Resolving the Richat enigma: Doming and hydrothermal karstification above an alkaline complex

Geology ◽  
2005 ◽  
Vol 33 (8) ◽  
pp. 665-668 ◽  
Author(s):  
Guillaume Matton ◽  
Michel Jébrak ◽  
James K.W. Lee
Keyword(s):  
Volcanic Rocks ◽  
Hydrothermal Fluids ◽  
Alkaline Complex ◽  
Late Proterozoic ◽  
Hydrothermal Origin ◽  
Internal Sediments

Abstract The Richat structure (Sahara, Mauritania) appears as a large dome at least 40 km in diameter within a Late Proterozoic to Ordovician sequence. Erosion has created circular cuestas represented by three nested rings dipping outward from the structure. The center of the structure consists of a limestone-dolomite shelf that encloses a kilometer-scale siliceous breccia and is intruded by basaltic ring dikes, kimberlitic intrusions, and alkaline volcanic rocks. Several hypotheses have been presented to explain the spectacular Richat structure and breccia, but their origin remains enigmatic. The breccia body is lenticular in shape and irregularly thins at its extremities to only a few meters. The breccia was created during karst dissolution and collapse. Internal sediments fill the centimeter- to meter-scale cavities. Alkaline enrichment and the presence of Cretaceous automorphous neoformed K-feldspar demonstrate the hydrothermal origin of these internal sediments and their contemporaneity with magmatism. A model is proposed in which doming and the production of hydrothermal fluids were instrumental in creating a favorable setting for dissolution. The circular Richat structure and its breccia core thus represent the superficial expression of a Cretaceous alkaline complex with an exceptionally well preserved hydrothermal karst infilling at its summit.

scholarly journals Geology of the orogenic Cheminis gold deposit along the Larder Lake – Cadillac deformation zone, Ontario

2015 ◽  
Vol 52 (12) ◽  
pp. 1093-1108 ◽  
Author(s):  
Bruno Lafrance
Keyword(s):  
Deformation Zone ◽  
Volcanic Rocks ◽  
Tholeiitic Basalt ◽  
Hydrothermal Fluids ◽  
South Side ◽  
Shear Sense ◽  
Abitibi Subprovince ◽  
Deformation Features ◽  
Shear Sense Indicators ◽  
Host Rocks

The Larder Lake – Cadillac deformation zone (LLCDZ) is one of two major, auriferous, deformation zones in the southern Abitibi subprovince of the Archean Superior Province. It hosts the Cheminis and the giant Kerr Addison – Chesterville deposits within a strongly deformed band of Fe-rich tholeiitic basalt and komatiite of the Larder Lake Group (ca. 2705 Ma). The latter is bounded on both sides by younger, less deformed, Timiskaming turbidites (2674–2670 Ma). The earliest deformation features are F1 folds affecting the Timiskaming rocks, which formed either during D1 extensional faulting or during early D2 north–south shortening related to the opening and closure, respectively, of the Timiskaming basin. Continued shortening during D2 imbricated the older volcanic rocks and turbidites and produced regional F2 folds with an axial planar S2 cleavage. D2 deformation was partitioned into the weaker band of volcanic rocks, producing the strong S2 foliation, L2 stretching lineation, and south-side-up shear sense indicators, which characterize the LLCDZ. Gold is present in quartz–carbonate veins in deformed fuchsitic komatiites (carbonate ore) and turbiditic sandstone (sandstone-hosted ore), and in association with disseminated pyrite in altered Fe-rich tholeiitic basalts (flow ore). All host rocks underwent strong mass gains in CO2, S, K2O, Ba, As, and W, during sericitization, carbonatization, and sulphidation of the host rocks, suggesting that they interacted with the same hydrothermal fluids. Textural relationships between alteration minerals and S2 cleavage indicate that mineralization is syn-cleavage. Thus, gold was deposited as hydrothermal fluids migrated upward along the LLCDZ during contractional, D2 south-side-up shearing. The gold zones were subsequently modified during D3 reactivation of the LLCDZ as a dextral transcurrent fault zone.

