Primary Assemblage Research Articles

The Geology of a Primary Millerite-Bearing Sulfide Assemblage and Supergene Alteration at the Otter Shoot, Kambalda, Western Australia

Otter Shoot differs from the other shoots at Kambalda in that it contains a significant proportion of millerite-bearing sulfides. The sulfides have been extensively altered by supergene processes. A study was initiated to decide whether the millerite was a product of supergene alteration or was of primary magmatic origin and also how its presence affects weathering of the sulfides.The nickel sulfides in the Otter Shoot occur as a massive sheet at the lower contact of ultramafic rock with metabasalt and as disseminations within the ultramafic. The contact sulfides consist largely of violarite, pyrite, and millerite, but textural evidence indicates that they originally consisted of two different primary assemblages---pentlandite-millerite-pyrite and pyrrhotite-pentlandite-pyrite---that have subsequently been subjected to supergene alteration. The hanging-wall sulfides consist of spherical blebs of millerite, pentlandite, pyrite, and magnetite. Closely associated with the sulfide blebs are blebs of silicate material consisting mainly of talc and chlorite. Both are presumed to have originated as immiscible liquid globules in an ultramafic magma. The ultramafic rock has been altered to an assemblage consisting of talc, serpentine, and carbonates in varying proportions. The millerite-bearing assemblage is preferentially associated with a serpentine-rich zone.Supergene alteration has completely converted the pyrrhotite and pentlandite of the contact ore into violarite and pyrite by means of a coupled reaction in which the nickel released by the replacement of pentlandite by violarite has reacted with the pyrrhotite to produce additional violarite; the remaining pyrrhotite has been converted to pyrite. Where pentlandite is associated with miilerite in the contact ore, the pentlandite has been converted into a nickel-rich violarite, and the iron released by this reaction has participated in the conversion of some of the millerite to violarite. Decomposition of the sulfides at the water table has resulted in the formation of gaspeite and reevesite.

Comments on “Precambrian Rocks South of Erquy and around St Cast, Côtes-du-Nord”: Reply

IT was not our intention1 to question the accuracy of the isotopic determinations of Vidal et al.2 but rather to stress the difficulties in an interpretation of their Rb/Sr isochron age of 466 ± 10 m.y. for the Erquy Spilites. Vallance3 has argued that the paragenesis of ancient spilites is one due to alteration in a hydrothermal environment which may be either diagenetic or in response to the lower grades of metamorphism. The typical feldspar of these ancient spilitic pillow lavas is albite with low-temperature optics in contrast to the more calcium-rich plagioclase with high or transitional optics found in pillow lavas of Recent age3. We reassert that the Erquy volcanics contain a metamorphic paragenesis (albite, chlorite and epidote) overprinting the original igneous assemblage—glassy pillow margins and crystalline pillow centres. The clinopyroxene present within these rocks is metastable and in no way contradictory with alteration under lower greenschist facies conditions. The presence of forked albites quoted by Auvray et al.4 as evidence for the presence of a primary assemblage derived from the crystallization of a sodium-rich basaltic magma we suggest is evidence of the response of these rocks to a low grade of metamorphism and is a common feature of spilitic rocks within proven greenschist facies terrains5.

Les pyroxenolites a grenat du massif de lherzolite de Moncaup (Haute Garonne - France); Caracteres communs avec certaines enclaves des basaltes alcalins

Abstract Pyroxenites at Moncaup display high temperature - high pressure primary assemblages (clinopyroxene + garnet; clinopyroxene + orthopyroxene + spinel) with remarkable exsolution features. Chemical composition of rocks and minerals, and the mode of occurrence, suggest in situ crystallization of anatectic melt extracted from peridotite; these rocks invite comparison with zoned dykes in alpine-type peridotites resulting from dynamic fractional crystallization. The Moncaup pyroxenites may be compared texturally, mineralogically and chemically with xenoliths in basalts and nephelinites of the Honolulu series. The Moncaup rocks and Honolulu xenoliths could have the same origin and petrogenetic history.

Quelques précisions sur les caractères minéralogiques et chimiques des pyroxénolites à grenat du massif des Beni Bousera (Maroc) et sur leur signification pétrogénétique

The primary liquid of partial fusion, in the Beni Bousera ultra-basic Massif (Morocco), gives, by fractionnal cristallization, various assemblages that can be distinguished on the basis of ore mineral associations and silicate paragenesis. Residual liquids, of variable compositions, would have cristallized as garnet clinopyroxenites, with granulite facies primary assemblages (Ca Tschermak rich cpx + garnet + ilmenite) or eclogite facies primary assemblages (omphacitic cpx + garnet + rutile).

