G4 key idea 1 Flashcards
(13 cards)
cumulate deposits (iron chromite)
chromite layer forms at the base of the convecting magma chamber due to magmatic segregation
when convection slows, the lighter silicate minerals become less buoyant and settle
repeated injection of new magma replenishes mineral supplies and renews convection, allowing only denser minerals to settle
hydrothermal mineralisation
vein formed in a fracture (joint/ fault)
disseminated mineralisation
fine grained ore minerals are scatters throughout host rock, occupying pore spaces and grain boundaries/ fractures
solution cavities in limestone
containing infills of ore minerals and gangue (waste deposits)
pegmatite deposits
form from the final water and element rich stages of cooling magma rich in volatiles (rare metals such as beryllium and uranium)
in granitic magma, minerals like feldspar, muscovite and quartz crystallise first
others are left behind in the residual melt enriched with volatiles
this melt then intrudes into cracks or edge zones of the main intrusion or country rock
due to cooling slowly, large crystals form, sometimes meters long
massive sulphide ores (black smokers)
seawater percolates down through fissures in the crust near spreading centres or subduction zones
hot mineral rich fluids meet cold sea water
the seawater does not boil due to extreme pressure at this depth
massive sulphide deposits are precipitated
black - mostly iron sulphide
white - calcium and silicon
residual deposits
minerals left behind after intense chemical weathering of rock
one product is Kaolinite - a clay which is the product of feldspar hydrolysis
useful product in the china paper and paint industries
also contains large quantities of aluminium
all soluble minerals are removed in percolating water by leaching (dissolving soluble minerals)
leaving behind surface ground layers rich in Al
laterite soil rich in insoluble hydrated aluminium oxides = Bauxite
for bauxite to develop there must be the correct bedrock and climate
placer deposits
in each case, there will be selective enrichment in the heavier (usually denser) particles
this may result in the deposition and accumulation of valuable minerals as a placer deposit through weathering and erosion and transport
characteristics:
high density
unreactive
lack of Cleavage
relatively hard
sites for deposition of placer deposits
meander bends
deposition on the inside of the bend where velocity is at its lowest
plunge pools
dense placer minerals become trapped in the sediments of a plunge pool
upstream of projections
projections of the riverbed will trap dense placer minerals on the upstream side
may be where a hard rock (dyke) projects upward
beaches
waves throw sediment up the beach on the swash and as the energy of the water reduces on the backwash, dense placer minerals can be left behind
aggregates : sand and gravel
an aggregate is a mass or cluster of mineral grains or crystals that form in a rock
e.g granite is an aggregate of quartz, feldspar, and mica
aggregates are sand, gravel, crushed stone used as bulk material in construction
used in concrete, road base and drainage systems
source rocks
oil and natural gas form from microscopic, planktonic marine algae
these floating organisms die and sink through the water column to settle and accumulate on the sea bed in a low energy environment
conditions must be anoxic so plankton will not decay or be scavenged and eaten by other organisms
anaerobic bacteria cause partial decay of the plankton to an organic mud
examples include black oil shales and mudstones, dark in colour due to the high organic carbon content
what makes a good source rock
contain 90% of all organic matter in sediments
anoxia can be created if water circulation is restricted
deeper waters become stagnant and anoxic
little bioturbation (no oxygen so few organisms)
organic-rich sediments are undisturbed
what makes a good reservoir rock
high porosity to store significant amounts of oil and natural gas
high permeability to allow them to migrate into it and then be extracted
include poorly cemented sandstones, jointed and fractured
cap rocks
further upward migration of hydrocarbons is prevented by an overlying impermeable caprock
otherwise they will continue to rise, forming seeps of gas and oil
include fine-grained clay, mudstone, shale and evaporites
traps
traps allow hydrocarbons to be concentrated in one place, making them economic to extract
migration from the reservoir rock must be prevented by impermeable barriers (caprocks) to the sides as well as above
