Lecture 13 resources 4 porphyry copper

Question 1

What happens in divergent plate boundaries?

Answer 1

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

Question 2

What happens at convergent plate margins

Answer 2

subduction zone

Question 3

Where do porphyry copper deposits form?

Answer 3

above Subduction zones
- oceanic or continental arcs

Question 4

Which deposits form at divergent plate boundaries?

Answer 4

Volcanogenic massive sulphides - mid ocean ridges, hydrothermal which causes big submarine deposits.

Question 5

Why is copper important

Answer 5

• principle use is for alloys,
• need it for construction - wiring in houses
• transport - average car has 1.5km of wire.

Question 6

Who has the most copper reserves?

Answer 6

Chile

Question 7

What are the types of copper deposits?

Answer 7

Porphyry copper deposits

stratiform sedimentary copper deposits

Volcanic massive sulfide (VMS) ore deposits

iron oxide- copper- gold (IOCG) ore deposits

copper skarn deposits

Question 8

What are porphyry deposits

Answer 8

• 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.

Question 9

What are stratiform sedimentary copper deposits

Answer 9

• 2nd most important type of deposits (1/4 of identified cu deposits)

Question 10

Volcanogenic massive sulfide (VMS)

Answer 10

• source of copper sulfide, formed through hydrothermal alteration in submarine environments

Question 11

Iron-oxide-copper-gold(IOCG) ore deposits

Answer 11

• high value ore deposits associate with iron oxides
• related to fault systems but no genetic relationship with exposed igneous intrusions

Question 12

copper skarn deposits (don’t really need)

Answer 12

• formed through chemical and physical mineral alterations at contacts, two separate lithologies.

Question 13

Porphry copper mineralisation - what are the deposits like?

Answer 13

• 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.

Question 14

The geology of a porphyry copper deposit

Answer 14

• 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.

Question 15

What is porphyry

Answer 15

used to describe texture of the rock
- source rock
copper porphyry deposits associated with veins

Question 16

what can expose copper deposits on the surface

Answer 16

erosion can expose surface copper
- over time further erosion and oxidation and dissolution so the copper deposits can disappear.

Question 17

Key facts about copper deposits

Answer 17

• 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.

Question 18

What does the model lowland and gilbert explain

Answer 18

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

Question 19

Where is the highest grade ore - Lowell guilbert model

Answer 19

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

Question 20

where is the ore located

Answer 20

• 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.

Question 21

What is hydrothermal alteration

Answer 21

Any alteration of rocks or minerals by the reaction of hydrothermal fluid with pre-existing solid phases.

Question 22

How does the hydrothermal alteration happen in the stock.

Answer 22

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

Question 23

What size and grade are porphyryry copper deposits

Answer 23

very large and low grade

Question 24

Porphyry copper deposits are associated with…

Answer 24

Porphyry copper deposits are associated with intermediate to acidic plutonic intrusives.

Question 25

What are Porphyry copper deposits characterized by

Answer 25

They are characterized by intense and extensive hydrothermal alteration
of the host rocks.

Question 26

What are Porphyry copper deposits associated with?

Answer 26

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.

Question 27

How many intrusive events for CU-porphyry

Answer 27

Commonly multiple intrusive events with mineralization often associated with
the youngest event

Question 28

What does Cu-Porphyry represent

Answer 28

Porphyritic reflects sudden crystallization due to rapid chilling or from
concentration and sudden release of fluid phases

Question 29

Cu-porphyry - zonation

Answer 29

Intrusions and host rocks are intensely fractured, and mineralization and
alteration form large zones, which exhibit lateral zonation.

Question 30

Economic deposits Cu-porphyry

Answer 30

Economic deposits commonly with signification zone of hi-grade (1-2%) Cu,
enriched by SUPERGENE processes in the weathering environment.