Pegmatite Dikes Research Articles

Lithium and its isotopes in tourmaline as indicators of the crystallization process in the San Diego County pegmatites, California, USA

In the lithium-cesium-tantalum-type pegmatite dikes of San Diego County, California, USA, tourmaline is the main reservoir for Li, except in the cores and the pockets of the dikes where other Li-bearing minerals also occur. Tourmaline from three subhorizontal dikes was analyzed for bulk Li concentrations and Li isotope ratios. The bottom portion of each dike includes rhythmically layered aplite called line-rock. Above the aplite is the lower pegmatite zone that crystallized upward whereas the hanging pegmatite zone crystallized downward. The lower and hanging pegmatite zones are joined at the core zone. Pockets that were once fluid-filled occur in the core zone. Tourmaline in the line-rocks and the upper border zones has 22–70 ppm Li and in the pegmatite zones 53–450 ppm Li. Large tourmaline blades in the cores have 174–663 ppm Li. Elbaite rims on prismatic tourmaline in the pockets have up to 5075 ppm Li. The progressive enrichment in Li from the wall-zones to the pockets is attributed to inward fractional crystallization of the dikes. The line-rock in each dike appears to have crystallized until the melt reached fluid saturation, at which point the melt and the fluid began to unmix to form the pegmatite zones and the pockets. The estimated initial Li concentration in the magma that produced the dikes is ~ 630 ppm. At this low concentration, Li has had much smaller effect on crystallization of the dikes than H 2 O. δ 7 Li in tourmaline in the line-rocks, the cores, and the pockets ranges from +11.2 to +16.1 ‰ with no systematic difference between these textural zones. However, in radial tourmalines δ 7 Li is > 19 ‰. The very elevated δ 7 Li may reflect Li isotope fractionation between the melt and the exsolving fluid at the time of crystallization of these tourmalines, with 7 Li preferring the more strongly-bonded occupancy in the silicate melt over a hydrated ion occupancy in the fluid. Alternatively, the elevated δ 7 Li may also have been caused by preferential accumulation of the slower-diffusing 7 Li ahead of the rapidly-growing radial tourmalines. The overall elevated δ 7 Li values of the dikes may have been acquired by Li isotope exchange with wall-rocks during passage of the pegmatite melts from their sources.

Timing of deformation and exhumation in the western Idaho shear zone, McCall, Idaho

Research Article| September 01, 2008 Timing of deformation and exhumation in the western Idaho shear zone, McCall, Idaho Scott Giorgis; Scott Giorgis † 1Department of Geological Sciences, State University of New York–Geneseo, 1 College Circle, Geneseo, New York 14454, USA †E-mail: giorgis@geneseo.edu Search for other works by this author on: GSW Google Scholar William McClelland; William McClelland 2Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844-3022, USA Search for other works by this author on: GSW Google Scholar Annia Fayon; Annia Fayon 3Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455-0219, USA Search for other works by this author on: GSW Google Scholar Brad S. Singer; Brad S. Singer 4Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Basil Tikoff Basil Tikoff 4Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2008) 120 (9-10): 1119–1133. https://doi.org/10.1130/B26291.1 Article history received: 25 May 2007 rev-recd: 21 Feb 2008 accepted: 14 Mar 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Scott Giorgis, William McClelland, Annia Fayon, Brad S. Singer, Basil Tikoff; Timing of deformation and exhumation in the western Idaho shear zone, McCall, Idaho. GSA Bulletin 2008;; 120 (9-10): 1119–1133. doi: https://doi.org/10.1130/B26291.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The western Idaho shear zone is one of several Cretaceous high-strain zones in the Cordillera that are thought to have been associated with the northward translation and/or docking of terranes presently in British Columbia. Located in west-central Idaho, this zone of intense deformation consists of a mid-crustal exposure of a lithospheric-scale, intra-arc dextral shear zone that overprints the Salmon River suture zone along the western edge of the Idaho batholith. U/Pb zircon geochronology constrains the main phase of deformation to between ca. 105 and 90 Ma. Cessation of movement on the shear zone occurred by 90 Ma, as determined by dating of the syntectonic Payette River tonalite and a crosscutting pegmatite dike in the Little Goose Creek complex. The 40Ar/39Ar thermochronology indicates that the shear zone passed through both the hornblende (~550 °C) and biotite (~325 °C) closure temperatures between 85 and 70 Ma. The 40Ar/39Ar biotite dates from an outcrop-scale, crosscutting shear zone are indistinguishable from that of the host rock, indicating that deformation occurred above the closure temperature of biotite. Apatite fission-track analysis suggests that exhumation to shallow crustal levels occurred ca. 40 Ma during mid-Tertiary regional exhumation or renewed tectonic activity along the Salmon River suture zone. Taken together, the 40Ar/39Ar results and apatite fission-track analyses indicate a two-stage uplift history for the western Idaho shear zone. Overall, the geochronology indicates that dextral transpressional movement on the western Idaho shear zone was temporally distinct from the Early Cretaceous suturing event. Additionally, the first stage of exhumation recorded by the western Idaho shear zone immediately followed transpressional deformation. Cessation of displacement on the western Idaho shear zone by ca. 90 Ma indicates that the exhumation did not solely occur as a result of ductile deformation on the shear zone itself. Moreover, dextral strike-slip movement on the western Idaho shear zone had also ceased by 90 Ma, indicating that terrane translation models for the Cordillera can only use the western Idaho shear zone to accommodate northward translation up to ca. 90 Ma. Lastly, the timing of movement on the western Idaho shear zone and contractional deformation recorded in the Insular terrane suggests a correlation between these events. This hypothesis implies that the deformation recorded in the western Idaho shear zone may have been linked to the oblique collision of the Insular superterrane with North America. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Primary oxide minerals in the system WO3Nb2O5TiO2Fe2O3FeO and their breakdown products from the pegmatite No. 3 at Dolni Bory-Hate, Czech Republic

