Showing papers on "Ankerite published in 1982"
TL;DR: The Douglas Creek Member of the Tertiary Green River Formation underlies much of the Uinta basin, Utah, and contains large volumes of oil and gas trapped in a complex of fractured low-permeability sandstone reservoirs as discussed by the authors.
Abstract: The Douglas Creek Member of the Tertiary Green River Formation underlies much of the Uinta basin, Utah, and contains large volumes of oil and gas trapped in a complex of fractured low-permeability sandstone reservoirs. In the southeastern part of the basin at Pariette Bench, the Eocene Douglas Creek Member is a thick sequence of fine-grained alluvial sandstone complexly intercalated with lacustrine claystone and carbonate rock. Sediments were deposited in a subsiding intermontane basin along the shallow fluctuating margin of ancient Lake Uinta. Although the Uinta basin has undergone postdepositional uplift and erosion, the deepest cored rocks at Pariette Bench have never been buried more than 9,800 ft (3,000 m). The sandstones, dominantly lithic arkoses and feldspathic litharenites, were derived from source terranes south of the Uinta basin. Secondary silica and several generations of authigenic calcite [Ca1.8-1.9(Mg0.02-0.06Fe0.02-0.06)(CO3)2], dolomite [Ca1.3-1.4(Mg0.6-0.7Fe0.02-0.04)(CO3)2], and ankerite [Ca1.2-1.3(Mg0.2-0.3Fe0.4-0.6)(CO3)2] form a replacive cement in the sandstones. Commonly, syntaxial overgrowths of late iron-bearing carbonate occur on detrital grains and preexisting relicts of iron-free carbonate cement. In sandstone where carbonate has been partly dissolved, abundant authigenic illite, partly ordered mixed- ayer illite-smectite, and small amounts of chlorite partly to completely fill secondary pores. Isotopic composition of carbonate cement and grain-supported rock range from -0.39 to -6.18 ^pmil for ^dgr13C and -7.80 to -13.98 ^pmil for ^dgr18O, indicating that authigenic carbonate formed at low temperatures in the presence of meteoric waters by a process of solution-precipitation. Enrichment of carbon and oxygen in early diagenetic calcite and fossiliferous rock relative to late diagenetic ankerite indicates a trend toward lighter isotopic carbonate compositions with increasing diagenesis. Kerogenous rocks at Pariette Bench are thermochemically immature and therefore are not the source of oil produced in the field. Hydrocarbons are compositionally similar to some of the oils produced from the Green River Formation in the Bluebell-Altamont field and are interpreted to have migrated from mature Green River source rocks through a network of open fractures. The occurrence of small amounts of hydrocarbon in secondary pores indicates that its emplacement postdated carbonate dissolution.
39 citations
TL;DR: Theoretical analysis of phase relations on isobaric T-X (sub CO 2 ) diagrams leads to the following conclusions: 1. Metamorphic reactions caused substantial, localized CO 2 enrichment in the coexisting fluid phase as mentioned in this paper.
Abstract: Progressive regional metamorphism of the Negaunee Iron Formation in the Marquette district from chlorite to sillimanite grade has resulted in the development of three metamorphic zones and two subzones. The diagnostic zone assemblages are, in order of increasing grade: 1, siderite + quartz + or - ankerite + or - minnesotaite + or - stilpnomelane + or - magnetite; 2, grunerite + or - quartz + or - ankerite + or - magnetite and grunerite + or - siderite + or - ankerite + or - magnetite; 3a, grunerite + or - garnet + or - quartz + or - magnetite and grunerite + or - Ca amphibole + or - quartz + or - calcite + or - magnetite; 3b, grunerite + or - olivine + or - clinopyroxene + or - quartz + or - calcite + or - magnetite and grunerite + or - Ca amphibole + or - olivine + or - magnetite. The grunerite, garnet, and Ca amphibole, and olivine and clinopyroxene isograds define the boundaries between the respective zones and subzones. Temperature estimates are approximately 300 degrees C for zone 1 and approximately 600 degrees C for subzone 3b. Pressure is estimated at between 200 to 300 MPa. Microprobe analyses of coexisting minerals indicate general attainment of exchange equilibrium and a strong localized influence of bulk composition on mineral chemistry.Theoretical analysis of phase relations on isobaric T-X (sub CO 2 ) diagrams leads to the following conclusions:1. Throughout zone 1, minnesotaitc-bearing assemblages are diagnostic of low X (sub CO 2 ) values, stilpnomelane-bearing assemblages represent low to medium values, and chlorite-bearing assemblages represent medium to high X (sub CO 2 ) values. Juxtaposition of these three assemblages at one locality indicates that fluid phase compositional gradients most likely existed.2. Throughout zone 3, actinolite-, hornblende-, and garnet-bearing assemblages reflect similar low to high X (sub CO 2 ) values. The continued juxtaposition of these various assemblages suggests the continued presence of fluid-phase compositional gradients into the highest grade metamorphic zones.3. Metamorphic reactions caused substantial, localized CO 2 enrichment in the coexisting fluid phase. A diversity of T-X (sub CO 2 ) paths was followed, typically differing on the scale of individual mesobands.
