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by Bruce Ryan
Newfoundland Department of Mines and Energy, Geological Survey
Branch, 1993
The rocky landscape of Labrador conceals many
treasures that are only recognizable to the trained or searching eye
or those with a need to be filled. For instance, to most of us the
flinty grey rock of northern Labrador that is known as Ramah chert
is little different from any other rock. Yet, to the prehistoric
inhabitants of Labrador in search of a superb stone from which to
fashion the cutting tools they needed to make survival a little more
probable in a hostile environment, the Ramah chert was an ideal
material. In 1770 a Moravian missionary in Nain was given a piece of
brightly coloured local rock, and became the first to appreciate the
potential of "labradorite" as a decorative stone of wide appeal. The
heavy red rocks of western Labrador were trodden over for generations
before they were recognized as a world-class iron ore deposit by
geologists in the late 1930's. Now Labrador is being viewed as a
potential source of other "rocks" -- diamonds, the mainstay of the
jewellery trade.
Diamonds have attracted humans for millennia.
They are hard, brilliant, dazzling gems that adorn fingers and
crowns, and evoke much emotion in their owners or would-be owners.
They have been the cause of much grief inflicted on the hearts and
pocketbooks of people and nations, as anyone who has read the story
of the famous Koh-i-noor diamond will tell you. Many diamonds come
from the ground as unattractive glassy knobs, but when crafted to
perfection in the hands of the expert diamond cutter, they hold even
the most indifferent person spellbound. All this from a mineral
composed of the element carbon, the very same element that makes up
the lead in your pencil and the graphite lubricant on your door
hinges.
Diamonds form only under exceptional conditions of
pressure and temperature deep under the surface, and they arrive at
the surface through very fortuitous circumstances. Hence they are
rare and are found only in certain parts of the Earth's crust. In
particular, the following facts are relevant to diamond discoveries.
The regions of the Earth where diamonds occur, such as South Africa,
are all crustal areas that are over two billion years old. The
diamonds grew in these old crustal regions because these rocks remain
relatively cool (900 - 1200EC) to great depths (150 - 200 km) below
the surface in comparison to nearby regions. The diamonds are plucked
from their deep crustal sites of formation and rapidly transported to
the surface by special types of magma rising from the liquid mantle;
these magmas give rise to small volcanic cones and to vertical
carrot-shaped zones of rock known as pipes. The volcanic and pipe
rocks are called kimberlite, after the diamond-rich district of
Kimberly in south Africa where they were first described. One of the
best ways to picture the process of diamond transport from 200 km up
to the surface is to think of diamonds as the "passengers" who jump
aboard the kimberlite "elevator" as it goes by.
How does
Labrador fit into the picture of a diamond region? Firstly, it is
made up of old crust, much of it older than two billion years.
Secondly, although only a few kimberlitic rocks are known from
Labrador, specifically in the region of Saglek Bay, other rocks are
present that are indicative of probably kimberlite occurrences. The
largest concentration of these can be found near Makkovik. So, even
thought diamonds are not yet known in Labrador, this may be a
reflection of the absence of the trained eye - you see what you look
for! Perhaps in the not too distant future, Labrador may be able to
claim the honour of having the province's first real "jewel in the
ground"!
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