Age of the Grenville dyke swarm, Ontario–Quebec: implications for the timing of lapetan rifting

1995 ◽  
Vol 32 (3) ◽  
pp. 273-280 ◽  
Author(s):  
S. L. Kamo ◽  
T. E. Krogh ◽  
P. S. Kumarapeli
Keyword(s):  
Long Range ◽  
Volcanic Rocks ◽  
Canadian Shield ◽  
Dyke Swarm ◽  
Alkaline Complex ◽  
Southeastern Part ◽  
Time Marker ◽  
Continental Breakup ◽  
Iapetus Ocean

U–Pb baddeleyite and zircon ages for three diabase dykes from widely spaced localities within the Grenville dyke swarm indicate a single age of emplacement at [Formula: see text] Ma. The 700 km long Grenville dyke swarm, located in the southeastern part of the Canadian Shield, was emplaced syntectonically with the development of the Ottawa graben. This graben may represent a plume-generated lapetan failed arm that developed at the onset of the breakup of Laurentia. Other precisely dated lapetan rift-related units, such as the Callander Alkaline Complex and the Tibbit Hill Formation volcanic rocks, indicate a protracted 36 Ma period of rifting and magmatism prior to volcanism along this segment of the lapetan margin. The age of the Grenville dykes is the youngest in a progression of precisely dated mafic magmatic events from the 723 Ma Franklin dykes and sills to the 615 Ma Long Range dykes, along the northern and northeastern margins of Laurentia, respectively. Thus, the age for these dykes represents a key time marker for continental breakup that preceded the formation of the Iapetus ocean.

scholarly journals Fracture Fillings and Implication of Fluid Activities in Volcanic Rocks: Dixi Area in Kelameili Gas Field, Junggar Basin, Northwestern China

Minerals ◽  
2019 ◽  
Vol 9 (3) ◽  
pp. 154 ◽  
Author(s):  
Mingyou Feng ◽  
Tian Liu ◽  
Tong Lin ◽  
Xiaohong Liu ◽  
Ningxin Li ◽  
...  
Keyword(s):  
Carbon Dioxide ◽  
Volcanic Rocks ◽  
Junggar Basin ◽  
Hydrothermal Fluids ◽  
Northwestern China ◽  
Volatile Components ◽  
Gas Field ◽  
Weakly Alkaline ◽  
Quartz Veins ◽  
Calcite Veins

The Carboniferous Batamayineishan Formation of the Kelameili Gas Field is a specific weathered crust-related volcanic reservoir that has a significant production rate in the Junggar Basin, Northwestern China, attributed to debatable processes of fluid evolution. The results suggest that various types of fluids occurring in volcanic rocks lead to the filling of quartz and calcite in fractures and their associated alteration haloes. The silica that formed quartz veins was mainly derived from deep hydrothermal fluids, while the carbon dioxide that formed calcite veins originated from sources characterized by mixing and alteration of deep hydrothermal and hydrocarbon fluids. Siliceous hydrothermal fluids rich in sulphur dioxide and other volatile components were driven by a pressure gradient and buoyancy, and circulated both laterally and vertically along the fractures, forming quartz veins and tension fractures under different temperature conditions. Moreover, changes in salinity, pressure, and carbon dioxide of deep fluids, varying from acidic to weakly alkaline, resulted in earlier calcite precipitation in contraction fractures and weathered fractures. Tectonic uplift resulted in the long-term exposure of volcanic rocks, where fresh water mixed with the partially alkaline fluid escaping the basin to form calcite cements, thus retaining the characteristics of a seepage environment in the weathered fractures. Structural fractures occurred due to tectonic movements during the burial period. Filling and leakage of hydrocarbons caused pore fluids to convert from acidic to alkaline, precipitating late sparry calcite in dissolution fractures. Late hydrothermal fluid metasomatism, brought about by infiltration into the permeable zone, caused partial dissolution of local calcite along cleavage cracks.

Hydrothermal origin of Late Proterozoic bedded chert at Gusui, Guangdong, China: petrological and geochemical evidence

Sedimentology ◽  
1994 ◽  
Vol 41 (3) ◽  
pp. 605-619 ◽  
Author(s):  
YONGZHANG ZHOU ◽  
E. H. CHOWN ◽  
JAYANTA GUHA ◽  
HUANGZHANG LU ◽  
GUANGZHI TU
Keyword(s):  
Geochemical Evidence ◽  
Late Proterozoic ◽  
Hydrothermal Origin ◽  
Bedded Chert

