Note: Descriptions are shown in the official language in which they were submitted.
CA 02827753 2013-09-12
GEOLOGICAL SAMPLE ANALYSIS USING SIZE FRACTION SEPARATION
BACKGROUND OF THE INVENTION
[0001] Mining for desired minerals, such as precious metals, requires an
efficient
means for evaluating a potential mine site, a process known as prospecting.
Although
different methods of prospecting exist, one of the traditional techniques
makes use of a
drill core analysis, that is, the drilling and extraction of a cylindrical
section of rock from
varying locations at a potential mine site. Ore in different samples may
differ
significantly in their mineral composition based on the original formation
process within
which the ore developed. The composition of a core sample, along with the
location
from which the sample was extracted, can be indicative of a primary ore zone,
a
surrounding alteration region and a likely concentration of a particular
mineral.
[0002] A variety of techniques exist for predicting the abundance of a
particular
mineral from a core sample. Most of these techniques are well-known in the
field of
prospecting, and are derived from experience and various theories on the
underlying
geological processes involved in the formation of ores. For example, the
mining of
precious metals typically involves first localizing an ore zone where the
highest
abundance of the material sought is likely to be found. A geologist will then
attempt to
determine the extent of the alteration surrounding the ore zone in which
lower, yet
important, concentrations of the mineral are located. Typically, vectorizing
of the ore
zone and the alteration are done by visual examination of core samples that
pass
laterally through the area. A more precise determination of the concentration
of a
desired mineral in a particular sample is obtained through a lab analysis that
includes
crushing of the stone and performing a variety of tests on it. While the use
of such
techniques by an experienced geologist can greatly improve the process of
predicting
mineral abundance at a particular site, there remains a great deal of
uncertainty.
Successful exploration frequently involves a combination of sound geological
principles,
extensive sampling and analysis, and a certain amount of luck.
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SUMMARY OF THE INVENTION
[0003] In accordance with an aspect of the invention, there is provided a
method
of determining a relative concentration of one or more minerals of interest
present in
a sample from a region of expected mineralization, the method comprising:
[0004] separating the sample into a plurality of size fractions,
including a first size
fraction having a relatively small average particle diameter and a second size
fraction having a relatively large average particle diameter; and
[0005] performing, on one of the size fractions, a content analysis that
includes
an optical spectral analysis so as to identify constituents thereof that are
indicative of
the presence of said one or more desired minerals.
[0006] In accordance with one embodiment, the method provides for
estimating
the quantity of one or more desired minerals present in a geological region of
expected
mineralization. Using a drill core sample extracted from the mineralization
region, a
portion of the sample is disaggregated into a plurality of size fractions,
including a first
size fraction having a relatively small average particle diameter and a second
size
fraction having a relatively large average particle diameter. In an exemplary
embodiment, the small size fraction includes material having an average
particle
diameter of less than 2 pm, while the large size fraction includes material
having an
average particle diameter of 40 pm to 100 pm. The separation of the size
fractions may
make use of one or more sieves to separate particles of varying size. Once the
size
fractions are separated, a content analysis, including an optical spectral
analysis, is
performed on one of the size fractions to identify constituents thereof that
are indicative
of the presence of one or more desired minerals. The use of the isolated size
fractions
provides a particularly high accuracy in identifying the sought-after
materials.
[0007] A third size fraction of interest may also be isolated, having an
average
particle diameter of 2 ¨ 20 pm. Analysis of this size fraction may make use of
a method
for separating the size fraction components by relative density. To accomplish
this, the
sample material may be crushed and mixed with a liquid and allowed to separate
by
precipitation. In this variation of the invention, the third size fraction
represents part of a
hydrothermal fraction of the sample, and denser components thereof are
particularly
indicative of the presence of the mineral of interest.
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[0008] In one embodiment, the optical spectral analysis of an isolated
size
fraction is performed with a hyperspectral imager. This analysis may include a
determination of the content of a mineral of interest within a sample, as well
as
molecular site occupancy of the transition metals in the sample so as to
estimate its
relative extractability. The content analysis may also include either or both
of a
geochemical analysis and a mineralogical analysis. The geochemical analysis
may
include any of a variety of techniques, such as inductively coupled plasma
emission
spectrometry. The mineralogical analysis may also include any of a variety of
techniques, such as transmission electron microscopy, scanning electron
microscopy or
x-ray diffraction.
