Alpine Peridotite Podiform Chromite Deposits
Alpine Peridotite Podiform Chromite Deposits...
Alpine Peridotite Podiform Chromite Deposits General Setting Major producers of much of the world’s chromium despite their small size relative to layered complexes. Generally, are more desirable because the Cr/Fe ratio of the chromite is higher than the LMIs and the layers of chromite although aerially less extensive are thicker and hence more easily minable. This deposit group takes its name from the association of the chromite ores with serpentinized peridotites. The association was first recognized in the Alps despite the fact that the Alpine deposits are too low in grade to constituent an economic reserve. Distribution Occur in all the major Paleozoic or younger tectonic belts of the world. Maps show the very close association between convergent plate boundaries and/or major crustal sutures and these deposits. Most important producing districts are in the Philippines, New Caledonia, Turkey and Cuba. Important low grade deposits occur in the Franciscan Complex of the California Coastal Ranges. Within any one district the actual number of chromite deposits might number in the thousands, but usually only a few are large enough to be economic.
Form In contrast to the layered complexes alpine peridotites are often highly deformed and/or attenuated. Ore bodies are usually quite small with lengths of a few thousand meters at
most with widths and thicknesses on the order of a few tens of meters. In cross section, many of the deposits appear tabular to pod-shaped, hence the term "podiform" deposits. Within a district the deposits may have many forms and orientations, either parallelling layering of the host or cross cutting it.
All deposits have been extensively altered, with the host rock often undergoing complete alteration to serpentine. In addition, faulting is ubiquitous. High angle faults are common with the ore zones sometimes appearing to be squeezed or plastered along fault zones. In other cases, faults offset ore zones, but do not appear to localize ore bodies and clearly post-date mineralization. The chromite occurs in a series of discontinuous bands. Layering in these deposits is indistinct, to absent. The chromite grains are often rounded and occasionally display pullapart structures. The former indicates post-depositional abrasion, possibly during squeezing into dilated fault zones and the latter indicates extensive post-depositional extensional deformation. Setting Occur in structurally complex setting with extensive post-ore deformation. Extensive alteration and chaotic nature of the host rocks has made further understanding of the geology of these deposits a matter of much speculation until quite recently. Discovery of relatively unaltered deposits in Cyprus and Onam has allowed more quantitative study. Now realized the host rocks consist of four distinct stratigraphic units (Figure). The uppermost unit (1) is comprised of deep water sediments, radiolarian chert, siliceous shale (umber), minor limestone and rare greywacke. This is underlain by serpentinized pillow basalt cross cut by diabase dikes (2) which in turn overlays gabbro with minor interbedded ultraniafics and chromitite (3). These units rest on a base of
peridotite and dunite (4) which contains most of the minable chromitite. This sequence was termed the Steinmann Trinity. The chromite layers themselves are intimately associated with the dunites, often completely encased in dunite or resting immediately on a layer of dunite. Ore bodies in the ultraniafic portion of the sequence (4) generally are more tabular and show less tendency toward deformation. The less common chromite lenses in the gabbro (3) are the products of extreme deformation.
Genesis Remained something of a mystery until quite recently. Dredging of the ocean basins and the subsequent reconstruction of the geology indicated the oceanic crust consisted of 4 layers; deep water sediments; pillow basalt; gabbro and diabase; and ultramafics. The term ophiolite was coined to define this sequence. Economic geologists were quick to recognize that the alpine peridotites were petrologically identical and suggested that these were the nothing more than slabs of oceanic material that had somehow gotten emplaced on continental crust. Suggested that an ophiolite sequence forms when mantle peridotite is partially melted generating a basalt magma. The magma rises through fissures at divergent plate margins to emplace itself on the seafloor as pillow lava (Figure). Beneath the pillow lavas slow cooling magmas differentiate to generate first ultramafic rocks and chromite layers and finally gabbro. Subsequent spreading tectonically disturbs the layers as does the emplacement on the continental land mass. (Some geologists argue the ultramafics are not differentiates, but residual material left after partial melting. This theory does not easily accommodate the formation of the chromite layers.)
One unanswered question is how the ophiolite sequences end up on the continents. Theoretically, their greater density should cause them to be subducted back into the mantle. The term obduction has been used to describe the emplacement on continents, but there is no general consensus how this occurs. Two suggestions are the most widely accepted. One (Figure) allows for the underthrusting of a continental land mass beneath oceanic crust. Its lower density causes it eventually to rise buoyantly bringing the intervening oceanic crust/mantle up with it. The second mechanism relies on inflation of oceanic crust during the serpentinization process, which involves a significant increase in volume, and the subsequent gravity sliding of the oceanic crust/mantle onto continental crust (Figure).
Troodos, Cyprus The island of Cyprus has attracted attention in recent years because it contains a mafic intrusive complex that is thought to represent an excellent example of an ophiolite slice. The ophiolite complex, known as the Troodos Massif, was emplaced during the Cretaceous period. It consists of pillow basalts overlying a sheeted dike complex, and then an intrusive complex of gabbro that grades downward into olivine gabbro, then into an ultramafic body of harzburgite and dunite. Some of the harzburgites are serpentinized. The upper parts of the gabbro and the lower basalts are cut by many closely spaced diabase dikes that form conspicuous sheeted masses. They are overlain by fine-grained ferruginous, siliceous, sulfide bearing sediments called umbers. The complex thus
includes all three layers of oceanic crust, the most important here being Layer 3, an assemblage of lower cumulate ultramafic rocks and upper gabbros.
The deformed tectonized harzburgites contain scores of podiform chromite deposits; 64 in a 16km2 area. They occur as isolated pods and layers in dunite near harzburgite, in harzburgite within dunite lenses, or in narrow dunite envelopes, or jackets. Chromitesilicate textures include primary cumulate textures and silicate overgrowths over chromite, and secondary ones such as younger schlieren, or sheared, "strung out" textures produced by deformation. The ore is in isolated pods and discontinuous layers of variable size and grade. The pods do not seem to fit a pattern. It appears that the upper part of the plutonic complex evolved as a differentiated body of magmatic cumulates laid down upon a basement of depleted-mantle harzburgite. The chromitites originated as isolated magmatic segregation ore deposits near the base of the cumulate dunite during episodic crystallization. Postcumulus mobilization and accompanying deformation of the lower ultramafic rocks caused the tectonic overprint of schlieren structures on earlier magmatic textures.
Characteristics of Alpine Peridotite Chromite Complexes 1. Occur as small pod-shaped bodies characterized by extreme deformation. 2. Associated with ophiolite sequences, occurring with dunites in the basal portion of the sequence. 3. Restricted to rocks of Phanerozoic age. Long axis of the district parallels the trend of the orogenic belt. 4. Close association with subduction zones and/or crustal sutures. 5. Layering often minor or absent. 6. Chromlte only ore mineral of significance, minor nickel. 7. Deposits thought to represent slices of oceanic lithosphere obducted unto continental land masses.