Marble Unit Research Articles

Zinc and iron metallogeny in the Maniwaki-Gracefield District, southwestern Quebec

Zinc and iron deposits in the Grenville Supergroup metasedimentary rocks of the Maniwaki-Gracefield district, Quebec, exhibit close similarities to the economically important Balmat-Edwards district of New York State. The mineralization is stratiform, it is consistently associated with dolomitic marble units, and it has ages similar to the New York deposits ( approximately 1,200-1,300 m.y.). At least two phases of intense Grenvillian deformation affected the host rocks, and regional metamorphism reached the granulite facies. Lateral and vertical zoning of major and trace elements observed within the zinc deposits indicate that they probably had a submarine exhalative origin.Small magnetite-rich iron-formations related to Mg-rich carbonates, and minor amounts of base metal sulfides including sphalerite, occur within metasediments closely resembling those which host the zinc deposits. The iron deposits are interpreted to occur at stratigraphic positions which are laterally equivalent to the zinc mineralization, and consequently the iron-formations form an important stratigraphic guide to zinc mineralization within the complexly deformed and strongly metamorphosed strata of the Grenville Supergroup.At the regional scale, the Grenville zinc deposits of Quebec, Ontario, and New York are located along two major structural lineaments which border the Central Metasedimentary belt and define a first-order basin hosting the metasedimentary rocks of the Grenville Super-group. These basin-margin lineaments also delineate two regional metallogenic zones characterized principally by zinc and iron deposits: a western zone which encompasses the Maniwaki-Gracefield showings as well as several minor to medium-sized stratiform base metal deposits in Ontario and adjacent portions of Quebec, and an eastern zone which includes the Balmat-Edwards and East Pierrepont deposits of New York as well as the stratiform gold-zinc deposits of Montauban, Quebec. These regional aspects are consistent with current models for the origin of sediment-hosted submarine-exhalative base metal mineralization.

Geochemistry and chemostratigraphical correlation of slates, marbles and quartzites of the Appin Group, Argyll, Scotland

ABSTRACTApproximately 700 samples from five slate, three marble and five quartzite units have been analysed for 26 elements to determine the use of geochemistry in stratigraphic correlation. Statistical treatment of the data has established that for all the slates and marbles, and certain of the quartzites, geochemistry is a viable method of distinguishing units of similar lithology. The most useful elements for distinguishing the four main slate units are P, Cr. Zn, Cu and, to a lesser extent, Rb, Sr, Y, Nb, Ba, La and Ce. Sr may indicate climatic changes or variable organic activity. The three marble units were considered as three limestone and three dolostone types. Dolostones are distinguished by high insoluble residue contents and the elements that distinguish between the dolostones are heavily influenced by these. Limestones, however, have very large differences in Sr. SiO2, Al2O3, K2O, Cr, Mn, Cu, Rb, Sr, Y and Zr have been used in discrimint function analysis. These parameters are strongly controlled by the insoluble residue with Al2O3, K2O, Cr and Rb being correlated with shaley impurities and SiO2, TiO2 and Zr with sandy impurities. Sr, Pb, Y and Zn, and to a lesser extent S and Mn, appear to be associated with the carbonate fraction.The quartzites were found to be of three major types: (1) a very variable deltaic deposit (Appin Quartzite Formation), (2) marine bodies of immature quartz sands (Glen Coe and Eilde Quartzite formations) and (3) highly mature quartz sands (Binnein Quartzite Formation and northerly outcrops of the Glen Coe Quartzite Formation). It is possible to distinguish these three types on the basis of some trace elements (Zn, Rb, Sr, Y, Ba, La, Ce) but it is not feasible to distinguish between the Glen Coe and Eilde quartzites purely on geochemistry. The Eilde Flags, an immature estuarine sandstone, has a geochemistry intermediate between that of the quartzites and the pelites, although with higher CaO, Zr and Ba than either.

Evolution of the first generation folds in a marble unit (Damara Orogenic Belt, Nambia)

An exhaustive field study of complex minor fold structures has been directed at a better understanding of the individual fold-like features of a complex structure, to gain insight into their development and into the evolution of the major folds. It has been found conceivable and in practice, enlightening to explain complexity by the retention of geometrical features from an earlier stage of deformation within the same folding phase. Irregularities in fold and layer geometry were, whenever possible, interpreted as intact or modified relicts from such an earlier stage. Results of experimental work on fold development have been used as an aid in interpretation.