[0009] Additional steps may also be performed prior to the separation of
the size
fractions. For example, in a first step, an initial visual examination of the
sample may be
performed so as to correlate it to an area from which it was extracted and to
determine
the location of an ore zone. An analysis may also be performed prior to the
size fraction
separation, this analysis including a determination of the mineralogical
chemistry of the
sample and the crystal shape.
[0010] In one embodiment of the invention, the analysis of an isolated
size
fraction includes crushing the size fraction and mixing it with a liquid. The
liquid may be
used to separate materials in the size fraction by precipitation. In
particular, a portion of
the size fraction may precipitate out more quickly based on its grain size
and/or its
relative density, and this portion may be removed and subsequently analyzed
using
optical spectral analysis. An optical spectral analysis may also be used to
identify the
presence of molecules of interest in the lixivia, which is a solution obtained
by leaching,
and may make use of wavelength signature analysis in both the visible and the
infrared
range.
[0011] In one embodiment, determining a relative concentration of one or
more
minerals of interest present in a sample from a region of expected
mineralization makes
use of optical spectral analysis after a series of sample preparation steps. A
drill core
sample is extracted from the geological region of interest and a portion of
the sample is
crushed to a relatively small granularity. The crushed sample portion is then
prepared
by mixing it with a liquid and placing it in a sample holder. The surface of
the prepared
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sample material is then smoothed with a straight edge so as to improve the
relative
alignment of its crystals. Once the sample is so prepared, it is subjected to
the optical
reflectance spectral analysis. Preparation of the sample in this manner
greatly reduces
the background optical noise during the optical analysis. This technique may
be
performed after separation of size fractions of the sample. A size fraction of
interest is
isolated before being prepared for analysis as described above.
[0012] Features of the present invention will be better understood upon a
reading
of embodiments thereof with reference to the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Figure 1 is a schematic view of an optical spectral imaging system
that
may be used in an analysis of geological drill core samples.
[0014] Figure 2 is a graphical view of an optical spectrum of a
geological sample
such as those analyzed using embodiments of the present invention.
[0015] Figure 3 is a schematic view of a cross section of a drill core
sample
showing a relatively large size fraction and a relatively small size fraction.
[0016] Figure 4 is a schematic/graphical view showing the different
mineralization
regions in a geological sample and the corresponding constituents of each.
[0017] Figure 5 is a schematic view of a sample tray housing crushed
geological
sample material, some of which has been prepared for spectral analysis.
[0018] Figure 6 is a flow diagram indicating the general steps involved
in an
analysis of a geological sample according to one embodiment.
[0019] Figure 6A is a flow diagram indicating the general steps involved
in a
process of separating different size fractions in a geological sample.
DETAILED DESCRIPTION
[0020] In a first embodiment of the present invention, a series of drill
cores are
examined for a variety of features. The drill cores are extracted in a
conventional
manner, the path of the drill intersecting the expected area of mineralization
in a section
at multiple depths along its extension. If the drilling areas are well-chosen,
examination
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of the drill cores allow for an estimation of the ore zone, as well as the
size and shape of
the surrounding alteration. Conventional analysis relies on visual inspection
of the drill
core along with possible assays of important sections. However, in the
embodiments
described herein, these techniques are greatly enhanced through the use of
spectral
analysis of the drill core. The following example refers to a determination of
the
occurrence of gold at a particular site, but those skilled in the art will
understand that the
techniques described apply to a wide variety of different minerals.
[0021] In the context of the present description, spectral analysis is
performed
using hyperspectral imaging of the drill core samples. This type of optical
spectral
analysis provides much more information than is gained from a simple visual
inspection,
and is much faster and less expensive than an assay. In the one embodiment,
multiple
sections of drill core are imaged simultaneously with the imager and the
resulting
spectra analyzed to determine the relative presence of one or more minerals of
interest.
This is shown schematically in Figure 1, in which a box 10 of drill cores 12
is imaged
spectrally by a hyperspectral imaging apparatus 14. Those skilled in the art
will
understand that Figure 1 is for explanatory purposes only, and is not intended
to
represent the details of the imaging system.