Uranium and thorium in Late Proterozoic volcanic rocks from northwestern Africa

1984 ◽  
Vol 42 (1-4) ◽  
pp. 297-306 ◽  
Author(s):  
J. Dostal ◽  
C. Dupuy ◽  
M. Chikhauoi ◽  
M. Zentilli
Keyword(s):  
Volcanic Rocks ◽  
Late Proterozoic ◽  
Northwestern Africa

scholarly journals Amethyst Occurrences in Tertiary Volcanic Rocks of Greece: Mineralogical, Fluid Inclusion and Oxygen Isotope Constraints on Their Genesis

Minerals ◽  
2018 ◽  
Vol 8 (8) ◽  
pp. 324 ◽  
Author(s):  
Panagiotis Voudouris ◽  
Vasilios Melfos ◽  
Constantinos Mavrogonatos ◽  
Alexandre Tarantola ◽  
Jens Gӧtze ◽  
...  
Keyword(s):  
Fluid Inclusion ◽  
Open System ◽  
Volcanic Rocks ◽  
Hydrothermal Fluids ◽  
Northern Greece ◽  
Quartz Veins ◽  
Low Salinity ◽  
Moderate Salinity ◽  
Host Rocks ◽  
Of Greece

Epithermally altered volcanic rocks in Greece host amethyst-bearing veins in association with various silicates, carbonates, oxides and sulfides. Host rocks are Oligocene to Pleistocene calc-alkaline to shoshonitic lavas and pyroclastics of intermediate to acidic composition. The veins are integral parts of high to intermediate sulfidation epithermal mineralized centers in northern Greece (e.g., Kassiteres–Sapes, Kirki, Kornofolia/Soufli, Lesvos Island) and on Milos Island. Colloform–crustiform banding with alternations of amethyst, chalcedony and/or carbonates is a common characteristic of the studied amethyst-bearing veins. Hydrothermal alteration around the quartz veins includes sericitic, K-feldspar (adularia), propylitic and zeolitic types. Precipitation of amethyst took place from near-neutral to alkaline fluids, as indicated by the presence of various amounts of gangue adularia, calcite, zeolites, chlorite and smectite. Fluid inclusion data suggest that the studied amethyst was formed by hydrothermal fluids with relatively low temperatures (~200–250 °C) and low to moderate salinity (1–8 wt % NaCl equiv). A fluid cooling gradually from the external to the inner parts of the veins, possibly with subsequent boiling in an open system, is considered for the amethysts of Silver Hill in Sapes and Kassiteres. Amethysts from Kornofolia, Megala Therma, Kalogries and Chondro Vouno were formed by mixing of moderately saline hydrothermal fluids with low-salinity fluids at relatively lower temperatures indicating the presence of dilution processes and probably boiling in an open system. Stable isotope data point to mixing between magmatic and marine (and/or meteoric) waters and are consistent with the oxidizing conditions required for amethyst formation.

scholarly journals Amethyst occurrences in Tertiary volcanic rocks of Greece: mineralogical and genetic implications

2013 ◽  
Vol 47 (1) ◽  
pp. 477 ◽  
Author(s):  
P. Voudouris ◽  
I. Psimis ◽  
C. Mavrogonatos ◽  
C. Kanellopoulos ◽  
M. Kati ◽  
...  
Keyword(s):  
Volcanic Rocks ◽  
Hydrothermal Activity ◽  
Hydrothermal Fluids ◽  
Calc Alkaline ◽  
Northern Greece ◽  
Volcanic Arc ◽  
Quartz Veins ◽  
High Sulfidation ◽  
Host Rocks ◽  
Of Greece

Epithermal-altered volcanic rocks in Greece host gem-quality amethyst veins in association with various silicates, carbonates, oxides, sulfides and halides. Host rocks are Oligocene to recent calc-alkaline to shoshonitic lavas and pyroclastics of intermediate- to acid composition. The amethyst-bearing veins occur in the periphery of porphyry-type and/or high-sulfidation epithermal mineralized centers in northern Greece (e.g. Sapes, Kirki, Kornofolia/Soufli, Lesvos island) and on Milos island in the active Aegean Volcanic Arc. Hydrothermal alteration around the quartz veins includes sericitic, K-feldspar (adularia), argillic, propylitic and zeolitic types. Precipitation of amethyst in the northern Greece occurrences, took place during the final stages of the magmatic-hydrothermal activity from near-neutral to alkaline fluids, as indicated by the presence of gangue adularia, calcite, smectite, chlorite, sericite, pyrite, zeolites (laumontite, heulandite, clinoptilolite), analcime and minor amounts of barite, halite, epidote and fluorite in the quartz veins. Amethyst at Milos Island (Chondro Vouno and Kalogries-Vani areas), is accompanied by barite, smectite and lepidocrocite. Colloform-crustiform banding with alternations of amethyst, chalcedony and/or carbonates is a common characteristic of the studied amethyst-bearing veins. Fluid inclusion- and mineralogical data suggest that the studied amethyst were formed at: 174-246 °C (Sapes area), 100-175 °C (Kirki and Kornofolia areas) and 223-234°C (Lesvos island). The amethyst formation requires oxidizing conditions and is probably the result of mixing between meteoric or seawater with upwelling hydrothermal fluids. The involvement of seawater in the studied mineralization is supported by the presence of halite and abundant barite in the veins. Finally, the studied amethyst deposits should be evaluated as potential gemstone sources in Greece.

