Geochemistry and origin of the Regan Intrusive Suite and other granitoids in the northeastern Slave Province, northwest Canadian Shield

1985 ◽  
Vol 22 (7) ◽  
pp. 1048-1065 ◽  
Author(s):  
R. A. Frith ◽  
B. J. Fryer
Keyword(s):  
Rare Earth ◽  
Regional Metamorphism ◽  
Volcanic Rocks ◽  
Parental Magma ◽  
Quartz Diorite ◽  
Heavy Rare Earth Elements ◽  
Flow Differentiation ◽  
Felsic Volcanic ◽  
Intrusive Suite ◽  
Host Rocks

The Regan Intrusive Suite of about 100 plutons of tonalite, granodiorite, and quartz diorite intruded the Yellowknife Supergroup and migmatite terrain in the northwest Slave Structural Province 2.59 Ga ago. Rare-earth-element (REE), trace-element, and major-element analyses from 39 representative whole rocks from the suite suggest it was derived by batch melting of the crust, producing a parental magma of tonalitic or granodioritic composition. By analysing REE from different parts of a zoned pluton, it was concluded that REE distribution was controlled by early separation of quartz diorite from the parent magma by flow differentiation and that the bulk of the REE were contained in early, cumulate, accessory apatite and monazite. The residual magma was further fractionated in pipelike magma chambers during ascent into more leucocratic rocks. Chondrite-normalized REE patterns of single-lithology plutons are similar to lithologies in zoned plutons, and it is proposed they initially segregated during ascent. It was found that granites, which were formerly grouped with the suite, formed in three ways, only one of which is related to the Regan Intrusive Suite.Study of 2.67 Ga old synvolcanic tonalite pluton revealed a strong covariance of light REE with those of the bimodal, calc-alkaline Hackett River Group of volcanic rocks. The data imply a common crustal source, but mass balance requires larger volumes of felsic volcanic rocks than are presently preserved, suggesting that much of the erupted felsic pyroclastic rocks were eroded. Partial melts from synvolcanic tonalite during subsequent regional metamorphism differentially depleted host rocks in REE and concentrated Eu and heavy rare-earth elements (HREE) in trondhjemite pegmatites.

Petrogenesis of the peralkaline Flowers River Igneous Suite and its significance to the development of the southern Nain Batholith

2021 ◽  
pp. 1-26
Author(s):  
Taylor A. Ducharme ◽  
Christopher R.M. McFarlane ◽  
Deanne van Rooyen ◽  
David Corrigan
Keyword(s):  
Trace Element ◽  
Volcanic Rocks ◽  
Second Phase ◽  
Discrete Events ◽  
Volcanic Event ◽  
Volcanic Succession ◽  
Intrusive Suite ◽  
Tectonic Boundaries ◽  
Host Rocks ◽  
Nain Plutonic Suite

Abstract The Flowers River Igneous Suite of north-central Labrador comprises several discrete peralkaline granite ring intrusions and their coeval volcanic succession. The Flowers River Granite was emplaced into Mesoproterozoic-age anorthosite–mangerite–charnockite–granite (AMCG) -affinity rocks at the southernmost extent of the Nain Plutonic Suite coastal lineament batholith. New U–Pb zircon geochronology is presented to clarify the timing and relationships among the igneous associations exposed in the region. Fayalite-bearing AMCG granitoids in the region record ages of 1290 ± 3 Ma, whereas the Flowers River Granite yields an age of 1281 ± 3 Ma. Volcanism occurred in three discrete events, two of which coincided with emplacement of the AMCG and Flowers River suites, respectively. Shared geochemical affinities suggest that each generation of volcanic rocks was derived from its coeval intrusive suite. The third volcanic event occurred at 1271 ± 3 Ma, and its products bear a broad geochemical resemblance to the second phase of volcanism. The surrounding AMCG-affinity ferrodiorites and fayalite-bearing granitoids display moderately enriched major- and trace-element signatures relative to equivalent lithologies found elsewhere in the Nain Plutonic Suite. Trace-element compositions also support a relationship between the Flowers River Granite and its AMCG-affinity host rocks, most likely via delayed partial melting of residual parental material in the lower crust. Enrichment manifested only in the southernmost part of the Nain Plutonic Suite as a result of its relative proximity to multiple Palaeoproterozoic tectonic boundaries. Repeated exposure to subduction-derived metasomatic fluids created a persistent region of enrichment in the underlying lithospheric mantle that was tapped during later melt generation, producing multiple successive moderately to strongly enriched magmatic episodes.

