WEEK 9 - Metal 2 Flashcards
(42 cards)
1
Q
How is magma generated at convergent boundaries?
A
- Formed by hydration melting.
- Magma is intermediate to felsic in composition.
- Contains lots of water and volatiles.
2
Q
What happens after magma crystallizes at convergent boundaries?
A
- The leftover fluid is hot, watery, and full of metal.
- It moves into cracks in nearby rocks.
- As it cools, it leaves behind metal minerals spread through the rock or in metal-rich veins (like cracks filled with metal).
3
Q
What is contact metamorphism and how is it related to metal deposits?
A
- Occurs when hot plutonic bodies heat nearby rock.
- Can also lead to the formation of mineral deposits.
- This is the next process in mineral deposit formation.
4
Q
How can hydrothermal processes during metamorphism create metal deposits?
A
- Contact metamorphism can affect chemical sedimentary rocks like limestone.
- Can create skarn deposits (a type of metal deposit).
- Magma heats the limestone → turns it into marble.
- Acidic fluids from magma dissolve holes in the marble.
- Metallic minerals can form in these holes.
5
Q
What is a skarn and how does it form?
A
- A skarn forms when hot fluids from a magma intrusion react with surrounding rock.
- Usually happens where limestone is turned into marble by contact metamorphism.
- The fluids dissolve parts of the rock and can lead to metal-rich deposits.
6
Q
What can be found in a skarn deposit?
A
- Red chalcocite (copper sulphide)
- Native copper
- Pyroxene crystals (green)
- Garnet crystals (red/brown)
- Often found as crystals or metallic veins in the marble
7
Q
What happens to magma at divergent boundaries?
A
- Magma forms by decompression melting
- It’s mostly mafic (rich in magnesium and iron)
- Metal deposits here are linked to mafic igneous rocks
8
Q
How are hydrothermal metals formed at divergent boundaries?
A
- Metal-rich brines (from magma) can move into cracks and cool
- These brines can form metallic mineral deposits
9
Q
What plays a major role in metal concentration at divergent boundaries?
A
- Seawater interacts with oceanic crust under the seafloor
- This seawater gets heated, dissolves metals, and helps concentrate them
10
Q
What happens to seawater at mid-ocean ridges?
A
- Salty seawater seeps into faults and cracks in ocean crust
- The water gets heated by nearby magma
11
Q
What does the heated seawater do at mid-ocean ridges?
A
- It dissolves sulphide ions and metal ions
- Metals include iron, copper, lead, and gold
- These leached ions are carried in the hot water
12
Q
How do metal-rich fluids rise and escape in VMS deposits?
A
- Heated water carrying metal ions rises through fractures
- Escapes at the seafloor through volcanic vents called black smokers
13
Q
How do metals form deposits near black smokers?
A
- When hot solution meets cold seawater, metals precipitate
- Metals form sulphide minerals or native metals (like gold)
- Fine crystals settle and build up in pods on the seafloor
14
Q
What is the “black smoke” from black smokers made of?
A
- Not actual smoke
- Made of fine-grained metal sulphide crystals
- Precipitated from hot vent water
15
Q
What forms around black smoker chimneys in VMS deposits?
A
- Black smoker chimneys build up from mineral deposits
- VMS pods (metal-rich deposits) form around the chimneys
16
Q
What is a Volcanogenic Massive Sulphide (VMS) deposit?
A
- Formed from seafloor volcanism
- Pod-like deposit of sulphides
- Not well defined in shape
17
Q
What metals are commonly found in a VMS deposit?
A
- Pyrite
- Chalcopyrite (copper ore)
17
Q
How does chemical weathering lead to metal deposits?
A
- Chemical weathering removes metals from rocks.
- Example: Bauxite (aluminum ore).
18
Q
What is bauxite and how is it formed?
A
- Bauxite is aluminum ore.
- Formed from the weathering of feldspar and mica.
- Gibbsite forms through chemical weathering.
19
Q
How is aluminum extracted from bauxite?
A
- Gibbsite is heated to remove water.
- Further heated to separate pure aluminum from oxygen.
20
Q
What is bauxite and how does it form?
A
- Bauxite is a highly weathered soil.
- Formed when most dissolved ions are washed away, leaving gibbsite.
21
Q
What is the composition of bauxite?
A
Bauxite is rich in gibbsite (Al₂O₃·3H₂O).
22
Q
What are examples of bauxite deposits?
A
- Field photo of bauxite deposit.
- Hand sample of bauxite.
23
Q
How can metallic mineral deposits form through sedimentary processes?
A
- Some metallic mineral deposits form by sedimentary processes.
- Erosion and physical transportation of minerals concentrate metals in clastic sediment deposits.
24
How can metals be concentrated in chemical sediments?
Chemical sediments can also concentrate metals of interest.
25
What are examples of sedimentary deposits with metals?
- Gold-bearing conglomerate from South Africa.
- Banded iron formation in Australia.
26
What are placer deposits?
Placer deposits form when rock fragments eroded from rocks are affected by moving water.
27
What are stream placers?
- Stream placers form when heavy minerals like gold are concentrated in areas affected by stream currents.
- Light particles are washed away, concentrating large/heavy particles.
28
Where are good areas to prospect for placer deposits?
- Point bars, waterfalls, and potholes in the stream bed are good prospect areas.
- These areas have fast-moving water that suddenly slows down, allowing heavier particles to settle.
29
What are beach placer deposits?
Beach placers are deposits that form on beaches where heavy minerals are concentrated due to wave action
30
What is magnetite concentration on the beach?
Magnetite (often called "black sand") is concentrated along beaches due to the wave action sorting out lighter minerals.
31
What are iron oxide placers?
Iron oxide placers are similar to beach placers, where iron-rich minerals like magnetite are deposited by natural sorting processes on the beach.
32
What is panning?
Panning is a method of separating gold from sediment using water and agitation.
33
How does panning work?
In panning, sediment and water are mixed and stirred, causing gold to sink to the bottom while lighter materials are washed away.
34
What is pay dirt?
Pay dirt is the gold-bearing (contains gold) sediment left behind after the panning process.
35
What is the largest gold nugget ever found?
- The Welcome Stranger Nugget, found in Moliagul, Australia, is the largest gold nugget ever discovered.
- Gross weight: 78.4 kilograms (about 173 pounds).
36
What is Banded Iron Formation (BIF)?
- Banded Iron Formation (BIF) is a major source of iron ore.
- During the Proterozoic era, iron dissolved from continental rocks and was carried to the sea.
- The lack of oxygen in the atmosphere kept iron dissolved in river water.
- Once bacteria in the sea produced enough oxygen, iron oxides (magnetite or hematite) precipitated out as chemical sediment.
37
What does Banded Iron Formation look like?
Example: from northern Ontario.
- Purple-grey bands: hematite and magnetite.
- Red bands: iron-stained chert (jasper).
38
What is Banded Iron Formation (BIF) made of?
- Banded Iron Formation (BIF) consists of alternating layers of iron oxide (hematite or magnetite) and chert.
- Both iron oxide and chert are precipitated as chemical sediments.
39
Why doesn’t Banded Iron Formation form today?
1. Oxygen in the atmosphere rusts iron on land, so iron is precipitated before reaching the sea.
2. Silica dissolved in the sea is taken up by organisms like sponges.
40
Why are ore deposits important?
- Ore deposits are formed through various geological processes.
- Knowledge of geology helps in understanding and finding these deposits.
- Geologists play a key role in producing metals used in everyday products.
41
Why is geology important for metals?
- Geologists help us access metals for the products we use daily.
- Without geologists, we wouldn’t have the metals in many of our everyday products.
