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.
Figures & Tables
This monograph on Magmatic Ore Deposits has resulted from a Symposium held at Stanford University on November 12 and 13, 1966. All except three of the papers that were presented are published in this volume as well as some of the discussion and the summation of the symposium. Unfortunately much of the discussion cannot be included because the volume is already so large. The best introduction to this volume is, perhaps, the introduction as it was presented at the symposium:
This symposium was conceived in 1962 when the Program Policy Committee recommended that the . Society of Economic Geologists should sponsor a symposium on magmatic ore deposits. The Committee under the chairmanship of John K. Gustafson believed this to be an effective method of advancing geologic thought. It is fitting that the symposium should finally be held during Gustafson’s presidential year. The proposal of the Program Policy Committee was approved by Council at its meeting in November, 1962. A special committee consisting of G. Kullerud, J. A. Noble, C. H. Smith, T. P. Thayer, with H. D. B. Wilson as chairman, was appointed by the President, Olaf N. Rove, in February 1963 to make arrangements for the symposium. E. N. Cameron, Secretary of the Society, was ex officio member of the special committee and remained as an active member when he resigned the secretaryship. C. H. Park, Jr. joined the committee shortly after its formation.
The Program Policy Committee was prompted to recommend the symposium by the realization that the underlying theory of the formation of magmatic ore deposits was formulated many decades ago., In the intervening years, much new data have been acquired from systematic research. It seemed to the Program Policy Committee that it was time for those with an abiding interest in the magmatic deposits to meet to assess this new data and to point out the unresolved problems.
The symposium was entitled “Symposium on Magmatic Ore Deposits.” The special committee accepted the terminology in the “Glossary of Geology and Related Sciences,” Edition 2, page 175.
Magmatic Deposits Certain kinds of mineral deposits form integral parts of igneous rock masses and permit the inference that they have originated, in their present form, by processes of differentiation and cooling in molten magmas. (Lindgren p. 863, 1929).
The symposium committee has added the term “ore” to attempt to keep the discussions centered on ore, or near ore material, or with