scholarly journals Silica enrichment, graphic granite and aquamarine growth: a new exploration guide

2002 ◽  
Vol 74 (3) ◽  
pp. 533-538
Author(s):  
A. BHASKARA RAO
Keyword(s):  
Granitic Pegmatite ◽  
Potash Feldspar ◽  
Granitic Pegmatites ◽  
Quartz Veins ◽  
Rock Types ◽  
Eutectic Crystallization ◽  
State Present ◽  
Through Diffusion ◽  
Texture Structure ◽  
Definition Of

Granitic pegmatites are traditionally known to contain graphic, perthitic and myrmekitic intergrowths related to quartz and K- and Na- feldspars. They are further considered to characterise the pegmatite types distinguishing them from the granites and other related plutonic rock types. Graphic granite is accepted also as a synonym to granitic pegmatite. Systematic studies, by the author and colleagues, on the granitic pegmatite gem deposits have permitted the definition of two aquamarine gem provinces in ENE Brazil, one in the NeoProterozoic and the other in the Archaean sequences. Potash feldspars in the pegmatites in the former show perthitic intergrowths, whereas in the latter graphic intergrowth dominates with anomalously coarse centimetric quartz along the cleavages of K-feldspar. Several granitic pegmatites hosted in Archaean complex, in Lages Pintadas Aquamarine Province, Santa Cruz, RN State, present this texture-structure. Graphic intergrowth is attributed to the eutectic crystallization, succeeded by hydrothermal fluids with silica enrichment permitting the growth through diffusion and nucleation of quartz and along cleavages of potash feldspar. In the Archaean terrain, the abundance of recycled chert forming metapsammitic migmatites traversed by numerous quartz veins and coarse graphic granites, has contributed to the growth of beryl and also the aquamarines.

Geochemistry of syntectonic, crustal fluid regimes along the Lithoprobe Southern Canadian Cordillera Transect

1995 ◽  
Vol 32 (10) ◽  
pp. 1699-1719 ◽  
Author(s):  
Bruce E. Nesbitt ◽  
Karlis Muehlenbachs
Keyword(s):  
Deep Convection ◽  
Canadian Cordillera ◽  
Depth Of Penetration ◽  
Crustal Fluid ◽  
Quartz Veins ◽  
Economic Significance ◽  
Rock Types ◽  
Brittle Ductile Transition ◽  
Fluid Convection ◽  
Convection Model

In conjunction with the Lithoprobe southern Canadian Cordillera program, an extensive examination of geochemical indicators of origins, movement, chemical evolution, and economic significance of paleocrustal fluids was conducted. The study area covers approximately 360 000 km2from the Canadian Rockies to Vancouver Island. Research incorporated petrological, mineralogical, fluid-inclusion, δ18O, δD, δ13C, and Rb/Sr studies of samples of quartz ± carbonate veins and other rock types. The results of the study document a variety of pre-, syn-, and postorogenic, crustal fluid events. In the Rockies, a major pre-Laramide hydrothermal event was identified, which was comprised of a west to east migration of warm, saline brines. This was followed by a major circulation of meteoric water in the Rockies during Laramide uplift. In the southern Omineca extensional zone, convecting surface fluids penetrated to the brittle–ductile transition at 350–450 °C and locally into the underlying more ductile rocks. A principal conclusion of the study is that most quartz ± carbonate veins in metamorphic rocks in the southern Canadian Cordillera precipitated from deeply converted surface fluids. This conclusion supports a surface fluid convection model for the genesis of mesothermal Au–quartz veins, common in greenschist-facies rocks worldwide. The combination of our geochemical results with the results of other Lithoprobe studies indicates that widespread and deep convection of surface fluids in rocks undergoing active metamorphism is a commonplace phenomena in extensional settings, while in compressional-thrust settings the depth of penetration of surface fluids is more limited.

scholarly journals Gold-hosting supracrustal rocks on Storø, southern West Greenland: lithologies and geological environment

2007 ◽  
Vol 13 ◽  
pp. 41-44 ◽  
Author(s):  
Christian Knudsen ◽  
Jeroen A.M. Van Gool ◽  
Claus Østergaard ◽  
Julie A. Hollis ◽  
Matilde Rink-Jørgensen ◽  
...  
Keyword(s):  
Volcanic Rocks ◽  
West Greenland ◽  
Metasedimentary Rocks ◽  
Quartz Veins ◽  
Rock Types ◽  
Minimum Age ◽  
Arc Setting ◽  
Basic Volcanic ◽  
Back Arc ◽  
Different Sources

