LOW‐AMPLITUDE AEROMAGNETIC ANOMALIES IN SOUTHEASTERN MISSOURI

Geophysics ◽  
1964 ◽  
Vol 29 (4) ◽  
pp. 537-552 ◽  
Author(s):  
John W. Allingham
Keyword(s):  
Volcanic Rock ◽  
Mineral Exploration ◽  
Volcanic Rocks ◽  
Igneous Rocks ◽  
Aeromagnetic Anomalies ◽  
Fine Grained ◽  
Mining Areas ◽  
Granite Batholith ◽  
Extrusive Rock ◽  
Low Amplitude

This study shows that aeromagnetic anomalies of less than 200 gammas are associated with topographic relief of exposed Precambrian granitic and volcanic rocks of the St. François Mountains. Anomalies resulting from hills coarsely crystalline granite are as high as 100 gammas in amplitude, whereas anomalies over comparable hills of fine‐grained rocks, such as granophyre or devitrified volcanic rock, are as much as 200 gammas. Anomalies caused by normal faulting or shearing of igneous rocks have amplitudes of less than 100 gammas; they are observed best in profile. These anomalies are superposed on larger magnetic features related to pendants of volcanic rock in the roof of a granite batholith and are distinguished from large lateral variations in magnetic intensity by their low amplitude and small areal extent. Analyses of the compound anomalies yielded the subsurface configuration of isolated roof pendants of resistant extrusive rock in some areas. As many of these pendants have negligible remanent magnetization, induction theory was used in the analyses. Application of these results to the lead‐mining areas shows that aeromagnetic patterns of low amplitude can guide mineral exploration in the region flanking the Ozark uplift, where isolated, buried hills of Precambrian igneous rocks controlled the development of some lead‐bearing sedimentary structures in the overlying Cambrian carbonate strata.

Mafic igneous rocks and mineralisation in the Palaeoproterozoic Ketilidian orogen, South-East Greenland: project SUPRASYD 1996

1997 ◽  
pp. 66-74
Author(s):  
Henrik Stendal ◽  
Wulf Mueller ◽  
Nicolai Birkedal ◽  
Esben I. Hansen ◽  
Claus Østergaard
Keyword(s):  
Mineral Exploration ◽  
Volcanic Rocks ◽  
Shear Zones ◽  
Field Work ◽  
Igneous Rocks ◽  
Border Zone ◽  
The South ◽  
East Greenland ◽  
North West ◽  
Arc Setting

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Stendal, H., Mueller, W., Birkedal, N., Hansen, E. I., & Østergaard, C. (1997). Mafic igneous rocks and mineralisation in the Palaeoproterozoic Ketilidian orogen, South-East Greenland: project SUPRASYD 1996. Geology of Greenland Survey Bulletin, 176, 66-74. https://doi.org/10.34194/ggub.v176.5064 _______________ The multidisciplinary SUPRASYD project (1992–96) focused on a regional investigation of the Palaeoproterozoic Ketilidian orogenic belt which crosses the southern tip of Greenland. Apart from a broad range of geological and structural studies (Nielsen et al., 1993; Garde & Schønwandt, 1994, 1995; Garde et al., 1997), the project included a mineral resource evaluation of the supracrustal sequences associated with the Ketilidian orogen (e.g. Mosher, 1995). The Ketilidian orogen of southern Greenland can be divided from north-west to south-east into: (1) a border zone in which the crystalline rocks of the Archaean craton are unconformably overlain by Ketilidian supracrustal rocks; (2) a major polyphase pluton, referred to as the Julianehåb batholith; and (3) extensive areas of Ketilidian supracrustal rocks, divided into psammitic and pelitic rocks with subordinate interstratified mafic volcanic rocks (Fig. 1). The Julianehåb batholith is viewed as emplaced in a magmatic arc setting; the supracrustal sequences south of the batholith have been interpreted as either (1) deposited in an intra-arc and fore-arc basin (Chadwick & Garde, 1996), or (2) deposited in a back-arc or intra-arc setting (Stendal & Swager, 1995; Swager, 1995). Both possibilities are plausible and infer subduction-related processes. Regional compilations of geological, geochemical and geophysical data for southern Greenland have been presented by Thorning et al. (1994). Mosher (1995) has recently reviewed the mineral exploration potential of the region. The commercial company Nunaoil A/S has been engaged in gold prospecting in South Greenland since 1990 (e.g. Gowen et al., 1993). A principal goal of the SUPRASYD project was to test the mineral potential of the Ketilidian supracrustal sequences and define the gold potential in the shear zones in the Julianehåb batholith. Previous work has substantiated a gold potential in amphibolitic rocks in the south-west coastal areas (Gowen et al., 1993.), and in the amphibolitic rocks of the Kutseq area (Swager et al., 1995). Field work in 1996 was focused on prospective gold-bearing sites in mafic rocks in South-East Greenland. Three M.Sc. students mapped showings under the supervision of the H. S., while an area on the south side of Kangerluluk fjord was mapped by H. S. and W. M. (Fig. 4).

