continuity in mineral inventory estimation

1
Prof. Dr. Hassan Z. Harraz
Geology Department, Faculty of Science,
Tanta University
hharraz2006@yahoo.com
Spring 2019
@Hassan Harraz 2019
Continuity in Mineral Inventory Estimation
1

@Hassan Harraz 2019
OUTLINE
 CONTINUITY:
 Geological Continuity
 Value (Grade) Continuity
Primary factors that affect the
estimation of value continuity
 Continuity Domains
 Reserves and Resources

CONTINUITY
Continuity is "the state of being connected or unbroken in space .
. . ". (Oxford Dictionary)
 "Resource/reserve estimation depends first and foremost on a
geological model that provides a sound, confident expectation that
a well defined volume (deposit/domain) is mineralized throughout.
Without this explicit decision regarding geological continuity of a
delimited mineralized zone, neither estimates nor classification of
mineral inventory is possible." (Sinclair and Blackwell, 2000).
 In mineral deposit appraisals, this spatial definition commonly is used in an
ambiguous way to describe both the physical occurrence of geological features that
control mineralization and for grade values. Such dual use of the term continuity
leads to ambiguity.
 To clarify this ambiguity Sinclair and Vallee (1993) define two types of continuity that
bear on the estimation of mineral inventories as defined in Mineral Inventory
Estimates (Table 1).

 Continuity is a topic of international concern in the study of mineral
deposits and the classification of mineral inventories. This
characteristic is an important parameter in several national
resource/reserve classification systems used to describe formally
those parts of a mineral deposit that can be regarded as being well
defined assets of mining and exploration companies. Examples of
such systems are: the United States system (Anon, 1991b) the
Australasian system (Anon, 1991a), and National Instrument 43-101
of the Canadian Security Administrators. These resource/reserve
classification schemes describe the near certainty with which the
best-defined reserve category should be known (by observation and
very limited interpolation) and the decreasing certainty of continuity
in other categories of resources/reserves.
 During the 1980's many gold exploration and producing companies
placed too little attention toward confirming the physical continuity
of mineralization prior to an actual production decision (e.g. Knoll,
1989; Clow, 1991). The resulting errors in estimating metal grades
and ore tonnages contributed to the early closing of several mines
and the abrupt termination of plans for production at others. More
reliable estimates of mineral inventories require better
understanding of continuity as a prelude to detailed mineral deposit
appraisal.
 Two types of continuity are recognized in mineral inventory studies
(Sinclair and Vallee, 1994), geological and value continuity;
definitions are summarized in Table 1.
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Table 1: Two Categories of Continuity in Mineral Inventory
Estimation (after Sinclair and Vallee, 1994)
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Figure 1. A simplistic illustration of the importance of geological continuity (from Rostad, 1986).
Interpretations concerning continuity clearly control the volume of ore (and therefore the
tonnage) as well as the way in which sample grades will be extended. Detailed study of the
geological form and controls of mineralization constrain the geometric model of a deposit which,
in turn, impacts on mine planning. In this case vein intersections in a shear zone are shown
misinterpreted as a simple vein rather than correctly as a series of sigmoidal veins
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i) Geological Continuity
 Geological continuity is the physical or
geometric occurrence of geological
feature(s) that control localization and
disposition of mineralization.
 These controlling features can be either
lithological or structural, primary or
secondary, and can be a complex
interplay of more than one control.
 Superimposed metamorphic, structural
or alteration processes can disrupt (or
enhance) an originally continuous body.
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Figure 2. Qualitative
relationship of
geological continuity as
a function of ore
mineral abundance. The
diagram is useful in
showing the relative
difficulties of obtaining
mineral inventory
estimations in
"average" deposits of
the various classes
shown. The concept of
geological continuity is
illustrated schematically
along the x-axis. (from
King et al, 1982).
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 Geological continuity is a geometric feature and a function of scale; increasing continuity
within a mineralized zone can be imagined (King et al , 1982) in the progression from
widely dispersed mineral grains through larger blebs and semi-massive ore to massive ore
minerals (x-axis in Figure 2). This is a useful if simplistic view because the relative scales
of sample size and the size of mineralized blebs also must be taken into account.
 For example, 10 m blastholes in a porphyry-type deposit are many orders of magnitude
larger than the individual blebs of ore minerals. Thus, physical continuity of mineralized
ground can be viewed in terms of the sample size rather than the ore mineral size; one
can think of the physical continuity of a zone of disseminated mineralization or dispersed
mineral blebs where the spacing between blebs is much less than the sample dimension.
