1 Chemostratigraphy and mass changes related to ore forming processes. Ajit Kumar Walter Tejada Rui Zhang

2 Rui Zhang Rui Zhang ChemostratigraphyChemostratigraphy Ajit Kumar Ajit Kumar Applications of chemostratigraphyApplications of chemostratigraphy Mass changes related to ore forming processesMass changes related to ore forming processes Walter Tejada Walter Tejada Case studiesCase studies ConclusionConclusion

3 Problems in VMS exploration Primary stratigraphic relations are obscured by hydrothermal alteration, or distorted by tectonic effects such as folding and fault offsets, making it difficult to locate target horizons. Ore Systems Consulting T.J. Barrett, W.H. MacLean & Associates

4 Sulfide lenses are generally located at the contacts between chemically different rhyolite units. So, it is important to identify individual volcanic units. This can be done using their lithogeochemical fingerprints. Their 3-D distribution can be also be plotted for interpretation purposes. Ore Systems Consulting T.J. Barrett, W.H. MacLean & Associates

5 Trace elements Incompatible elements: (I) Smaller, highly charged HFSE: REE, Th, U, Ce, Pb4+, Zr, Hf, Ti, Nb, Ta and (II) Large ILE: K, Rb, Cs, Ba, Pb2+, Sr, Eu2+ Incompatible elements: (I) Smaller, highly charged HFSE: REE, Th, U, Ce, Pb4+, Zr, Hf, Ti, Nb, Ta and (II) Large ILE: K, Rb, Cs, Ba, Pb2+, Sr, Eu2+ Incompatible elements are concentrated in the melt Incompatible elements are concentrated in the melt KD or D « 1 Compatible elements are concentrated in the solid Compatible elements are concentrated in the solid KD or D » 1 For dilute solutions can substitute D for KD: For dilute solutions can substitute D for KD: D =C(s)/C(l) (Where CS = the concentration of some element in the solid phase) Identification of the source rock or a particular mineral involved in either partial melting or fractional crystallization processes. Identification of the source rock or a particular mineral involved in either partial melting or fractional crystallization processes.

6 Fractionation monitors Incompatible trace elements are concentrated in liquid, these reflect the proportion of liquid at a given state of crystallization or melting. Incompatible trace elements are concentrated in liquid, these reflect the proportion of liquid at a given state of crystallization or melting. Their activity varies in direct relation to their concentration in the system (Henry’s Law). Their activity varies in direct relation to their concentration in the system (Henry’s Law). Because of this, the ratios of trace elements are often superior to the concentration of a single element in identifying the role of a specific mineral. Because of this, the ratios of trace elements are often superior to the concentration of a single element in identifying the role of a specific mineral. Zr Harker Diagram for Crater Lake. From Winter (2001)

7 By applying the mass change technique to rock samples with varying chemistry, it is possible to detect chlorite alteration zones outlining conduits for ascending hydrothermal fluids responsible for the formation of a VMS deposit. From discrimination analysis, percent mass change of “Mobile Oxides” (Na2O Fe2O3, MgO, CaO), Cu, and Zn can be determined, which outline discordant zones of chlorite alteration. By applying the mass change technique to rock samples with varying chemistry, it is possible to detect chlorite alteration zones outlining conduits for ascending hydrothermal fluids responsible for the formation of a VMS deposit. From discrimination analysis, percent mass change of “Mobile Oxides” (Na2O Fe2O3, MgO, CaO), Cu, and Zn can be determined, which outline discordant zones of chlorite alteration. Mass change values are compared for each sample relative to its precursor composition. Mass change values are compared for each sample relative to its precursor composition. Mass Change

8 Percent mass change values for mobile oxides and raw Cu and Zn are the most effective indicators of primary chlorite alteration associated with the bulk of the ore forming event. Percent mass change values for mobile oxides and raw Cu and Zn are the most effective indicators of primary chlorite alteration associated with the bulk of the ore forming event. Percent mass change K2O and SiO2 effectively outline zones of early low temperature silicification and sericitization. Percent mass change K2O and SiO2 effectively outline zones of early low temperature silicification and sericitization. Contouring the mass change data for MgO, SiO2, Fe2O3, Na2O and other elements can illustrate different alteration zones. For example, a core of footwall massive rhyolite becomes pervasively silicified (+SiO2) and the surrounding ultramafic and rhyolite units get pervasively chloritized (+ Fe). Contouring the mass change data for MgO, SiO2, Fe2O3, Na2O and other elements can illustrate different alteration zones. For example, a core of footwall massive rhyolite becomes pervasively silicified (+SiO2) and the surrounding ultramafic and rhyolite units get pervasively chloritized (+ Fe).

9 Immobile elements Identification of immobile elements is an essential first step in the chemical study of alteration. This begins with the selection of several probable immobile elements (perhaps Ti, Zr, Nb, Y, Sc and Th) and plotting their values against the values for probable mobile elements (perhaps Ca, Mg, K and Na). A definite immobile- mobile pair will plot on a simple binary graph as a strongly correlated straight line, which passes through or near the origin. Identification of immobile elements is an essential first step in the chemical study of alteration. This begins with the selection of several probable immobile elements (perhaps Ti, Zr, Nb, Y, Sc and Th) and plotting their values against the values for probable mobile elements (perhaps Ca, Mg, K and Na). A definite immobile- mobile pair will plot on a simple binary graph as a strongly correlated straight line, which passes through or near the origin.

