Showing papers on "Ankerite published in 1984"

Regional Controls on Diagenesis and Reservoir Quality in Lower Tertiary Sandstones along the Texas Gulf Coast: Part 1. Concepts and Principles

Abstract: Reservoir quality trends in Lower Tertiary sandstones along the Texas Gulf Coast are a product of regional variations in intensity of diagenesis The major controls on diagenesis were detrital mineralogy and regional geothermal gradient Porosity and permeability in sandstones shallower than 3350 m (11,000 ft) are generally adequate for hydrocarbon production, whereas reservoir quality in deeper sandstones in the onshore Lower Tertiary section is highly variable Many of these sandstone reservoirs have permeability values of less than 1 millidarcy (md), but in a few areas permeability values are higher than 1000 md Wilcox sandstones are poorly to moderately sorted, fine-grained, quartzose lithic arkoses, becoming more quartz-rich from the upper to the lower Texas Gulf Coast Most rock fragments are metamorphic or volcanic in origin Wilcox sandstones exhibit no systematic regional reservoir quality trends Along the lower and parts of the middle and upper Texas Gulf Coast, deep Wilcox sandstones are tight, but in other parts of the middle and upper Texas Gulf Coast, porosity exists at depth Vicksburg sandstones are poorly sorted, fine-grained lithic arkoses Rock fragments are mainly volcanic clasts with lesser carbonate and minor metamorphic clasts The deep Vicksburg Formation has low-quality reservoirs Frio sandstones range from poorly sorted, fine-grained, feldspathic litharenites to lithi arkoses along the lower Texas Gulf Coast to poorly sorted, fine-grained, quartzose lithic arkoses to subarkoses along the upper Texas Gulf Coast Volcanic and carbonate rock fragments are common in the lower Texas Gulf Coast and decrease in abundance up the coast Frio sandstones show a systematic improvement in reservoir quality from the lower to the upper Texas Gulf Coast that is related to grain composition and geothermal gradient Reservoir quality trends in Tertiary sandstones have been substantiated by acoustic log analysis In spite of variations in composition, Lower Tertiary sandstones exhibit similar diagenetic sequences generalized as follows: Surface-to-shallow-subsurface diagenesis (0 to 1200 m ±; 0 to 4000 ft ±) began with the formation of clay coats on framework grains, dissolution of feldspar, and replacement of feldspar by calcite Minor amounts of kaolinite, feldspar overgrowths, and Fe-poor calcite was locally precipitated Porosity was commonly reduced by compaction and cementation from an estimated original 40% to less than 30% Intermediate subsurface diagenesis (1200 to 3400 m ±; 4000 to 11,000 ft ±) involved dissolution of early carbonate cements and subsequent cementation by quartz overgrowths and later by carbonate cement Cementation commonly reduced porosity to 10% or less, but this trend could be reversed by later dissolution of feldspar grains, rock fragments, and carbonate cements Restoration of porosity to more than 30% occurred, but some porosity was later reduced by kaolinite, Fe-rich dolomite, and ankerite cementation Deep subsurface diagenesis (>3400 m±; >11,000 ft±) was a continuation of late Fe-rich and Fe-poor carbonate cement precipitation Plagioclase was albitized during this stage Differences in intensity of diagenetic events and depths at which they first occurred correspond to the chemical and mechanical stability of the original detrital mineralogy and to regional variations in geothermal gradient

Hydrothermal alteration of felsic volcanic rocks at the Helen siderite deposit, Wawa, Ontario

Abstract: Within the footwall volcanic sequence five chemically and mineralogically distinct alteration types have been defined: least altered, sericite, chlorite, chloritoid, and ankerite. An alteration model is proposed in which shallow circulating seawater (1-3 km), heated by a subvolcanic intrusion, encountered rocks of felsic composition within the upper part of the volcanic succession. Seawater-rock reactions evolved an acidic, potassium-rich solution which migrated upward, reacting with overlying felsic rocks, converting plagioclase to sericite and quartz. Porosity of the altered rocks decreased and a widespread sericitic alteration zone formed. At depths greater than 3 km, circulating seawater came into contact with basaltic rocks and ensuing reactions enriched the seawater in Fe, Mg, Mn, Ca, and CO 2 . This second hydrothermal solution moved upward and encountered sericite-rich rocks. Near the sea floor, boiling of the solution and/or mixing with relatively cold seawater rapidly decreased CO 2 solubility causing mass precipitation of iron carbonate. This led to the formation of the ankerite assemblage in the immediate footwall rocks and the siderite iron-formation on the sea floor.--Modified journal abstract.

