Hydrothermal ore forming processes Flashcards
What are hydrothermal ores?
Deposits of ore generated from hot, ion-rich fluids
What are the main types of hydrothermal ore?
Vein
Disseminated
WHat are hydrothermal vein deposits?
Localised zone of fracture filling materials that often form in rock fractures and fissures close to fault zones
What are desseminated hydrothermal ore deposits?
Minerals dispersed in igneous rock
What hydrothermal processes form ores?
Hydrothermal fluid generation
Fluid migration
Mineral precipitation
Ore mineral deposition
Post-deposit alteration
What is needed to allow water to dissolve large amounts of metal?
Something else must be dissolved in there as well so circulated through a large volume of rock
What conditions need to change for precipitation to occur?
Temperature
pressure
composition
What are some examples of non-magmatic fluid generation?
Sea
Meteoric
Connate
Metamorphic
How are minerals from river rocks thermally deposited in the sea?
The thermal circulation - mid-ocean ridges then reword through black smokers
What is meteoric fluid generation?
Rain and ground water which is low temperature and contaons different H and O isotopes depending on evaporation strength
What is connate fluid generation?
water trapped in subsurface rocks high temperature and pressure alters rock properties
How is the release of water controlled from connates?
Depth and the changing temp and pressure on pore spaces
How does fluid migration generate ores?
Large water mobalisation needed to form ores, controlled by pressure and permeability with minerals being dissolved from host rock
How does thermal fluid migration occur?
Sea water drawn down into Basaltic oceanic crust (convection)
Heat up scavenge metals and then exhaled at mid ocean ridges
How does orogenic fluid migration occur?
Tectonic loading and compression
Squeezing fluids out ahead of Orogenic front
How does gravity driven fluid migration occur?
Hydraulic head water wants to flow down under force of gravity
Headed up as drawn down – able to scavenge metals
May remerge as hot springs
What are fluid pathways?
Spaces/ conduits for fluid to flow through (pore space or permeability)
Connected pores, joints or fractures reopened by tectonics
What are the anomalous properties of water?
Liquid at room temp
High heat capacity
High surface tension
Density maximum just above freezing
High dielectric constant
What is the critical point with water?
Temperature and pressure beyond which no boiling occurs (374c and 225 bar)
What is the density of liquid and vapour at the critical point?
0.3 g cm-3
WHat are the subcatergories of ore mineral deposition?
Porphyry
Skarn
Volcanogenic massive sulphide (VMS)
Sedimentary exhalative (SEDEX)
Mississipi Valley Type (MVT)
What are porphyries a global source of?
60% of global copper
What are porphyries associated with?
porphyritic felsic intrusions
WHat type of deposits are formed from porphyries?
Form large, low-grade deposits
Copper
Copper- Gold
Copper- Molybdenum
Molybdenum
How do porphyries form?
Partial melting of Oceanic crust. Melt rises and collect in magma chambers this rises and erupts forming volcanic cone magam solidifies but vent allows magma to keep accumulating and cooling
What are alterations like for porphyry?
Magam chamber cools out to in causing cystallisation (feldspar and quarts) left melt rich in volatiles and metals volatiles migrate to top (steam) increasing pressure cuaing rupture into country rock carrying metals into cooler rock altering surroundings
What are skarn deposits?
Silicate rocks produced by the interaction between the hot hydrothermal fluids released by crystallizing magma with carbonate rocks
What is the name of the process which produces skarn deposists?
metasomatism
Where are skarn deposits often found?
Contact zones between intrusive igneous rocks
What are the types of skarn?
Endoscarns
Exoscarns
How do exoscarns form?
Form outside an intrusive body
Fluids left over from crystallisationof intrusion are ejected from the mass (boiling)
Common
What are endoscarns?
Form within fractures, cooling joints and stockworks inside intrusive body
Rare
What are metasomatic fluids commonly rich in?
Si, Fe, and Mn react with Ca and/or Mg-bearing carbonate rocks
Causes release of CO2 and producing a wide range of Ca-Mg-Fe-Mn silicates
What are contact skarns?
Occur at the contact between carbonate and silicate rocks
What are vein skarns?
Occur as veins or dikes
What are SEDEX deposits?
Sedimentary Exhalative deposits
Where do SEDEX deposits form?
Fault bounded sedimentary basins on the earth’s crust
Where is the heat that dirves SEDEX from?
Dominantly from depth and burial rather than felsic intrusion
Where are SEDEX metals derived from?
From the leaching of the rocks
Gold and copper absent
What do volcanic massive sulphide (VMS) deposits mainly consist of?
Metal sulphide ore deposition mainly copper-zinc
What process forms VMS deposits?
Volcanic hydrothermal events in submarine
environments Predominantly accumulations of sulphide minerals that precipitate from hydrothermal fluids on or below the seafloor
What forms VMS deposists in modern oceans?
Black smokers
What exhalative deposits?
