Eruption and emplacement dynamics of coarse-grained, wall rock–rich beds in the Keanakāko‘i Tephra, Kīlauea, Hawai‘i

2019 ◽  
Author(s):  
Samantha J. Isgett* ◽  
Bruce F. Houghton ◽  
Donald A. Swanson
Keyword(s):  
Wall Rock ◽  
Coarse Grained

Pétrologie et gîtologie d'un filon-couche différencié et minéralisé archéen : le gisement aurifère Sigma-2, canton de Louvicourt, Québec

1991 ◽  
Vol 28 (11) ◽  
pp. 1731-1743 ◽  
Author(s):  
Réjean Hébert ◽  
Michel Rocheleau ◽  
Christine Giguère ◽  
Benoît Perrier ◽  
Roch Gaudreau
Keyword(s):  
Structural Feature ◽  
Gold Mineralization ◽  
Wall Rock ◽  
Coarse Grained ◽  
Fault System ◽  
Tectonic Event ◽  
The North ◽  
Geochemical Study ◽  
Main Fault ◽  
East West

The Archean Sigma-2 orebody is hosted in the felsic granophyric zone of the differentiated Vicour sill. The sill contains anomalous gold valves and is intrusive into the uppermost part of the Val-d'Or Formation. A geochemical study shows that the Vicour sill has evolved from a ferriferous tholeiitic melt and is comagmatic with the Héva Formation to the south. The competent granophyric zone has been affected by several ductile–brittle deformation events. Three systems of faults and fractures are recognized. Each of these systems is composed of two to three subsystems. The main fault system is oriented east–west with subvertical dip and has a dextral component of movement. Two east–west oriented fault subsystems, moderately dipping (45°) towards north and south, are associated with this feature. The second major structural feature consists of northeast and north-northwest conjugate fractures superimposed on structures of the first tectonic event. The shear movement is sinistral for the northeast fractures and dextral for the north-northwest fractures. The third structural feature is the most interesting with respect to gold mineralization. It consists of east–west-trending, moderately dipping fractures that could be genetically linked with the first structural feature and resulted from a northwest–southeast compression. These fractures increased the tectonic permeability of the granophyre, which allowed Cl- and Na-rich and Ca- and CO2-poor hydrothermal fluids to circulate through the rock and produced subhorizontal mineralized quartz lenses. The lenses are composed of quartz–tourmaline ± carbonate and of pyrite–pyrrhotite ± chalcopyrite. Arsenopyrite is observed in the bleached wall rock surrounding the lenses as well as in east–west faults and northeast and north-northwest conjugate fractures. Bleaching is the result of metasomatic sericitization, albitization, silicification, and low carbonatization of the wall rock and decreases away from the mineralized lenses. Gold is associated with pyrite and arsenopyrite and occurs as inclusions and veinlets crosscutting sulfide grains. It was deposited at a late stage along with quartz and, locally, chalcopyrite. Metasomatism was responsible for the formation of arsenopyrite, coarse-grained pyrite, pyrrhotite, and chalcopyrite while ilmenite recrystallized in the veins. Fractures within arsenopyrite and pyrite are filled with late deposits of pyrrhotite and chalcopyrite. The tholeiitic composition and anomalous gold values of the mafic section of the sill could be additional valuable guidelines in the exploration for similar orebodies.

scholarly journals Strain localization in brittle–ductile shear zones: fluid-abundant vs. fluid-limited conditions (an example from Wyangala area, Australia)

Solid Earth ◽  
2015 ◽  
Vol 6 (3) ◽  
pp. 881-901 ◽  
Author(s):  
L. Spruzeniece ◽  
S. Piazolo
Keyword(s):  
Shear Zone ◽  
Bulk Composition ◽  
Shear Zones ◽  
Wall Rock ◽  
Coarse Grained ◽  
Bulk Rock ◽  
Rock Composition ◽  
Fine Grained ◽  
Ductile Shear ◽  
Zone Centre

