mountains, somewhere near Big Pine, CA. I'm not sure why actinolite schist would pop into my dreaming mind; I haven't worked in those kinds of schists much before. Blueschists yeah, greenschists not so much.

I realized, looking through a batch of rocks lying about our little house, brought from here and there, often from mine dumps, sometimes from close by, other times from places as far away as northeastern Idaho or southcentral Alaska, that I have photos of what I was calling actinolite skarn, but which may really be tremolite skarn. I think the difference might only be apparent by either doing petrography or x-ray work, neither of which will be done on these rocks anytime soon.

So, on with the photos!
The tremolite-actinolite in this skarn is a pale yellowish green, a color my camera seems to want to either distort with too much green, too much blue, or not enough color at all.
The skarn is cut by numerous quartz-calcite veinlets containing pyrite, magnetite, and chalcopyrite. The veinlets are up to a half centimeter wide, usually more like a millimeter in width. Tiny.
The gray, vaguely bluish metallic mineral in the central part of the veinlet is magnetite. The brownish to yellowish mass of sulfides is mostly made of pyirte — you can barely make out a few cubic forms. Some of the shiny golden yellow grains are chalcopyrite. It's hard to say what the pyrite:chalcopyrite ratio is, even when looking at this rock in person.
This photo, especially when enlarged, shows the generally felted look of the fine-grained tremolite-actinolite.
Zooming in a bit, we can see the sheaf-like to radiating crystals of tremolite-actinolite, which are hosted in nearly invisible calcite and are possibly intergrown with other very fine-grained skarn minerals such as wollastonite.
A couple of the larger tremolite-actinolite crystals can be seen near the center of the way zoomed in photo above. The largest of these elongate sheaves is probably no more than a millimeter in length. Fine-grained indeed!

Actinolite is the iron-rich end member of the tremolite-actinolite series. I suspect that these very pale green to white crystals are tremolite. In hand sample, I usually just go by color, prefering my tremolite to be white to clear, but it can be light green, light yellow, brown, and even pinkish.

UPDATE: I've indicated the general area of where I most likely found this skarn rock in the Location shown below.

17 comments:

hollis said...

I had to look up "skarn" ... first I've come across it (amateur geologist here). Simplest definition says calcium-bearing silicates (hmmm) often formed at the contact zone between granitic intrusions and carbonate rocks (now that I can relate to). What's your definition?

NIce photos.

Silver Fox said...

Oh, sorry. Basically a skarn is a contact-metamorphosed rock near an intrusive (can be quite a ways away), often with some metasomatism (addition of elements including, hopefully, some ore metals, often from magmatic fluids going out into the wallrock or because of related hydrothermal/mesothermal activity). Skarns are generally coarser grained than their contact-metamorphic cousins, "hornfels." Hornfelses are more often formed from fine-grained aluminosilicate rocks like shales; skarns are more often formed from calcium-magnesium rich rocks like limestones and dolomites.

Skarn Web Page

And, people who work with these rocks for a living argue heatedly at times about what consitutes a skarn v. hornfels v. tactite (which is which, whether skarns v. tactites are mineralized or waste rock, is the grain size significant, does there need to be addition of major and minor elements, either or both). Aarrgh. And I won't get into the skarn-tactite question here!

Calc-silicates are minerals, and they are often the major skarn-forming minerals, but can sometimes form in other environments; these include some garnets, diopside, actinolite, tremolite, wollastonite, and others. Epidote is a calcium-bearing silicate that can form under skarn and non-skarn environments.

I've never thought about writing about them before, could be a whole book! :)

hollis said...

nice ... thanks. So are skarns only found underground? maybe where the intrusion/wallrock relationship still exists? Or is it possible to go somewhere and see some? (my real question :)

Silver Fox said...

Uplift and erosion will bring the originally deeper rocks to the surface. So there are places to see skarns, for example Yerington, NV. I didn't post much in the way of exoskarn photos (skarn outside the intrusive; skarn minerals formed inside, often just inside the intrusive-wallrock contact is sometimes called endoskarn).