Secondary ilvaite in a dolerite dyke from Rogaland, SW Norway

Abstract: A BST R ACT. llvaite is an alteration product of carbonate, which itself replaced clinopyroxene, in a Precambrian tholeiitic dyke. Additional secondary minerals are ferroactinolite, cummingtonite, chlorite, biotite, prehnite, and epidote. Microprobe analyses demonstrate that the ilvaite is close to the ideal end-member composition [CaFe22" Fe3+Si2Os(OH)] and that only cummingtonites, Mg/(Mg+Fe 2+) 0.77-0.39, and carbonates (solid solutions between calcite and dolomite, dolomite and ankerite, magnesite and siderite) show significant compositional variations. It is estimated that ilvaite formed at temperatures < 470 ~ and at pressures < 2 kbar, probably during a low-grade metamorphic event (or events) associated with the Caledonian orogeny. The fluids involved in the alteration process contained both CO2 and HzO, initial fluids probably being richer in CO2 than later ones. Ilvaite may be more common in hydrothermally altered igneous rocks than previously supposed. ILVAITF, CaFe22+Fea+Si2Os(OH), is a relatively rare mineral which typically occurs in Ca-Fe-Si skarn deposits (Fonteilles, 1962; Bartholom6 and Dimanche, 1967; Burt, 1971a; Verkaeren, 1974; Plimer and Ashley, 1978). It is also found in contact metamorphosed iron formations (Ramdohr, 1927; Dietrich, 1972) and as a late-stage alteration product in some igneous rocks, perhaps the bestknown examples of which are the Upper Border Group of the Skaergaard intrusion (Wager et al., 1957; Naslund et at., 1983), and the Basistoppin sill (Douglas, 1964) which intrudes the upper part of the Skaergaard intrusion. The mineral occurs as an alteration product in a dolerite dyke from Rogaland, SW Norway, and has not been previously reported from Rogaland despite the occurrence in this region of iron-rich metamorphic assemblages containing andradite and hedenbergite (minerals which are frequently associated with ilvaite in skarn deposits). Moreover, its mode of occurrence is quite unlike that in most other igneous rocks in that other extremely Fe-rich silicates are absent. In this paper we discuss the paragenesis and mineral chemistry of the assemblage in the dykes.

Genesis of the Cumbrian hematite deposits

Abstract: (Resume of an evening lecture given during the field meeting on the West Cumberland Hematite ore field, 15th – 17th July, 1983) West Cumbria and Furness have provided about 250 million tons of high-grade iron ore since mining began in earnest about the middle of last century. Iron contents in the range 50–60 per cent, with P2O5 less than 0.02 and S less than 0.06 have been normal as a result of the very simple mineralogy of the deposits, composed of massive and botryoidal hematite with dolomite and quartz as the only gangue minerals of quantitative importance. In a few places, a little chalcopyrite occurred as an early phase, while in others, small amounts of blue fluorite and of baryte were associated with specularite in late cavities. As J. D. Kendall (1881–82) first conclusively demonstrated, most orebodies were emplaced by metasomatism of limestone, in the course of which fossils were replaced molecule by molecule by hematite or quartz, and layers of chert nodules were preserved in situ in the orebodies, particularly those, like the example seen at Florence Mine, having the form of flats. The ores have been wrongly classified as of Bilbao type by United Nations investigators (United Nations 1970); this type is derived from siderite/ankerite protores by oxidation; careful determination of large numbers of carbonates has failed to show any evidence for such protores in West and South Cumbria. The hematite is unquestionably a primary mineral; in this respect the origin of the ores is radically different …

Carbonates of Chiknayakanhalli Schist Belt, Karnataka

Abstract: Carbonates of the Chiknayakanhalli (CN halli) area include (i) dolomite, (ii) limestone, (iii) ankerite and (iv) calc-schist. Of these, dolomite is the most widely developed rock. chemically the CN halli carbonates cover a wide range from almost pure calcite and dolomite-ankerite to Mg-limestone, shaly limestone and cherty limestone/dolomite/ankerite. In their trace element content, the rocks are typically much depleted in comparison with carbonatites. Available evidence indicates overall primary chemical sedimentary deposition of the carbonates in a barred basin with a gradual rise in salinity, pH, temperature and Mg:Ca ratio of the environment. Evidence of dolomitization is limited to the border zones between limestone and dolomite and there is no recognizable indication of organic activity.

Sedimentological, mineralogical and geochemical definition of oil-shale facies in the lower Parachute Creek Member of Green River Formation, Colorado

Abstract: Sedimentological, mineralogical and geochemical studies of two drill cores penetrating the lower Saline zone of the Parachute Creek Member (middle L-4 oil-shale zone through upper R-2 zone) of the Green River Formation in north-central Piceance Creek basin, Colorado, indicate the presence of two distinct oil-shale facies. The most abundant facies has laminated stratification and frequently occurs in the L-4, L-3 and L-2 oil-shale zones. The second, and subordinate facies, has ''streaked and blebby'' stratification and is most abundant in the R-4, R-3 and R-2 zones. Laminated oil shale originated by slow, regular sedimentation during meromictic phases of ancient Lake Uinta, whereas streaked and blebby oil shale was deposited by episodic, non-channelized turbidity currents. Laminated oil shale has higher contents of nahcolite, dawsonite, quartz, K-feldspar and calcite, but less dolomite/ankerite and albite than streaked and blebby oil shale. Ca-Mg-Fe carbonate minerals in laminated oil shale have more variable compositions than those in streaked and blebby shales. Streaked and blebby oil shale has more kerogen and a greater diversity of kerogen particles than laminated oil shale. Such variations may produce different pyrolysis reactions when each shale type is retorted.

The radiogenic hypothesis of formation of periore zones of hematitization in hydrothermal uranium deposits

Abstract: Halos of hematitization generally occur in the peripheral, more dense areas surrounding impregnated pitchblende ores, in rocks surrounding veinlets of pitchblende and in the gouge of these veinlets. The breadth of the reddening halo in the rocks can range from fractions of a millimeter to tens of meters. Developing at the edges of oxides and silicates of uranium (IV) and forming the outermost zone, hematite and goethite are present as finely dispersed particles impregnating grains of feldspar and sometimes quartz, chalcedony, calcite and replacing ferrous iron minerals such as chlorite, ankerite, siderite, biotite, pyrite, magnetite and ilmenite. In the immediate vicinity of the ore veins and phenocrysts of pitchblende, an additional quantity of modal iron is often found as oxide, indicating that this element has been carried in and precipitated as a result of oxidation or alkalization of hydrothermal solutions. 15 references, 1 figure.