Abstract. This study focuses on physiochemical processes occurring in a brittle–ductile shear zone at both fluid-present and fluid-limited conditions. In the studied shear zone (Wyangala, SE Australia), a coarse-grained two-feldspar–quartz–biotite granite is transformed into a medium-grained orthogneiss at the shear zone margins and a fine-grained quartz–muscovite phyllonite in the central parts. The orthogneiss displays cataclasis of feldspar and crystal-plastic deformation of quartz. Quartz accommodates most of the deformation and is extensively recrystallized, showing distinct crystallographic preferred orientation (CPO). Feldspar-to-muscovite, biotite-to-muscovite and albitization reactions occur locally at porphyroclasts' fracture surfaces and margins. However, the bulk rock composition shows very little change in respect to the wall rock composition. In contrast, in the shear zone centre quartz occurs as large, weakly deformed porphyroclasts in sizes similar to that in the wall rock, suggesting that it has undergone little deformation. Feldspars and biotite are almost completely reacted to muscovite, which is arranged in a fine-grained interconnected matrix. Muscovite-rich layers contain significant amounts of fine-grained intermixed quartz with random CPO. These domains are interpreted to have accommodated most of the strain. Bulk rock chemistry data show a significant increase in SiO2 and depletion in NaO content compared to the wall rock composition. We suggest that the high- and low-strain microstructures in the shear zone represent markedly different scenarios and cannot be interpreted as a simple sequential development with respect to strain. Instead, we propose that the microstructural and mineralogical changes in the shear zone centre arise from a local metasomatic alteration around a brittle precursor. When the weaker fine-grained microstructure is established, the further flow is controlled by transient porosity created at (i) grain boundaries in fine-grained areas deforming by grain boundary sliding (GBS) and (ii) transient dilatancy sites at porphyroclast–matrix boundaries. Here a growth of secondary quartz occurs from incoming fluid, resulting in significant changes in bulk composition and eventually rheological hardening due to the precipitation-related increase in the mode and grain size of quartz. In contrast, within the shear zone margins the amount of fluid influx and associated reactions is limited; here deformation mainly proceeds by dynamic recrystallization of the igneous quartz grains. The studied shear zone exemplifies the role of syn-deformational fluids and fluid-induced reactions on the dominance of deformation processes and subsequent contrasting rheological behaviour at micron to metre scale.

The Alteration of the Pore Spaces in Sandstones

1973 ◽  
Vol 31 ◽  
pp. 210-211
Author(s):  
R. E. Ferrell ◽  
G. G. Paulson
Keyword(s):  
Electron Microscope ◽  
Scanning Electron Microscope ◽  
The Other ◽  
Coarse Grained ◽  
Depositional Fabric ◽  
Soluble Components ◽  
Scanning Electron ◽  
Shape And Size

The pore spaces in sandstones are the result of the original depositional fabric and the degree of post-depositional alteration that the rock has experienced. The largest pore volumes are present in coarse-grained, well-sorted materials with high sphericity. The chief mechanisms which alter the shape and size of the pores are precipitation of cementing agents and the dissolution of soluble components. Each process may operate alone or in combination with the other, or there may be several generations of cementation and solution.The scanning electron microscope has ‘been used in this study to reveal the morphology of the pore spaces in a variety of moderate porosity, orthoquartzites.

On the Origin of the Microscystalline Structure in Rapidly Solidified IN-100 Powder

1977 ◽  
Vol 35 ◽  
pp. 260-261
Author(s):  
J. M. Walsh ◽  
K. P. Gumz ◽  
J. C. Whittles ◽  
B. H. Kear
Keyword(s):  
The Other ◽  
Coarse Grained ◽  
Microcrystalline Structure ◽  
Routine Examination ◽  
Atomization Process ◽  
Centrifugal Atomization ◽  
Splat Quenching ◽  
Powder Particles ◽  
Dendritic Microstructures ◽  
Rapidly Solidified

During a routine examination of the microstructure of rapidly solidified IN-100 powder, produced by a newly-developed centrifugal atomization process1, essentially two distinct types of microstructure were identified. When a high melt superheat is maintained during atomization, the powder particles are predominantly coarse-grained, equiaxed or columnar, with distinctly dendritic microstructures, Figs, la and 4a. On the other hand, when the melt superheat is reduced by increasing the heat flow to the disc of the rotary atomizer, the powder particles are predominantly microcrystalline in character, with typically one dendrite per grain, Figs, lb and 4b. In what follows, evidence is presented that strongly supports the view that the unusual microcrystalline structure has its origin in dendrite erosion occurring in a 'mushy zone' of dynamic solidification on the disc of the rotary atomizer.The critical observations were made on atomized material that had undergone 'splat-quenching' on previously solidified, chilled substrate particles.

