• Shuffle
    Toggle On
    Toggle Off
  • Alphabetize
    Toggle On
    Toggle Off
  • Front First
    Toggle On
    Toggle Off
  • Both Sides
    Toggle On
    Toggle Off
  • Read
    Toggle On
    Toggle Off
Reading...
Front

Card Range To Study

through

image

Play button

image

Play button

image

Progress

1/12

Click to flip

Use LEFT and RIGHT arrow keys to navigate between flashcards;

Use UP and DOWN arrow keys to flip the card;

H to show hint;

A reads text to speech;

12 Cards in this Set

  • Front
  • Back

2 deposit subtypes

1. Stratiform (monominerallic horizons in layered intrusions)


2. Podiform (lenses, pods in ophiolites)

Grade and tonnage

100-1000 Mt @ 45-50% Cr2O3

Source of Cr

- partitions only in chromite


- can form solid solution with magnetite


- always oxide, never natural

Ore textures

- host rocks with cumulate textures (compaction of cumulate pile and main chamber composition still able to enter the system, additional growth around grains)



- growth boundary between initial and subsequent is distinct (poikilitic texture - interstitual minerals join to form large, background matrix)


Sequence of ultrabasic cumulates

Tectonic settings

- continental rifts (stratiform)


- subduction/collision zones; thrusted oceanic crust - ophiolites (podiform)


Genesis

- crystallisation in a mafic intrusion by gravitational settling


- earliest freactional crystallisation produced pure chromite



- arguments against:


- can't explain lateral continuity across whole chamber


- cant explain 99% chromite, normal crystallisation produces layers too small

1. Increase FO2 (oxygen fugacity)

- oxidising conditions, favourable to oxide precipitation


- mechanism of increase: decomposition of siderite ( siderite - hematite, carbon monoxide and O could potentially flood chamber with O2)

2. Magma Mixing (+arguments against)

- early magma fractionates in chamber


- injection of primitive basaltic magma into earlier fractionated magma


- mechanism of chromite precpitation can be explained through phase diagram (ol crystallises first, gets used up reaches boundary and precipitates both, triple point crystallises all 3)


- reinjection of more felsic magma drives the reaction into the chromite field, allows for precipitation of pure oxide long enough to produce a thin seam



Arguments against


- injection of magma is localised, should produce zonation around intrusion and does not explain lateral continuity

3. Assimilation of country rock (+arguments against)

- Hot basaltic magmas (>1300C) intrude upper crustal rocks with low melting points


- causes melting of country rock and mixing of melt with primitive basalt



Arguments against


- would produce downward plumes or chromite not long stratiform layers

4. Pressure change in chamber

- at low pressures, reaction is driven from the phase boundary into the chromite field

Exploration (both Stratiform and Podiform)

Stratiform


- identification of well layered mafic-ultrabasic intrusion


- prospect below cumulate portion of intrusion



Podiform


- carefully prospect within all dunite portions of Alpine-type peridotites