Strain and folding mechanisms in a similar style fold from the northern Selkirks of the Canadian Cordillera

Minor structures and intragranular strain in calcite have been examined in a folded marble unit near Mica Dam, 130 km north of Revelstoke, British Columbia. The fold forms part of a train of polyclinal folds in the core of the major Mica Dam Antiform which formed in a third folding phase, F 3 Metamorphism had reached kyanite grade before F 3, biotite quartz and calcite were recrystallizing early in F 3 but no longer did so in the later stages. Pressure of 4.0–5.0 kbar and temperatures of 300–400°C are estimated to have prevailed during folding. The fold is an asymmetric, approximately cylindrical antiform with axial surface dipping southwest and hinge plunging southeast at 12°. The parameters t′ α and T′ α determined on a right section indicate the fold to be approximately similar. Calc-silicate layers and pegmatite sheets which outline the fold are both buckled and boudiné and the adjacent synform is limited by an upper detachment surface. Other F 3 minor folds, related to the Mica Dam Antiform, exhibit the same characteristics and all this indicates that there was a buckle component in the folding history. Using the least-squares strain-gage technique, intragranular twinning strains have been determined for five samples of marble. The maximum extensional strain, ϵ 3, is subparallel to layering and at a high angle to the fold hinge. All of the samples exhibit a compression strain nearly parallel to the fold hinge. These strains do not reflect the buckling mechanism suggested by field observations. The late-stage three-dimensional strains observed in this fold are not compatible with existing two-dimensional, theoretical models for the origin of similar style folds.

Carbon, hydrogen, and oxygen isotope studies of the regional metamorphic complex at Naxos, Greece

At Naxos, Greece, a migmatite dome is surrounded by schists and marbles of decreasing metamorphic grade. Sillimanite, kyanite, biotite, chlorite, and glaucophane zones are recognized at successively greater distances from the migmatite dome. Quartz-muscovite and quartz-biotite oxygen isotope and mineralogie temperatures range from 350 to 700°C. The metamorphic complex can be divided into multiple schist-rich (including migmatites) and marblerich zones. The δ 18O values of silicate minerals in migmatite and schist units and quartz segregations in the schist-rich zones decrease with increase in metamorphic grades. The calculated δ 18O H 2O values of the metamorphic fluids in the schist-rich zones decrease from about 15‰ in the lower grades to an average of about 8.5‰ in the migmatite. The δD values of OH-minerals (muscovite, biotite, chlorite, and glaucophane) in the schist-rich zones also decrease with increase in grade. The calculated δD H 2O values for the metamorphic fluid decrease from −5‰ in the glaucophane zone to an average of about −70‰ in the migmatite. The δD values of water in fluid inclusions in quartz segregations in the higher grade rocks are consistent with this trend. Theδ 18O values of silicate minerals and quartz segregations in marble-rich zones are usually very large and were controlled by exchange with the adjacent marbles. The δD values of the OH minerals in some marble-rich zones may reflect the value of water contained in the rocks prior to metamorphism. Detailed data on 20 marble units show systematic variations of δ 18O values which depend upon metamorphic grade. Below the 540°C isograd very steep δ 18O gradients at the margins and large δ 18O values in the interior of the marbles indicate that oxygen isotope exchange with the adjacent schist units was usually limited to the margins of the marbles with more exchange occurring in the stratigraphic bottom than in the top margins. Above the 540°C isograd lower δ 18O values occur in the interior of the marble units reflecting a greater degree of recrystallization and the occurrence of Ca-Mg-silicates. Almost all the δ 13C values of the marbles are in the range of unaltered marine limestones. Nevertheless, the δ 13C values of most marble units show a general correlation with δ 18O values. The CO 2 H 2O mole ratio of fluid inclusions in quartz segregations range from 0.01 to 2. Theδ 13C values of the CO 2 range from −8.0 to 3.6‰ and indicate that at some localities CO 2 in the metamorphic fluid was not in carbon isotopic equilibrium with the marbles.

Dedolomitized Marble Lenses in Shear Zone Tectonites, Medicine Bow Mountains, Wyoming

Lenses of dedolomitized marble occur interspersed with Precambrian cataclastic rocks of the northeast-trending Mullen Creek-Nash Fork shear zone in the east-central portion of the Medicine Bow Mountains of southeast Wyoming. Eight lenses of tremolite marble (dolomitic) and one small body of serpentinized forsteritic marble (calcic) were observed. The Mullen Creek-Nash Fork shear zone separates a thick sequence of low grade metasedimentary rocks, including two locally siliceous dolomitic marble (metadolomite) units of the Nash Fork Formation, from a higher grade felsic and amphibolitic complex. The marble lenses that occur within the shear zone are considered to be fault slices of the dolomitic units that were dedolomitized during mylonitization. The formation of tremolite and forsterite in the lenses was probably facilitated by the presence of a water-rich fluid in the shear zones. Minor occurrences of talc-dolomite-calcite-quartz and tremolite-dolomite-calcite-quartz assemblages in the main sequence of Nash Fork Formation metadolomite are probably also a function of reactions in zones locally enriched in fluids. The presence of assemblages of Tc + Dol + Cc + Q and Tr + Dol + Cc + Q without the intermediate assemblage Tc + Tr + Dol + Cc + Q in the Nash Fork Formation and Tr + Dol + Cc and Fo + Cc + Dol without Tr + Fo + Cc + Dol in shear zone marble slices suggests that each set of reactions may have been controlled by different fluid paths. The stable talc and forsterite assemblages apparently formed under conditions of higher than the tremolite assemblages. Parent dolomite in the marble slices, although recrystallized, only partly decomposed to form tremolite; the tremolite-dolomite ratio increasing with increasing silica content. Under local conditions of higher tremolite apparently reacted with much of the excess dolomite to form forsteritic olivine (). Both tremolite and olivine have since been partially altered to talc and antigorite respectively (most pseudomorphically). This alteration represents a secondary or late retrogressive event superposed upon the upper greenschist or lowermost almandine amphibolite grade dedolomitization process that accompanied the development of the mylonites.