The nature and origin of authigenic smectites in some recent marine sediments

Clay Minerals ◽  
1983 ◽  
Vol 18 (3) ◽  
pp. 239-252 ◽  
Author(s):  
T. G. Cole ◽  
H. F. Shaw
Keyword(s):  
Marine Sediments ◽  
Biogenic Silica ◽  
Volcanic Rocks ◽  
Hydrothermal Fluids ◽  
The Third ◽  
Direct Precipitation ◽  
Atlantis Ii Deep ◽  
Fe Oxyhydroxides ◽  
New Evidence ◽  
Deep Sediments

AbstractThree principal modes of formation are apparent for authigenic smectites in Recent marine sediments: alteration of volcanic rocks and glass, low-temperature combination of biogenic silica and Fe-oxyhydroxides, and direct precipitation from hydrothermal fluids. The latter two mechanisms are discussed with reference to new evidence from studies of sediments from the Bauer Deep of the equatorial eastern Pacific and the Atlantis II Deep in the Red Sea. In the Atlantis II Deep sediments, three sub-environments of smectite formation from hydrothermal fluids are recognized. In two of them nontronites are formed, whilst in the third an Fe-poor smectite, intermediate in composition between beidellite and montmorillonite, occurs.

Geochemistry and tectonic discrimination of Late Proterozoic arc-related volcaniclastic turbidite sequences, Antigonish Highlands, Nova Scotia

1993 ◽  
Vol 30 (12) ◽  
pp. 2273-2282 ◽  
Author(s):  
J. Brendan Murphy ◽  
Deborah L. MacDonald
Keyword(s):  
Nova Scotia ◽  
Tectonic Setting ◽  
Volcanic Rocks ◽  
Sedimentary Rocks ◽  
Widespread Application ◽  
Isotopic Signatures ◽  
Late Proterozoic ◽  
Felsic Volcanic ◽  
Clastic Sedimentary ◽  
End Members

The Late Proterozoic (ca. 618–610 Ma) Georgeville Group of northern mainland Nova Scotia lies within the Avalon Composite Terrane and consists of subgreenschist- to greenschist-facies mafic and felsic volcanic rocks overlain by volcaniclastic turbidites that were deposited in an ensialic basin within a rifted volcanic arc. Geochronological data indicate that the volcanic and sedimentary rocks are coeval. The geochemical and isotopic signatures of the sedimentary rocks are attributed to erosion of the coeval Avalonian volcanic rocks that flank the basin and are consistent with synorogenic deposition. There is no evidence of significant chemical contribution from Avalonian basement.Knowledge of the tectonic setting facilitates the testing of published geochemical discriminant diagrams for clastic sedimentary rocks. Discrimination diagrams using ratios such as K2O/Na2O and Al2O3/(CaO + Na2O) give inconclusive results, probably due to elemental mobility during secondary processes. Plots involving MgO, TiO2, and Fe2O3 detect the chemical contribution of mafic detritus, give much tighter clusters of data, and plot between Aleutian- and Cascade-type arc-derived sediments, suggesting a moderate thickness of continental crust beneath the arc.The arc-related signature of the Georgeville sedimentary rocks is clearly recognizable on ternary plots involving inter-element ratios of high field strength elements (e.g., Ti–Y–Zr, Nb–Y–Zr, and Hf–Ta–Th) in which the samples plot as mixing trends between mafic and felsic end members. Diagrams of this type may have widespread application to tectonic discrimination of sedimentary rocks because in most suites these ratios are relatively insensitive to sedimentary and metamorphic processes.

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