Correlation between dislocation, grain boundary and interface of duplex SS in stress corrosion cracking(SCC)

1990 ◽  
Vol 48 (4) ◽  
pp. 928-929
Author(s):  
Wang Zheng-fang ◽  
Z.F. Wang
Keyword(s):  
Stainless Steel ◽  
Thermal Cycle ◽  
Secondary Phase ◽  
Corrosion Cracking ◽  
Coarse Grained ◽  
Test Environment ◽  
Fine Grained ◽  
Evaluation Test ◽  
Welding Thermal Cycle ◽  
Micro Analysis

The main purpose of this study highlights on the evaluation of chloride SCC resistance of the material,duplex stainless steel,OOCr18Ni5Mo3Si2 (18-5Mo) and its welded coarse grained zone(CGZ).18-5Mo is a dual phases (A+F) stainless steel with yield strength:512N/mm2 .The proportion of secondary Phase(A phase) accounts for 30-35% of the total with fine grained and homogeneously distributed A and F phases(Fig.1).After being welded by a specific welding thermal cycle to the material,i.e. Tmax=1350°C and t8/5=20s,microstructure may change from fine grained morphology to coarse grained morphology and from homogeneously distributed of A phase to a concentration of A phase(Fig.2).Meanwhile,the proportion of A phase reduced from 35% to 5-10°o.For this reason it is known as welded coarse grained zone(CGZ).In association with difference of microstructure between base metal and welded CGZ,so chloride SCC resistance also differ from each other.Test procedures:Constant load tensile test(CLTT) were performed for recording Esce-t curve by which corrosion cracking growth can be described, tf,fractured time,can also be recorded by the test which is taken as a electrochemical behavior and mechanical property for SCC resistance evaluation. Test environment:143°C boiling 42%MgCl2 solution is used.Besides, micro analysis were conducted with light microscopy(LM),SEM,TEM,and Auger energy spectrum(AES) so as to reveal the correlation between the data generated by the CLTT results and micro analysis.

Input of coarse-grained siliciclastics into the Pyrenean Basin during the PETM (1): overview

2014 ◽  
Vol 31 ◽  
pp. 177-178
Author(s):  
Victoriano Pujalte ◽  
Juan Ignacio Baceta ◽  
Birger Schmitz
Keyword(s):  
Coarse Grained

Patterns in diversity and composition of the microbenthos of subarctic intertidal beaches with different morphodynamics

2020 ◽  
Vol 648 ◽  
pp. 19-38
Author(s):  
AI Azovsky ◽  
YA Mazei ◽  
MA Saburova ◽  
PV Sapozhnikov
Keyword(s):  
Species Abundance ◽  
Abundance Ratio ◽  
Wave Action ◽  
Coarse Grained ◽  
Sediment Properties ◽  
Β Diversity ◽  
Α Diversity ◽  
Wide Range ◽  
Similar Mode ◽  
Abundance And Diversity

Diversity and composition of benthic diatom algae and ciliates were studied at several beaches along the White and Barents seas: from highly exposed, reflective beaches with coarse-grained sands to sheltered, dissipative silty-sandy flats. For diatoms, the epipelic to epipsammic species abundance ratio was significantly correlated with the beach index and mean particle size, while neither α-diversity measures nor mean cell length were related to beach properties. In contrast, most of the characteristics of ciliate assemblages (diversity, total abundance and biomass, mean individual weight and percentage of karyorelictids) demonstrated a strong correlation to beach properties, remaining low at exposed beaches but increasing sharply in more sheltered conditions. β-diversity did not correlate with beach properties for either diatoms or ciliates. We suggest that wave action and sediment properties are the main drivers controlling the diversity and composition of the intertidal microbenthos. Diatoms and ciliates, however, demonstrated divergent response to these factors. Epipelic and epipsammic diatoms exhibited 2 different strategies to adapt to their environments and therefore were complementarily distributed along the environmental gradient and compensated for each other in diversity. Most ciliates demonstrated a similar mode of habitat selection but differed in their degree of tolerance. Euryporal (including mesoporal) species were relatively tolerant to wave action and therefore occurred under a wide range of beach conditions, though their abundance and diversity were highest in fine, relatively stable sediments on sheltered beaches, whereas the specific interstitial (i.e. genuine microporal) species were mostly restricted to only these habitats.

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