Distribution and Composition of Sulfide Minerals at Balmat, New York

Distribution and Composition of Sulfide Minerals at Balmat, New York

CONTACT METAMORPHISM OF MAGNESIAN LIMESTONES AT CRESTMORE, CALIFORNIA

The contact-metamorphic rocks at Crestmore, California, occur between magnesian marbles and a plutonic mass of quartz diorite (Bonsall tonalite) and between the same marbles and a relatively small pipelike hypabyssal mass of quartz monzonite porphyry. The marbles are preserved as two crudely lenticular bodies about 400 and 500 feet thick, respectively, that occur as screens in the quartz dioritic intrusive rocks of the southern California batholith. Both bodies, which are very similar petrographically, are composed of alternating layers of predazzite and coarsely crystalline calcite marbles and are nearly free of silica, alumina, iron, and the alkalies. The quartz diorite that engulfed the marbles is contaminated locally very near the contacts where minor monzonitic and gabbroic variants are present. Its exomorphic effects on the carbonate rocks consist mainly of: (1) the formation of metasomatic silicate contact rocks that generally are less than 1 foot thick and are composed of diopside, wollastonite, and grossularite; and (2) the conversion of nearly all the magnesium-bearing carbonate beds to periclase marbles, which subsequently were altered to predazzites. In contrast, the younger quartz monzonite porphyry, which was injected into the upper or Sky Blue marble unit, is nearly all contaminated as a result of reactive assimilation of marble. Moreover, its exomorphic silicate aureole is as much as 50 feet thick and contains the numerous complex mineral assemblages for which Crestmore is famous. Several lines of evidence indicate that much of this aureole was formed prior to the final consolidation of the porphyry intrusive mass. The thicker parts of the silicate contact aureole that surrounds the quartz monzonite porphyry exhibit the following well-defined zonal distribution of mineral assemblages, as traced outward from the intrusive body: (1) a garnet zone that is composed of grossularite and lesser amounts of wollastonite and diopside; (2) a zone characterized by only one mineral, idocrase; and (3) a zone in which monticellite is the most abundant mineral, but in which there are various amounts of clinohumite, cuspidine, ellestadite, forsterite, melilite, merwinite, perovskite, spinel, spurrite, tilleyite, and xanthophyllite. This zonation of mineral assemblages reflects a corresponding zonation in the bulk chemical composition of the rocks, as shown by changes in the ratio of metasomatic to indigenous constituents (Si + Al + Fe/Ca + Mg) from 0.66 in the monticellite zone, through 1.15 in the idocrase zone, to 1.62 in the garnet zone. There also is a strong tendency toward a mineralogical zonation within the monticellite zone, such that clinohumite, forsterite, spurrite, and spinel are concentrated in the silica-poor and calcite-rich outer part, and merwinite, cuspidine, and melilite in the more silica-rich inner part. Hence, a sequential occurrence of mineral assemblages and a change in bulk chemical composition can be traced inward toward the intrusive mass from un-metasomatized marbles. Moreover, textural features reveal a corresponding paragenetic sequence in which the more highly metasomatized assemblages appear to have formed at the expense of those that were less metasomatized. Available evidence indicates that: (1) the contact-metamorphic mineral assemblages at Crestmore and their zonation are largely the compositionally controlled products of silica, alumina, and iron metasomatism of relatively pure magnesian limestones; (2) temperatures of 625°C. or higher were reached prior to the introduction of silica into the present monticellite-zone rocks; and (3) the so-called “high-temperature” assemblages, such as monticellite, spurrite, and melilite, formed directly from the magnesian marbles without the intervention of “lower-temperature” steps that involve diopside, wollastonite, and grossularite. Therefore, it is proposed that contact metamorphism at Crestmore should be viewed as progressive metasomatism with consequent decarbonation at elevated temperatures rather than as progressive decarbonation attendant simply upon rising temperature.