Symmetrically zoned barren pegmatite dike No. 3 cuts granulite at Dolni Bory-Hatě in the Czech Republic. It contains minor to accessory biotite, schorl, muscovite, sekaninaite, andalusite, diaspore, apatite and several rare accessory minerals. Black, tabular crystals of ferberite-wolframoixiolite occur almost exclusively in an andalusite-diaspore aggregate with sequential accumulations of late pyrophyllite, kaolinite, and muscoviteillite. Complex zoning of the individual crystals of ferberite-wolframoixiolite shows primary, coarse oscillatory zoning consisting of narrow zones of ferberite (5.01-6.00 apfu W) and dominant niobian ferberite I (5.00-3.01 apfu W) to wolframoixiolite ( tungstenian-titanian ferrocolumbite >> niobian and tungstenian rutile > ScPO4 phase > scheelite. Primary and secondary phases are characterized by large variation in W/(W+Nb+Ta) and calculated Fe2+/(Fe2++Fe3+) but low and almost constant Mn/(Mn+Fetot) and Ta/(Ta+Nb). The contents of P, U, Ti, Zr, Si, Sc, Al and Ca vary from negligible in ferberite to minor in wolframoixiolite and its U-rich variety: UO2 2.71 (19.82 U-rich variety), P2O5 0.30, TiO2 4.81, ZrO2 2.84, SiO2 0.93, Sc2O3 4.36, Al2O3 0.87, CaO 1.02 (all in wt.%). The combined exchange vector: (R3+)8(R4+)6(R5+)16(R2+)−13(R6+)−17, where R2+ = Fe2+ > Mn, Ca; R3+ = Fe3+ > Sc, Al; R4+ = Ti > Zr, Si, U; R5+ = Nb > Ta, P; R6+ = W, seems to be the best expression of the actual substitution mechanism from ferberite to wolframoixiolite with the theoretical end composition (R2+1.1R3+3.4R4+1.9R5+5.6)∑12O24. The homovalent substitutions expressed by the exchange vectors: Fe2+Mn−1, TaNb−1 and ScFe3+−1 are rather negligible. Formation of primary W, Nb, Fe-oxide minerals is closely related to the assemblage andalusite+diaspore, formed at T < ~ 400 °C for P = 2 kbar. The breakdown process probably proceeded at slightly lower temperatures of about 350-300 °C. Textural relations of the breakdown products characterized by the total absence of any depleted primary phase in the BSE images indicate that the secondary assemblage did not originated from an exsolution-type process. Complete recrystallization and reconstitution of the primary minerals seems to be responsible for its origin. The chemical composition of the primary niobian ferberite to wolframoixiolite from Dolni Bory-Hatě is distinct from all other W-rich Nb,Ta-oxide minerals described to date except niobian wolframite from the granitic pegmatite at Nuaparra, Mozambique.

PHOSPHATE ASSEMBLAGES FROM THE BRISSAGO GRANITIC PEGMATITE, WESTERN SOUTHERN ALPS, SWITZERLAND

We describe a first significant occurrence of graftonite–triphylite-bearing LCT-type granitic pegmatite in the Alps. In 1999, in the Brissago area, in the western Southern Alps, Canton Ticino, Switzerland, we found a boulder containing an almost complete section of a phosphate-bearing pegmatite dike. The zoned pegmatite is composed by four main units: 1) an aplite unit, 2) a plagioclase + quartz graphic unit, 3) a coarse-grained plagioclase unit, and 4) a quartz core containing several phosphate masses up to 10 cm in diameter. On the basis of textural features and results of chemical analyses, we distinguish two phosphate associations. Association I, of magmatic origin, is represented by lamellar intergrowths of graftonite and triphylite. The leaching of Li and oxidation of Fe have led to the formation of ferrisicklerite and heterosite in replacement of triphylite, whereas the origin of kryzhanovskite and stanekite remains unclear. Association II, of metamorphic origin, consists of a polycrystalline aggregate of graftonite and triphylite. Exceptionally, primary maghagendorfite grains are present. Non-oxidizing low-temperature processes led to the formation of ludlamite and vivianite in replacement of triphylite. Locally, at the rim of phosphate masses, in close association with apatite-(CaOH), jahnsite and mitridatite formed at the expenses of graftonite. The chemical characteristics of the phosphate assemblages and associated mafic silicates indicate that this phosphate-bearing pegmatite dike belongs to a geochemically primitive system. Our U/Pb dating of zircon (242 ± 2.8 Ma) allows us to relate the formation of this dike to the Mid-Triassic tectonomagmatic cycle, characterized by the evolution of a magmatic arc well represented in the Southern Alps. In this geodynamic environment, a pegmatite-forming melt formed after partial melting of metapelites belonging to the Kinzigitic Complex of the Ivrea–Verbano zone.