A gold prospect on central Storø in the Nuuk region of southern West Greenland is hosted by a sequence of intensely deformed, amphibolite facies supracrustal rocks of late Mesoto Neoarchaean age. The prospect is at present being explored by the Greenlandic mining company NunaMinerals A/S. Amphibolites likely to be derived from basaltic volcanic rocks dominate, and ultrabasic to intermediate rocks are also interpreted to be derived from volcanic rocks. The sequence also contains metasedimentary rocks including quartzites and cordierite-, sillimanite-, garnet- and biotite-bearing aluminous gneisses. The metasediments contain detrital zircon from different sources indicating a maximum age of the mineralisation of c. 2.8 Ga. The original deposition of the various rock types is believed to have taken place in a back-arc setting. Gold is mainly hosted in garnet- and biotite-rich zones in amphibolites often associated with quartz veins. Gold has been found within garnets indicating that the mineralisation is pre-metamorphic, which points to a minimum age of the mineralisation of c. 2.6 Ga. The geochemistry of the goldbearing zones indicates that the initial gold mineralisation is tied to fluid-induced sericitisation of a basic volcanic protolith. The hosting rocks and the mineralisation are affected by several generations of folding.

Fe-Mn phosphate associations as indicators of the magmatic-hydrothermal and supergene evolution of the Jálama batholith in the Navasfrías Sn-W District, Salamanca, Spain

2012 ◽  
Vol 76 (1) ◽  
pp. 1-24 ◽  
Author(s):  
T. Llorens ◽  
M.C. Moro
Keyword(s):  
Hydrothermal Fluids ◽  
Granitic Pegmatite ◽  
Metamorphic Rocks ◽  
Low Grade ◽  
Magmatic Differentiation ◽  
Metasedimentary Rocks ◽  
Quartz Veins ◽  
Ore Minerals

AbstractThe residual melts that remained after the consolidation of the Jálama batholith crystallized to form a group of intra-granitic pegmatite dykes, which are hosted by its outermost facies (the External Unit), and the most evolved residual melts migrated through fractures to form the Cruz del Rayo field of pegmatite dykes, which are hosted by pre-Ordovician low-grade metasedimentary rocks. The increasing activity of phosphorus as magmatic differentiation took place led to the crystallization of primary phosphates, including members of the triplite–zwieselite and the amblygonite–montebrasite series. A strong albitization of the granitic and pegmatite rocks led to the replacement of the primary assemblage by other phosphates such as alluaudite. The influx of post-magmatic hydrothermal fluids, produced quartz veins, gave rise to the crystallizationof ore minerals and triplite, and altered the granites, aplites and pegmatites, replacing some of the phosphate minerals and feldspars and depositing goyazite, montebrasite and childrenite–eosphorite. The interaction of the residual hydrothermal fluids with those from the surrounding metamorphic rocks during later alteration events resulted in the influx of large quantities of Ca and Mg, and produced phosphate assemblages enriched in those elements. Finally, late goyazite, hydroxylapatite and an unidentified Fe-rich phosphate were formed as a result of supergene alterationby percolating meteoric waters, which added Ca, Sr and other elements into the system, and increased fO2.

scholarly journals A review on the definitions of terms of sedimentary facies

2019 ◽  
Vol 8 (1) ◽  
Author(s):  
Zeng-Zhao Feng
Keyword(s):  
Sedimentary Facies ◽  
English Edition ◽  
Thin Sections ◽  
Rock Types ◽  
Definition Of