And it should be possible to find skarns in the Sierra Nevada, and I've seen them in quite a few places in the Mojave Desert.

hollis said...

thanks for the info, web page. I love to plan my vacation travels (western US) around geology, it lends itself so well to the purpose. From road guides, the Geology Underfoot series, online resources and several classes at the U. I've learned quite a bit and it has been so enjoyable. Anyway, I usually go through the Mojave at least once a year so now I have something else to keep my eye out for while I'm there :)

hollis said...

and ... just finished going through the Yerington posts. Wow! In Accretionary Wedge 38 re "Back to School" I wrote about the value of virtual tours and online resources to amateurs. Your Yerington posts are a fantastic example, thanks for all the photos and explanations.

A good skarn to you said...

No micropetrographic photos available....

Silver Fox said...

Hollis, glad you enjoyed the read. If you drive through Nevada, especially on Highway 50, check my "roadside" tags, and also tags for specific roads like Highway 50, 722, 8A, 93 - and others in other states nearby. Some are in the numbers part of the tags on the sidebar (before the alphabetical tags), others are down under H for "Highway" 50, 36, etc.

And, you can see a skarn at the Linka Mine in central Nevada if you go to Spencer Hot Springs just off Highway 50 and S.R. 367 (old 8A). Lots of garnet.

Silver Fox said...

@A Good Skarn, yeah, no photos through a petrographic scope, just these macros.

hollis said...

I checked out the road tags, cool! I will try to put them to good use. Spencer Hot Springs ... brings back memories. A friend and I camped there one winter night in the late 70s, it was gorgeous looking west over the basin in the morning, everything had a light coating of fresh snow.

thanks again for the info

Coconino said...

One of my favs is from the Sierras, along the Rock Creek Trail in between Bishop and Mammoth. I miss those hikes, and could only wistfully remember them as I drove by the area on my way home this last August.

Silver Fox said...

Skarn in the Sierra? I probably haven't been to the Rock Creek Trail.

Good to hear from you! :)

coconino said...

roof pendant area near Mammoth - nice to read your blog again, even though I'm taking time out that I probably shouldn't!

Silver Fox said...

I've driven by a roof pendant area over Tioga pass, never stopped to look at the non-granitic rocks, however. Always am so focused on the granite when I'm in the Sierra!

George C said...

Great post! Coming from a recently graduated geologist who has little experience in igneous/metamorphic field work, how do you tell actinolite/tremolite from other metamorphic assemblages like hornblende, chlorite, epidote, etc when it's this fine-grained??
Cheers,
-George

George C said...

Great post! Coming from a recently graduated geologist who has little experience in igneous/metamorphic field work, how do you tell actinolite/tremolite from other metamorphic assemblages like hornblende, chlorite, epidote, etc when it's this fine-grained??
Cheers,
-George

Silver Fox said...

Thanks, George! Sorry for the comment moderation; I find it helps with spam, especially on older posts.

I suppose there could be some hornblende present rather than just tremolite and/or actinolite, although none has been reported in the area. Mostly I'm going on crystal shape and very light greenish color in this particular sample as seen in a scope combined with sometimes being able to get the hardness, and on thin-sections and xrd from other rocks of the area. The crystal shape narrows it down to most likely an amphibole. The color makes hornblende unlikely (not enough iron). At this grain size, under a scope one should be able to flake off chlorite xtls. Epidote is more likely to be confused with idocrase (and maybe some uncommon and obscure minerals).

In finer grained samples, one can sometimes use color, when working in a particular area where the color of certain minerals has been seen to be consistent. This method can result in incorrect IDs.

Another very fine-grained green skarn mineral to get confused in this mess is diopside. It's only slightly harder than actinolite, but has a higher density.

I use mindat's locality search to find the area of interest (in this case, here). These known lists can be helpful, though don't rule out minerals not yet found or listed.


And yeah, sometimes you just call it unknown green skarn mineral!