[0022] The steps of analysis of the drill core may include those followed
in
conventional analyses, but the additional information provided by the spectral
data can
provide a previously unavailable level of detail, and enables the use of new
techniques
for estimating both the ore region and the scope and content of the
alteration. Some of
these techniques are discussed in more detail below.
Identification of Proximal and Distal Alterations using Transitional Metal
Variations
[0023] It is known in gold prospecting to perform a visual inspection of
core
samples in the region of an alteration to identify variations in
phyllosilicates that may be
indicative of the presence of gold. The spectral data collected using imaging
apparatus
14 can be used to identify redox (reduction-oxidation) fronts in mineral
phases that
contain traces of transitional metals. Many of these indicators are difficult
or impossible
to detect by visual inspection, and the higher precision of the spectral
analysis allows for
a much more accurate determination of the alteration region. Examples of the
iron
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compounds of interest that are detected using spectral imaging include, but
are not
limited to, pyrite, pyrrhotite, magnetite, hematite, siderite and chlorite.
The location and
concentration of these and other minerals is used to characterize an
alteration halo.
[0024] In the present embodiment, a particular library of spectra is used
with the
imaging apparatus to focus the detection on the specific assemblages of
minerals of
interest. This allows the system to identify and localize these materials
within the core
sample. As part of this method, the sample may be separated into multiple
"size
fractions" before being subjected to spectral analysis. This process is
described in
more detail below.
Size Fraction Analysis
[0025] Minerals commonly differ in particle size and density depending on
whether they are rock-forming, ore-forming or alteration minerals. This is of
particular
interest in the "economic ore" region of a potential mine site, as well as in
the "sub-
economic alteration." The chemistry of hydrothermal regions of such a zone may
be
indicative of different geological processes that led to the ore-forming
process. The
differentiation between materials based on average particle size and relative
particle
density is referred to as a "size fraction" analysis and is used herein as
part of a
prospecting technique. Separation of the size fractions, in combination with
other
techniques, allows for a more precise determination of the sample
characteristics and,
as such, a more accurate evaluation of the occurrence of minerals of interest
in the
sample material.
[0026] The separation of size fractions is based primarily on particle
size. This
may involve disaggregation of a sample and subsequent sieving, but separation
may
also make use of separation by density using, for example, precipitation in a
liquid
medium. In such a case, sedimentation will be affected by not only particle
size but by
density as well. As such, smaller particles may settle faster than larger ones
if they
have a higher relative density. For example, particles that contain an
abundance of a
heavy metal, such as gold (19 g/cm3), will settle faster than similarly-sized
particles
having a lower overall density.
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[0027] Figure 2 is a graph that shows the spectral response of several
components of a mineral sample. In the figure, the spectrum for gold is shown
as a
solid line. The spectrum for a lixivia containing a small size fraction (i.e.,
one having
particles with a diameter of less than 2 pm), which is obtained by leaching of
the
disaggregated sample material, is shown as a dashed line. Finally, the
spectrum for a
larger size fraction, having particles with a diameter of between 2 and 20 pm,
is shown
as a dotted line. As can be seen from these curves, the spectral response for
the lixivia
portion of the sample is much closer to that of gold than the larger size
fraction,
indicating the advantage of performing the size fraction separation prior to
performing
the spectral analysis. The other size fractions reveal different signals,
demonstrating
that they are indicative of different assemblages of minerals. In practice,
the
comparison of the spectra may be done using spectral similarity metrics and
algorithms,
such as Euclidean Distance ( ED), Normalized Euclidean Distance (NED),
Spectral
Correlation Mapping (SCM), Spectral Angle Mapping (SAM), Spectral Information
Divergence, or any combination of these techniques.
[0028] As part of the sampling process, both the whole rock and different
size
fractions thereof may be examined. Figure 3 is a schematic depiction of a
cross section
of a core sample, showing different size fractions therein. Those skilled in
the art will
understand that the shapes of the particles shown in the figure are schematic,
and are
not intended to represent the actual particle shapes. In one example, a large
size
fraction 30 has average particle diameters of 40-100 pm, which typically
consists of a
mix of metamorphic, igneous and hydrothermal components. A medium size
fraction 33
has average particle diameters of 2-20 pm, and consists primarily of
hydrothermal
components that include high-density ore-forming minerals. A small size
fraction 32,
having average particle diameters less than 2 pm, consists almost entirely of
lighter
hydrothermal components. In an exemplary embodiment, particles in the size
range of
20-40 pm are not used in the analysis.