 Geological observations regarding the nature of primary or secondary features are the
input from which the physical continuity of a mineral deposit is interpreted. This
geological information is based on some combination of surface observations, drilling and
underground information which provide the basis for observing and recording the main
features of the mineral concentration of interest (mode of occurrence and spatial
distribution) and the major features controlling mineral distribution: intrusion, volcanic or
sedimentary layer, faults and/or shear zones and/or folds, stockwork, …etc.
 The methods that can be used and their effectiveness depend on the level of information
available and on the geological framework and deposit type present, but have much in
common with techniques of stratigraphic correlation and include theoretical studies,
alteration patterns, chemical profiles across mineralized structures, mineral association
patterns, and so on, all of which also contribute to the development of an 'ore deposit
model'.
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Figure 3. Example of the importance of using geological information to
interpret physical continuity of ore. This idealized example shows the
vertical projection of a vein. Recognition of the local discontinuity in the
vein depends on (a) a knowledge of lithological control on presence or
absence of mineralization, and (b) detailed geological mapping and
interpretation (from Rostad, 1986).
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 Geological information is used to interpret explicitly,
systematically and in three dimensions (Sides, 1992b),
the general geological environment, the general extent
and character of mineralized ground (lithological
variations and limits, structures,)…. etc. Then follow
assumptions (interpolations and extrapolations) about
the presence, extent and limits of a mineralized
structure or mass in relation to the sample control sites
and the known geology (e.g. Figure 3).
 These assumptions are based on an understanding of
continuity that is derived from a geological framework
that is known only within limits. For convenience, deposit
types can be grouped into a few basic categories, for
example, King et al (1982) propose a useful geometric
scheme as follows: massive and/or disseminated,
stratiform (or planar/tabular), vein systems, surficial
(residual), and alluvial (placer) deposits. These
descriptive categories can be further subdivided if
necessary. Direct geological observations and
correlations are supplemented by indirect geophysical
evidence to assist in developing a 3-dimensional image
of the geology in and around a mineral deposit.
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 In many cases a particular geological character persists in much the same manner in all directions within a domain, that is,a
feature is isotropic. However, many geological attributes are directional or anisotropic in nature and differ in their character as
a function of direction in space. Several examples will emphasize the importance of this attribute of anisotropy.
 Consider a zone of sheeted veins: it is evident that the physical continuity of a single vein is more extensive within the plane of a
vein than across the vein. Similarly, it is common that the regular array of sheeted veins has greater physical continuity parallel
to the plane of the vein than across that plane.
 As a second example, consider a syngenetic massive sulphide deposit. Generally, such deposits are physically more extensive
parallel to bedding than across bedding. Similarly, mineralization in shear zones is generally more elongate within the planeof the
shearing rather than across the shear zone. Anisotropy of shapes of mineralized zones is a common product of the processes that
form such zones and reflects underlying anisotropic geological attributes.
 This concept of anisotropy is fundamental in the application of geology to obtaining high quality resource/reserve estimates.
Experience suggests that preferred directions of geological continuity commonly are also preferred directions of grade continuity
as illustrated in Figure 4 for the South Tail zone of Equity Silver mine, central British Columbia.
 It is important to realize that adopting a deposit model introduces implicit assumptions about both geological continuity andvalue
continuity as implied in Figures 2 and 3. For example, in many stratiform deposits the well-established physical and grade
continuity parallel to bedding contrasts markedly with the highly irregular geometric form and "erratic" grade distribution
characteristic of many skarn deposits. These model-related assumptions, built into early resource/reserve estimates, must be
documented explicitly as work progresses. Once deposit delineation has reached a sufficient level of confidence, physical
continuity can be studied effectively through the use of many traditional procedures.
 In tabular deposits, for example, the use of structure contours and isopach maps for evaluating trends and physical disruptions to
trends, are well-established procedures. Similarly, Connelly diagrams, contoured distances from an arbitrary plane that is near
and subparallel to the tabular form, is a useful way of recognizing changes in orientation and disruptions in the tabular body
(Connelly, 1936).
 Contoured maps of such variables as fracture density, vein density and grade (one or more elements) and mineral or metal
zoning maps for successive levels or vertical sections are other useful procedures. They are particularly useful for evaluating
continuity of equidimensional deposits and for comparing spatial distributions of various metals. For example, two metals canbe
deposited simultaneously with the result that they have a similar spatial distribution, or they may be deposited at different
paragenetic stages of deposition in which case there is a possibility that their spatial distributions will be significantly different.
