Geochemistry

3.3.2.1

ICP-AES (ICP-OES)

The fabrication of the plasma torch by T.B. Reed (1962) and the conception of plasma emission source spectroscopy by Greenfield et al., (1964) has led to over 50 years of analytical improvement of inductively coupled plasma atomic (optical) emission spectrometry (ICP-AES or ICP-OES). The method is a form of emission spectrometry measuring the wave-length and intensity of photons given off by of excited electrons returning to a stable energy state (Dean, 2005). Elemental quantification is allowed because the photon intensity at a given wavelength is proportional to concentration (Dean, 2005). Plasma itself is a neutral ion gas containing equal number of ions and electrons. The plasma energy source is formed by an electrically ignited Argon gas pumped through a magnetic field, induced by radio frequencies between 27.12 to 40.68 MHz (Dean, 2005). Operating temperatures range between 5,000°K and 10,000° K (Hill, 1999). Dissolved aliquot solutions are passed through a nebulizer creating an aerosol. The atomized-aerosol solution is then injected into the plasma torch. Atoms in the aerosol absorb and emit photons that are focused through a lens and into a spectrometer (Hill, 1999). Wave length (λ) is measured and processed to identify elemental

composition. Intensity is compared to standards to quantify concentration.

ALS Global carried out ICP-AES analysis of 46 drill core samples and 35 grab samples for major elemental oxides listed in Table 3.1. Geochemical sample preparation followed several steps. First 70% of the whole sample was crushed to less than 2 mm in size. A riffle split separated 250 g of crushed sample for pulverization to a consistency of 80% less than 70 μm. Three grams of pulverized sample material was mixed with

lithium borate to create a fused bead allowing homogenization of all elemental

components from major, trace and refractive minerals. A complete four acid digestion followed. The solution was analyzed using an Agilent 720 ICP-OES. Detection limits are supplied in Table 3-1.

Major Oxides Detection Range Wt% Major Oxides Detection Range Wt% Al2O3 0.01-100 MnO 0.01-100 BaO 0.01-100 Na2O 0.01-100 CaO 0.01-100 P2O5 0.01-100 Cr2O3 0.01-100 SiO2 0.01-100 Fe2O3 0.01-100 SrO 0.01-100 K2O 0.01-100 TiO2 0.01-100 MgO 0.01-100 LOI 0.01-100

Table 3-1 List of major elemental oxides and their detection ranges analyzed by ICP-AES.

3.3.2.2

ICP-MS

Inductively coupled mass spectrometry uses the same ionization argon plasma source but measures atomic mass (M) to charge (Z) ratios (M/Z) to identify and quantify elemental and isotope concentration. Injected analytes are ionized from the argon plasma and passed through a sample cone and skimmer. Sample ions reach supersonic speeds while passing through the skimmer due to a vacuum change and increased enthalpy from adiabatic expansion (Hill, 1999). Ion lenses emitting a controlled and focused electric field accelerate electrons to the mass analyzer (Hill, 1999). Several mass analyzers exist but the quadrupole is the most common and the type used for this thesis. A quadrupole mass analyzer contains four rods in which two opposing rods carry an applied and controlled direct current to filter selected elements to an ion detector. The applied direct current forms a hyperbolic electric field filtering out unwanted elemental masses (Hill, 1999). Aliquots used in ICP-AES were also subjected to ICP-MS for trace element analysis. A second aliquot was analyzed for base metals, rare earth metals and rare earth elements via ICP-MS. Detection limits of ICP-MS are listed in Table 3.2.