Volcanic pebbles from Pleistocene gravels in Norfolk and Essex

1977 ◽  
Vol 114 (3) ◽  
pp. 219-225 ◽  
Author(s):  
R. W. Hey ◽  
P. J. Brenchley
Keyword(s):  
Volcanic Rocks ◽  
Igneous Rocks ◽  
East Anglia ◽  
Source Areas ◽  
Fine Grained ◽  
North Wales ◽  
Almost All

SummaryIn the pre-Anglian Pleistocene of East Anglia and Essex are unfossiliferous gravels with pebbles of flint, quartz, quartzite, chert and volcanic rocks. All appear to belong to the Kesgrave Sands and Gravels of Rose & Allen (1976), which are largely or wholly fluviatile and probably of Beestonian age. Forty-six of the volcanic pebbles, from three different localities, have been examined microscopically. Almost all are fine-grained acid igneous rocks or vitric tuffs, evidently derived from a single suite. Various possible source-areas are considered, of which North Wales, with its Ordovician volcanics, is thought to be the most likely.

New Insights into Interpretation of Aeromagnetic Data for Distribution of Igneous Rocks in Central Iran

2021 ◽  
Author(s):  
M. Mohammadzadeh Moghaddam ◽  
S. Mirzaei ◽  
M. Abedi
Keyword(s):  
Magnetic Susceptibility ◽  
Volcanic Rocks ◽  
Igneous Rocks ◽  
Aeromagnetic Data ◽  
3D Inversion ◽  
Aeromagnetic Anomalies ◽  
Susceptibility Data ◽  
Lut Block ◽  
Wide Range ◽  
Intrusive Rocks

Abstract —New insights in the aeromagnetic data over the Central Iranian Microcontinent (CIM) have revealed interesting results for future studies and exploration. This work presents the interpretation of different magnetic analyses and the calculated 3D inversion model to provide important insights into the distribution of igneous rocks in the area that may be traced under significant cover. By analyzing several hundred magnetic susceptibility data points and aeromagnetic anomalies of known igneous rocks over the area, it was determined that mafic–ultramafic intrusive rocks generally have a high magnetic susceptibility and produce a strong magnetic response. Intermediate–felsic intrusive rocks have a low magnetic susceptibility and show a smooth gradient variation and commonly regular shape. Volcanic rocks show a wide range of magnetic susceptibility; therefore, the aeromagnetic anomalies are often random or show strong amplitude with high frequency signals and are rapidly eliminated when an upward continuation is applied. Based on the results of analysis of different magnetic maps and 3D inversion of data, and combining this information with known outcropped of igneous rocks, we revealed 1215 concealed intrusive rocks and 528 volcanic rocks in the area. We also renewed the boundaries of tens outcropped igneous rocks. The known and new mapped igneous rocks can be identified as 12 regions (or zones) for intrusive rocks and 4 regions for volcanic rocks. The results indicate that the mafic–ultramafic rocks are mainly located in the Sistan suture zone of eastern Iran along the Nehbandan fault zone. They also show that the many parts of the Lut block as the main structure of CIM have been under magmatic events, so that most of concealed igneous rocks are distributed in the middle and southern part of the Lut block. Volcanic rocks are widespread in the southeastern and northern parts of the area such as the Urumieh-Dokhtar Magmatic Arc, North Lut, and Bam region.

