Assaying
and
Ore Reserve Estimation
INTRODUCTION
• When a deposit is proved by exploration, it becomes more
or less a commercial proposition.
• The deposit is now viewed in terms of investment, returns
in capitals, daily rate or production.
• In order to take investment decision, knowledge of an
accurate estimation of ore reserves together with their
grade is essential.
• More precise the exploration, more accurate the reserve
estimation will be.
Estimation of the ore reserve comprises the following;
Ø Determination of quantity of the mineral and all its
valuable constituents.
Ø
determination of grade and quality of the mineral.
Ø
Determination of tonnage of ore and average grade
or value per ton.
What is an ore deposit?
An occurrence of minerals or metals in sufficiently high
concentration to be profitable to mine and process
using current technology and under current economic
conditions.
A zone that has concentration of minerals of economic
interest.
What is ore grade?
Ore grade is the concentration of economic mineral or
metal in an ore deposit.
Cut off grade
It is the minimum permissible grade of the ore at which
the value of the recovered ore equals direct operating
cost per ton of ore.
Mineral Resource
§ Mineral Resource is a concentration or occurrence of
natural, solid, inorganic, or fossilized organic material in
or on the Earth’s crust in such form and quantity and of
such a grade or quality that it has reasonable prospects
for economic extraction.
§ The location, quantity, grade, geological characteristics,
and continuity of a mineral r esource are known,
estimated or interpreted from specific geological
evidence and knowledge.
§ Mineral Resources are sub divided in order of
increasing geological confidence into;
§ Inferred
§ Indicated and
§ Measured categories.
Classification of resource
According to IBM (Indian Bureau of Mines), mineral
resources are placed under three categories;
Possible
Probable
Proved
According to USGS, USBM, 1949, mineral resources are
placed under three categories;
inferred
Indicated
Measured
Inferred /Possible Resource
• Part of a mineral resource where the tonnage and
grade can be estimated with a low level of confidence
and certainty.
• It is based on information gathered through
appropriate techniques from location such as
outcrops, trenches, pits, workings and widely spaced
(>100m) drill holes which may be of limited or
uncertain quality and reliability.
Indicated/Probable Resource
• Part of a mineral resource that have been sampled
(from locations such as outcrops, trenches, pits and
drill holes) to a point where an estimate has been
made, at a reasonable level of confidence and
certainty of their contained metal, grade, tonnage,
shape, densities, and the physical characteristics.
• An indicated resource usually serves as a basis for
mine planning.
• In this the behavior of ore body can be determined
with reasonable accuracy on the basis of detailed
geological map.
Measured/Proved Resource
• Estimated based on sufficient data which is not vary
from the actually tonnage and grade at the time of
mining.
• Part of a mineral resource that have undergone
enough further sampling that a 'competent person'
(usually a geologist) declared them to be an
acceptable estimate , at a high degree of confidence
and certainty, of the grade, tonnage, shape, densities,
physical characteristics and mineral content of the
mineral occurrence.
Reserve
Ore Reserves are those portions of Mineral Resources
that, after the application of all Modifying Factors,
result in an estimated tonnage and grade
Ore Reserves are sub- divided in order of increasing
confidence into
Probable Ore Reserves and
Proved Ore Reserves
Probable Reserve
A probable ore reserve is the part of indicated
resources that can be mined economically.
It includes factors like dilution, recovery, and
commodity price. It is of sufficient quality to serve as
the basis for decision on the development of deposit.
Proved Reserve
A proven ore reserve is the part of measured resources
that can be mined in an economically viable fashion.
It includes diluting materials and allowances for recovery
and the commodity price. A proven ore reserve
represents the highest confidence category of reserve
estimate.
Reserve Estimation Methods
Depending upon the nature of ore bodies. The area of a
block can be determined by various methods.
The methods are broadly paced under two categories
1. Polygonal method
2. Triangular method
Polygonal method
• This method can be applied to a continuous tabular ore body with an
irregular shape and variable grade or very regular ore body where no
correlation can be made from bore hole to bore hole.
• Polygons are constructed by drawing perpendicular to the mid points
on the lines joining each of the pits or bore hole with the surrounding
pit or borehole.
• The area of influence of each polygon or pit is half the distance to all
surrounding boreholes or pits. Polygon are so constructed that each
grid or borehole point occupies the central part of the polygon.
• The volume and tonnage of the deposit is then determined using
simple geometrical equations.
Polygonal Method
Bore hole/Pit
The area of a regular polygon is given by the formula below.
Area = (½)(apothem)(perimeter)
Triangular Method
The method used when sampling points pits are not in grid
pattern. The area is divided into numbers of triangles. In this
method lines are drawn connecting adjacent boreholes on a plan
to form mosaic of triangular blocks. The average thickness and
grad of each block is determined from the values of the bore
holes at the corners of each triangle.
S= a+b+c
2
Area =
Where
a, b, c are sides of triangle
S = Perimeter of triangle
S(S-a)(S-b)S(S-c)
Average Thickness =
Average Grade =
The plan area of triangle is multiplied by its average thickness and
then with specific gravity to give tonnage of the triangular sub
block. By adding the tonnage of all sub blocks the total tonnage
of the ore can be obtained
Volume = Area x Average thickness
Tonnage = Volume x Specific gravity