Skip to Main Content
Book Chapter

Gravity Differentiation and Magmatic Re-emplacement of Podiform Chromite Deposits1

T. P. Thayer
T. P. Thayer
U. S. Geological Survey, Washington, D. C.
Search for other works by this author on:
January 01, 1969


Relict cumulate features preserved in podiform chromite deposits include some textures found in stratiform complexes and, in addition, nodular and orbicular textures. Nodules of chromite are shown to have grown while freely suspended in magma, either by crystallization of chromite alone, or of chromite plus plagioclase and/or olivine in varying proportions. Some chromite nodules have the external form of crystals, but internally all are aggregates of grains or crystals in random orientation. Regardless of form, the nodules constitute packed structures like piles of marbles. In most podiform deposits the relict textures have been modified or destroyed by flowage at magmatic temperatures; extreme deformation produces gneissic silicate rocks and schlieren-banded disseminated chromitites.

The podiform chromitites are believed to have formed as layers in supercomplexes analogous to those known to have produced stratiform type chromitite, by gravitational differentiation of fluid magma in the upper part of the mantle. The chromite of podiform deposits is characterized by high MgO: FeO ratio and reciprocal variation in Cr2O3 and A12O3; primary variation in total Fe content is much smaller than in the stratiform deposits.

The layered differentiates appear to have been re-emplaced into the crust as hot crystal mushes. During re-emplacement chromite was solid and relatively rigid compared to olivine, pyroxene, and plagioclase which, although also solid, yielded more or less plastically by crushing and recrystallization. However, some interstitial magma able to form dilation dikes was present.

Random distribution of gabbro and peridotite in many alpine complexes and erratic compositional variation of chromite in neighboring deposits indicate mixing of the various differentiates during re-emplacement. A few masses of feldspathic chromitite found in feldspar-free peridotite are attributed to sinking of broken-up dense layers of chromitite (sp. gr. 3.8–4.2) from troctolitic into peridotitic crystal mushes (sp. gr. 2.8–3.2) during intrusion. To produce the thicknesses of massive chromitite and volumes of dunite and olivine-rich peridotite in alpine complexes requires very large volumes of magma, or primary magma more mafic than tholeiite, and differentiation trends very different from those of known stratiform complexes.

You do not currently have access to this article.

Figures & Tables


Society of Exploration Geophysicists Geophysical Monograph Series

Magmatic Ore Deposits

H. D. B. Wilson
H. D. B. Wilson
Search for other works by this author on:
Society of Economic Geologists
ISBN electronic:
Publication date:
January 01, 1969




Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal