Amethyst from Boulder Creek Mine

“Rock of the Month # 73, posted for July 2007” —

This sample is a particularly intensely purple amethyst,
on which are superposed rosettes of tabular bladed crystals of
green to orangey barite. 16x14x8 cm, 3467 grams.
This sample was mined and prepared by

Boulder Creek Amethyst Mines.

Amethyst is an attractive semi-precious member of the
quartz family, bulk composition SiO₂.
As such it is hard,
and suitable for tumbling and cutting to yield beads
and other shapes for jewellery and lapidary purposes.

The occurrences in the Thunder Bay region are particularly well-known,
and have been described in detail (Vos, 1976; Garland, 1994).
On a world scale, amethyst is widely distributed, with
a majority of specimens originating in Brazil (including some huge
banded geodes) and Ontario.
It occurs also throughout the USA (e.g., White and Cook, 1990),
in several states of Mexico, and elsewhere.

Most amethyst mined in the Superior region is
recovered near the lakeshore northeast of the city of Thunder Bay,
east from Loon to
Black Bay on the north side of the Sibley Peninsula (Elliott, 1982).
Much of the material is collected in the vicinity of
McTavish and MacGregor townships (including
Loon, Pearl and Dorion),
with additional specimens
extracted both closer to Thunder Bay and also further eastward
to Rossport and the Schreiber-Hemlo district.
This “amethyst belt”,
is roughly 40×200 km in extent, with amethyst
in crystal-lined cavities in fault-controlled
veins and breccias (Wilson, 2007).
Amethyst is the official “provincial stone” of Ontario.

The amethyst-bearing silica veins are typically
in fault and breccia zones separating Archean granitoid rocks
(monzonite, granodiorite, etc) from Proterozoic Sibley Group sediments
(Garland, 1992, 1994). Thus the amethyst may enclose
blocks of diverse rock types, such as banded
stromatolite fossils from the Sibley sediments, and
reddened granitoid host rock. In addition to the
attractive barite, accessory minerals may include
small shiny cubes of galena, and small equant crystals
of a pale green sphalerite.
Coarse rhombohedral calcite is also abundant in
some of the cavities.
Stable-isotope and fluid-inclusion studies
suggest that the amethysts formed from basinal brines
expelled from the Sibley Group sediments
at quite low temperatures, <100°c>et al., 1993).
The amethyst encloses fragments of Sibley sediment of estimated
age 1339 Ma, which is thus an upper estimate for the age of the
amethyst. Given the importance of voluminous flood basalt magmatism in
both the southern Brazilian amethyst-mining region and
in the Thunder Bay district, it seems reasonable to suggest
that the amethyst was formed during the
Midcontinent Rift magmatic episode (circa 1100 Ma),
a prolific generator of
hard basalt, diabase and related intrusive rocks.
These igneous rocks dominate the
scenery from Duluth, Minnesota to the Lake Nipigon region northeast
of Thunder Bay, and are also prominent in the landscape of the
Keweenaw copper-mining region of northern Michigan.

The colour of amethyst has long been attributed to substitution
of small amounts of iron for a trace of the
silicon, and to low-level natural irradiation of the silica
(Dennen and Puckett, 1972; Xiong and Kissin, 1993).
The colour of the best amethyst is naturally intense, as demonstrated by the
specimen illustrated here. However, the world of semi-precious and precious
stones is full of ingenious human “enhancements” to the colour, clarity,
and of course cut (shape, size) of a finished gemstone.
Such treatments are generally considered legitimate, provided that
buyer and seller are both aware of the process involved, and
much sub-standard material may thereby be
rendered attractive and suitable for use
in jewellery.
Possible processes of enhancement for amethyst in the Superior region
have been investigated (e.g., Dutrizac, 1995).

References

DENNEN,WH and PUCKETT,AM (1972) On the chemistry and color of amethyst.
Can.Mineral. 11, 448-456.

DUTRIZAC,JE (1995) Comprehensive research to colour enhance Canadian amethyst by heat
treatment and irradiation. In `Summary Report 1994-1995′ (Owsiacki,L, Walters,M, Brown,GH
and Shannon,R editors), Northern Ontario Development Agreement (Ontario Ministry of Natural
Resources / Natural Resources Canada), 27-29.

ELLIOTT,DG (1982) Amethyst from the Thunder Bay region, Ontario. Mineral.Record 13
no.2, 67-70.

GARLAND,MI (1992) Amethyst in northwestern Ontario – 1991. In `Report of Activities
1991, Resident Geologists’, OGS Misc.Pap. 158, 181-194.

GARLAND,MI (1994) Amethyst in the Thunder Bay Area. OGS OFR 5891, 197pp. plus 6
maps.

McARTHUR,JA and KISSIN,SA (1988) Stable isotope, fluid inclusion, and mineralogical
studies relating to the genesis of amethyst, Thunder Bay amethyst mine, Ontario, Canada. GSA
Abs.w.Progs. 20 no.7, 423pp., 40, Annual Meeting, Denver.

McARTHUR,JR, JENNINGS,EA, KISSIN,SA and SHERLOCK,RL (1993) Stable-isotope,
fluid-inclusion, and mineralogical studies relating to the genesis of amethyst, Thunder Bay
amethyst mine, Ontario. Can.J.Earth Sci. 30, 1955-1969.

VOS,MA (1976) Amethyst Deposits of Ontario. ODM Geol.Guidebook 5, 99pp.

WHITE,JS and COOK,RB (1990) Amethyst occurrences of the eastern United States.
Mineral.Record 21 no.3, 203-213.

XIONG,J and KISSIN,SA (1993) Cathodoluminescence studies on amethyst. GSA
Abs.w.Progs. 25 no.6, 489pp., 217, Boston.

WILSON,BS (2007) Colored gemstones from Canada.
In `Geology of Gem Deposits’ (Groat,LA editor), MAC Short Course
37, 270pp., 255-270.