AbstractIn recent years, in some papers and manuscripts published in and submitted to the Journal of Palaeogeography (Chinese Edition and English Edition), the authors named the rocks or rock types as “microfacies” or “lithofacies”, named the microfeatures in thin-sections under microscope as “microfacies”, and named the macrofeatures of rocks as “macrofacies”. I wrote two short papers “Words of the Editor-in-Chief — Rocks are not microfacies” (Feng, Journal of Palaeogeography 19(5):II 2017) and “Words of the Editor-in-Chief — Rocks are not lithofacies” (Feng, Journal of Palaeogeography 20(3):452–452, 2018) which were in Chinese and published in the Journal of Palaeogeography (Chinese Edition). However, they did not attract much attention of readers in China and outside China. In addition, in 1980s, some Chinese sedimentologists proposed “subfacies” and “microfacies” based on the macrofeatures of rocks from outcrops and drilling cores. However, the definition of this “microfacies” is totally different from the “microfacies” proposed by foreign sedimentologists in 1940s based on the microfeatures in thin-sections under microscope.These problems appeared repeatedly and forced me, as the Editor-in-Chief of the Journal of Palaeogeography (Chinese Edition and English Edition), to observe the policy of “A hundred flowers blossom and a hundred schools of thought contend” , to write new papers “A review on the definitions of terms of sedimentary facies” both in Chinese and in English, to clarify the definitions of the terms of sedimentary facies, i.e., “facies”, “lithofacies”, two “microfacies”, “macrofacies”, “subfacies”, etc. I hope that the new papers will attract attention of readers worldwide and they can write papers and participate in the discussion and contending of these problems, strive for getting some common understandings, and therefore promote the progress and development of sedimentology and palaeogeography.

Definition of an Archean – Proterozoic crustal suture by isotopic studies of basement intersections from offshore wells in the southern Labrador Sea

1997 ◽  
Vol 34 (2) ◽  
pp. 209-214 ◽  
Author(s):  
Andrew Kerr ◽  
Richard J. Wardle
Keyword(s):  
Seismic Profile ◽  
Hydrocarbon Exploration ◽  
Labrador Sea ◽  
Early Proterozoic ◽  
The North ◽  
Rock Types ◽  
Basement Rocks ◽  
Archean Crust ◽  
Marine Seismic ◽  
Definition Of

Eight hydrocarbon exploration wells in the southern Labrador Sea penetrated Precambrian basement rocks, adjacent to the course of a Lithoprobe marine seismic reflection survey. The rock types are mostly Early Proterozoic (1870–1800 Ma) calc-alkaline plutonic rocks or their metamorphic derivatives, but one example is a Middle Proterozoic (1270 Ma) anorogenic intrusion. These samples of buried basement document a transition from juvenile Proterozoic crust in the southeast (shown by εNd from +0.8 to +2.2) to ancient Archean crust in the northwest (shown by εNd from −2.7 to −7.0). However, initial Sr isotope ratios do not show such a clear pattern. The transition in εNd is similar to results of onland studies in Labrador and formerly adjacent south Greenland, and delimits the hidden edge of the North Atlantic Archean craton within the Early Proterozoic Makkovikian–Ketilidian Orogen. These results from close to the seismic profile confirm that regional reflectivity contrasts on either side of the isotopic boundary record fundamental differences between local Archean and Proterozoic crust. Southeast-dipping reflectors that broadly correspond with the boundary zone probably represent a collisional suture zone, along which juvenile terranes were probably placed structurally above the Archean craton.

scholarly journals A new statistical approach to the geochemical systematics of Italian alkaline igneous rocks

2020 ◽  
Vol 12 (1) ◽  
pp. 133-147
Author(s):  
Francesco Antonio Ambrosio
Keyword(s):  
Latent Variables ◽  
Principal Component ◽  
Alkaline Rock ◽  
Elemental Content ◽  
Discrete Groups ◽  
Rock Composition ◽  
Rock Types ◽  
Definition Of ◽  
Rock Suite ◽  
Two Parameters

AbstractThe ultra-alkaline rocks have exotic features that frustrated many attempts to group them in a single classification diagram. A consistent classification would be very useful to define a possible consanguinity, an argument that feed a living debate. This paper investigates the petrologic characteristics of the Cenozoic Italian ultra-alkaline rock-suite using -Rank Entropy Anentropy (RHA) and Principal Component Analysis (PCA)-. The RHA formula is the succession of component’s symbols arranged according to the diminishing of their elemental content in the analysis (whole rock composition). Other two parameters considered are an expression of the anentropy and entropy of the system. The PCA allows the definition of new latent variables based on geochemical compositions through linear combinations of the major oxides. Using both statistical methods was possible to create discrete groups of rock-types, associated by genetic relationship. The groups plotted in the Mg-Ca-Al ternary diagram depicts two main evolutionary arrays. We interpret the chemical variance in term of a general magmatic processes based on immiscibility and crystal fractionation. A comparison with similar association worldwide give a more general prospective to the magmato-tectonic assignment of these rocks, which is highly controversial in Italy.

