week 3

Question 1

where does the water come from

Answer 1

• some of the water is derived from the dehydration of minerals that were melted to form the magma e.g. hydrous minerals muscovite, biotite and amphibole
• some of the water in arc-related magmas derives from the subduction of oceanic crust that has been highly altered and hydrated by percolating seawater

Question 2

S-type vs I-type granites

Answer 2

I-type granites:
- relatively ‘dry’
- metaluminous (Na2O+K2O+CaO>Al2O3)
- formed by melting of a biotite or biotite + hornblende-bearing source rock (i.e. meta-igneous rocks)
- associated with Cu-Mo ore deposits
S-type granites
- relatively ‘wet’
- peraluminous (Al > Na + K)
- formed by melting of a muscovite or muscovite + biotite-bearing source rock (i.e. metasediments)
- associated with SN-W-U ore deposits

Question 3

describe arc magmatism

Answer 3

• seawater introduces chlorine into the crust (lots of Cl in arc magmas that must have come from seawater). sedimentary units e.g. limestones can introduce CO2. SO2 is also likely to have come from seawater
• the fluids produce basaltic melts in the mantle wedge that rise through the crust and differentiate as they go, ending up as shallow, highly evolved, felsic magmas

Question 4

how do these evolved melts make a mineral deposit

Answer 4

• arc magmas tend to be pretty similar. it is the fluid and the host rocks they interact with that influence whether a mineral deposit is formed and what kind of deposit
• understanding the composition of the fluid is key

Question 5

describe proto-ore fluids

Answer 5

• magmatic gases exsolve from arc-magmas and extract metals such as Cu, Au, Mo etc.
• ore metals are extracted from magmas with varying efficiencies depending on the gas chemistry, available metals/volatiles and other physical parameters

Question 6

describe H2O phases with salt

Answer 6

• above the critical point, the higher boiling point of saline fluids means that saline liquid (brine) and less-saline vapour can coexist
• this is extremely important for ore formation because metals partition differently between the salty brine (liquid) and less salty H2O vapour (gas) phase
• the brine is more dense than the vapour so will settle while the less dense than the vapour so will settle while the less dense vapour keeps moving up and out of the system
• solubilities of many metals in a magmatic-hydrothermal fluid are strongly dependent on T, pH and chloride anion (Cl-) concentration
• certain metals will only readily dissolve in an aqueous solution if the solution contains appreciable amounts of Cl-

Question 7

describe the role of sulphur

Answer 7

• despite the importance of NaCl-H2O, PCDs are sulphide deposits
• so where does the S come from?
• we know it is sourced from a magma, not from sedimentary host rocks
• but magmas that produce lots of Cl cannot hold much S. if we put lots of S in them, we’d just make sulphides and thus strip the metals out of the system
• in volcanic systems, despite having evolved melts with very little S, fumaroles produce lots of S. it is thought the gases leave the magma and steam through the crustal differentiation reservoir and emerge from a magma that is not associated with the volcano