[0029] The small and medium size fractions can be of great interest for
gold
prospecting because hydrothermal-based formation is the most likely area to
contain
gold deposits. Moreover, depending on the geological chronology of events
having
affected the sample material, there may be a specific composition of different
minerals
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that is highly predictive of the presence of a significant concentration of
gold. The
exclusively hydrothermal fractions are therefore subjected to any or all of
inductively
coupled plasma (ICP) emission spectrometry, scanning electron microscopy,
energy
dispersive x-ray spectroscopy (EDX) and x-ray diffraction (XRD) analysis (24).
[0030] Isolation of the different size fractions of a sample may be done
in a
number of different ways. In the present embodiment, a multiple step process
is used.
First, a portion of the core sample is disaggregated using a tool such as a
rotary mortar
and pestle, which shears apart the sample to separate the smaller fraction
components
without crushing the larger ones. Following the disaggregation of the sample,
a sieve is
used to remove all particles larger than 100 pm, and the rest of the sample is
mixed with
an appropriate liquid, such as distilled water. Over time, the larger and
heavier particles
settle to the bottom of the liquid, while the small size fraction components
remain in
suspension.
[0031] Once the larger/heavier size fractions have settled, the
precipitate in the
chamber is removed and dried, after which it is passed through a sieve having
a hole
size of approximately 40 pm, thus separating the large size fraction from the
medium
size fraction. The small size fraction components that remain in suspension
are also
extracted from the chamber for subsequent analysis. Moreover, as described in
more
detail below, surfactants that float to the surface of the chamber liquid are
also isolated
and analyzed using spectral analysis.
[0032] Once the different size fractions have been separated, each may be
independently analyzed. The small size fraction may be examined using non-
optical
techniques such as a geochemical analysis (using, for example, inductively
coupled
plasma (ICP) emission spectrometry) and a mineralogical analysis (using, for
example,
scanning electron microscopy (SEM) or transmission electron microscopy (TEM)
and x-
ray diffraction). The sample may also be subjected to optical spectral
analysis using a
hyperspectral imaging apparatus as described above in conjunction with Figure
1.
[0033] The medium size fraction can be of significant interest because it
often
consists of small molecules containing heavy metals, the density of which
caused these
medium sized particles to settle out with the larger size fraction. As such,
this fraction
may be very rich in the target mineral of interest (e.g., gold, copper, zinc
or nickel). In
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contrast, the small size fraction consists of primarily phyllosilicates, while
the large size
fraction tends to be one of several lithologies typical of the indigenous
rock.
[0034] An example of how size fraction analysis may be beneficial in
mineral
prospecting may be better understood when considering a particular geological
system.
In an area of rock containing a significant abundance of phyllosilicates, the
smaller and
medium size fractions typically reside in a hydrothermal region of the area of
mineralization. A fault at this region of mineralization represents a pathway
that would
have been pervaded by superheated water, which induces chemical and/or
physical
mineralogical changes. Within these fractions are also deposited
phyllosilicates such as
illite, which may be subjected to age analysis. In accordance with one
embodiment,
such age analysis may be used with empirical data to make a prediction of the
relevant
minerals in a certain region of the fault. Using size fraction analysis
combined with illite
dating, the relevant samples may be categorized according to age, such as a
"late"
mineralization event and an "early" mineralization event.
[0035] Shown in Figure 4 is an example of an early and a late
mineralization
event recorded in samples taken from the same potential mine site. Although
these
samples came from the same general region, the mineral composition of each is
quite
different, demonstrating that the mineralogical changes produced by the
superheated
water were quite different from one geological event to another. The cross-
sectional
image of the rock structure presented in the figure indicates the different
regions of
respective mineralization, that is, the early mineralization (shown at 46) and
the late
mineralization (shown at 48). Figure 4 further shows the breakdown of the
primary
components of each sample. Both show a relatively high quantity of albite, but
the
remaining constituents vary more widely in percentage. While the early
mineralization
sample contains 13% quartz, the late mineralization sample contains 40%
quartz. The
level of iron-rich chlorite (Chi-fe) is 30% in the early mineralization
sample, but much
lower (1%) in the late mineralization sample. In addition to 2% of smectite in
the early
mineralization sample, there is 10% of illite-smectite. In contrast, the late
mineralization
sample contains only 1% illite.