Uranium distribution in ultramafic inclusions from Victorian basalts

The uranium distribution in all phases of nine Iherzolite inclusions from the Newer Volcanics of western Victoria has been determined using fission tracks. Primary clinopyroxene has a mean content of 0.30 ppm U in five of the inclusions, and two contain primary apatite with 35 ppm U. Secondary clinopyroxene and apatite crystallizing in equilibrium with glass formed from partial melting of the Iherzolites have a much lower abundance. Partition coefficients calculated from the uranium contents in these secondary phases and the glass indicate that a liquid in equilibrium with the primary assemblage would have had to contain 25 to 75 ppm U. Since these abundances are at least ten times those of normal basalts, these inclusions could not have formed as accumulates from a basaltic magma, neither are they the residue from a previous episode of complete magma extraction. However other inclusions containing clinopyroxene with a low uranium abundance could be accumulates or residua. A model for the uranium distribution in the upper mantle is based on the uranium abundances in the high U primary assemblage. It is consistent with estimates of the upper mantle uranium content, and the uranium contents of basalt magma series calculated from it are consistent with the reported abundances. The petrology and geochemistry of inclusions illustrate a mechanism for limited wall rock reaction, and suggest that potassium may be moved without uranium.

SULFIDES IN THE MUSKOX INTRUSION

The Muskox intrusion is a layered, dominantly ultramafic pluton, in northern Canada. Sulfide phases belonging to the Ni–Cu–Fe–S system are concentrated along its margins and in chromite-rich horizons within its central layered series. They are sparsely disseminated through the rest of the intrusion, except in a core zone, which is almost barren of sulfides.The primary sulfide assemblages in the layered series range from nickel-rich at the base, through copper-rich at intermediate levels, to iron-rich near the roof. Nickel:copper ratios in the sulfides vary sympathetically with those of the host silicates. A similar but inverted sequence of sulfide assemblages occurs in the marginal zone. This and textural evidence indicate that the disseminated sulfides in the main body of the intrusion formed by segregation of sulfides from inter-cumulous magma and that they crystallized in equilibrium with their respective host silicates. The ultimate origin of the sulfur is not known. Sulfides in the upper part of the layered series and upper border zone appear to have formed by late replacement of the host rocks at temperatures below 675 °C.The concentration of sulfides along the intrusive contacts, and in adjacent wallrocks, developed during the primary cooling cycle by a process of desulfidization of the Muskox magma. Sulfide transfer in these locations was controlled primarily by the temperature gradient, not by gravity settling of a separate sulphide fluid.During subsequent serpentinization of the intrusion, the introduced fluid phase became progressively more oxygen depleted. The effect was most pronounced in the central regions, where primary sulfides ultimately decomposed to form native metals. Sulfur so released migrated as H2S to more peripheral parts of the intrusion, where it entered into various reactions with primary sulfides, oxides, and silicates.Separate processes thus operated to achieve desulfidization and desulfurization at magmatic and submagmatic temperatures, respectively, of the main body of the intrusion and to favor development of sulfide-enriched zones towards its margins.Sulfides in both the Muskox feeder dyke and the diabase dykes of the area are disseminated relatively uniformly through these rocks. These sulfides appear to have crystallized in situ and show no obvious zoning relationships.

The problem of reworked pollen and spores in marine sediments

Some pollen-and-spore assemblages found in ocean-bottom sediments contain many reworked plant microfossils. In the search for a method to separate the recycled pollen grains and spores from the primary assemblage recovered from Pleistocene and Recent ocean-bottom-core samples, it was found that morphologically distinct Tertiary and older pollen grains and spores accept Safranin O stain differently than do most of the Pleistocene or younger plant microfossils in the same assemblage. Variations in stain acceptance may be a reflection of exine-chemistry changes that took place during the reworking process. High percentages of reworked pollen and spores in sediments formed during periods of continental glaciation are thought to be the result of increased erosion resulting from base-level lowering. Large numbers of recycled plant microfossils in Pleistocene and even pre-Pleistocene sediments may be indicative of decreasing base levels associated with orogenic movements or former periods of continental glaciation. Secondary plant microfossils can offer important clues on the location of the source areas of marine sediments.

Contribution to the study of ore minerals in some igneous rocks from Assynt

SummaryThe parageneses of opaque minerals in three Assynt rocks are described. A biotite-diorite dyke-rock of Lewisian age is characterized by a primary igneous assemblage of Fe-Ti oxides, pyrrhotine, and pentlandite. Effects of metamorphism are recognized, particularly in the oxidation of ilmenite in a Lewisian epidiorite dyke-rock. Cromaltite contains a primary assemblage of oxide minerals together with a later hydrothermal sulphide assemblage. Evidence from the opaque minerals cannot be reconciled with a theory of limestone assimilation. The correlation of the distribution of Co, Ni, and Cu in the rocks with the opaque minerals present is considered.