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Dont substitute geochemistry for geology spatial

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Unformatted text preview: – this provides framework for alteration geochemical sampling. • • DON’T SUBSTITUTE GEOCHEMISTRY FOR GEOLOGY!!!!!!!!!!!! • Spatial sampling (i.e., every XX meters). Sample stratigraphic units proximal to distal from ore and alteration zones, and with varying intensity of alteration from least-altered to most-altered with gradations in between, and with different alteration index minerals (e.g., sericite, chlorite, carbonate, etc.). Wednesday, 15 August, 12 Philosophical Approach to Alteration Sampling • In many camps there are not fresh rocks, thus we must use the leastaltered of the altered. • It’s a relative alteration scaling – same as setting up what background is in a soil, till, etc. survey and measuring changes relative to the background. • From here we collect samples, do petrography, and lithogeochemistry – then initial screening of geochem, then more rigorous quantitative treatment of data. Wednesday, 15 August, 12 Initial Screening of Samples for Alteration • Initially use field calls and thin sections chemistry then • Withhow altered: we can do a quick overview of the chemistry to see • • • Check LOI, H2O, CO2 contents – high values = altered. • Look at metal contents and alteration indexes in rock relative to fresh ones, high metals = altered. • Compare immobile-mobile pairs to see if there is correlation or scatter (i.e., Zr-SiO2, Na2O-Zr, etc.); scatter = alteration • • High MgO, FeO* in felsic rocks = altered. • Normative corundum in CIPW norm = altered. Al2O3/Na2O, A/NK, A/CNK - high values = alkali mobility Look at rock call and then compare SiO2 = if higher than expected then silicified, if less than maybe sericite or chlorite altered. Check out Hughes (1973) plot -spillite, keratophyre fields = altered Wednesday, 15 August, 12 Low Na2O in most rocks = altered Establishing Co-genetic Samples • In order to monitor alteration, we must prove that an altered sample came from some parent. • i.e., a series of rocks have a fresh precursor (or least altered sample) that was variably altered. • To do this we must use immobile elements, particularly the HFSE, REE, Al2O3 and TiO2. • Cogenetic samples, even altered, should form linear trends on binary plots of immobile elements. Wednesday, 15 August, 12 Establishing Cogenetic Behaviour From Barrett and MacLean (1994) Wednesday, 15 August, 12 Establishing Cogenetic Samples From Piercey et al. (2008) Wednesday, 15 August, 12 Quantifying Hydrothermal Alteration • We use immobile elements to break out populations of samples and we establish precursors or least altered samples for each population. • We have samples separated as a function of alteration intensity and type within each population (e.g., chlorite, quartz, intense, weak, etc.). • • We can now use various methods to quantify alteration. Now we can move forward and calculate mass changes using a variety of methods: Gresens (1967), Grant (1986), MacLean (1990), Huston (1993), Stanley and Madeisky (1994), etc. Wednesday, 15 August, 12 Sample Alteration Density SiO2...
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