Crystal Sample Preparation
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[0036] Spectral analysis of certain sections of the drill core may also
benefit from
some preparation of the sample. While an initial analysis of the drill core
may be
performed directly on a box of core samples (as shown in Figure 1), detection
of an
optical signal from some of the crystalline material (particularly the small
fraction
material in the alteration regions) can suffer from a relatively high degree
of noise due to
optical scattering phenomena. For this reason, the present embodiment makes
use of a
sample preparation technique that greatly reduces this noise level.
[0037] One variant of this technique is performed for the small size
fraction,
which contains abundant phyllosilicates, characterized by their octahedral
crystal shape.
If simply transferred to a sample dish, the random crystal alignment would
result in a
high degree of noise in the optical spectral signal. However, in the present
embodiment, the small size fraction material, while still moist from the
precipitation
liquid, is placed in an appropriate sample container, such as a dish with a
negligible
spectral signature over the detection range. A completely dry small size
fraction may
also be moistened with distilled water before being placed in the sample
container. The
powder material is then smeared with an appropriate instrument, such as a
straight
edge, while being pressed into the sample container.
[0038] An example of this is shown in Figure 5, which is a schematic top
view of
a sample tray having twenty-five sample locations. In some of the sample
locations the
unprepared powder 50 has been simply placed in the dish, and, as a result, the
crystal
orientation of the sample material is random, and the spectral signal will be
rather noisy
as a result. For other samples, however, the material has been smoothed and
pressed
into place, and these prepared samples 52 are also shown in some of the sample
locations of the sample dish in Figure 5. This preparation technique has the
effect of
better aligning the crystals of the sample material, greatly reducing the
scattering noise.
As a result, the signal-to-noise ratio during spectral analysis of the sample
is much
lower, allowing for a cleaner, clearer signal.
[0039] A similar sample preparation technique may be used for the medium
size
fraction. As the particle sizes are significantly larger than those of the
small size
fraction, this portion is first dried and crushed to homogenize it. Because
the crystal
shape of this size fraction tends to be cubic, there is not subsequent
smearing with a
CA 02827753 2013-09-12
straightedge, as is done for the small size fraction. Rather, a press pallet
technique is
used to press the sample material into the desired sample container. As with
the small
size fraction preparation, the result is a lower signal-to-noise ratio during
the
subsequent optical spectral analysis.
Surfactant Analysis
[0040] Embodiments of the present invention may make use of another
analytical
technique that involves a detection of a surfactant component of a
disaggregated
sample material. During a precipitation separation of the sample, the
surfactant
molecules are lixiviated from the sample material and float to the surface of
the liquid
where they are easily skimmed off. The organic ligands in this sample may be
strongly
indicative of the presence of certain minerals. In particular, materials such
as nitrates
and methane, which are extremely reactive to infrared light, may be detected
using fast
Fourier transform spectroscopy. The detection of certain molecules is
particularly
indicative of the presence of certain minerals. For example, it has been shown
that
nitrates can carry about ten times more gold than chlorine or thiosulfate.
Other
materials present in the surfactant can likewise be strong indicators of the
presence or
absence of a mineral of interest.
Example
[0041] Given a particular set of drill core samples, there are different
ways to
analyze the sample material. What follows is one example of doing such an
analysis
according to an embodiment of the present invention. The analysis makes use of
some
conventional techniques, as well as some novel methodologies unknown in the
prior art.
The general steps of this example are depicted in Figure 6.
[0042] First, the ore zone is localized from the drill core (step 60).
This first step
follows the conventional steps of visual examination of the samples by a
geologist,
followed by assays on the critical sections. Such assays (such as a gold assay
in the
case of gold exploration) tend to be expensive and slow, and the experienced
visual
examination is therefore used to minimize the number of assays. Nevertheless,
such
assays are very valuable indicators of the ore quality, and may be performed
if desired.
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straightedge, as is done for the small size fraction. Rather, a press pallet
technique is
used to press the sample material into the desired sample container. As with
the small
size fraction preparation, the result is a lower signal-to-noise ratio during
the
subsequent optical spectral analysis.
