This article originally appeared in the March
2002 issue of The BACKBENDER'S GAZETTE
the official monthly publication of The Houston Gem & Mineral Society
A Page from a Collector's Note Book:
Hunting for Uranium Minerals in the
Hams-Weeks Pegmatite, Wakefield,
Carroll County, New Hampshire
by ART SMITH
Member of the Houston Gem & Mineral Society
The Hams-Weeks pegmatite -- also known as
the Province Lake quarry or just Hams or
Weeks mine pegmatite or quarry -- is located
about six miles from my summer quarters on
Round Pond in Wakefield, New Hampshire. It
is situated just south of the dirt road that
leads from Route 153 in East Wakefield to
Granite. The location has been a collecting
area for many years, but it is currently
leased by the Maine Mineralogical and Geological
Society. You must be a member or guest to
be able to collect there. Less known and
collected are the shafts on the opposite
side of the road on Mineral Hill. These shafts
produce mostly microminerals.
In June of 2001, Gene Bearss located a small area with a radioactive anomaly in the Hams-Weeks pegmatite quarry. One day after collecting across the road and up the hill on Mineral Hill, we strolled down to the pegmatite and confirmed the location, cleared away the rubble, and did a small amount of preliminary digging in the quarry floor. The digging yielded the tell-tale signs of red-stained albite and the small anhedral pieces of a black mineral we assumed was a mixture of samarskite and columbite. The word is that this material may actually be ishikawaite and columbite. They are fairly close in composition with ishikawaite, being monoclinic and has calcium not in samarskite. Samarskite is orthorhombic and has some titanium not in ishikawaite. Well, whatever it actually is, some large though somewhat crude crystals over 5 inches long of the mineral were found in the past, so maybe we could find some more. We were a little pooped and did not have the proper equipment, including digging bars and pry bars to work in the floor of the abandoned quarry. So we reburied the area with rubble and left.
When I returned to the cottage, I examined the material we collected with a microscope. I had eight small, 1 inch or less, pieces of the gray-to-black metallic masses that had small areas of brown, opaque, somewhat glassy areas. Typical of the columbite with metamict samarskite from the mine. A metamict mineral, in this case the samarskite, is a mineral that had its crystal structure at least partly altered or destroyed by the included radioactive elements uranium or thorium. There were also some minute irregular cavities containing a pale yellow botryoidal uranium mineral. The botryoids are composed of tightly-packed thin crystals. We seemed to be on the right track.
The last commercial operations at the quarry were during World War II for beryl. For a complete history and description of minerals from the quarry, see Smith and Bearss (1991). The Maine Mineralogical and Geological Society has previously done some blasting, most of it at least 5 years ago, in the hope of finding some gem beryl. But as far as I know all of the beryl here, though often a pleasing blue color, is only cabochon-grade material. One mineral new to the locality that they encountered was chalcopyrite in small black-coated masses up to almost an inch across. Additional library research has also confirmed that there were pegmatite mining operations south of the present quarry during the late 1800s, and they moved to the current quarry site after they were flooded out by ground water (Smith in press).
About a week after our initial investigations in July, we were back with the proper tools and renewed vigor. We commenced to dig out the area we buried (after delineating it with the geiger counter) and set to work. When we hit solid rock and started digging in the red-stained feldspar, we initially got a little of the same material as that from the previous dig -- small anhedral fragments of columbite-samarskite. Then nothing, though the red-stained feldspar persisted as did the radioactive anomaly. Eventually we dug-out most of the anomaly, making an irregular shallow pit less than 2 feet deep and about four to five feet across. Further investigation found that the radioactivity in the dug-out material came from thin muscovite seams in the feldspar, all much less than half inch thick. Our initial examination showed nothing else apparent in the seams but the muscovite which was in thin, pale yellow books about half inch or less across and arranged in an irregular jumbled pattern. However, when the muscovite was brushed with the hand and when part of it fell out, some small pale brown-gray subhedral crystals of zircon in the quarter of an inch size-range were revealed.
Some of the zircons are elongated and extend from the feldspar into the muscovite, but others are more equidimensional and seem to be on the feldspar and in the muscovite. The broken zircons are zoned with a darker outer rim around a lighter core. We assume that these are the source of much of the radiation. Looking again at the small black masses that are mostly irregular and anhedral, I noticed one that had flat areas, but they do not appear to be distinct faces. Though this mineral is deep black, much of the rest is a very dark gray. It is a bit more lustrous than much of the typical columbite. I assumed it was due to the reflection from the flat areas, or maybe it was attacked less by the radiation of the samarskite. I sent a specimen of the material off to Excalibur Mineral Company for an EDS, mostly to see if it was ferrocolumbite or manganocolumbite.
The results were an unexpected surprise. It is actually a thorian-rich uraninite. So is was probably this uraninite that was also initially giving us part of the radioactivity anomaly. Re-examining the pieces again, it seems that there are probably three minerals present--the opaque dark brown glassy metamict masses of samarskite, dull metallic very dark gray to black columbite, and the brighter metallic black thorian-rich uraninite. A fourth mineral, now represented by the tiny irregular cavities, could also have once been present or it may just be selected removal of one of the three minerals already mentioned. Obviously to fully understand what these masses represent, additional study and analysis need to be made. However, it gave us an interesting if not rewarding day of digging and an additional discovery by microscopic examination and analysis of the results.
Smith, A.E. in press "Through the 'scope: The Mineral Hill mine, Wakefield, Carroll County, New Hampshire". Rocks & Minerals.
Smith, Arthur and Bearss, Gene 1991 "The Weeks pegmatite quarry, Wakefield, Carroll County, New Hampshire". Rocks & Minerals. 66:129-135.