Ore Deposits Related to Mafic Igneous Rocks

1. Rheabelle E. Palajos

2. I. Introduction
II. Layered Mafic Intrusions
III. Anorthosites
IV. Kimberlites
V. Carbonatites
VI. Ultramafic Volcanic Rock Associations

3.  MAGMATIC SEGREGATION DEPOSITS
- Ore deposits that are direct crystallization products of a magma
- Usually form in the magma chamber
- Can also be driven into magma chamber walls and roofs to form
dikes, sills and extrusive flows
The ore mineral could be early or late fractionation products concentrated by:
-gravitative settling of crystals or liquids
-liquid immiscibility or filter pressing

4. Mafic rocks: chromite, ilmenite, apatite, diamonds, nickel,
copper, and platinum.
Intermediate rocks : magnetite, hematite, ilmenite, and
vanadium.
Felsic rocks : magnetite, hematite, with accessories
mineral such as zircon, monazite, uraninite, and
cassiterite.

5.  Rock types of this system are considered to have formed in cratonic
settings, but in cratons that were undergoing rifting or proto rifting.
This cratons are deeply rifted that upper mantle partial melting is
tapped or generated.
 They are uncommon in orogenic belts, occurring only along
lineaments within stable continental interiors.

6.  Largest ore-forming magmatic system
 Major source of chromium, nickel, copper, platinum metals, titanium, iron,
vanadium, tin, and by-product sulfur
Name Age Location Area (km2)
Bushveld Precambrian S. Africa 67,000
Dufek Jurassic Antarctica 50,000
Duluth Precambrian Minnesota, USA 4,700
Stillwater Precambrian Montana, USA 4,400
Muskox Precambrian NW Terr. Canada 3,500
Great Dike Precambrian Zimbabwe 3,300
Kiglapait Precambrian Labrador 560
Skaergård Eocene East Greenland 100

7. BIC holds the
earth’s greaterst
reserves of:
• Chromium
• Platinum
• Palladium
• Osmium
• Iridium
• Rhodium
• Ruthenium
• Iron
• Titanium
• Vanadium
• tin
Figure . Simplified geological map of the
Bushveld Igneous Complex (BIC)
Bushveld Igneous Complex (BIC) is
measuring 375-km east-west by 300-km
south-west. It extends over 67,000 km2.

9.  Layer- sheet-like cumulate unit having uniform compositional and/ore textural
features.
Uniform chromite layers
alternate with plagioclase-
rich layers, BIC

10.  Layering (stratification)- the structure and fabric of sequences of multiple layers
1. PHASE LAYERING
-appearance or
disappearance of minerals
in crystallization sequence
developed in modal layers

11. 2. CRYPTIC LAYERING
-not obvious to the eye
-systematic variation in
chemical composition of certain
minerals with stratigraphic
height in layered sequence
Cumulate orthopyroxenite –
Cumulus orthopyroxene (brown),
intracumulus plagioclase (white)
and the large, poikilitic
intracumulus augite (green)

12. Regularity of layering
1. RHYTHMIC
LAYERING
-layers systematically
repeat
• Macrorhythmic
-several meters thick
• Microrhyhtmic
-only a few centimeters
thick

13. 2. INTERMITTENT LAYERING
-less regular patterns
Common type consists of
rhythmic graded layers
punctuated by uniform layers

14.  Graded Bedding- any gradual variation in mineralogy or grain size
Modal and size graded
layers.

15. Graded bed between
homogeneous layers,
SIC

16.  The layering has been ascribed to:
1. Magma mixing
2. Reversals in the thermal or chemical regime
3. Variation in oxygen fugacity
4. Convective circulation
5. Differential magmatic sedimentation and winnowing of minerals
6. Sheeted, turbidite-like spreading of crystal mushes
7. Evolution and gravitative separation of immiscible silicate and oxide magmatic liquids
8. Varied bottom crystallization
9. Differential flotation of lighter phases during sedimentation of denser solids
10. Variation in total pressure
11. Combinations of all these explanations

17. Anorthosites are monomineralic rocks composed of 90% or
more intermediate to calcic plagioclase bodies which
contain the world’s most significant igneous titanium
orebodies as rutile, ilmenite, and titanomagnetite.
Anorthosites can result from partial melting of tholeiitic
composition, rather than deep eclogate ones, at fairly
shallow depths in the asthenosphere and again probably
in response to shallow rifting (Simmons and
Hanson,1978).