10 Reconstituted composition is calculated as Reconstituted composition is calculated as RC = immobile component (precursor) / immobile component (altered) * % mobile component (altered) Absolute mass change is calculated as Absolute mass change is calculated as Mass Change = RC - element’s abundance in precursor composition A normalization calculation is added in order to take into consideration differing initial elemental abundances between lithologies. A normalization calculation is added in order to take into consideration differing initial elemental abundances between lithologies. Percent Mass Change = Mass Change / Element’s Abundance in Precursor Composition Calculating Mass Change

11 Fig: Effects of net mass gains and losses of mobile elements are shown for a dacite to rhyolite series. Samples plotting in the mass loss field have generally experienced leaching due to hot fluids, leaving sericite-chlorite- rich residues. Samples plotting in the mass gain field have experienced precipitation of components such as silica, carbonates and sulfides, commonly at cooler temperatures. Fractionation trend and Mass Change Ore Systems Consulting T.J. Barrett, W.H. MacLean & Associates

12 Mass change From the immobile element relations, mass changes can be calculated for the mobile elements for each sample, as schematically shown. This allows quantitative assessment of hydrothermal effects. The results often give a picture of alteration which differs considerably from taking the untreated lithogeochemical data at face value. Ore Systems Consulting T.J. Barrett, W.H. MacLean & Associates

13 Immobile element relationships for altered volcanic rocks, Phelps Dodge deposit. The samples lie essentially along a single alteration line corresponding to variably altered rhyolite. Samples which experienced mass loss due to leaching by hot fluids have apparently higher (residually concentrated) Al2O3 and Zr contents relative to the precursor rhyodacite. By contrast, samples which experienced mass gain due to precipitation from cooler fluids have apparently lower (diluted) Al2O3 and Zr contents. Immobile element relationships for altered volcanic rocks, Phelps Dodge deposit. The samples lie essentially along a single alteration line corresponding to variably altered rhyolite. Samples which experienced mass loss due to leaching by hot fluids have apparently higher (residually concentrated) Al2O3 and Zr contents relative to the precursor rhyodacite. By contrast, samples which experienced mass gain due to precipitation from cooler fluids have apparently lower (diluted) Al2O3 and Zr contents. Ore Systems Consulting T.J. Barrett, W.H. MacLean & Associates

14 Single Precursor The upper plot shows highly altered volcanic rocks at the Phelps Dodge deposit, in the Matagami area, which were all derived from one homogeneous unit. The magmatic affinity of a series of volcanic rocks can also be determined, as in the lower plot, where the Ansil mafic to felsic volcanic rocks, in the Noranda area, are of transitional affinity. Ore Systems Consulting T.J. Barrett, W.H. MacLean & Associates

15 Immobile element plots for footwall rhyolites and hangingwall basalts at Eskay Creek. The variably altered rhyolites show a great range of mass change effects, but were derived from an essentially homogeneous, low-TiO2 rhyolite. The basalts are little altered. Immobile element plots for footwall rhyolites and hangingwall basalts at Eskay Creek. The variably altered rhyolites show a great range of mass change effects, but were derived from an essentially homogeneous, low-TiO2 rhyolite. The basalts are little altered. Ore Systems Consulting T.J. Barrett, W.H. MacLean & Associates

16 Multiple Precursor Commonly, VMS deposits are hosted by only two or three main volcanic units, and thus single-precursor treatments can be applied. However, more complex volcanic systems also exist which are related by continuous fractionation. These are treated using the multiple precursor system, which is similar in principle, but requires a few extra steps to obtain proper mass change results. Ore Systems Consulting T.J. Barrett, W.H. MacLean & Associates

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18 Two immobile element ratios (Al2O3/TiO2 and Zr/TiO2), which are not mobile to any significant degree in the alteration zones of typical VMS deposits, can be plotted downhole in order to examine lithological variation. Two immobile element ratios (Al2O3/TiO2 and Zr/TiO2), which are not mobile to any significant degree in the alteration zones of typical VMS deposits, can be plotted downhole in order to examine lithological variation. Ore Systems Consulting T.J. Barrett, W.H. MacLean & Associates

19 Binary plots of one immobile- element ratio vs. another. Altered samples are also included. The ratios remove the effects of mass changes caused by alteration. Globally, a majority of VMS deposits are hosted in sequences of tholeiitic affinity (e.g. the Matagami camp) or transitional affinity (e.g. the Noranda camp). The Kristineberg (Sweden) deposit probably formed in an evolved island-arc setting. Binary plots of one immobile- element ratio vs. another. Altered samples are also included. The ratios remove the effects of mass changes caused by alteration. Globally, a majority of VMS deposits are hosted in sequences of tholeiitic affinity (e.g. the Matagami camp) or transitional affinity (e.g. the Noranda camp). The Kristineberg (Sweden) deposit probably formed in an evolved island-arc setting. Ore Systems Consulting T.J. Barrett, W.H. MacLean & Associates

20 Comparison of Y-Zr (upper) and Nb- Zr (lower) relations for rhyolites from Kutcho Creek versus those from rifted continental margin settings. The Kutcho Creek rhyolites have much lower contents of these incompatible elements, consistent with an origin in a primitive island arc setting. Comparison of Y-Zr (upper) and Nb- Zr (lower) relations for rhyolites from Kutcho Creek versus those from rifted continental margin settings. The Kutcho Creek rhyolites have much lower contents of these incompatible elements, consistent with an origin in a primitive island arc setting. Ore Systems Consulting T.J. Barrett, W.H. MacLean & Associates