 Geological features that affect physical continuity of a mineralized mass can predate, postdate or be synchronous with the
mineralization process; hence, a detailed geological history is essential to sorting out all possible complexities that mightaffect an
interpretation of continuity. Pre-existing structures can themselves be physically continuous but this does not guarantee the
existence of a continuously mineralized zone. Undetected en echelon structures can cause uncertainty in developing models of
physical and/or grade continuity (e.g. Leitch et al (1991)).
 The effect of superimposed structures, which potentially disrupt already mineralized ground, e.g., faulting or folding, alsomust be
considered. Clearly, a detailed geological evaluation, with particular attention to mineralization control and possible subsequent
disruption, contributes to the understanding of physical continuity of geological bodies and is an essential prelude to mineral
inventory studies.
 Generally, the limiting scale on which we need to define geological continuity is the size of the selective mining unit. In the case
of value continuity the required scale of knowledge is substantially less than the dimensions of the smu. The question of scale
clearly is important for samples used in reserve estimation, if for no other reason than the constraints of possible mining methods
and the implications to ore/metal recovery. Composites that are large relative to the size of original samples (e.g. 3m core
samples vs. 12m composites) have a smoothing effect on original grade values; consequently, a mineral distribution pattern that
is highly irregular based on contouring grades of short samples might appear much more regularly distributed based on much
larger composites.
@Hassan Harraz 2019

Figure 4. Open pit limits, 1310 level, South Tail
zone, Equity silver deposit, central British
Columbia. The dashed line separates the
deposit into two domains, each characterized
largely by stockwork mineralization. In the
northern (smaller) domain the predominant
veins strike roughly easterly; in the southern
domain the predominant vein direction is
parallel to the length of the open pit. These
different directions of strong continuity of veins
is illustrated schematically by the ellipses (the
axes of which are proportional to
semivariogram ranges). After Giroux et al
(1986).
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ii) Value (Grade) Continuity
Value continuity is a measure of the spatial character of metal
grade distribution, mineral abundances or some other value or
quality (or impurity) measure, throughout a specified field (domain)
of interest. Grades, for example, are said to be continuous over
distances where they show a recognizable degree of similarity.
Hence, continuity of grade is linked closely with the concept of
homogeneity of mineralization (Figure 1). Whereas, a geological
attribute is commonly a "present or absent" feature, value
continuity is a question of degree. Mineralization may extend
between control points; the problem is to ascertain how
representative the grades of the control points are, of the
intervening ground.
Generally, the structural and/or lithological zones that localize or
control mineralization (i.e. zones of geological continuity) are the
limits within which value continuity will be defined. It is one thing to
have identified the structure(s) controlling mineralization, but
another one to have reasonable expectation that the structure, or a
particular part of it, is continuously mineralized (and of ore grade)
between control points. Grades normally will be continuous over
much shorter distances than the dimensions of the controlling
geological structure.
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Figure 1. Homogeneity of mineralization versus ore mineral abundance. As used here the term homogeneity
is akin to the concept of grade continuity. Highly homogeneous ores are relatively easy to estimate with
confidence; less homogeneous ores are more difficult to estimate (from King et al , 1982).
Symbols are as follows: E = evaporite; C = coal; Fe = bedded iron ore; P = phosphate; B = bauxite; Pb Zn
= Stratiform lead-zinc; Ni = Nickel; SSn = stratiform tin; PC = porphyry copper; VSn = tin veins; V = gold,
silver veins; U = uranium.
The diagram is highly schematic; exceptions exist.
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Figure 2. Example of a grade profile (5m composite grades) along a drill
hole in an epithermal gold deposit. The drill hole is entirely within
mineralized/altered volcanic rocks and illustrates different physical
continuity for lower grades versus higher grades. A low grade population of
grades is more continuous over greater distance (on average) than a high
grade population.
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 In the past, grade continuity has been examined subjectively by using such traditional techniques as grade profiles (Figure
2) and grade contour maps/sections; both are useful techniques and should form part of the data evaluation stage in
preparation for a mineral inventory study. Grade profiles along linear samples (drill holes, trenches, etc.) are useful because
they illustrate the spatial character of grades of contiguous (or nearly so) and closely spaced samples over short to
intermediate distances.
 Obviously, it is useful where possible, to examine grade profiles for drill holes with different orientations through a deposit.