19. Two types :
1. layered mafic rock near the upper portion of LMIs which
formed after the mafic mineral crystallized and sunk
2. Anorthosite massifs – plutons that typically contain
plagioclase.
a. labradorite anorthosite massifs
-magnetite-ilmenite-rich anorthosites
b. andesine anorthosite massifs (Adirondack type)
-ilmenite-rich anorthosites
*Andesine-type may be either more shallowly derived
by partial melting or contaminated by lower continental crustal
materials.

20. a. Mantle-derived magma underplates the crust as it becomes density equilibrated.

21. b. Crystallization of mafic phases (which sink), and partial melting of the crust
above the ponded magma. The melt becomes enriched in Al and Fe/Mg.

22. c. Plagioclase forms when the melt is sufficiently enriched. Plagioclase rises to the top of the chamber
whereas mafics sink.

23. d. Plagioclase accumulations become less dense than the crust above and rise as crystal mush plutons.

24. e. Plagioclase plutons coalesce to form massif anorthosite, whereas granitoid
crustal melts rise to shallow levels as well. Mafic cumulates remain at depth
or detach and sink into the mantle.

25.  KIMBERLITES- volatile-rich, potassic ultramafic igneous rock dominated by olivine,
with subordinate minerals of mantle derivation.
 Kimberley, South Africa, site of the first and probably the most dramatic diamond rush
 Kimberlites have been found on all continents except Antarctica.
 Mineralogical guide for kimberlites:
- chromian diopside
- magnesian ilmenite
- high-chromium, low calcium Mg-Al (pyrope) garnets

27. Typical model of a kimberlite pipe
• Not all kimberlite pipes contain diamonds at
levels currently exposed.
• They represent a rapid, violent upward rush
of deep-mantle material from the
asthenosphere in the form of a diatreme.
• Diamond ores are perhaps the “lowest grade”
mined anywhere.
Three texturally distinctive kimberlite facies:
1. Hypabyssal kimberlites
- deeper seated, porphyritic, and result from
crystallization of kimberlite magma beneath the
diatreme
2. Diatreme kimberlites
-represent the bulk of the intrusive body
-contain mantle- and crustal-derived rock
fragments, with kimberlite minerals.
-dominated by tuffisite in their upper
portions
*TUFFISITE- a rock that looks like a tuff and
has the characteristic of one, but that was
intruded into position
3. Crater kimberlites
- May be pyroclastic fallback breccias or
epiclastic, water-lain material.

28.  They are rare igneous rocks formed predominantly of carbonate, whose only modern
expression is a single active volcano that erupts strongly alkaline carbonate lavas with
no direct match in Earth’s geological record.
 Mafic rocks that appears to be closely related to kimberlites
 They are derived from the mantle, showing almost no sign of contamination by the
crust.
 They contain calcite, dolomite, siderite which has been concluded to be truly igneous.
 Most carbonatites have unambiguous origins in the mantle and the limit to their depth
is not known, but the likelihood that they may exist in the lower mantle (Kaminsky et
al. 2009, 2012; Stoppa et al. 2009) needs to be appraised since they may exert a
fundamental control on the mobility and long-term storage of deep carbon in Earth.

29.  Carbonatite complexes are concentrically arranged
- rock types become progressively poorer in silica toward the core which is
commonly occupied by carbonatite.
 Typical succession of rocks from the rim to the core would consist of:
-nepheline syenite
-ijolite
-carbonatite with all rocks being cut by lamprophyric dikes

31. 1. With alkalic ring complexes
2. With alkali complexes not of the ring type
3. Not associated with alkali rocks
4. As flows and pyroclastic rocks
 Rare earth minerals: niobium-tantalum, zirconium-hafnium, iron-
titanium-vanadium, uranium-thorium
Industrial minerals: apatite, vermiculite, and barite

32.  Common association of nickel sulfides with both plutonic and
volcanic ultramafic rocks.
 Komatiites- ultramafic, high MgO volcanic flows
-the extrusive equivalents of peridotites, harzburgites, and
even dunites
-they are characterized by spinifex texture
*Spinifex texture- skeletal quench crystals of olivine and
pyroxene which resembles the spinifex grass

34.  Most of the ore is at the base of komatiite in contact with metabasalt, though some is in
lenses within the ultramafic flows.
 Localization of ores at the basal contacts is caused by structural depressions.
*Net texture-texture formed by the freezing of the sulfide liquid while olivine
crystals float upward within it and form a loose meshwork when immobilized.
 The ore consists of both massive and disseminated sulfides.
Primary mineralization:
-pyrrhotite-pentlandite assemblage with subordinate pyrite and minor amounts of
chalcopyrite; platinum metals are present but in low amounts.