VIGEZZITE AND ASSOCIATED OXIDES OF Nb-Ta FROM EMERALD-BEARING PEGMATITES OF THE VIGEZZO VALLEY, WESTERN ALPS, ITALY

Two sequences of crystallization involving oxides of Nb–Ta occurred during the albitization process of three emerald-bearing pegmatites that cut serpentinites at Mount Pizzo Marcio and Alpe Rosso, in Vigezzo Valley, Western Alps, Italy: 1) a higher-temperature sequence, characterized by an alkaline micro-environment [high a (Na + )], allowing the crystallization of Mg-rich columbite, vigezzite, thorian vigezzite and U-rich microlite, and 2) a lower-temperature sequence, characterized by high a (Ca 2+ ), producing replacements of tapiolite-(Fe) and ferrowodginite by microlite, of tantalite-(Mn) by microlite and (later) fersmite, and of columbite-(Mn) by fersmite. Chromium-bearing beryl crystals (emerald) occur along the exocontacts characterized by a green to grey, sheared micaceous unit composed of phlogopite + talc + tremolite. Beryl crystals commonly show a green Cr-rich vitreous rim and a whitish milky core, partially replaced by beryllium silicates such as bavenite, bertrandite and bityite. Tourmaline crystals vary from schorl–dravite to Fe-rich dravite. In the model of evolution envisaged, we propose an anatectic origin of the pegmatite-forming melts during the latest stages of the Alpine tectonometamorphic event. The emplacement of the pegmatitic dikes in the serpentinites was followed by intense circulation of supercritical to hydrothermal fluids enriched in Mg, B and Be, and depleted in F, producing albitization of pegmatites and generating the aforementioned mineral reactions and replacements.

Mineral chemistry of Ti-rich biotite from pegmatite and metapelitic granulites of the Kerala Khondalite Belt (southeast India): Petrology and further insight into titanium substitutions

Precise chemical composition, including Fe 3+ and H, of biotite from a pegmatite dike and its host granulite from the Kerala Khondalite Belt of SE India has been determined using a multi-technique approach involving EMP, SIMS, Mössbauer, and C-H-N elemental analysis. Biotite in these rocks formed at T &gt; 800-850 °C and P = 5 ± 1 kbar. The full analyses were normalized on the basis of [O 12-(x+y+z) (OH) x Cl y F z ]. Biotite in the pegmatite is Ti-, F-, and Cl-rich (0.33, 0.46, and 0.16 apfu, respectively), H 2 O-poor (OH = 0.86 pfu), has XMg = 0.49 and Fe 3+ /Fe tot ≤ 3%. The low octahedral vacancies (0.06 pfu) and the high oxygen content in the hydroxyl site (OH + F + Cl = 1.49 pfu) confirm the role of the Ti-oxy substitution as a major exchange vector in these high-T biotites. In the host granulite, fine-grained biotite is Fe 3+ -free, has low Cl (0.03 apfu), and more variable composition, with Ti, F, and X Mg in the ranges 0.26-0.36, 0.52-0.67, and 0.67-0.77, respectively. The number of octahedral vacancies is relatively large (0.10-0.18 pfu) and the sum of volatiles (OH + F + Cl) varies from 1.71 to 2.06 pfu. Systematic variations of X Mg are a function of the microstructural position and are in agreement with retrograde exchange reactions: biotite included in or in contact with garnet has the maximum values, whereas crystals in the matrix have the minima. Titanium has systematic negative correlations with F, X Mg , and (OH + F + Cl), whereas Al and octahedral vacancies are virtually constant. These trends indicate that the Ti-vacancy, along with substitutions involving Al, cannot explain the observed short-scale variations. Conversely, the Ti-oxy exchange appears to be active, resulting from combination of two vectors: the more conventional hydroxylation Ti 4+ + 2O 2- = (Fe,Mg) 2+ + 2OH - and the “fluorination” Ti 4+ + 2O 2- = (Fe,Mg) 2+ + 2F-. The systematic retrograde redistribution involves not only Fe and Mg as commonly observed, but also Ti, F, and H, in a way such to eliminate the primary Ti-oxy component of biotite.

Age and kinematics of ductile deformation in the Cerro Durazno area, NW Argentina: Significance for orogenic processes operating at the western margin of Gondwana during Ordovician – Silurian times

The Cerro Durazno Pluton belongs to a suite of Paleozoic granitoid intrusions in NW-Argentina, that are central for understanding the tectonic setting of the western margin of Gondwana in Ordovician and Silurian times. The pluton and its host rocks were tectonically overprinted by metamorphic mineral shape fabrics formed under middle greenschist-facies metamorphic conditions and associated with the nearby Agua Rosada Shear Zone. Kinematic analysis of the shear zone based on the geometric relationship between individual segments of the shear plane and principal axes of mineral fabric ellipsoids indicates reverse-sense of shear with a minor component of left-lateral displacement. This is compatible with the kinematics of other ductile deformation zones in this area, collectively forming a network, which accomplished orogen-parallel extension in addition to vertical thickening. Using the Rb–Sr isotopic system, an undeformed pegmatite dike of the Cerro Durazno Pluton was dated at 455.8 ± 3.6 Ma and mineral fabrics of the Agua Rosada Shear Zone formed at middle greenschist-facies metamorphism gave deformation ages of 437.0 ± 3.8 Ma and ⩽428.4 ± 4.5 Ma. Thus, tectonic overprint at low metamorphic grade occurred about 20–30 Ma after terminal magmatism in the Cerro Durazno area. Our data from the Cerro Durazno area and regional considerations suggest that the western margin of Gondwana was characterized by orogen-parallel extension in addition to crustal thickening as well as episodes of magmatism and ductile deformation that varied greatly in time and space.