TEM/AEM characterization of fine-grained clay minerals in very-low-grade rocks: Evaluation of contamination by empa involving celadonite family minerals

1996 ◽  
Vol 54 ◽  
pp. 694-695
Author(s):  
Gejing Li ◽  
D. R. Peacor ◽  
D. S. Coombs ◽  
Y. Kawachi
Keyword(s):  
Electron Microscopy ◽  
Volcanic Rocks ◽  
Analytical Electron Microscopy ◽  
Metamorphic Rocks ◽  
New Mineral ◽  
Chemical Compositions ◽  
Low Grade ◽  
Fine Grained ◽  
Depth Analysis ◽  
Mineral Aggregates

Recent advances in transmission electron microscopy (TEM) and analytical electron microscopy (AEM) have led to many new insights into the structural and chemical characteristics of very finegrained, optically homogeneous mineral aggregates in sedimentary and very low-grade metamorphic rocks. Chemical compositions obtained by electron microprobe analysis (EMPA) on such materials have been shown by TEM/AEM to result from beam overlap on contaminant phases on a scale below resolution of EMPA, which in turn can lead to errors in interpretation and determination of formation conditions. Here we present an in-depth analysis of the relation between AEM and EMPA data, which leads also to the definition of new mineral phases, and demonstrate the resolution power of AEM relative to EMPA in investigations of very fine-grained mineral aggregates in sedimentary and very low-grade metamorphic rocks.Celadonite, having end-member composition KMgFe3+Si4O10(OH)2, and with minor substitution of Fe2+ for Mg and Al for Fe3+ on octahedral sites, is a fine-grained mica widespread in volcanic rocks and volcaniclastic sediments which have undergone low-temperature alteration in the oceanic crust and in burial metamorphic sequences.

Quantitative Determination of some Carbonate Minerals in Greenschist Facies Meta-volcanic Rocks

1972 ◽  
Vol 9 (1) ◽  
pp. 36-42 ◽  
Author(s):  
Calvert C. Bristol
Keyword(s):  
Standard Deviation ◽  
Quantitative Determination ◽  
Volcanic Rocks ◽  
Internal Standard ◽  
Greenschist Facies ◽  
Carbonate Mineral ◽  
Mineral Contents ◽  
X Ray ◽  
Fine Grained

X-ray powder diffraction methods, successful in quantitative determination of silicate minerals in fine-grained rocks, have been applied to the determination of calcite, dolomite, and magnesite in greenschist facies meta-volcanic rocks. Internal standard graphs employing two standards (NaCl and Mo) have been determined.Carbonate mineral modes (calcite and dolomite) for 6 greenschist facies meta-volcanic rocks obtained by the X-ray powder method have been compared to normative carbonate mineral contents calculated for the same rocks. This comparison showed a maximum variation of 7.7 wt.% between the X-ray modes and the normative carbonate mineral contents of the rocks. Maximum standard deviation for the X-ray modes of these rocks was equivalent to 4.4 wt.%.

scholarly journals VII.—The Late Palæozoic Alkaline Igneous Rocks of the West of Scotland

1912 ◽  
Vol 9 (3) ◽  
pp. 120-131 ◽  
Author(s):  
G. W. Tyrrell
Keyword(s):  
Thin Section ◽  
Basic Type ◽  
Igneous Rocks ◽  
The West ◽  
Fine Grained ◽  
Late Palaeozoic

2. Essexite. (a) Carskeoch type.—A small boss of essexite, of rather basic type, pierces the agglomerate of the Carskeoch vent, near Patna, Ayrshire. Macroscopically it is light grey, compact, and fine-grained. In thin section it shows a plexus of fluxionally-arranged plagioclase laths of the composition Ab An, the interstices being filled with subhedral augite of a nearly colourless variety, and fresh olivine. Here and there minute angular interspaces are occupied with turbid isotropic matter, the form and arrangement of the particles of which suggest the former presence of nepheline. A few broad plates of pale augite and crystals of olivine interrupt the general trachytoid fabric. Ilmenite and biotite occur sparsely, and a little orthoclase may be detected on the margins of the plagioclases, extending irregularly into the interspaces. This rock has a distinct individuality, and resembles neither the essexites of Lochend and Craigleith in the Lothians, nor the Crawfordjohn type described below. It is poor in alkali-felspars and felspathoids, is devoid of purple augite, and has a well-developed trachytoid fabric.

