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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.
  8.  Layer- sheet-like cumulate unit having uniform compositional and/ore textural features. Uniform chromite layers alternate with plagioclase- rich layers, BIC
  9.  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
  10. 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)
  11. Regularity of layering 1. RHYTHMIC LAYERING -layers systematically repeat • Macrorhythmic -several meters thick • Microrhyhtmic -only a few centimeters thick
  12. 2. INTERMITTENT LAYERING -less regular patterns Common type consists of rhythmic graded layers punctuated by uniform layers
  13.  Graded Bedding- any gradual variation in mineralogy or grain size Modal and size graded layers.
  14. Graded bed between homogeneous layers, SIC
  15.  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
  16. 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).
  17. 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.
  18. a. Mantle-derived magma underplates the crust as it becomes density equilibrated.
  19. 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.
  20. c. Plagioclase forms when the melt is sufficiently enriched. Plagioclase rises to the top of the chamber whereas mafics sink.
  21. d. Plagioclase accumulations become less dense than the crust above and rise as crystal mush plutons.
  22. 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.
  23.  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
  24. 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.
  25.  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.
  26.  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
  27. 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
  28.  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
  29.  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.