A natural example of crystal-plastic deformation enhancing the incorporation of water into quartz

Water content of quartz in and around a greenschist facies mylonitic shear zone located in the western Adirondacks was analyzed by micro-FTIR spectroscopy. The shear zone is within a pegmatitic dike, which cuts across a granitic gneiss. The thickness of the shear zone varies along strike from 15 cm wide and encompassing all of the pegmatite dike at its northern most exposure to 5 cm wide approximately 10 m south, along strike. Microstructures, including quartz ribbons and recrystallized grains, indicate quartz and feldspar within the mylonite underwent dislocation creep. Infrared spectral analysis was carried out using a Nicolet micro-FTIR on mylonitic quartz ribbons, pegmatitic quartz and gneissic quartz. A small aperture size (56 μm by 50 μm) for the IR beam allowed optically clear regions of the quartz grains to be analyzed without any contribution from grain boundaries. The smallest dimension of the quartz ribbons is 0.3 mm, whereas the pegmatitic quartz has a grain size of 3 to 5 cm. Results show mylonitic quartz ribbons contain the most water (320 H:10 6 Si average, range of 50 to 1120 H:10 6 Si); pegmatite quartz contains much less water (30 H:10 6 Si average, range of 20–40 H:10 6 Si) and the gneissic quartz contained an intermediate amount (200 H:10 6 Si average, range of 20 to 870 H:10 6 Si). These data indicate that water was preferentially incorporated into the deformed quartz ribbons.

AN UNUSUAL TOURMALINE COMPOSITION FROM SITHONIA PENINSULA (NORTHERN GREECE)

The variety and wideness of substitutions shown by tourmaline group, mainly in the Ζ, Y and X sites, make possible a high number of real and hypothetical endmembers. However in some cases it is not so obvious to determine to what endmembers to refer to, but the wide stability field of tourmaline and its occurrence in different geological environments make useful to define the composition as percentages of end-members to trace back to its genesis. In this paper the followed procedure to identify the components of a complex tourmaline is given. During a study on the granitoid intrusions outcropping on Sithonia, Chalkidiki Peninsula (Northern Greece), a swarm of very small crystals of tourmaline, subhedral or more often allotriomorphic, has been found as stuffing of subparallel micro fractures in the quartz core of a pegmatite dike. Their composition, analysed by EMPA, turns out to be rather unusual and not satisfactorily referable to so far proposed solid solutions. It is characterized by remarkable Al deficiency (ZAl &lt; 6 apfu), high amounts of magnesium and iron with Mg &gt; Fe and Να dominating X site. On the basis of compositional and crystal-chemical evidences, supported by previous literature, it was hypothesized that Mg and Fe have to be referred to an uvite — feruvite component, while the considerable part of total iron in its trivalent oxidation state yields a povondraitic component according also with Na in X site. These data, coupled with a theoretical evaluation of bond valence sum (B VS) that allowed inferring the presence of OFT anions in the W site, make reasonable to consider Sithonia tourmaline as the new variety sodian povondraitic hydroxil uviteferuvite. On the basis of its composition, of its fabric and of the oscillatory trends shown by the main elements of Y and X sites, this tourmaline may be considered post magmatic likely of hydrothermal genesis.

Geologic relationships and mineralization of peralkaline/alkaline granite-syenite of the Zargat Na’ am ring complex, Southeastern Desert, Egypt

The Zargat Na’ am ring complex crops out 90 km NW of Shalatin City in the Southeastern Desert of Egypt. The ring complex forms a prominent ridge standing high above the surrounding mafic-ultramafic hills. It is cut by two sets of joints and faults which strike predominantly NNW-SSE and E-W, and is injected by dikes, porphyritic alkaline syenites, and felsite porphyries. It consists of alkali syenites, alkali quartz syenites, and peralkaline arfvedsonite-bearing granitic and pegmatitic dikes and sills. The complex is characterized locally by extreme enrichments in REEs, wolframite and rare, high field strength metals (HFSM), such as Zr and Nb. The highest concentrations (1.5 wt% Zr, 0.25 wt% Nb, 0.6 wt% Σ REEs) occur in aegirine-albite aplites that formed around arfvedsonite pegmatites. Quartzhosted melt inclusions in arfvedsonite granite and pegmatite provide unequivocal evidence that the peralkaline compositions and rare metal enrichments are primary magmatic features. Glass inclusions in quartz crystals also have high concentrations of incompatible trace elements including Nb (750 ¢ 10−6), Zr (2500 × 10−6) and REEs (1450 × 10−6). The REEs, Nb and Zr compositions of the aegirine-albite aplites plot along the same linear enrichment trends as the melt inclusions, and Y/Ho ratios mostly display unfractionated, near-chondritic values. The chemical and textural features of the aegirine-albite aplites are apparently resultant from rapid crystallization after volatile loss from a residual peralkaline granitic melt similar in composition to the melt inclusions.

U-Pb ages (3.8–2.7 Ga) and Nd isotope data from the newly identified Eoarchean Nuvvuagittuq supracrustal belt, Superior Craton, Canada

We report new U-Pb ages and Sm-Nd isotope data for an Eoarchean volcano-sedimentary sequence in the northeast Superior Province of northern Quebec, Canada. The Nuvvuagittuq supracrustal sequence is characterized by mafi c amphibolite rocks with rare felsic schists, accompanied by ultramafi c sills, oxide-rich and quartz-rich iron formations, and possible conglomeratic units, with peak metamorphism having reached upper amphibolite facies. The sequence crops out as an open, shallow, south-plunging synform with tonalites in the center and around the exterior margins of the fold structure. A 3817 ± 16 Ma age from a felsic schist is interpreted to represent a maximum age for the sequence, whereas a 3661 ± 4 Ma age from the enclosing tonalites suggests a minimum age. Detrital zircons from a conglomeratic unit yielded similar old ages of ca. 3.78 Ga. Younger ages of ca. 3.66 and 3.3 Ga from a conglomeratic unit and a felsic schist, and 2.7 Ga ages from an amphibolite gabbro and a pegmatite dike, underline the polymetamorphic history of the sequence. Initial Nd isotope compositions (eNd 3.8 = −0.5 to + 2.3) for mafi c amphibolites and felsic schists are indicative of a moderately depleted mantle. Sm-Nd analyses of a 2.7 Ga pegmatite dike and the ca. 3.66 Ga tonalites both yield Nd depleted mantle model ages (TDM) of ca. 3.9 Ga, indicating that they were derived from a crust of juvenile Eoarchean origin. We propose that fragments of Paleoarchean to Eoarchean terranes that rim the Superior Province represent remnants of a larger terrane(s) that was rifted and dismembered prior to the formation of the 3.0‐2.7 Ga Superior Craton.

