The Contouring Accuracy of Polymetallic Nodules Ore Fields
in the Interoceanmetal (IOM) Area, East Pacific Ocean
Jacek Mucha and Monika Wasilewska-Błaszczyk
AGH University of Science and Technology, Faculty of Geology, Geophysics and Environment Protection, Department of Economic Geology,
Mining Geology Unit
Kraków, Poland
ABSTRACT
The accuracy of area estimations was evaluated for the ore fields within
the East Pacific polymetallic nodules deposit licenced to the
Interoceanmetal Joint Organization (IOM). The ore fields were
contoured using various cut-off values of nodules abundance. It was
found that accuracies of both the contouring and the area estimations of
high-grade ore fields are low, if based solely on the results of ocean
floor sampling. Comparison was made of ore fields areas estimated
using two parameters: the minimum abundance of nodules and the
minimum abundance of nickel equivalent. An attempt was made to
apply the geostatistical methods to evaluate the accuracy of estimations
of ore fields areas. The difficulties were highlighted of the usage of
geostatistical methods in exploration of polymetallic nodules deposits
due to irregular sampling grids.
KEY WORDS: Nodules; abundance; sampling; geostatistics;
estimation; accuracy.
INTRODUCTION
The resources estimations of polymetallic nodules deposits require the
knowledge of the areas of ore fields for which the nodules abundance
values exceed the presumed cut-off values for particular ore field. The
resources can be estimated using the simple formula:
Q=KF (1)
where:
K – average abundance of nodules calculated with geostatistical or
statistical methods,
F – area of ore field contoured by the boundary value (cut-off) resulted
from economic evaluations.
The resources estimation error depends on the estimation errors of both
values. If the values are independent the relative error of resources
estimation is given by the formula:
2 2
RF RK RQ
(2)
where:
σ
RK
– relative estimation error of nodules average abundance [%],
σ
RF
– relative estimation error of ore field area [%].
The evaluation of estimation accuracy of ore fields areas based upon
the ocean floor sampling is difficult due to complicated pattern of ore-
fields contours and low density of sampling sites. Generally, the fully
credible evaluation is attained as late as during the development of
mining operations.
The standard estimation error of ore field area (geometric error) can be
preliminary prognosed theoretically using the geostatistical method
(Journel, Huijbregts 1978). However, this method requires the regular
(rectangular) or, at least, the pseudo-regular sampling grid, which is
rather difficult to obtain in exploration practice of the ocean-floor
polymetallic nodules. Some opportunities of provisional estimation of
this error are provided by densification of a part of sampling grid in an
ore field. This is the case of a fragment of polymetallic nodules deposit
licensed to the Interoceanmetal Joint Organization (IOM) in the Clarion
– Clipperton fracture zone, East Pacific (Kotliński 2010). The fragment
is named below as the N11 polygon (Fig. 1). In that area two sampling
stages were executed: primary sampling in 2006 and secondary
sampling (grid densification) in 2009. The contours of ore fields were
determined using the ordinary kriging procedure with the Yamamoto
correction for the two cut-off values of wet nodules abundance: 10 and
15 [kg/m
2
]. Depending on demand and prices of metals contained in
nodules, these richer fragments of deposit area can be profitably
exploited.
Additionally, the contours of ore fields were determined with an
alternative criterion based upon the minimum abundance of nickel
equivalent – a parameter which takes advantage of the contents of four
principal metals present in the nodules: Ni, Cu, Co and Mn.
53
Proceedings of the Tenth (2013) ISOPE Ocean Mining and Gas Hydrates Symposium
Szczecin, Poland, September 22-26, 2013
Copyright © 2013 by The International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-92-0; ISSN 1946-0066
www.isope.org