Contoured values (commonly widely spaced control points) reflect an implicit assumption that the variable is continuous
between control points. Hence, plots of contoured grade values, while instructive, must be viewed critically in the study of
value continuity; there must be geological reason to believe that mineralization is continuous within the contoured area. For
example, grade contours for a bench of a porphyry-type deposit will not necessarily correctly depict that part of a
mineralized field cut locally by barren dykes.
 More recently value continuity has been studied by the use of autocorrelation functions such as semivariograms and
correlograms that quantify a statistical or 'average' continuity in various directions throughout a deposit or a significant
domain within a deposit. In general, these models show an increasing average disparity between samples as the distance
between samples increases. Many such measures level off at a sample spacing referred to as the range (of influence of a
sample). Ranges can be the same in all directions (isotropic continuity) or can vary with direction (anisotropic continuity).
Relative ranges can be used to construct ellipses that demonstrate variations in continuity in different directions and from
place to place in a deposit.
 These quantitative measures of continuity are built on an assumption concerning the physical continuity of a mineralized
body. Commonly this statistical continuity is determined with greatest confidence along the main axis of sampling, e.g.
along drill hole axes. Sampling in the other two dimensions is commonly much more widely-spaced, that is, distance
between sections and distance between drill holes along these sections are much greater than sample spacing along drill
holes. For these less well sampled directions, a conceptual understanding of continuity is very dependent on geological
interpretation.
 A semivariogram, the basic tool of geostatistics, is particularly useful for quanitifying value continuity, independent of its
applications in geostatistics. An example is illustrated in Figure 3. Continuity is commonly represented by an ellipse
showing relative lengths of semivariogram ranges as a function of direction as shown in Figure 4. Geostatistical texts (e.g.
Journel and Huijbregts (1978), Sinclair and Blackwell (2002)) should be referenced for methodology of estimating
semivargiogram models for domains for which resources/reserves are being estimated.
 In Figures 3 and 4 the long axes of the ellipses are parallel to the principal directions of geological and value continuity in
the various domains indicated. All ellipse axes are proportional to the ranges of influence as determined from
autocorrelation functions, ranges of semivariograms in this case, which characterize average value continuity as a function
of direction. The use of autocorrelation functions as a tool with which to characterize, compare and contrast value continuity
quantitatively from one domain to another is evident. Such ellipses are also useful in a relative sense in depicting
changing geological continuity as a function of direction in space.
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Figure 3. Experimental semivariograms (autocorrelation functions) for horizontal and
vertical directions for the east domain of the East Zone, Huckleberry porphyry copper
deposit, central British Columbia (after Postolski, 1998). Note that the ranges
(distances at which the experimental semivariograms level off) differ with direction in
the deposit; that is, value continuity is strongly anisotropic (after Postolski, 1998).
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Figure 4. Main zone, Huckleberry porphyry Cu-Mo deposit, central British Columbia.
The zone is divided into 3 domains (North, Central and South), each characterized by
a particular continuity model illustrated schematically for horizontal directions by an
ellipse. The radii of the ellipses represent ranges of influence for Cu as a function of
direction. A circular pattern indicates isotropic continuity of grades; ellipses indicate
anisotropic continuity of grades (after Postolski and Sinclair, 1999a).
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Primary factors that affect the estimation of value continuity
Primary factors that affect the estimation of value
continuity in a particular geological environment are:
mineral/metal concentrations
mineral distribution patterns and controls at various
scales
Sample sizes (supports) interact with these primary
factors.
In certain cases value continuity is approximately
related to the concentrations of the metals/minerals of
interest, and the geological deposit model. In particular,
the average local variability of grades is directly
proportional to average grade. This generality is
consistent with the concept of "Proportional Effect"
where the absolute value of an autocorrelation function
(e.g. the level of average differences between samples)
varies systematically with mean grade discussed in,
Dilution.
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CONTINUITY DOMAINS
Different parts of a single deposit can be distinctive geologically and, thus, can be
characterized by different models of physical and statistical continuity (Vallee and
Sinclair, 1993). Consequently, for mineral inventory purposes it may be desirable, even
necessary, to subdivide a deposit into separate domains, using as a basis the geological
features that control or characterize mineralization. Even a simple vein can give way over
a short distance to a zone of horsetail veins. Similarly, where conjugate fractures control
mineralization, one fracture direction can predominate in one part of the deposit and the
second fracture direction can predominate elsewhere in the deposit (e.g. Figure 4).