A Review of Rare-Element (Li-Cs-Ta) Pegmatite Exploration Techniques for the Superior Province, Canada, and Large Worldwide Tantalum Deposits

Research Article| January 01, 2005 A Review of Rare-Element (Li-Cs-Ta) Pegmatite Exploration Techniques for the Superior Province, Canada, and Large Worldwide Tantalum Deposits JULIE B. SELWAY; JULIE B. SELWAY Precambrian Geoscience Section, Ontario Geological Survey 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Search for other works by this author on: GSW Google Scholar FREDERICK W. BREAKS; FREDERICK W. BREAKS Precambrian Geoscience Section, Ontario Geological Survey 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Search for other works by this author on: GSW Google Scholar ANDREW G. TINDLE ANDREW G. TINDLE Department of Earch Sciences, Open University Milton Keynes, Buckinghamshire, UK MK7 6AA Search for other works by this author on: GSW Google Scholar Exploration and Mining Geology (2005) 14 (1-4): 1–30. https://doi.org/10.2113/gsemg.14.1-4.1 Article history received: 16 Feb 2004 accepted: 20 Sep 2004 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation JULIE B. SELWAY, FREDERICK W. BREAKS, ANDREW G. TINDLE; A Review of Rare-Element (Li-Cs-Ta) Pegmatite Exploration Techniques for the Superior Province, Canada, and Large Worldwide Tantalum Deposits. Exploration and Mining Geology 2005;; 14 (1-4): 1–30. doi: https://doi.org/10.2113/gsemg.14.1-4.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyExploration and Mining Geology Search Advanced Search Abstract Rare-element pegmatites may host several economic commodities, such as tantalum (Ta-oxide minerals), tin (cassiterite), lithium (ceramic-grade spodumene and petalite), and cesium (pollucite). Key geological features that are common to pegmatites in the Superior province of Ontario and Manitoba, Canada, and in other large tantalum deposits worldwide, can be used in exploration.An exploration project for rare-element pegmatites should begin with an examination of a regional geology map. Rare-element pegmatites occur along large regional-scale faults in greenschist and amphibolite facies metamorphic terranes. They are typically hosted by mafic metavolcanic or metasedimentary rocks, and are located near peraluminous granite plutons (A/CNK > 1.0). Once a peraluminous granite pluton has been identified, then the next step is to determine if the pluton is barren or fertile. Fertile granites have elevated rare element contents, Mg/Li ratio < 10, and Nb/Ta ratio < 8. They commonly contain blocky K-feldspar and green muscovite. Key fractionation indicators can be plotted on a map of the fertile granite pluton to determine the fractionation direction: presence of tourmaline, beryl, and ferrocolumbite; Mn content in garnet; Rb content in bulk K-feldspar; and Mg/Li and Nb/Ta ratios in bulk granite samples. Pegmatite dikes with the most economic potential for Li-Cs-Ta deposits occur the greatest distance (up to 10 km) from the parent granite.Metasomatized host rocks are an indication of a nearby rare-element pegmatite. Metasomatic aureoles can be identified by their geochemistry: elevated Li, Rb, Cs, B, and F contents; and by their mineralogy: presence of tourmaline, (Rb, Cs)-enriched biotite, holmquistite, muscovite, and rarely garnet.Once a pegmatite dike has been located, the next step is to assess its potential to contain Ta mineralization. Pegmatites with the highest degree of fractionation (and thus the most economic potential for Li-Cs-Ta) contain blocky K-feldspar with >3,000 ppm Rb, K/Rb < 30, and >100 ppm Cs; and coarse-grained green muscovite with >2,000 ppm Li, >10,000 ppm Rb, >500 ppm Cs, and >65 ppm Ta. Pegmatites with Ta mineralization usually contain Li-rich minerals (e.g., spodumene, petalite, lepidolite, elbaite, amblygonite, and lithiophilite) and may contain Cs-rich minerals (e.g., pollucite, Cs-rich beryl). The ore minerals of Ta are commonly manganotantalite, manganocolumbite, wodginite, and microlite; Ta-rich cassiterite is also commonly present. Tantalum mineralization tends to occur in albitic aplite, mica-rich (lepidolite, cleavelandite ± lepidolite), and spodumene/petalite pegmatite zones. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Nature and Distribution of Tantalum Mineralization in Pegmatite Dikes, Lilypad Lakes Property, Fort Hope, Northwestern Ontario

