MINERAL DEPOSITS EPSC-452
MID-TERM TEST
INSTRUCTIONS: Answer only one question.
If asked to write reactions among silicate minerals, use the stoichiometric
compositions provided in the following list: alunite (KAl3(SO4)2(OH)6;
biotite (KAl(Fe,Mg)3Si3O10(OH)2); chlorite (Fe,Mg)6AlSi3O10(OH)8);
epidote (Ca2Al2Si3O4(OH)); kaolinite (Al2Si2O5(OH)4); K-feldspar
(KAlSi3O8); muscovite (KAl3Si3O10(OH)2);
Marks:
25
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1)
In contrast to the presidents of many mining companies, you are a geologist and
believe that model-driven grass-roots mineral exploration holds the most hope of
success for your company, “New World Mining Inc.” One of your current objectives
is to discover a large copper deposit that can be mined by open pit methods in the
newly independent country of Nueva Esperanza.
a) Your geological staff have reported showings of copper with traces of gold in
rocks containing large phenocrysts in a fine-grained matrix. What is the likely
composition and petrochemical affinity of these rocks? In what tectonic
environment are they likely to have formed? Why are these rocks particularly
favourable hosts for the type of mineralization that you are seeking? Explain your
answers.
b) You believe that mapping hydrothermal alteration is essential to developing
targets for drilling. What type of alteration should your staff look for in
developing these targets? What was the composition of the fluid that likely caused
this alteration and how would you determine this composition with the aid of a
microscope? Explain your answers.
c) From experience, you know that ore stage hydrothermal alteration is commonly
obscured by later hydrothermal alteration. What is the nature of this later
alteration? Write a balanced reaction that relates this later alteration to the earlier
ore stage alteration, show this reaction on a phase diagram and use it to discuss
the physicochemical evolution of the hydrothermal system.
d) Using information gained from a study of samples in the field and under the
microscope and your understanding of the physicochemical controls of metal
transport and deposition, develop and support a model of ore genesis hat you
believe will lead to the discovery of a large economic copper deposit.
2) A junior Canadian mining company, “Utopia Minerals” has employed you to
provide practical training for their geological staff and help them to evaluate an
epithermal gold-bearing prospect that they have discovered in the Oro Grande
region of Chile.
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a) The Utopia Minerals geologists are confused over the usage of the terms “high
sulphidation” and “low sulphidation”. Explain these terms to them and indicate
the fO2-pH conditions under which these two deposit types form.
b) The prospect that Utopia Minerals is evaluating in Oro Grande is characterized by
extensive kaolinite alteration that passes northwards into muscovite/illite
alteration and southwards into alunitic alteration. What type of epithermal deposit
is Utopia Minerals evaluating? Write a balanced reaction involving the minerals,
muscovite and kaolinite, and show how this reaction can be used to determine the
direction of pH change in the prospect. In which direction (relative to the kaolinite
zone) would you recommend that Utopia minerals concentrate their exploration
for economic gold mineralization. Explain your recommendation.
c) Based on your recommendation, Utopia minerals have undertaken a stable isotope
study of alteration and fluid inclusions to help them understand the genesis of the
deposit. On a plot of D vs 18O show the distribution of the estimated
compositions of the fluids responsible for alteration relative to those of commonly
encountered natural fluids. What conclusion(s) do you draw from the analysed
compoisitions?
d) Develop and support a model for the genesis of the deposit that Utopia Minerals is
hoping to find at Oro Grande.