scholarly journals The Geological Work of the Baudin Expedition in Australia (1801-1803): The Mineralogists, the Discoveries and the Legacy

2009 ◽  
Vol 28 (2) ◽  
pp. 293-324 ◽  
Author(s):  
Wolf Mayer
Keyword(s):  
Sea Level ◽  
Sedimentary Rocks ◽  
Left Behind ◽  
Rock Types ◽  
Geological Surveys ◽  
Scientific Staff ◽  
Unconsolidated Sediment ◽  
Geological Work ◽  
Definition Of ◽  
Mistaken Identification

The Baudin expedition to Australia included among its scientific staff Louis Depuch and Joseph Charles Bailly, the first professionally educated geologists to visit this country. Together with the zoologist François Péron, they carried out the earliest geological surveys along large parts of its coast. Their views on the origin of the major rock types were mainly guided by Neptunist thinking. However, in line with the beliefs of a number of French geologists at the time, they recognized basalt as a volcanic rock. Their identification of earth materials was hampered by the still imprecise definition of the physical properties of minerals and rocks. Their work provided the first detailed descriptions of the major rock types and their distribution along the margins of the continent and its islands, and led to some tentative conclusions with regard to the presence of mountains in the country's interior. The three investigators concluded that Australia was built on a foundation of granite, overlain by a variety of sedimentary rocks and fringed by extensive deposits of largely unconsolidated sediment, left behind by a retreating sea. Their mistaken identification of dolerite as basalt led them to believe that they had discovered evidence of volcanic activity in Australia. Issues such as the presence of marine organisms in rocks now above sea level, and the finding of various species of mollusks on Australia's shores, known to be extinct in Europe, led to lively discussions among a number of European naturalists.

Geophysical imaging of Silurian carbonates by use of ground and airborne electromagnetic and radiometric methods on the island of Gotland, Sweden

2015 ◽  
Vol 3 (3) ◽  
pp. SY1-SY11 ◽  
Author(s):  
Lena Persson ◽  
Mikael Erlström
Keyword(s):  
Carbonate Platform ◽  
Low Frequency ◽  
Rock Types ◽  
Transient Electromagnetic ◽  
Geophysical Surveys ◽  
Airborne Electromagnetic ◽  
The Difference ◽  
Geophysical Imaging ◽  
Definition Of ◽  
The Baltic

The island of Gotland in the Baltic Sea exhibits rock types dominated by limestone and marlstone, deposited in a Silurian carbonate platform environment. The strata are extensively exposed in outcrops, quarries, and coastal cliff sections. Low-lying marlstone-dominated areas are separated by highs dominated by limestone. The surface bedrock is well known, but the subsurface composition down to a 100-m depth has, until recent deployment of modern geophysical measurements, been largely unknown. Our geophysical surveys and methods were performed with the aim to evaluate their usefulness in characterizing the upper 100 m of the bedrock in a carbonate-dominated bedrock setting. The surveys were performed in connection to a bedrock mapping project including geologic and geophysical investigations. The airborne techniques complemented by ground measurements and correlation with outcrops and boreholes resulted in a definition of bedrock boundaries and especially subsurface characterization of bedding sequences. The airborne electromagnetic (very low frequency) survey was accompanied by radio magnetotelluric (RMT) measurement on the ground along several profiles. The difference in resistivity between high-resistive limestone overlaying a more conductive marlstone gave favorable conditions for this type of survey. The results have so far given new and valuable information on the thickness of the limestone as well as the distinction of lateral changes of the bedrock related to the depositional setting. The RMT measurements have made it possible in a new way to remotely map out bedding sequences in carbonates and to detect saline groundwater and fracture zones. In addition, a recently performed airborne transient electromagnetic survey contributed to the interpretation of the RMT measurements that we evaluated. The applied geophysical methodology and correlation with outcrops revealed carbonate bedrock with geophysical properties, enabling a better understanding concerning the groundwater conditions, limestone resources, and radon situation on the island of Gotland.

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