Week 9 Flashcards
(38 cards)
Skarns
Can host metals and metal-rich minerals due to reaction of hot fluids with the rock into which the intrusion was emplaced
divergent boundaries
Magma generated at divergent boundaries (by decompression melting) tends to be mafic in composition.
Metallic mineral deposits in divergent boundary settings tends to be associated with mafic igneous rocks
Volcanogenic Massive Sulphide (VMS) deposits
Seawater enters cracks, heats up, and dissolves metal and sulphide ions.
Heated water rises and exits through black smokers.
When it hits cold water, metals precipitate as sulphide minerals, forming pods of fine crystals on the seafloor.
How does weathering lead to aluminum (bauxite) deposits?
Chemical weathering breaks down feldspar and mica, releasing aluminum.
Aluminum bonds with water to form gibbsite.
Gibbsite is heated to remove water and extract pure aluminum from oxygen.
Bauxite
A highly weathered soil
How do sedimentary processes form metal deposits?
Metals can concentrate through physical sorting of eroded minerals or by chemical precipitation in chemical sediments.
Placer Deposits
Many rock fragments eroded from rocks are ultimately affected by moving water
Stream Placers
Particles of gold and other heavy minerals can be concentrated in areas affected by stream currents
(remember light particles are preferentially washed away, concentrating large/heavy particles).
Panning
Panning separates gold (dust to nugget size) by agitating sediment in water. Lighter material is washed away, leaving heavier “pay dirt” (gold) behind—same principle as placer formation.
How did Banded Iron Formations (BIFs) form?
In the Proterozoic, iron dissolved from rocks was carried to the sea.
With little oxygen, it stayed in solution until bacteria produced enough oxygen for iron oxides (magnetite/hematite) to precipitate as chemical sediments.
Banded iron formation does not form today because …
1) The oxygen in the atmosphere rusts iron on land (so is precipitated before it can reach the sea).
2) Most silica dissolved in the sea is taken up by some organisms (e.g. sponges)
The Stone Age
Prior to the use of metals, humans relied on materials such as obsidian, chert and quartzite for the fashioning of tools.
Late stone age
Humans used soft, malleable native gold (likely from placer nuggets) to make decorative objects.
The Copper Age
humans used native copper (pure form) by coldworking it into tools like hooks and needles
limitation of cold-worked copper
brittleness and softness
Why is accidental smelting in campfires unlikely?
1) Campfires don’t reach copper’s melting point (>1000°C).
2) High oxygen levels would oxidize the metal, making it brittle and unworkable.
How might ore minerals have ended up in a pottery kiln?
Potters may have added colorful minerals like malachite or galena to decorate pottery.
Firing the kiln could produce molten metal beads, which observant potters realized could be collected and refined into larger metal pieces.
The invention of bronze
As malachite became scarce, copper was extracted from sulphide minerals like chalcopyrite.
These ores contained arsenic, which mixed with copper during smelting, creating the first metal alloy—bronze.
The advantage of bronze
bronze is harder than pure copper, and is therefore much more useful for tools.
The iron age
Iron blooms, unlike copper, retained slag that was removed by reheating and pounding on an anvil
What are the drawbacks of working with pure iron?
1) Pure iron is softer than bronze.
2) It can’t hold a sharp edge.
3) Iron oxidizes (rusts) easily.
Steeling
Adding carbon to iron increases hardness, creating steel
Tempering
involves quenching hot metal in water to increase hardness, then reheating to reduce brittleness.
Casting
In China developed furnaces to melt iron, which was then poured into molds.
The cast iron was reheated to remove excess carbon, making it more elastic and malleable for use