The Lilypad Lakes property is host to a group of Ta-mineralized dikes that occur over an area of at least 10 km2. Field relationships are consistent with a syn- to late syn-tectonic timing for their emplacement (with respect to the major regional D1 episode of deformation), with several of the dikes exhibiting a steeply dipping, open, S- to M- to Z-shaped fold geometry. Individual dikes range up to 30 m wide (Rubellite Dyke, Pollucite Dyke), have strike lengths of up to 750 m (e.g., JJ Dyke), and are continuous to depths of greater than 250 m (e.g., Rubellite Dyke, Pollucite Dyke). Typically, they display sharp intrusive contacts characterized by the development of thin, holmquistite-rich haloes. The Ta-mineralized dikes are comprised of silica- and alumina-rich, sodic, granitic pegmatites that display extreme levels of geochemical fractionation (K/Rb typically 0.045 wt.% Ta2O5) in both the Rubellite and the Pollucite Dykes. Three Ta-oxide minerals (microlite, wodginite, manganotantalite) of primary origin predominate in the Ta-mineralized dikes, where they occur as fine, disseminated grains in the albite-rich pegmatite matrix. The sequence of crystallization is typically manganotantalite wodginite microlite, which conforms to a well-established paragenetic sequence for rare-element granitic pegmatites in general. All three of the primary Ta oxides are characterized by a high degree of chemical purity (e.g., average Ta concentrations in microlite from 77–79 wt.% Ta2O5), a compositional feature consistent with the extreme levels of geochemical fractionation characteristic of the host granitic pegmatites, and one that indicates that the Lilypad Lakes pegmatites are likely to produce Ta mineral concentrates of the very highest quality. Locally, a second generation of Ta minerals is present, restricted to small vugs in the pegmatite matrix that are surrounded by fine aggregates of K-rich mica or clay. The secondary Ta minerals are dominated by microlite, which is typically intergrown with fluorite and Fe-sulphides, and is distinguished from its primary counterpart by its lower Ta content (around 70 wt.% Ta2O5) and much higher concentration of uranium (up to 9 wt.% UO2). Also, secondary microlite consistently yields low analytical totals from electron microprobe analysis, suggesting that it is H2O-rich. The presence of this second, vug-related generation of microlite, together with the occurrence of localized networks of crosscutting fractures that host high concentrations of Cs and Rb (in the minerals pollucite and Rb-K-feldspar), suggests that a late-stage, fluid-dominated metasomatic/hydrothermal event may have been important in certain sections of the dikes, and has effected a redistribution of the economically important elements Ta and Cs.

Re-Os Model Age Constraints on the Genesis of the Moss Molybdenite Pegmatite-Aplite Deposit, Southwestern Grenville Province, Quyon, Quebec, Canada

The Moss molybdenite deposit, which was mined during both world wars, is hosted in late Grenvillian granitic pegmatite dikes. These dikes are locally aplitic, and intrude the late tectonic Onslow syenite (probably correlative with the ~1090–1070 Ma Kensington suite), which is in turn intrusive into the 1170–1155 Ma Chevreuil intrusive suite of southwestern Quebec. A Re-Os model age of 1053 ± 4 Ma (8.3 ppm Re) was obtained for molybdenite from a pegmatitic veinlet within an aplitic phase of the dike. A much younger Re-Os date of 860.9 ± 9.2 Ma (6.9 ppm Re) obtained previously from a pyrrhotite-and pyrite-rich metasomatic vein might reflect Os diffusion during slow cooling of the Grenville province, because this age is consistent with K-Ar dates reported from the region. Alternatively, the similarity between these younger dates may be fortuitous, and may instead relate to sampling issues for molybdenite. The Mo-rich granitic dikes at the Moss deposit are similar to other Mo-rich A-type pegmatite-aplite granitic dike systems that formed immediately after the peak of the Ottawan orogeny (1070–1090 Ma), and possibly are more closely related to the Kensington suite than previously thought.

The Nevoria Gold Skarn Deposit, Southern Cross Greenstone Belt, Western Australia: II. Pressure-Temperature-Time Path and Relationship to Postorogenic Granites

The Nevoria deposit (production 11.88 t Au, 0.41 t Ag) is one of more than 20 gold skarns mined in the 3.1 to 3.0 Ga Southern Cross greenstone belt. The belt constitutes part of the continental foreland of the 2.7 to 2.6 Ga Norseman-Wiluna fold belt, the youngest Archean orogen of the Yilgarn craton. The Nevoria skarns are confined to iron formations at the limb of a regional F 1 anticline bent around the Ghooli orthogneiss dome. The refolding (F 2 ) took place at 2775 to 2724 Ma during batholith emplacement and contact metamorphism at estimated P-T conditions of 610° ± 50°C and 400 ± 100 MPa (14-km burial depth). This structural setting differs from that of the mezozonal lode gold deposits in the adjacent fold belt, which are controlled by fault-vein arrays in crustal-scale shear zones. The Nevoria orebodies (6–7 g/t Au), composed of pyrrhotite-rich hedenbergite-actinolite and almandine-hornblende skarns, are dated by a concordant U-Pb age (2635.7 ± 1.2 Ma) and by a less precise Pb-Pb errorchron age (2630 ± 13 Ma, MSWD = 5.9, n = 7), both defined by allanite-bearing almandine and by cogenetic scheelite. These ages demonstrate that the skarns formed 90 m.y. after amphibolite-facies metamorphism in the Southern Cross greenstone belt and at least 20 m.y. after transpressional faulting related to late-orogenic deformation in the Norseman-Wiluna fold belt. The peak fluid temperature during skarn formation is estimated at 550° to 600°C, based on the Fe-Mg exchange thermometry of almandine-biotite and almandine-hornblende pairs. Calcic-potassic mineral assemblages in skarn hosted by amphibolite constrain the pressure to 300 to 400 MPa, based on the reaction of biotite + cummingtonite + anorthite to almandine + grossular + biotite. The pressure estimate confirms that the Nevoria deposit formed at considerable depth (11–14 km) in a midcrustal environment. Substantial uplift did not take place until 2565 ± 25 Ma, when biotite in the associated granite closed to Rb-Sr diffusion at 300°C ambient temperature. The Nevoria orebodies are cut by pegmatite dikes and are underlain by a 500-m-thick pluton of peraluminous two-mica granite. The concordant zircon U-Pb age (2634 ± 4 Ma) of the granite is indistinguishable from the age of the skarn. The abundance of pegmatite in the main intrusion, increasing upward to a more than 80-m-thick roof zone, indicates that an aqueous fluid separated from the melt. The enrichment of aluminum, Au, As, Bi, Cu, Nb, W, and Zn in the upper pegmatite suggests that these elements were transported to the roof by an early magmatic fluid (700°–600°C), prior to the development of disseminated muscovite-epidote-carbonate alteration at a lower temperature (400°C). The major and trace elements elevated in the pegmatite are also enriched in the ore skarn, implicating the granite as the fluid source. Fluid inclusion data indicate that the magmatic fluid was CO 2 bearing, aqueous, and of moderate salinity (about 10 wt % CaCl 2 + NaCl equiv). Gold skarn formed in the iron formations above when the infiltrating fluid became progressively reduced during the replacement of grunerite. Other gold skarn deposits in the continental foreland of the Norseman-Wiluna fold belt show a similar space-time relationship to differentiated I-type granites (Thornton-Tuttle index >90). These granites (2.66–2.62 Ga) postdate the compressional deformation in the fold belt and are thus classified as postorogenic. The scheelite-bearing Archean gold skarns share characteristic features such as gangue-sulfide mineralogy and granite association with the deep-seated (150–250 MPa), reduced tungsten skarns located on the continental side of the North American Cordillera. Cretaceous deposits such as MacTung contain up to 2 g/t gold, locally.