Element Detection Range ppm Element Detection Range ppm Element Detection Range ppm Ba 0.5-10,000 Hf 0.2-10,000 Sn 1-10,000 Ce 0.5-10,000 Ho 0.01-1,000 Sr 0.1-10,000 Cr 10-10,000 La 0.5-10,000 Ta 0.1-2,500 Cs 0.01-10,000 Lu 0.01-1,000 Tb 0.01-1,000 Dy 0.05-1,000 Nb 0.2-2,500 Th 0.05-1,000 Er 0.03-1,000 Nd 0.1-10,000 Tm 0.01-1,000 Eu 0.03-1,000 Pr 0.03-1,000 U 0.05-1,000 Ga 0.1-1,000 Rb 0.2-10,000 V 5-10,000 Gd 0.05-1,000 Sm 0.03-1,000 W 1-10,000 Y 0.5-10,000 Yb 0.03-1,000 Zr 2-10,000

Table 3-2 List of trace and rare earth elements analyzed by ICP-MS accompanied with their detection limits in ppm.

3.3.2.3

Pycnometer Measurements

Particle density measurements of geochemically analyzed samples were conducted on returned 70 μm pulps in a gas displacement density analyzer known as a pycnometer. The pycnometer measures volume by filling the sample chamber with a non-reactive gas (He). The machine then empties the sample chamber gas into an empty chamber with a known volume and calculates the volume using the difference in

pressure. Taking the mass of sample over its volume gives its particle density negating the original bulk porosity. An AccuPyc II 1340 pycnometer was used under the

supervision of Ying Zhang at the Spence Engineering building at the University of Western Ontario. Samples were measured three times and averaged with recalibration of the machine after every five samples. Porosity in silicate rocks was absent inferring particle density is equivalent to bulk rock density. Hematite ores contained 5% porosity. Hematite ore density was calculated by measuring the mass and volume of a precision cut tile by scale and micrometer.

3.3.2.4

Mass Balance Calculations

Two main methods of calculation exist for quantification of mass changes in rocks and minerals; Gressens’ method (1967) and Grant’s isocon method (1986). Gressens’ method is preferred as it takes density and volume changes into account.

Gressens’ method is illustrated in Equation 3.1. The variable xi is the element

concentration of either the parent (A) or an altered unit (B) while ρ is the density of the respective unit and Fv is the volume factor. The result is elemental mass change (Δxi) reported as grams or ppm change.

Δ𝑥𝑖 = 𝐹𝑣 (ρB

ρA) 𝑥𝑖𝐵− 𝑥𝑖𝐴

Equation 3-1 Gressen`s equation mass balance equation (Gresens, 1967).

All variables in the equation are measured with the exception of Fv. To calculate Fv, elements considered immobile are plotted using theoretical Fv values of 0.5 and 2.0 to their respective parent and altered concentrations. This produces two points on a Δxi versus Fv plot known as a composition-volume (C-V) diagram. The intersection of the line between the two points across the Fv axis is considered the elemental volume change. A grouping of lines (immobile elements) intersecting the same Fv value represents the units volume factor or change in volume. Other criteria needed to aid in the interpretation of mass balance changes are:

1. Supposition of an Fv value can only be determined alongside petrographic evidence.

2. Only component ratio comparisons of parent and altered unit can show if the component was removed or added.

Elements considered immobile are listed in Table 3.3. Major elements, aluminum and titanium, are generally considered immobile because their low solubility in water and low diffusion coefficients. Rare earth elements, (REEs) and especially heavy rare earth elements (HREEs) are considered immobile because their intermediate ionic potential, odd number valence states and encapsulation in refractory trace minerals such as zircon. All elements considered immobile are plotted on a Fv vs X intercept plot to determine correct volume factors. After an Fv value has been determined for the lithology, it is applied and recalculated for the suite of analyzed elements.

Atomic Number 57 58 59 60 61 62 63 64

Element Symbol La Ce Pr Nd Pm Sm Eu Gd

Element Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium

Atomic Number 39 65 66 67 68 69 70 71

Element Symbol Y Tb Dy Ho Er Tm Yb Lu

Element Yttrium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium

Atomic Number 13 22 41 31

Element Symbol Al Ti Nb Ga

Element Aluminum Titanium Niobium Gallium

Table 3-3 Tables listing elements considered immobile in geologic systems. Light rare earth elements are in blue and heavy rare earth elements are in green.