Fluids that emerge onto land surface (or seafloor)
On land degassing (geysers)
What is the source of exhalative deposits (land surface)?
continental crust
What are exhalative deposits (sea floor)?
Ocean floor: mix with cold water precipitation
May not degas (pressure)
What is the source of seafloor exhalative deposits?
Oceanic crust (basaltic)
How do submarine hydrothermal deposits form?
Fluids emerge onto ocean floor at 300°C mixes with cool water precipitates metal sulphides/sulphates
* E.g., Anhydrite (Ca sulphate); Barytes (Ba sulphate)
What are VMS black smokers like?
Chimney-like structures, rich in metal sulphides
What is an example of a present day submarine hydrothermal deposit?
Red sea
How do Mississippi Valley Type (MVT) deposits form?
Sedimentary basins subside and thicken
Compaction and convection
Plug- or pipe-like replacement of host rock
WHat are ore fluids of MVTs typically like?
low temperature
with composition of basal brines
WHat are residual desposits (chemical leaching)?
‘Leftovers’ after chemical weathering E.g., tropical climate:
- Warm and humid
- surface rock weathers rapidly
- Laterite (residual soil)
- Al, Fe (only Al economic)
What is secondary enrichment?
Metal ions in solution - supergene process
What is the supergene process of secondary enrichment?
Groundwater is oxidising (acid) at surface, reducing beneath water table sulphides dissolved close to surface metals deposited below water table
What is chemical transport and deposition by surface water?
Surface waters contain ions which can be evaporated off forming evapourites
What characteristics make copper a major industrial metal?
high ductility,
malleability,
thermal and electrical conductivity
resistance to corrosion
How is copper commonly extracted?
open pit Porphyry copper deposits
How does the distribution of copper ore affect the viability of mining?
Ore distributed so large low- grade ore bodies
Not good return when building a mine
How is copper typically processed?
hydrometallurgy
What is hydrometallurgy?
Use aqueous solutions to extract copper from copper-oxide
Involves:
* Heap leaching
* Solvent extraction
* Electrowinning
What are the impacts of copper mining?
Stability of open pit mines
Sulphide-ore copper mining can release numerous chemicals
Heap leaching processes uses sulphuric acid
What chemicals can be released from sulphide ore copper mining?
Mercury
Arsenic
Lead
Asbestos like fibres
What are the uses of zinc?
Used to galvanise other metals such as iron to prevent rusting
Galavanised steel is used for car bodies
Die-casting – important for Automobile, elecetircal and hardware industries
How is zinc mined?
Some open pit but mostly underground (drilling to deposits and using adits and shafts)
What is the impact of underground mining?
subsidence
What are the processes of zinc extraction?
Separation and zinc-lead blasting furnace
What are the steps of zinc separation?
- Isolate sulphide mineral
- Ore crushed into water slurry
- Placed in flotation tank with chemical reagents which form bubbles
- Mineral particles cling to bubbles at top of tank are skimmed
What is the impact of zince mining?
Releases toxic cadmium
Impacts:
* Water resources
* Soils
* Vegetables and crops
* Human health
Where can palladium be found?
Can be found in deposits alloyed with gold and platinum
What are the uses of palladium?
- Catalytic converters (extremely heat resistant)
- Electronics
- Fuel cells
- Dentistry
- Jewellery
How is palladium mined?
Mined through drilling blasting and crushing ore deposits
What are the impacts of palladium mining?
Significant waste
Energy intensive
Environmental pollution
What are the environmental impacts of palladium mining?
soil erosion, deforestation
Heavy metals into air
Large water usage
Low toxicity metal
Allergic reactions
Tests on roadent show potentially carinogenic
Compounds regarded as highly toxic
where is aluminium used?
Transport
COnsumer goods
Architecture
Electricals
How is aluminium used in transport?
Light weight and strong makes vehicles easier to
manoeuvre and better fuel efficiently (lighter)
How is aluminum used in consumer goods?
Household tech i.e. phones
How is aluminuim used in architecture?
Can be curved, punched, folded etc.
Cladding panels (radiates heart, less fixings than steel in buildings)
How is the electrical industry using aluminium?
Long distance power lines
How is aluminium mined?
Open pit mining of Bauxite
Ore is refined to recover alumina
Alumina smelted to produce aluminium
What are the processes of aluminium production?
Digestion
Clarification
Precipitation
Calcination
WHat are the imapcts of aluminum mining?
Energy and emission intensive
Toxic agent in high concentrations
Acid precipitation and aciification of catchments
WHat are the environemtnal impacts of gold mining?
Wastewater leaks include toxic heavy metals and cyanide
Heating cyanide produces air pollution
Cyanide pollution/ contamination
Why is cyanide used in heap ore extrcation?
Crushed ore on water proof membrane with cyclic pumping of sodium cyandie to dissolve gold into solution run through carbon filter where cyandie bound gold sticks to carbon which is then burnt for extraction