In certain cases a uniform sampling grid size and/or orientation may not be appropriate
for all domains or zones of a deposit. The Kemess South porphyry-type, copper-gold
deposit described by Copeland and Rebagliatti (1993) is characterized by five distinct
continuity domains with differing geological characteristics. These authors strongly
emphasize the importance of geological control in optimizing continuity assumptions for
mineral inventory purposes. Similarly, five distinctive lithological domains at the Golden
Sunlight gold deposit each has its own characteristic autocorrelation model for gold grade
continuity (Sinclair et al , 1983).
In practice, many problems in establishing physical continuity are related to shortcomings
of the geological information base. For example, basic information dealing with the
geological framework and the actual stratigraphy or structure of the rocks hosting a
deposit may be missing or very sparse because only limited drill intersections are
available. In such a case, the geological model, the deposit (geometric) model, the
derived continuity assumptions and the interpreted grade and tonnages are all vulnerable
to large changes as new information is obtained.
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Fig.5:
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 Some of the types of domains that can be anticipated in a
porphyry-type deposit are illustrated in Figure 5 where
distinction is made between leached, supergene and
hypogene zones, zones whose geological (and ore)
character might differ from one rock type to another. In
order to integrate such information into a mineral inventory
estimation it is apparent that the basic geological
characteristics must be mapped spatially and examined in
conjunction with assay information.
 The different character of "ores" within leached, supergene
and hypogene zones is well known; what is not so
commonly taken into account is the very different nature
that any one of these zones might show in passing from
one host rock type to another. Even where the host rock
appears uniform in a porphyry environment, different
domains might result because of different intensities or
directions of predominant structures that control primary
mineralization.
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Reserves and Resources
 Mineral inventory is commonly considered
in terms of resources and reserves.
Definitions at present vary from one
jurisdiction to another although there are
increasing efforts being directed toward
internationally acceptable definitions. In the
absence of such international agreement,
there is an increasing tendency both in
industry and in technical literature for an ad
hoc agreement centering on definitions
incorporated in the "Australasian Code for
Reporting of Identified Mineral Resources
and Ore Reserves" (Anon. (1991a). Thus,
the Australasian terminology is summarized
here (Figures 6a and 6b).
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Figure 6. Examples of two published classification schemes for
resources/reserves.
(a) A proposed classification of the Society of Mining Engineers
(USA)
(b) Classification of the Australasian Institute of Mining and
Metallurgy, in use in Australia for more than two decades.
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 A "resource" is an in situ (i.e. on surface or underground) mineral
occurrence quantified on the basis of geological data and a geological
cutoff grade only.
 The term ore reserve will be used only if a study of technical and
economic criteria and data relating to the "resource" has been
carried out and it will be stated in terms of mineable tonnes or
volume and grade. The public release of information concerning
mineral resources and ore reserves and related estimates must
derive from reports prepared by qualified or "competent" persons.
 Prior to mineral inventory estimation, a variety of exploration
information is available. As exploration continues, the information
base increases and the level of detailed knowledge of a deposit
improves. The estimation of reserves or resources depends upon this
constantly changing data and the continually improving geological
interpretations that derive from the data. Thus, the continuous
progression of exploration information first permits the estimation of
resources and eventually, the estimation of reserves of different
categories. Reserve estimation is thus seen as continually changing
in response to a continually improving data base. An indication of the
wide range of data affecting mineral inventory estimation and
classification is presented in Table 1.
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 On the international scene it is becoming
increasingly common to progress from resources
to reserves by conducting a feasibility study.
A feasibility study of a mineral deposit is "an
evaluation to determine if the profitable mining
of a mineral deposit is.....plausible“ (Kennedy
and Wade, 1972).
The term covers a broad range of project
evaluation procedures yielding detailed insight
into the geological and quantitative data base,
resource/reserve estimation procedures,
production planning, mining and milling
technology, operations management, financing
and environmental and legal concerns.
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Recoverable Reserves
 Recoverable reserves are defined as that mineable volume for
which each block (normally the selection mining unit) is above
a specified cutoff grade.
 Many methods are described in the literature for estimating
recoverable reserves; the problem can be treated locally or
globally.
 The smoothing of block grades (i.e. generating groups of
blocks with very similar grades), endemic in many
estimation procedures using widely spaced exploration data,
is well documented in the literature and has led to the
development of a variety of methods that try to reproduce
local block grade distributions that are realistic.
 These procedures are known collectively as "estimation of
recoverable reserves" (e.g. Journel, 1985). The terminology
is somewhat unfortunate because the methods have been
widely applied to what are now referred to as resources. The
smoothing problem decreases as the data spacing decreases
(i.e. adding infill data) and the block size increases (i.e.
increasing the scale of selection).
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