Lecture 13 resources 4 porphyry copper Flashcards
What happens in divergent plate boundaries?
New (oceanic) crust is formed - can move (pushed) then causes subduction zones at continental crust because the plate is moving away from oceanic gets pushed into a convergent plate margin the other side
What happens at convergent plate margins
subduction zone
Where do porphyry copper deposits form?
above Subduction zones
- oceanic or continental arcs
Which deposits form at divergent plate boundaries?
Volcanogenic massive sulphides - mid ocean ridges, hydrothermal which causes big submarine deposits.
Why is copper important
- principle use is for alloys,
- need it for construction - wiring in houses
- transport - average car has 1.5km of wire.
Who has the most copper reserves?
Chile
What are the types of copper deposits?
Porphyry copper deposits
stratiform sedimentary copper deposits
Volcanic massive sulfide (VMS) ore deposits
iron oxide- copper- gold (IOCG) ore deposits
copper skarn deposits
What are porphyry deposits
- most common and most important deposits (50% of copper production)
- large-scale deposits with chalcopyrite mineralisation
- mined via open-pit mining
- low grade (0.4 - 1% Cu)
- Associated with other metals, e.g. molybdenum, gold, silver.
What are stratiform sedimentary copper deposits
- 2nd most important type of deposits (1/4 of identified cu deposits)
Volcanogenic massive sulfide (VMS)
- source of copper sulfide, formed through hydrothermal alteration in submarine environments
Iron-oxide-copper-gold(IOCG) ore deposits
- high value ore deposits associate with iron oxides
- related to fault systems but no genetic relationship with exposed igneous intrusions
copper skarn deposits (don’t really need)
- formed through chemical and physical mineral alterations at contacts, two separate lithologies.
Porphry copper mineralisation - what are the deposits like?
- very large
- low grade deposits (0.4-1% copper concentration)
- associated with intermediate to acidic plutonic intrusions - mainly granites.
- Magmatic activity forms granitic body in the subsurface - this (igneous) activity causes the porphyry deposits.
- Intense and extensive hydrothermal alteration of the host rock.
example - Escondida, Chile - reserve 1.8 billion tonnes - average grade roughly 1.6% total investment - very large mine is 4 billion dollars.
The geology of a porphyry copper deposit
- Mid ocean ridges and divergent plate boundaries with subduction zones.
- new oceanic crusts are formed - hydrothermal - altered oceanic crust through sea water intrusion.
- enriched by base metals and copper
- the rock moves, subduction zone - west coast of North America/south America - increasing temperature and pressure, because of the water within the minerals and sediments - dehydration forms weight - lowers the melting point = partial melting which forms magma and rises into crustal magma chamber - metals enriched within oceanic crust sediment.
- generation of hydrous melts in sub arc mantle.
- oxidized melts transport volatiles to upper crustal magma chamber where fractional crystallisation and volatile exsolution result in porphyry ore formations.
What is porphyry
used to describe texture of the rock
- source rock
copper porphyry deposits associated with veins
what can expose copper deposits on the surface
erosion can expose surface copper
- over time further erosion and oxidation and dissolution so the copper deposits can disappear.
Key facts about copper deposits
- spatially and genetically related to igneous intrusions (mainly granitic) - I-type granitoids with primitive Sr ratios (0.702 to 0.705).
This suggests origins of either the upper mantle or recycled oceanic crust. - commonly multiple intrusive events, not all events cause mineralisation.
Eg. Ray Deposit AZ: - Early Qtz-diorite 70 million years ago
porphyritic phase 63 million years ago
mineralized porphyry 61 million years ago - Porphyritic - reflecting sudden crystallization due to rapid chilling or from concentration and sudden release of fluid phase.
- host rocks can vary (island/oceanic vs continental Arcs)
- Intrusions and host rocks are intensely fractured
- Mineralization and alteration from large zones which exhibit lateral zonation.
What does the model lowland and gilbert explain
Stock = ground and source rock
potassic zone = rock around this - secondary mineralogy (secondary orthoclase and biotite, chlorite, anhydrite.
Phyllic Zone - sercitization/advanced argillic alteration) quartz, sericite and pyrite with minor chlorite, illite and rutile.
Argillic Zone - clay minerals - dominantly kaolin
Propylitic zone - hydrothermal alteration mineral - chlorite, pyrite, calcite and epidote
Where is the highest grade ore - Lowell guilbert model
Copper is related to the sulfurization of the rock.
copper ore occurs between the potassic and the phyllic zone.
main copper mineral is chalcopyrite
Stock
Potassic zone - low total sulfide innermost zone
Phyllic Zone - high pyrite intermediate zone
Propylitic zone - low pyrite outer zone
very low concentrations found here
where is the ore located
- totally within the host rock
- partially within the stock and the country rock
- entirely within the country rock
associated with fractured vein system - hydrothermal alteration and where sulphates occur.
ore invariably disseminated but fracture controlled.
ore zones also in concentric zones
base metal veins with gold and silver usually found in radial fracture zones peripheral to the pyrite halo.
What is hydrothermal alteration
Any alteration of rocks or minerals by the reaction of hydrothermal fluid with pre-existing solid phases.
How does the hydrothermal alteration happen in the stock.
Three stages in the development of Crackle Brecciation
magma intrusion into shallower crust
crystallisation causes an increase in the vapour pressure - increases weight of pressure in magma. rocks crack and fluid moves
1) Vapour pressure building up in and around the upper portion of magmatic fraction
2) Retrograde boiling occurs causing expansion
3) distribution of resulting brecciation.
This is key to forming the magmatic pluton above
What size and grade are porphyryry copper deposits
very large and low grade
Porphyry copper deposits are associated with…
Porphyry copper deposits are associated with intermediate to acidic plutonic intrusives.
What are Porphyry copper deposits characterized by
They are characterized by intense and extensive hydrothermal alteration
of the host rocks.
What are Porphyry copper deposits associated with?
they are spatially and genetically related to igneous intrusions in island or continental arc environment. Generally of associate with granitoids with primitic Sr ratios, indication upper mantle or recycled oceanic crust origin.
How many intrusive events for CU-porphyry
Commonly multiple intrusive events with mineralization often associated with
the youngest event
What does Cu-Porphyry represent
Porphyritic reflects sudden crystallization due to rapid chilling or from
concentration and sudden release of fluid phases
Cu-porphyry - zonation
Intrusions and host rocks are intensely fractured, and mineralization and
alteration form large zones, which exhibit lateral zonation.
Economic deposits Cu-porphyry
Economic deposits commonly with signification zone of hi-grade (1-2%) Cu,
enriched by SUPERGENE processes in the weathering environment.
