Thanks Keiko
I guess one never really know what the black stuff is as many of us have
seen similar things. One mineral that I have seen that commonly occurs as a
black fracture and shear surface coating is chlorite. Interestingly, in the
Wallaby deposit in the Yilgarn of Western Australia, we had a black mineral
that we couldn't identify initially and which got called 'black shit'.
However, we decided that a) the name wasn't really suitable and b) we
should identify it. It turned out to be a mixture of magnetite and
chlorite. The implications and genesis of this alteration(?) are still not
entirely clear.
Cheers
Brett
-----Original Message-----
From: Keiko Hattori [SMTP:[log in to unmask]]
Sent: Friday, 24 November 2000 23:50
To: [log in to unmask]
Subject:
Hi,
I am not answering your question, but I thought that the information
from Kirkland Lake may be of your interest.
Gold veins in the Kirkland gold camp, a major gold camp in Abitibi belt,
commonly contain "graphite seams", black greasy material. People called
"graphitic seam". The gold deposits are hosted by volcanic-intrusive
rocks of ~ 2680 Ma that have undergone sub-greenschist facies
metamorphism, essentially not metamorphosed. There is no graphite shale
in the vicinity of the camp
More than 15 years ago when students were doing projects, I thought that
I should properly identify the material. I was skeptical whether it is
really graphite. We tried to identify the material using XRD, but it was
difficult to obtain sufficient amount because the material occurs as a
film along fractures and within recrystallized quartz. We could not
scrape the black film very easily. Anyhow, we did not get the peaks of
graphite from chunky black material. One black silvery material was
found to be a mixture of fine-grained molybdenite and pyrite. We tried
to do electron probe analysis of the black seam, but it was again
difficult because the seam is along fractures, we could not get polished
surface. But we put a beam on the area and got no peaks-- could be
carbon (carbon cannot be detected).
I should have done more, but identifying minor black seam was not our
main interest and we did not do anything further.
Keiko Hattori
This may be a naive question, but it stems from a paper I was just
reading
on a gold deposit in the Northern Territory of Australia. The deposit,
Gold
Creek, has been interpreted as having a phase of graphite alteration. My
question is: How common is graphite alteration? I don't know of the
metamorphic grade of the rocks, but since it is in the Pine Creek Inlier
I
don't think it is that high. I was under the impression that graphite
alteration was a high-grade phenomena. Most graphite I've seen in gold
deposits occurs as a residual concentration due to dissolution of more
soluble elements and because we were originally dealing with
carbon-bearing
sediments eg the argillaceous rocks of the Hodgkinson Province in north
Queensland.
Thanks in advance for any info.
Cheers
Brett
Dr Brett Davis
Group Structural Geologist
Delta Gold Ltd
PO Box 152
Kalgoorlie
Western Australia 6430
Australia
Ph 61-8-9021 7622
Fax 61-8-9021 3200
Men are from earth. Women are from earth. Deal with it.
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Keiko H. Hattori
Professor, Department of Earth Sciences
University of Ottawa,
140 Louis Pasteur
Ottawa, K1N 6N5, Canada
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