Columbite-tantalite minerals from Nanping granitic pegmatites, South China: compositional trends and genetic implications

The No31 pegmatite group is the largest pegmatite cluster of the Nanping pegmatite field and represents one of the most important Ta-Nb deposits in China. The columbite-tantalite occurring in No31 pegmatites showed faint oscillatory zonation and complex patchy zonation patterns in back-scattered electron images. Microprobe analyses revealed that columbite-tantalite in the pegmatites has a wide range of composition. Neither Mn/(Mn + Fe) nor Ta/(Ta + Nb) atomic ratio has obvious trend of compositional variation along the profile of a single pegmatite dike. However, in the quadrilateral diagrams the compositional trend in columbite-tantalite varies front ferrocolumbite to tantocolumbite, then to manganotantalite. The features of zonation and composition in columbite-tantalite imply that the formation of Nanping pegmatites underwent original differentiation stage and later metasomatism stage.

SHRIMP U-Pb Zircon Ages of Granitoids in the Wulashan Gold Deposit, Inner Mongolia, China: Timing of Mineralization and Tectonic Implications

The Wulashan gold deposit, situated along the northwestern margin of the North China craton (NCC), is hosted by ductile-brittle faults within Archean metamorphic volcano-sedimentary rocks of the Wulashan Group. This deposit is characterized by gold-bearing quartz-K feldspar and quartz veins. Both granitoid batholiths and pegmatite dikes intruded the metamorphic basement rocks, and are spatially associated with gold mineralization. Contrasting genetic models have been proposed for the deposit due to lack of reliable age data. Our new SHRIMP U-Pb zircon ages for these intrusions now reveal important constraints on the mineralization time and tectonic evolution of this region. These intrusions contain inherited zircons of about 2.55 Ga, probably from the Wulashan basement that was intruded by pegmatite dikes at about 1.84 Ga; the latter probably are related to the major tectonic event leading to the final amalgamation of the NCC. The basement subsequently underwent at least three tectono-thermal events during Phanerozoic time (at 353 ± 7, 169 ± 7, and 132 ± 2 Ma). Combined with previous Ar-Ar and K-Ar ages, we suggest two gold mineralization episodes for the Wulashan gold deposit. The first episode occurred at about 350 Ma, indicated by ages of a goldrelated fuchsite and the Dahuabei granitoid batholith. This supports a previously proposed model that relates gold mineralization to the Dahuabei granite that formed during collision of the Paleo-Mongolian block with the NCC. The second one occurred in the late Yanshanian period, as indicated by the mineralized K-feldspar-rich vein of 132 ± 2 Ma. This episode is simultaneous with those in the eastern NCC, indicative of a widespread late Yanshanian metallogenic event that was a response either to the subduction of the Izanagi-Pacific plate beneath eastern China or to the removal of the Early Cretaceous lithosphere in the eastern NCC.

Fluid overpressure in layered intrusions: formation of a breccia pipe in the Eastern Bushveld Complex, Republic of South Africa

Fluids and volatile fluid overpressures in layered intrusions are becoming recognized as having important functions in magma chamber processes for the formation of magmatic structures and for the movement and concentration of economically important ore elements. We re-examine a breccia pipe in the Bushveld Complex to investigate the role of volatile fluid overpressure in its emplacement. This 10-m-diameter breccia pipe was emplaced vertically in the anorthosite of the Upper Critical Zone of the Bushveld Complex and contains blocks of Bushveld anorthosite, norite, and pyroxenite, apparently derived from lower in the stratigraphy. The blocks are commonly elongate, and are tightly packed within an ultramafic pegmatite groundmass. Ultramafic pegmatitic dikes radiate bilaterally from the pipe. Microprobe analyses of plagioclase in the breccia pipe blocks reveal that these grains are unzoned and have higher anorthosite content (~An85) than host rock plagioclase (~An75), indicating that the breccia pipe blocks were transported considerable vertical distances. Whole-rock major- and trace-element data show enrichment in fluid-soluble incompatible elements, supporting the role of volatile fluids in the formation of the pipe. A model is examined in which the segregation of a fluid phase from interstitial liquids in the cumulate section below the breccia pipe leads to the development of sufficient overpressure for its explosive emplacement. Fluid overpressure can develop by compaction and during crystallization of plagioclase-saturated silicate liquids as fluids exsolve from the solidifying interstitial liquid. If fluids separate faster than they can escape from the crystal pile, significant overpressures can develop. The minimum fluidization velocity which would be required to emplace blocks of the size and density found within the breccia pipe is found to be 10–110 m/s. The crystallization model predicts the development of about 400 bar of overpressure, an amount calculated to be sufficient for pipe emplacement.