Petrogenesis of the Upper Cretaceous volcanism in the Kefken region, Western Pontides, NW Turkey

2021 ◽  
Author(s):  
Turgut Duzman ◽  
Ezgi Sağlam ◽  
Aral I. Okay
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Partial Melting ◽  
Upper Cretaceous ◽  
Volcanic Rocks ◽  
Early Eocene ◽  
Heavy Rare Earth ◽  
Heavy Rare Earth Elements ◽  
Cretaceous Volcanism ◽  
Oceanic Slab

<p>The Upper Cretaceous volcanic and volcaniclastic rocks crop out along the Black Sea coastline in Turkey. They are part of a magmatic arc that formed as a result of northward subduction of the Tethys ocean beneath the southern margin of Laurasia. The lower part of the Upper Cretaceous volcanism in the Kefken region, 100 km northeast of Istanbul, is represented by basaltic andesites, andesites, agglomerates and tuffs, which have yielded Late Cretaceous (Campanian, ca. 83 Ma) U-Pb zircon ages. The volcanic and volcanoclastic rocks are stratigraphically overlain by shallow to deep marine limestones, which range in age from Late Campanian to Early Eocene.  Geochemically, basaltic andesites and andesites display negative anomalies in Nb, Ta and Ti, enrichment in large ion lithophile elements (LILE) relative to high field strength elements (HFSE). Light rare earth elements (LREE) show slightly enrichment relative to heavy rare earth elements (La<sub>cn</sub>/Yb<sub>cn</sub> =2.51-3.63) and there are slight negative Eu anomalies (Eu/Eu* = 0.71-0.95) in basaltic andesite and andesite samples. The geochemical data indicate that Campanian volcanic rocks were derived from the partial melting of the mantle wedge induced by hydrous fluids released by dehydration of the subducted oceanic slab.</p><p>There is also a horizon of volcanic rocks, about 230 m thick, within the Late Campanian-Early Eocene limestone sequence.  This volcanic horizon, which consists of pillow basalts, porphyritic basalts,  andesites and dacites, is of Maastrichtian age based on paleontological data from the intra-pillow sediments and U-Pb zircon ages from the andesites and dacites (72-68 Ma).  The Maastrichtian andesites and dacites are geochemically distinct from the Campanian volcanic rocks. They show distinct adakite-like geochemical signatures with high ratios of Sr/Y (>85.5), high La<sub>cn</sub>/Yb<sub>cn </sub>(16.4-23.7) ratios, low content of Y (7.4-8.6 ppm) and low content of heavy rare-earth elements (HREE). The adakitic rocks most probably formed as a result of partial melting of the subducting oceanic slab under garnet and amphibole stable conditions.</p><p>The Upper Cretaceous arc sequence in the Kefken region shows a change from typical subduction-related magmas to adakitic ones, accompanied by decrease in the volcanism.</p><p> </p><p> </p>

The Duck Pond and Boundary Cu–Zn deposits, Newfoundland: new insights into the ages of host rocks and the timing of VHMS mineralization

2010 ◽  
Vol 47 (12) ◽  
pp. 1481-1506 ◽  
Author(s):  
Vicki McNicoll ◽  
Gerry Squires ◽  
Andrew Kerr ◽  
Paul Moore
Keyword(s):  
Volcanic Rocks ◽  
Sedimentary Rocks ◽  
Isotopic Data ◽  
Sulphide Ores ◽  
Intrusive Rocks ◽  
Felsic Volcanic ◽  
Host Rocks ◽  
Felsic Volcanic Rocks