Surfactant Analysis
[0040] Embodiments of the present invention may make use of another
analytical
technique that involves a detection of a surfactant component of a
disaggregated
sample material. During a precipitation separation of the sample, the
surfactant
molecules are lixiviated from the sample material and float to the surface of
the liquid
where they are easily skimmed off. The organic ligands in this sample may be
strongly
indicative of the presence of certain minerals. In particular, materials such
as nitrates
and methane, which are extremely reactive to infrared light, may be detected
using fast
Fourier transform spectroscopy. The detection of certain molecules is
particularly
indicative of the presence of certain minerals. For example, it has been shown
that
nitrates can carry about ten times more gold than chlorine or thiosulfate.
Other
materials present in the surfactant can likewise be strong indicators of the
presence or
absence of a mineral of interest.
Example
[0041] Given a particular set of drill core samples, there are different
ways to
analyze the sample material. What follows is one example of doing such an
analysis
according to an embodiment of the present invention. The analysis makes use of
some
conventional techniques, as well as some novel methodologies unknown in the
prior art.
The general steps of this example are depicted in Figure 6.
[0042] First, the ore zone is localized from the drill core (step 60).
This first step
follows the conventional steps of visual examination of the samples by a
geologist,
followed by assays on the critical sections. Such assays (such as a gold assay
in the
case of gold exploration) tend to be expensive and slow, and the experienced
visual
examination is therefore used to minimize the number of assays. Nevertheless,
such
assays are very valuable indicators of the ore quality, and may be performed
if desired.
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[0043] Following localization of the ore zone, the alteration is analyzed.
Thus,
steps 62-70 are directed primarily, although not exclusively, to examination
of the drill
core regions in the expected region of the alteration. In step 62, a "whole
rock" analysis
is performed. This includes a determination of the mineralogical chemistry
(step 62a)
using, for example, energy dispersive x-ray spectroscopy, and a determination
of the
mineralogical habitus (step 62b) using, for example, scanning electron
microscopy. The
size fractions of selected portions of the sample are then separated into
three distinct
groups (step 64). The specific cut-offs for the relative size of the fractions
depends on
the application, but in the present example, which pertains to gold
exploration in
Archean terrain, the smaller size fraction consists of material having an
average particle
diameter of less than 2 pm, the medium size fraction consists of material
having an
average particle diameter of 2 pm to 20 pm and the large size fraction has an
average
particle diameter of 40-100 pm.
[0044] The manner of separating the different size fractions may change
from
one application to the next but, in the present embodiment, the method makes
use of
the steps shown in Figure 6A. First, the sample material is "pre-crunched" to
reduce it
to pieces of a manageable size (step 72) using a conventional means of sample
crushing, without pulverizing the material. One or more of the "pre-crunched"
pieces
are then disaggregated (step 74) using, for example, a mortar grinder set to
shear apart
the different mineral grains. The largest particles are then separated from
the rest of the
sample by sieving using a sieve having an average hole size of 100 pm (step
75). In this
example, the precrunching of the sample is to such an extent that 90% of the
material
passes through the 100 pm sieve. The sample material is then washed (step 76)
before
the size fractions are separated. Weak acid leaching (using 10% HCI) may be
used to
both decarbonate the material and to leach transition metals and salts
adsorbed on the
surface of the phyllosilicate material. The decarbonation serves to free
phyllosilicates
which may be entrapped in a matrix of carbonate minerals. If this step is
skipped, the
phyllosilicates would be flocculated, or form clumps, during suspension due to
the
presence of carbonate minerals which would bias the expected size fraction
separates
and their densities. Weak acid leaching will allow leaching of transition
metals and salts
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adsorbed on phyllosilicate surface, which can then be precipitated and
analyzed
spectrally.
[0045] To separate the size fractions, the material is first mixed with an
appropriate liquid in a precipitation chamber, where the larger, heavier
particles tend to
sink to the bottom, while the smaller particles remain in suspension (step
78). After
precipitation, the suspension containing the small size fraction (i.e., the
lighter particles
of < 2 pm) is removed. The precipitate is then removed and dried, before being
passed
through a first sieve having an average hole size of approximately 40 pm (step
80). The
larger size-fraction is thereby isolated from the rest of the precipitated
sample material.
