EAS368 Study Guide - Final Guide: Feldspar, Bushveld Igneous Complex, Apatite

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EAS 368: Ore Deposits Geology Example Final Lecture
Exam Questions
The exam usually offers a choice of 7 short-answer
questions from which you should pick 5 to answer. Each
question is worth 10% of the exam mark
What are the main ore minerals and economic metals
mined from orthomagmatic (a) oxide and (b) sulfide
deposits? (Lucas)
Oxide Deposits
Important ore minerals in orthomagmatic oxide
deposits are chromite, magnetite, and ilmenite.
Have stratiform siderophile elements and compatible
lithophile elements (Cr-PGE-Ti-Fe) which are
found within the above ore minerals. PGE =
Platinum Group Elements
Stratiform deposits of chromite (FeCr2O4) occur in
LMI’s or anorthositic intrusive complexes.
Chromitite (chromite bearing rock) can also occur in
podiform deposits (typically ophiolites of back arc
basin origin)
PGE’s occur in stratiform chromite layers as
arsenides, sulfides, antimonides, with some Pt and
Primary ilmenite and magnetite deposits may have Ti,
Fe, and V which occur in layered gabbroic
Sulfide Deposits
Ore minerals are Ni-sulfides (pentlandite), with
pyrrhotite, chalcopyrite, and magnetite
Fe, S, O, Ni are the main constituents
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Deposits with gabbroic rocks will have high Cu:Ni
(e.g. Sudbury)
Briefly describe two suggested mechanisms for the
formation of chromitite layers in layered mafic
-Look at the chart under Orthmagmatic deposits page 4
last slide, drawing those two diagrams would easily get
100% for this question. From what I understand The
two primary mechanisms are; Assimilation of country
rock by the magma which causes an increase in silica,
this in turn prevents olivine from crystallizing, leaving
chromite to be the only crystallizing phase. The second
mechanism is mixing of new magma in the magma
chamber. This works similar to the first mechanism, by
potentially increasing the the SiO2 content you again
prevent minerals other than chromite from crystallizing.
(If anyone “Ryan” understands this better they should
edit this).
Diamonds are currently being recovered off the Namibian
coastline. From where were these diamonds derived,
and how were they deposited? Why has this process
resulted in deposits of particularly high diamond
Theses diamonds were derived from kimberlites
inland. They were deposited by fluvial processes in
fluvial terraces and submarine deltas. This has
resulted in high quality diamonds due to the transport
process breaking down and destroying shitty
diamonds and therefore selectively concentrating high
quality diamonds.
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What is the “diamond pipeline”? Briefly explain how it
works to control the price of rough diamonds.
→ 1st mining companies conduct exploration, on
which economic projects proceed to the mining stage
→ 2nd rough diamonds extracted at the mines are
bought by the central selling organization(CSO)
→ 3rd is where “single channel” or “supplier of choice”
marketing, where stones are available for sale to a
select group of clients (~160) from the main cutting
centers worldwide, who submit requests for the type
and quantity they want. given proper market
conditions, the CSO will prepare packages of stones
for the clients, which are offered to them at 10 annual
“sights” in london
→ 5th, once jewellers acquire the packages of
diamonds, cutting and polishing is carried out in one
of the four main centers worldwide (antwerp, Mumbai-
(particularly skilled at polishing lower quality gem
stones) , New York, and Tel Aviv.
Briefly describe the mechanism by which gem-quality
rough diamond prices are maintained at levels well
above the utilitarian value of this mineral.
→ primarily the price for gem quality diamonds is controlled
by market controls, in which preferable sell rates are
typically maintained by the major producers. the major
producers control the supply variable of the diamond
market to produce favourable conditions for
themselves. The controlling stipulations of what makes
a gem-quality diamond and how they are brought to
market is as follows:
→ first, the value of diamonds depends on its weight,
and also its quality.
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