The Duck Pond Cu–Zn–Pb–Ag–Au deposit in Newfoundland is hosted by volcanic rocks of the Cambrian Tally Pond group in the Victoria Lake supergroup. In conjunction with the nearby Boundary deposit, it contains 4.1 million tonnes of ore at 3.3% Cu, 5.7% Zn, 0.9% Pb, 59 g/t Ag, and 0.9 g/t Au. The deposits are hosted by altered felsic flows, tuffs, and volcaniclastic sedimentary rocks, and the sulphide ores formed in part by pervasive replacement of unconsolidated host rocks. U–Pb geochronological studies confirm a long-suspected correlation between the Duck Pond and Boundary deposits, which appear to be structurally displaced portions of a much larger mineralizing system developed at 509 ± 3 Ma. Altered aphyric flows in the immediate footwall of the Duck Pond deposit contained no zircon for dating, but footwall stringer-style and disseminated mineralization affects rocks as old as 514 ± 3 Ma at greater depths below the ore sequence. Unaltered mafic to felsic volcanic rocks that occur structurally above the orebodies were dated at 514 ± 2 Ma, and hypabyssal intrusive rocks that cut these were dated at 512 ± 2 Ma. Some felsic samples contain inherited (xenocrystic) zircons with ages of ca. 563 Ma. In conjunction with Sm–Nd isotopic data, these results suggest that the Tally Pond group was developed upon older continental or thickened arc crust, rather than in the ensimatic (oceanic) setting suggested by previous studies.

Coeval sedimentation, magmatism, and fold-thrust belt development in the Trans-Hudson Orogen: geochronological evidence from the Wekusko Lake area, Manitoba, Canada

1999 ◽  
Vol 36 (2) ◽  
pp. 293-312 ◽  
Author(s):  
Kevin M Ansdell ◽  
Karen A Connors ◽  
Richard A Stern ◽  
Stephen B Lucas
Keyword(s):  
Regional Metamorphism ◽  
Volcanic Rocks ◽  
Sedimentary Rocks ◽  
Detrital Zircons ◽  
Lake Area ◽  
Thrust Belt ◽  
Fold And Thrust Belt ◽  
Minimum Age ◽  
Flin Flon ◽  
Felsic Volcanic

Lithological and structural mapping in the east Wekusko Lake area of the Flin Flon Belt, Trans-Hudson Orogen, suggested an intimate relationship between magmatism, fluvial sedimentation, and initiation of fold and thrust belt deformation. Conventional U-Pb geochronology of volcanic rocks in fault-bounded assemblages provides a minimum age of 1876 ± 2 Ma for McCafferty Liftover back-arc basalts, and ages of between 1833 and 1836 Ma for the Herb Lake volcanic rocks. A rhyolite which unconformably overlies Western Missi Group fluvial sedimentary rocks has complex zircon systematics. This rock may be as old as about 1856 Ma or as young as 1830 Ma. The sedimentary rocks overlying this rhyolite are locally intercalated with 1834 Ma felsic volcanic rocks, and yield sensitive high resolution ion microprobe (SHRIMP) U-Pb and Pb-evaporation detrital zircon ages ranging from 1834 to 2004 Ma. The Eastern Missi Group is cut by an 1826 ± 4 Ma felsic dyke, and contains 1832-1911 Ma detrital zircons. The dominant source for detritus in the Missi Group was the Flin Flon accretionary collage and associated successor arc rocks. The fluvial sedimentary rocks and the Herb Lake volcanic rocks were essentially coeval, and were then incorporated into a southwest-directed fold and thrust belt which was initiated at about 1840 Ma and active until at least peak regional metamorphism.

Shock brecciation around the Kidd Creek deposit, Abitibi belt, Canada

2008 ◽  
Vol 45 (8) ◽  
pp. 871-878
Author(s):  
I. K. Pitcairn ◽  
N. T. Arndt
Keyword(s):  
Volcanic Rocks ◽  
Massive Sulfide ◽  
Chemical Compositions ◽  
Fine Grained ◽  
Volcanic Processes ◽  
The Matrix ◽  
Major Fault ◽  
Heavy Rare Earth Elements ◽  
Felsic Volcanic