A second sieve having an average hole size of approximately 20 pm is used to
further
filter the remaining sample material (step 82) and separate out the medium
size fraction
of 2-20 pm. In this embodiment, the portion of the sample between 20pm and
40pm is
discarded, as it is typically a mix of lithologies and hydrothermal material,
and not as
useful in the process of analysis.
[0046] After the 20 pm sieving step, this portion of the sample is further
processed using gravity extraction to separate the heavy minerals from the
lighter
materials in the sample portion (step 84). The gravity extraction is
performed, according
to Stoke's Law (describing the rate at which particles of a certain size will
fall within a
fluid), by sedimentation or centrifugation of the samples. This results in two
medium
size fraction portions, the "heavy" portion and the "light" portion. Referring
again to
Figure 6, once the size fractions are separated, the smaller size fraction and
the
medium size fraction, both generally considered to have a hydrothermal origin,
are
analyzed using non-optical techniques (step 66). This includes a geochemical
analysis
(step 66a) using, for example, inductively coupled plasma (ICP) emission
spectrometry
and a mineralogical analysis (step 66b) using, for example, energy-dispersive
x-ray
analysis (EDX), scanning electron microscopy (SEM), transmission electron
microscopy
(TEM) and x-ray diffraction analysis (XRD). Sample preparation of the small
size
fraction (step 68) is then performed (in the manner described above), and
optical
spectral analysis (step 70) is performed on the small and medium size
fractions using a
hyperspectral imaging apparatus such as that shown in Figure 1.
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CA 02827753 2013-09-12
[0047] As mentioned above, the preparation of the medium size fraction for
optical spectral analysis may include some additional steps. In the present
embodiment, this heavy fraction is first ground to a soft and homogeneous
powder
using a mortar and pestle. This powder is then introduced to a sample
container using
a press pallet technique, that is, by forcibly pressing it into the sample
container to
compact it. An analysis of the host lithologies may also be performed on the
large size
fraction. Depending on the grain size and the nature of the rock, this may
involve either
the grinding of the sample material to a homogenous powder and its
introduction by
press pallet to a sample container, or the direct spectral sampling of the
drill core before
any sample preparation.
[0048] An analysis such as that described above provides significantly
more
information than is available using conventional methods. In particular, the
use of size
fraction analysis allows a much more detailed picture of the alteration plume.
By taking
into account the importance of the small and medium size fractions, materials
of interest
can be much more accurately identified, including the target mineral itself
(e.g., gold) as
well as indicators such as redox fronts. Moreover, the spectral analysis
provides
information not just on the material content, but also the molecular makeup of
the
sample material. This information can be of great value in determining how
difficult
extraction of a target mineral may be.
[0049] Although a target mineral of interest may be present in different
samples,
the molecular structure of the sample material has a significant effect on the
difficulty of
extraction. For example, in a small, phyllosilicate size fraction there is
typically a crystal
structure that has the form of a sandwich of alternating octahedral and
tetrahedral
molecules. These molecules may include transition metals of interest, but the
position
of the metals in the molecular structures has a direct impact on how easily
the metal
may be extracted. When the metal is located at the center of the tetrahedral
molecule,
it is very strongly bonded with the surrounding atoms, and is very difficult
to liberate.
When located at the center of the octahedral molecule, the bond is weaker and
not so
difficult to break. In some cases, the metals will also attach to sites along
the exterior of
the crystal. In such a case, the bonds are very weak and the metal is very
easy to
extract.
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[0050] Classic analysis of a sample of interest involves the pulverization
and fire
assay of a sample to determine the relative quantity of a mineral of interest
in an ore
sample. However, the fire assay necessarily breaks down the molecular
structure of the
sample, and it is therefore not possible to determine what portion of the
mineral of
interest is in an easily extractable molecular position, and what portion may
be much
more difficult to extract. In contrast, optical spectral analysis as described
herein
provides information regarding not just the presence of the mineral of
interest, but also
information regarding the molecular structure of the crystal. Thus, in
addition to
providing an estimate of the relative quantity of the mineral of interest, the
analysis
offers valuable information regarding how difficult it may be to extract. In
addition,
spectral imaging of the sample material also provides additional information
that
contributes to a more accurate mapping of the alteration.
[0051] Of course, numerous modifications could be made to the embodiments
above without departing from the scope of the present invention.