2. On the Carboniferous Volcanic Rocks of the Basin of the Forth

1862 ◽  
Vol 4 ◽  
pp. 582-583
Author(s):  
Archibald Geikie
Keyword(s):  
Volcanic Rocks ◽  
Igneous Rocks ◽  
Previous Communication ◽  
Sedimentary Matter ◽  
Simple Cone

After referring to a previous communication to the Society, in which the author had given an outline of the chronology of the igneous rocks of Scotland, he proceeded in the present paper to describe in detail the character of the volcanic phenomena in one district—that of the carboniferous system of the Forth basin. The igneous rocks of this district consist partly of doleritic and felspathic lava-form masses, and partly of various kinds of ash or traptuff. These materials present a considerable diversity in their modes of arrangement. But the author had found that all the volcanic hills of the district might be reduced to three types of structure:—1. A simple cone of ash, round and over which the ordinary sedimentary accumulation of the carboniferous period had been deposited. 2. A cone of ash with the crater filled up by a neck or plug of basalt. 3. Sheets of different lavas with intercalated ash or sedimentary matter.

scholarly journals Preferred orientation of ferromagnetic phases in rock-forming minerals: insights from magnetic anisotropy of single crystals

2019 ◽  
Vol 56 (9) ◽  
pp. 994-1001 ◽  
Author(s):  
Ann M. Hirt ◽  
Andrea R. Biedermann
Keyword(s):  
Magnetic Field ◽  
Magnetic Anisotropy ◽  
Single Crystals ◽  
Remanent Magnetization ◽  
Igneous Rocks ◽  
Flow Structures ◽  
Silicate Minerals ◽  
Fine Grained ◽  
Crystallographic Axes ◽  
Paleomagnetic Direction

In the early days of paleomagnetism, David Strangway was interested in understanding why igneous rocks are faithful recorders of the Earth’s magnetic field. He recognized that ferromagnetic (s.l.) grains that could be discerned by optical microscopy were too large to carry a stable remanent magnetization, and speculated whether fine-grained, ferromagnetic (s.l.) inclusions or exsolutions in silicate minerals are responsible. When these inclusions or exsolutions are randomly oriented, or the silicate hosts are randomly oriented in a rock, they can be a good recorder of the field. If these minerals, however, show an alignment within the silicate host, and the host is preferentially aligned due to flow structures or deformation, then the paleomagnetic direction and paleointensity could be biased. We examine the magnetic anisotropy arising from the ferromagnetic (s.l.) phases in silicate-host minerals. Single crystals of phyllosilicate, clinopyroxene, and calcite show most consistent ferrimagnetic fabric with relation to the minerals’ crystallographic axes, whereas olivine and feldspar display only a weak relationship. No discernable relationship is found between the ferrimagnetic anisotropy and crystallographic axes for amphibole minerals. Our results have implications when single crystals are being used for either studies of field direction or paleointensity or in cases where silicate minerals have a preferential orientation. Phyllosilicate minerals and pyroxene should be screened for significant magnetic anisotropy.

scholarly journals Dominant Weathering Profile Assessment of Kebo-Butak Volcanic Rocks in Gedangsari and Ngawen area, Yogyakarta, Indonesia

2021 ◽  
Vol 6 (1) ◽  
Author(s):  
Fathan Hanifi Mada Mahendra ◽  
I Gde Budi Indrawan ◽  
Sugeng Sapto Surjono
Keyword(s):  
Volcanic Rocks ◽  
Sedimentary Rocks ◽  
Coarse Grained ◽  
Residual Soil ◽  
Geological Condition ◽  
Weathering Profile ◽  
Fine Grained ◽  
Degree Pattern ◽  
Weathering Degree ◽  
East West

The Gedangsari and Ngawen area is predominantly composed of volcanic and volcaniclastic sequencesdistributed east – west direction of the northern parts of Southern Mountain. The massive tectonism as well as tropical climatein this region have been producing weathering profiles in varying thickness which inevitably affects thegeotechnical properties. This study aims to assess the dominant weathering profileof the lower part of Kebo-Butak Formation as well as evaluating the distribution of the discontinuity. In order to know the dominant weathering profile and discontinuity evaluation, this study utilizes a total of  26 panels from five stations investigated through a geotechnical data acquisition including the geological condition, weathering zones, joint distribution, and discontinuity characteristics. The result shows four types of dominant weathering profiles in lower part of Kebo-Butak Formation called as dominant weathering profile A, B, C, and D. Profile A, B, C consisted of a relatively identical weathering degree pattern of fresh, slightly, moderately, completely weathered zone with the variation of thicknesses. However, the weathering degree in profile D reached the residual soil degree controlled by more intensive joints. The fine-grained sedimentary rocks also tends to have smaller spacing, shorter persistence, and higher weathering degree of discontinuities as compared to coarse-grained sedimentary rocks.

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