The Kidd–Munro assemblage, Abitibi belt, Canada, is an ultramafic–mafic–felsic volcanic sequence that contains the giant Kidd Creek volcanic-hosted massive sulfide (VMS) deposit. The Kidd basin, 1.6 km northeast of the deposit, contains pervasively brecciated pillowed and massive basalts. The breccia is distinctly different from most breccias in volcanic rocks, which form through volcanic processes or during later deformation or alteration. The Kidd Creek breccia occurs pervasively through otherwise undeformed pillow interiors and margins, and also in localized corridors of particularly intense brecciation. Clasts are angular, up to 4 cm wide, hosted in a very fine-grained matrix, and commonly show jig-saw fit texture. The chemical compositions of the breccia fragments and matrix are generally similar, although the matrix is slightly enriched in high field-strength elements (HFSE) and heavy rare-earth elements (HREE) and depleted in some mobile elements, such as Rb and Ba. The breccia contains altered basaltic clasts and fragments of in-filled amygdales and is crosscut by late-stage quartz–carbonate–sulfide veins. The observations imply that the breccia was formed in-situ, with minimal transport of material, and developed after solidification of the volcanic rocks. In-situ breccias, such as these, are known to form proximal to major fault zones, but no such structure occurs in the vicinity of the Kidd Basin. We suggest the brecciation was caused by the propagation of shock waves from explosive volcanic eruption, perhaps related to the emplacement of felsic volcanic rocks observed in the Kidd Creek Mine. The breccia was subject to enhanced hydrothermal fluid flow, perhaps linked to the formation of the ore deposit.

Paleoproterozoic carbonatitic ultrabasic volcanic rocks (meimechites?) of Cape Smith Belt, Quebec

2001 ◽  
Vol 38 (9) ◽  
pp. 1313-1334 ◽  
Author(s):  
W RA Baragar ◽  
U Mader ◽  
G M LeCheminant
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Hydrothermal Alteration ◽  
Regional Metamorphism ◽  
Volcanic Rocks ◽  
Light Rare Earth ◽  
Shallow Marine ◽  
Incompatible Elements ◽  
Thick Lens ◽  
Light Rare Earth Elements

A 500 m-thick lens of carbonatitic ultrabasic lapilli tuffs and lavas interbedded with platformal Povungnituk sediments in the foreland of the Cape Smith Belt is its earliest known magmatism and may relate to its initial rifting. The sequence comprises tuffs capped in part by effusives. Accretionary and cored lapilli in the tuffs and pillows in the lavas suggest emplacement in a shallow marine environment. Its current assemblage of antigorite, chlorite, talc, and (in part primary?) carbonate, magnetite, ilmenite, minor chromite, and phlogopite results from probable concurrent hydrothermal alteration and subsequent greenschist regional metamorphism. Surviving accessory minerals: apatite, monazite, zircon, rutile, and aeschenite(?) are widespread but scarce. Carbonate (mostly dolomite) is a major and integral component of the rock and interpreted as an original, albeit recrystallized, magmatic constituent. Magnetite is conspicuous in the tuffs: as lapilli and lapilli cores, locally as giant crystals, and as stringers. Except in subhedral groundmass crystals, its negligible TiO2 is evidence of its hydrothermal reconstitution. Compositions of chromite cores and rare relicts of phlogopite crystals are consistent with mantle derivation. Rock compositions are low in SiO2 (<35%) and Al2O3 (<3%), high in MgO (>25 wt.%) and alkaline. The immobile incompatible elements (e.g., Zr, average 260 ppm; Nb, average 130 ppm) and the light rare-earth elements are enriched. The rocks are compositionally similar to type Siberian meimechites and closely resemble the "meimechite"–carbonatite eruptives of Castignon Lake, Labrador Trough. Based on experimental evidence, Lac Leclair magmas are interpreted as originating by minor partial melting of carbonated mantle at ~100 km depths and reaching the surface via conduits opened by deep rifting that initiated the Cape Smith segment of the Trans-Hudson Orogen.

Alteration mineralogy and pathfinder element inventory in the footprint of the McArthur River unconformity-related uranium deposit, Canada

2021 ◽  
Vol 59 (5) ◽  
pp. 985-1019
Author(s):  
Nicholas Joyce ◽  
Daniel Layton-Matthews ◽  
Kurt Kyser ◽  
Matthew Leybourne ◽  
Kevin Ansdell ◽  
...  
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Aluminum Phosphate ◽  
Mineral Chemistry ◽  
Chemical Mapping ◽  
Fe Oxide ◽  
Element Concentrations ◽  
Heavy Rare Earth Elements ◽  
Host Rocks ◽  
Pathfinder Elements

