Dear
List,
I have a rather
interesting problem with my Kriged estimates for a base metal mine. I am
estimating Zn, Pb and Fe, as percentage, the sum of which should total to
no more than 100% total sulphides.
All Zn comes from
sphalerite (ZnS) at 0.671 proportion. Sphalerite SG
= 3.80
All Pb comes from
galena (PbS) at 0.866 proportion. Galena SG = 7.40
All Fe comes from
pyrite (FeS2) at 0.466 proportion. Pyrite SG
= 4.80
Total Sulphides =
(Zn estimate x 1.4903) + (Pb estimate x 1.1547) + (Fe x
2.1459)
What I have
discovered is that I have areas in which the total sulphides are greater than
100% - with very few exceptions, the total is no more than 105% total
sulphide.
My
estimation domains for Zn and Pb are well constrained and
validated, and the variogram models and estimation parameters are
robust, and have been tested and validated to ensure they match the
geological expectations. My domains for Fe are less well constrained but
the variogram models are robust, as are the estimation parameters, and
these also match the geological expectation. So, at the time of the estimation,
there was very little I could do to improve on these. The estimation database
(composited drillhole samples) have upper data value limits (or cut-offs if you
wish to use that terminology) imposed on them such that :
Zn > 40% is never
used to estimate a block
Pb > 10% is never
used to estimate a block
Fe > 46.6% is
never used to estimate a block
Thus, there is no
combination of Zn, Pb and Fe in the estimation database that totals more
than 100% total sulphide
The areas with the
anomalous (erroneous?) total sulphide summation all correlate, without
fail, to areas of thick ore with very dominant pyrite content - there are
individual blocks scattered across the mine that buck this trend. This leads me
to suspect that the Fe estimates may be erroneous, or simply speaking, the Fe
content is being overestimated, hence the total sulphide count exceeds the
theoretical limit.
The only solution
to this problem is modifying the Fe variograms and estimation parameters,
but currently, in my judgment, there is nothing I can modify that would
lead to better variograms or estimation parameters. Of course there may be
blocks where the total sulphide is actually underestimated, but that is
impossible to determine, so the overestimates may balance the underestimates in
which case there is no bias, but that needs to be tested.
Has anyone heard of
similar issues on other base metal mines? In the absence of revisiting the
estimation parameters, is there anything I can do to realistically address this
issue?
Regards,
Colin
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