Brittle structures and their role in controlling porosity and permeability in a complex Precambrian crystalline-rock aquifer system in the Colorado Rocky Mountain Front Range

Research Article| November 01, 2003 Brittle structures and their role in controlling porosity and permeability in a complex Precambrian crystalline-rock aquifer system in the Colorado Rocky Mountain Front Range Jonathan Saul Caine; Jonathan Saul Caine 1U.S. Geological Survey, P.O. Box 25046, MS 973, Denver, Colorado 80225, USA Search for other works by this author on: GSW Google Scholar Stephanie R.A. Tomusiak Stephanie R.A. Tomusiak 2Department of Geological Sciences, Campus Box 250, University of Colorado, Boulder, Colorado 80309, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2003) 115 (11): 1410–1424. https://doi.org/10.1130/B25088.1 Article history received: 14 Dec 2001 rev-recd: 08 Nov 2002 accepted: 08 May 2003 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jonathan Saul Caine, Stephanie R.A. Tomusiak; Brittle structures and their role in controlling porosity and permeability in a complex Precambrian crystalline-rock aquifer system in the Colorado Rocky Mountain Front Range. GSA Bulletin 2003;; 115 (11): 1410–1424. doi: https://doi.org/10.1130/B25088.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Expansion of the Denver metropolitan area has resulted in substantial residential development in the foothills of the Rocky Mountain Front Range. This type of suburban growth, characteristic of much of the semiarid intermountain west, often relies on groundwater from individual domestic wells and is exemplified in the Turkey Creek watershed. The watershed is underlain by complexly deformed and fractured crystalline bedrock in which groundwater resources are poorly understood, and concerns regarding groundwater mining and degradation have arisen. As part of a pilot project to establish quantitative bounds on the groundwater resource, an outcrop-based geologic characterization and numerical modeling study of the brittle structures and their controls on the flow system was initiated. Existing data suggest that groundwater storage, flow, and contaminant transport are primarily controlled by a heterogeneous array of fracture networks. Inspections of well-permit data and field observations led to a conceptual model in which three dominant lithologic groups underlying sparse surface deposits form the aquifer system—metamorphic rocks, a complex array of granitic intrusive rocks, and major brittle fault zones. Pervasive but variable jointing of each lithologic group forms the “background” permeability structure and is an important component of the bulk storage capacity. This “background” is cut by brittle fault zones of varying structural styles and by pegmatite dikes, both with much higher fracture intensities relative to “background” that likely make them spatially complex conduits. Probabilistic, discrete-fracture-network and finite-element modeling was used to estimate porosity and permeability at the outcrop scale using fracture network data collected in the field. The models were conditioned to limited aquifer test and borehole geophysical data and give insight into the relative hydraulic properties between locations and geologic controls on storage and flow. Results from this study reveal a complex aquifer system in which the upper limits on estimated hydraulic properties suggest limited storage capacity and permeability as compared with many sedimentary-rock and surficial-deposit aquifers. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structural History and Timing of Gold Mineralization in the Northern East Strelley Belt, Pilbara Craton, Western Australia

The McPhees and Iron Stirrup deposits, recent gold producers in the Pilbara Craton, Western Australia, are located within a succession of Early to Middle Archean metavolcanic rocks of the northern part of the East Strelley belt. Deformation occurred during five stages (D1-D5), with mineralization late in the tectonic cycle (D4). D1 structures only affected rocks below a regional, Early Archean unconformity. D2 deformation produced a ubiquitous, variably developed S2 foliation, defined by peak metamorphic minerals. Mineralization is hosted within, but postdates a 100- to 400 m-wide, D2 high-strain zone of talc-chlorite-carbonate schist (ultramafic protolith) that contains competent boudins of actinolite-chlorite schist (mafic protolith). D3 deformation produced localized open folds, outlined by lithological layering and S2 foliation. Regional, open folding, and sinistral shearing of reactivated D2 structures occurred during D4 deformation, which controlled synkinematic intrusion of granitic pegmatite at ca. 2880 Ma. At the McPhees deposit, narrow, sinistral, north-northeast–trending D4 shear zones were conduits for mineralizing fluids, which reacted with, and preferentially mineralized, mafic rocks. Gold occurs in altered schistose wall rock and within veins composed of actinolite, quartz, and pyrrhotite, and minor tourmaline, albite, talc, and magnesite; vein selvages consist of albite and tourmaline. Mineralization at Iron Stirrup was also controlled by north-trending D4 structures that reactivated D2 fabrics, and was concentrated at a sheared contact between serpentinite and talc-chlorite ± carbonate schist. Similar north-trending D4 shear zones overprint earlier structures and control ca. 2890 Ma gold mineralization nearby in the Mount York gold district, suggesting a single mineralizing event for the entire belt, which is the same age (within analytical error) as granitic pegmatite intrusions. Gold was deposited under amphibolite metamorphic conditions at Mount York and greenschist conditions at Iron Stirrup and McPhees, indicating a decrease in metamorphic conditions away from the western granite-greenstone contact, where granitic pegmatite dikes are abundant. Therefore, mobilization and transport of gold occurred in response to thermal perturbations related to granitic pegmatite dike emplacement during D4 deformation.