ABSTRACT Pathfinder elements associated with the exploration footprint of the McArthur River unconformity-related U deposit include U, radiogenic Pb, V, Ni, Co, Cu, Mo, As, Zn, and rare earth elements. In this study, the mineralogical and paragenetic context for their occurrence was established by integrating in situ mineral chemistry and laser ablation mass spectrometry chemical mapping of interstitial assemblages, detrital grains, and cements with whole-rock analyses of drill core samples from the diagenetically altered background and the hydrothermally altered sandstone host rocks. Diagenetically altered background sandstones contain a matrix assemblage of illite and dickite, with trace to minor aluminum-phosphate-sulfate (APS) minerals, apatite, and Fe-Ti oxide minerals. Aluminum-phosphate-sulfate minerals account for the majority of the Sr and light rare earth element concentrations, whereas early diagenetic apatite, monazite, and apatite inclusions in detrital quartz and detrital zircon contribute significant U and heavy rare earth elements to samples analyzed with an aggressive leach (partial digestion) such as aqua regia. Hydrothermally altered sandstone host rocks also contain variable assemblages of Al-Mg chlorite (sudoite), alkali-deficient tourmaline, APS minerals, kaolinite, illite, Fe-oxide, and sulfide minerals. Late pre-mineralization chlorite accounts for a significant portion of the observed Ni concentrations, whereas Co, Cu, Mo, and Zn occur predominantly in cryptic sub-micron sulfide and sulfarsenide inclusions within clay mineral aggregates and in association with Fe-Ti oxides. Elevated concentrations of U were observed in cryptic micro-inclusions associated with sulfides in quartz overgrowths, with Fe-Ti oxide micro-inclusions in kaolinite, and in post-mineralization Fe-oxide veins. The distribution of pathfinder elements throughout the deposit footprint appears to be less related to the primary dispersion of alteration minerals from the hydrothermal system than to the secondary dispersion of elements post-mineralization. Their occurrence throughout pre-, syn-, and post-mineralization assemblages further demonstrates the limitations to defining geochemical footprints from pathfinder element concentrations expressed in lithogeochemical data sets without structural, lithological, and mineralogical context.

Tectonic evolution of southwestern Newfoundland as indicated by granitoid petrogenesis

1985 ◽  
Vol 22 (7) ◽  
pp. 1080-1092 ◽  
Author(s):  
Derek H. C. Wilton
Keyword(s):  
Fault Zone ◽  
Point Group ◽  
Volcanic Rocks ◽  
Early Carboniferous ◽  
Late Devonian ◽  
Partial Melt ◽  
Show Evidence ◽  
Partial Melts ◽  
Felsic Volcanic ◽  
Host Rocks

Four granitoid suites are recognized in the region of the Cape Ray Fault Zone of southwestern Newfoundland. The two oldest (Ordovician–Silurian (?)) suites represent partial melts of their enclosing host rocks. The Port aux Basques granite is modelled as a partial melt of the gneissic component of its host, Port aux Basques Complex. The Cape Ray granite forms a dominantly tonalitic terrane derived by partial melting of ophiolitic material. The Red Rocks granite and a megacrystic phase of the Cape Ray granite form coherent lines of geochemical descent from the parental tonalite but show evidence of some continental crust contamination.The Late Devonian Windowglass Hill granite is a subvolcanic equivalent of felsic volcanic rocks in the Windsor Point Group. Both units were derived as partial melts of continental crust.The post-tectonic, Late Devonian to Early Carboniferous Strawberry and Isle aux Morts Brook granites constitute the youngest granitoid suite in the region. These A-type granitoids were derived as partial melts of an underlying depleted granulitic (felsic) crust. The depleted nature of the source may have resulted from previous generation of the Windowglass Hill granite and Windsor Point Group. The only possible protolith for the granulitic source is Precambrian Grenvillian gneiss. The presence of this gneiss beneath the Cape Ray Fault Zone of southwestern Newfoundland implies that the complete series of lithologies is allochthonous.