38 citations
TL;DR: The Salgadinho copper mine in Portugal is located in the southernmost linear belt of outcropping low metamorphic grade deformed Palaeozoic rocks (Famennian) in the SW part of the Iberian Pyrite Belt as discussed by the authors.
Abstract: The recently discovered Salgadinho copper deposit, 7 km NNE of S Luis, Portugal is located in the southernmost linear belt of outcropping low metamorphic grade deformed Palaeozoic rocks (Famennian) in the SW part of the Iberian Pyrite Belt The stratabound replacement pyrite-chalcopyrite mineralisation is present in variably altered felsic pyroclastic rocks which are overlain by pyritic graphitic shales and tuffs which have undergone alteration in the lowermost 5m The altered pyroclastic rocks are characterised by pale green celadonitic fluoro-muscovite and, in the most intense zone of alteration, quartz, ankerite, and ore minerals are present (pyrite, chalcopyrite, sphalerite, tetrahedrite, galena, bournonite) No exhalative Cu-Zn or Zn-Pb ore has been discovered associated with the submarine volcanic stratabound Cu-rich alteration zone Alteration of feldspars and groundmass has involved a net loss from the system and gains in Fe2+, F, H2O, Mg, Fe3+, Ca, Mn, P, Ti, S, Cu, Zn, Pb, As, Sb, Ag and Au at the expense of Si, K, Al and Na The great enrichment of F in the altered rocks, the association of zonal alteration facies with coarse grained pyroclastic masses and the intimate association of pale green celadonitic fluoro-muscovite with mineralisation at Salgadinho and most other deposits of the Iberian Pyrite Belt represents a powerful exploration guide for submarine exhalative ores
22 citations
TL;DR: In this article, the distribution of rare earth elements in Fazenda Varela carbonatites was discussed as a part of a petrological and geochemical study, showing that the rare earth element concentration is very high relative to the concentration in chondrites, the mean in the earth crust and even in most carbonatite.
Abstract: As a part of a petrological and geochemical study the distribution of the rare earth elements in Fazenda Varela carbonatites was discussed. These carbonatites were the first described in Lages Dome. Santa Catarina State, Brazil. They intruded and fenitized sedimentites of the Tubarao Group; the fenitizing fluids are dominantly potassic. The main constituents of the carbonatites are ankerite, barite, K feldspar, and synchisite; secondary apatite is rare. High Ba/Sr, Nb/Ta, and Th/U ratios are conspicuous. The concentration of the rare earth elements is very high relative to the concentration in chondrites, the mean in the earth crust and even in most carbonatites; normalized distribution curves show higher concentration of the light lanthanides (Ce group) in comparison with the heavy lanthanides (Y group). In addition to other petrological and geochemical features, high absolute and relative concentrations of the light lanthanides indicate that these carbonatites represent the last phases of carbonatitic fractionation and reinforce, the possibility of occurence of carbonatites from the early phases of crystallization at the Lages alkaline district.
9 citations
Journal Article•
TL;DR: Two types of carbonatite dykes - alvikite and beforsite are associated with (the Mundwara alkaline igneous complex) as mentioned in this paper, and the presence of both metasomatic as well as intrusive syenites are noticed.
Abstract: Two types of carbonatite dykes - alvikite and beforsite are associated with (the Mundwara alkaline igneous complex. Petrochemical and mineralogical studies indicate enrichment of alkalies and niobium. Pyrochlore is noted in beforsites along with dolomite, apatite, barite and flourite, whereas calcite, ankerite, heamatite, aegirine, apatite and barite are noted in the alvikites. The presence of both metasomatic as well as intrusive syenites are noticed. Higher content of Nb is noted in the syenites containing sphene, aegirine and magnetite. The presence of annular fractures around the plutons and the arcuate shatter zone with breccia on the outer margins of the Mer pluton indicate upwarping and later intrusion of carbonatites through the Erinpura granite. High P 2 O 5 and Nb content of the soil samples around Mer Pluton suggest possible existence of a carbonatite plug below.
2 citations