The Tar Heel Club ventured forth Saturday to northern Guilford County
to check out the Vulcan Minerals Stokesdale Quarry.
This quarry is located so far north in Guilford Co. you can
almost throw a rock into Rockingham Co.
We had never been to this site before so weren't sure what we
would find. Here are a few pics of our first look at it.

There was good mineralization all thru the nice gneissic host rock.
We were seeing garnet, schorl tourmaline, muscovite, calcite,
epidote, and chalcopyrite in many areas. Most of it was in
small seams that were now exposed on the faces of the
blasted boulders and it was very hard to extract any in one
display quality piece. Very few, if any, vugs were found in
the bottom levels of the quarry. I did see a few nice dime
size garnets and a couple schorl sprays found.
Here are a couple pics of the host rock.

As is usually usual for these fieldtrips, just when you start
to figure out what and where to look, it's about time to go.
We started with the bottom two levels and about noon,
an hour before time to leave, we decided to check out one of
the upper levels on the way out.
That is where we started to find tiny seams that opened up
enough to allow for the growth of some decent collectible
looking mineral specimens and the zeolites that we were
expecting to find at this site. I don't have any good pics
of them yet, and as I said, as soon as we started finding
them, it was time to go. But here are a couple pics of some
pretty dark green olivine crystals, light green prehnite,
with some of the small vugs containing minor stilbite and
possibly apophyllite.
So if your club goes to this quarry, I would suggest you
start at the top levels and work your way down.
This quarry is going to be very active for the next few years
as NCDOT is making a 4-lane out of Hwy 220 north of Greensboro
which runs right by this site.

Great report Joe.  It figures that you start finding the good stuff when its time to go.  The good news is that the next time a trip is planned there, we'll all know where to start.  Thanks for the great report.

No doubt it is epidote, it is certainly not olivine.

Joe, I agree with Laurie that the green is epidote, but a possible green amphibole would be actinolite. 

Olivine occurs in ultramafic rock like the dunite area at Buck Creek (Chunky Gal) in Clay Co. and the ring d**e near Webster in Jackson Co.  The mafic rock I see in your photos is similar to the amphibolite/hornblende gneiss commonly found in Ashe and Mitchell counties.

I always wondered where the term mafic came from.  It's a loose acronym for Magnesium, Aluminum, Fluorine, Iron, and Calcium.  I'm not sure about the fluorine, but I am the other elements.

Dennis

That material looks very similar to the material from the Thomasville quarry.

Thanks for the correction, Joe.  Several years ago a botanist friend told me what the word mafic meant, but I took him at his word and never really researched it.  Here is a nice discussion of the various types of magmas from the Tulane University website:

Chemical composition of magma is controlled by the abundance of elements in the Earth. Si, Al, Fe, Ca, Mg, K, Na, H, and O make up 99.9%. Since oxygen is so abundant, chemical analyses are usually given in terms of oxides. SiO2 is the most abundant oxide.

1. Mafic or Basaltic--  SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na.

2. Intermediate or Andesitic--  SiO2 55-65 wt%, intermediate in Fe, Mg, Ca, Na, K.

3. Felsic or Rhyolitic--  SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na.

My experience with mafic rocks has been mostly in Ashe Co. where the mountains around Jefferson are referred to as the Black Mountains due to the dark rock that is exposed on the many rocky bluffs and outcrops.  The fairly recently described ecoregions for North Carolina call these mountains the Amphibolite Mountains.  The gneissic rock that I have examined contain, in varying percentages, quartz, biotite, hornblende (or other pyroxene like actinolite), epidote, feldspar, muscovite,  calcite, and other minerals.  The soils are higher in pH than other regions and support plants that prefer the rich soil types.

I love the bands creating by the folding the rock underwent during the slamming of Africa into North Carolina!

Dennis