Geochemistry of two metavolcanic arc suites from the Central Metasedimentary Belt of the Grenville Province, southeastern Ontario, Canada

1991 ◽  
Vol 28 (9) ◽  
pp. 1429-1443 ◽  
Author(s):  
Luc Harnois ◽  
John M. Moore
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Trace Element ◽  
Partial Melting ◽  
Volcanic Rocks ◽  
Parental Magma ◽  
Major Elements ◽  
Grenville Province ◽  
Element Abundances ◽  
Felsic Rocks

Samples of two subalkaline metavolcanic suites, the Tudor formation (ca. 1.28 Ga) and the overlying Kashwakamak formation, have been analysed for major elements and 27 trace elements (including rare-earth elements). The Tudor formation is tholeiitic and contains mainly basaltic flows, whereas the Kashwakamak formation is calc-alkaline and contains mainly andesitic rocks with minor felsic rocks. The succession has been regionally metamorphosed to upper greenschist – lower amphibolite facies. Trace-element abundances and ratios indicate that rocks of the Tudor and Kashwakamak formations are island-arc type. Geochemical modelling using rare-earth elements, Zr, Ti, and Y indicates that the Tudor volcanic rocks are not derived from a single parental magma through simple fractional crystallization. Equilibrium partial melting of a heterogeneous Proterozoic upper mantle can explain the trace-element abundances and ratios of Tudor formation volcanic rocks. The intermediate to felsic rocks of the Kashwakamak formation appear to have been derived from a separate partial melting event. The data are consistent with an origin of the arc either on oceanic crust or on thinned continental crust, and with accretion of the arc to a continental margin between the time of extrusion of Tudor volcanic rocks and that of Kashwakamak volcanic rocks.

Nova-Bollinger Ni-Cu Sulfide Ore Deposits, Fraser Zone, Western Australia: Petrogenesis of the Host Intrusions

2021 ◽  
Author(s):  
Valentina Taranovic ◽  
Stephen J. Barnes ◽  
Steve Beresford ◽  
Morgan Williams ◽  
Colin MacRae ◽  
...  
Keyword(s):  
Western Australia ◽  
Regional Metamorphism ◽  
Parental Magma ◽  
Ore Deposits ◽  
Sulfide Deposit ◽  
Sulfide Liquid ◽  
Sulfide Ore ◽  
Primary Water ◽  
Magma Pulses ◽  
Host Rocks

Abstract The Nova-Bollinger Ni-Cu sulfide ore deposit is the first economic Ni-Cu-Co sulfide deposit to have been discovered in the Albany-Fraser orogen in Western Australia. The host rocks are mafic-ultramafic intrusive cumulates subdivided into two connected intrusions, designated Upper and Lower. The Upper Intrusion is bowl-shaped and modally layered with alternating peridotite and norite mesocumulate layers, with a Basal Series of dominantly orthocumulate mafic lithologies. The Lower Intrusion is a much thinner, semiconformable chonolith (flattened tube-shaped intrusion) consisting of mostly unlayered mafic to ultramafic orthocumulates. The Lower Intrusion hosts all the high-grade mineralization and most of the disseminated ores. A distinctive plagioclase-bearing lherzolite containing both orthopyroxene and olivine as cumulus phases is a characteristic of the Lower Intrusion and the Basal Series of the Upper. The intrusions differ slightly in olivine and spinel chemistry, the differences being largely attributable to the more orthocumulate character of the Lower Intrusion. Sector zoning in Cr content of pyroxenes is observed in the Lower Intrusion and in the lower marginal zone of the Upper and is attributed to crystallization under supercooled conditions. Symplectite pyroxene-spinel-amphibole coronas at olivine-plagioclase contacts are ubiquitous and are attributed to near-solidus peritectic reaction between olivine, plagioclase, and liquid during and after high-pressure emplacement, consistent with high Al contents in igneous pyroxenes and estimates of the peak regional metamorphism. Original cumulus olivines had compositions around Fo86 and were variably Ni depleted, interpreted as the result of preintrusion equilibration with sulfide liquid. The Upper and Lower Intrusion rocks represent cumulates from a similar parental magma, a high-Al tholeiite with MgO between 10 and 12%, low TiO2 (0.5–0.6%), and high Al2O3 (14–17%). Modeling using alphaMELTS indicates a primary water content of around 2 wt %. The cumulates of both intrusions were derived via multiple magma pulses of liquid-olivine-sulfide slurries with variable amounts of orthopyroxene, emplaced into the deep crust at pressures of around 0.7 GPa during the peak of regional metamorphism. The intrusions developed initially as a bifurcating sill, the lower arm developing into the ore-bearing Lower Intrusion chonolith and the upper arm inflating into the cyclically layered Upper Intrusion.

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