CA2827753A1 - Geological sample analysis using size fraction separation - Google Patents

Abstract

A method of mineral prospecting uses analysis of a drill core sample by separating the sample into different size fractions and examining it using optical spectral analysis.
Additional focus on small and medium size fractions during subsequent analysis steps provides a large amount of information regarding both an ore region and a surrounding alteration. The optical spectral analysis may be used to identify relative content and molecular position for certain materials. The crushed sample material may be prepared by mixing it with a liquid, locating it in a sample holder, and smoothing it with a straight edge to better align the sample crystals. Size fraction separation may include mixing sample material with a liquid and allowing separation by precipitation. A
surfactant may also be skimmed from the surface of the liquid and analyzed as well.

Description

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.

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.

[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 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 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 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.

[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 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 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 [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 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.

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.

[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 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.

[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.

[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.

Claims (23)

1. 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:
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 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.

2. A method according to Claim 1 wherein the first size fraction comprises material having an average particle diameter of less than 2 µm.

3. A method according to Claim 1 wherein performing a content analysis on one of the size fractions further comprises performing a geochemical analysis and a mineralogical analysis.

4. A method according to Claim 3 wherein the geochemical analysis comprises inductively coupled plasma emission spectrometry.

5. A method according to Claim 3 wherein the mineralogical analysis comprises at least one of energy-dispersive x-ray analysis, scanning electron microscopy and transmission electron microscopy.

6. A method according to Claim 1 further comprising performing a whole rock analysis on the sample prior to the size fraction separation, the whole rock analysis comprising a determination of the mineralogical chemistry and the mineralogical habitus of the sample.

7. A method according to Claim 1 wherein performing an optical spectral analysis comprises performing an optical spectral analysis of the first size fraction.

8. A method according to Claim 1 wherein the optical spectral analysis is performed using a hyperspectral imager.

9. A method according to Claim 1 further comprising determining a molecular site occupancy of said one or more minerals of interest so as to estimate their relative extractability.

10. A method according to Claim 1 further comprising sample preparation prior to performing the optical spectral analysis, the sample preparation comprising placing the separated first size fraction material in a sample location and smoothing the material with a straight edge.

11. A method according to Claim 1 wherein the first size fraction has an average particle diameter of less than 2 µm and the second size fraction has an average particle diameter of greater than 40 µm, and wherein the separated size fractions of the sample further comprise a third size fraction having an average particle diameter of approximately 2 µm to 20µm.

12. A method according to Claim 1 wherein the optical spectral analysis comprises an optical spectral reflectance analysis.

13. A method according to Claim 1 wherein separating the sample into size fractions comprises mixing the sample with a liquid and allowing separation thereof by precipitation.

14. A method according to Claim 1 wherein an analysis of the first size fraction comprises mixing the sample material with a liquid, removing a floating portion of the sample material and performing an optical spectral analysis on the removed portion.

15. A method according to Claim 1 wherein separating the sample into a plurality of size fractions comprises disaggregating the sample.

16. A method according to Claim 15 wherein separating the sample into a plurality of size fractions comprises passing the disaggregated sample through a sieve.

17. A method according to Claim 1 wherein separating the sample into a plurality of size fractions comprises:
mixing the sample with a liquid; and removing a floating portion of the small size fraction from a surface of the liquid.

18. A method according to Claim 1 wherein performing an optical spectral analysis comprises performing an optical spectral analysis in an infrared wavelength range.

19. A method according to Claim 1 wherein performing an optical spectral analysis of the removed portion comprises identifying organic molecules present in the removed portion.

20. 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:
converting the sample material to a homogeneous powder form;
pressing the sample material into a sample location; and performing an optical reflectance spectral analysis of the prepared sample material.

21. A method according to Claim 20 further comprising smoothing a surface of the sample material with a straight edge so as to improve the relative alignment of crystals in the sample material.

22. A method according to Claim 20 wherein the sample material is a size fraction from a geological sample, wherein the geological sample has an average particle size of less than 2 µm.

23. A method according to Claim 20 wherein in the sample material is a size fraction from a geological sample, wherein the geological sample has an average particle size of between 2 µm and 20 µm, and wherein the sample material is crushed to convert it to said homogeneous powder form.