Daniel J. Kontak

Integrated model for Meguma Group lode gold deposits, Nova Scotia, Canada

Meguma Group lode gold deposits represent concentrations of a variety of auriferous quartz veins in lower Paleozoic metaturbidites of the Meguma terrane of southern Nova Scotia. Our studies, incorporating extensive field (regional and detailed mapping) and laboratory (petrography, fluid inclusions, geochemistry, isotopes) studies of 30 of these deposits in the eastern Meguma terrane, suggest that mineralization occurred at ca. 370 Ma ( 40 Ar/ 39 Ar dating of hydrothermal vein minerals) during widespread incursion of mafic and felsic magmatism into the crust concurrent with subvertical shear-zone development, thus postdating regional Acadian deformation by some 30-40 m.y. Isotopic data (S, C, O, Sr) indicate that vein-forming fluids were not solely magmatic and were in part exotic to the Meguma Group. Thus, earlier magmatic and metamorphogenic-lateral-secretion models that considered either late-stage magmatic fluids or the enveloping Meguma Group metasedimentary rocks as the sole source of vein components are invalidated.

Syn-Acadian emplacement model for the South Mountain batholith, Meguma Terrane, Nova Scotia: Magnetic fabric and structural analyses

The anisotropy of magnetic susceptibility and the structural geology of the Late Devonian South Mountain batholith (Meguma Terrane, Nova Scotia) are used to characterize its emplacement and structural evolution. The South Mountain batholith is the largest peraluminous granitoid complex within the Appalachian orogen. It is a composite batholith made up of stage 1 granodiorites and monzogranites and stage 2 leucomonzogranites and leucogranites. The anisotropy of magnetic susceptibility was used to map the biotite petrofabric at the scale of the batholith. The biotite fabric pattern is similar to the pattern of folding and shearing of the feldspar megacryst foliation, documented at the outcrop scale. In the stage 1 plutons, the magnetic foliation is deformed into a girdle about the horizontal, northeast-southwest—trending magnetic lineation, which is parallel to regional fold axes and extension lineations in the country rocks. In the stage 2 plutons, a horizontal northeast-southwest– to east-west–trending magnetic lineation is also present, but the horizontal orientation of the magnetic foliation is widely preserved. The folding and shearing of the foliations and the horizontal northeast-southwest–trending lineation that is pervasive throughout the South Mountain batholith are interpreted to record Acadian tectonic deformation of the plutons as they crystallized. Acadian deformation continued during the latest stages of crystallization, resulting in the localization of mineral deposits and greisens along shear zones, faults, and joints. A model is presented for the syntectonic emplacement of the South Mountain batholith as a laccolithic complex.

Modeling fluid–rock interaction during greisenization at the East Kemptville tin deposit: implications for mineralization

Interaction between a tin-bearing greisenizing fluid and a leucogranitic rock was modeled in order to evaluate the possible role of alteration induced changes in fluid chemistry in cassiterite deposition. The interaction was modeled using simple titration and multiple-pass fluid flow models with the program CHILLER, modified to permit calculations at 450°C and 4.0 kb. The compositions of the mineralizing fluid and the unaltered leucogranite were based, respectively, on analyses of fluid inclusions and whole-rock chemistry, of samples from the East Kemptville greisen-hosted tin deposit, Nova Scotia. The starting fluid contained 4.85 M Cl, 3.7 M Na, 0.53 M Fe, and minor K, Si, F and Sn, and was at a pH of 3.0, while the leucogranite contained 35 wt.% quartz, 29 wt.% albite, 19 wt.% muscovite and 17 wt.% K-feldspar. In both models, the calculations show that with increasing alteration, K-feldspar was the first mineral consumed, and that this was followed by replacement of albite by muscovite. With further alteration, muscovite was replaced by topaz and pyrrhotite, and cassiterite precipitated in quartz–topaz rocks. In the most altered rocks, only topaz and quartz are present. These changes accurately reproduce those determined for natural samples at East Kemptville. However, only the multiple-pass fluid flow model predicts the changes in fluid inclusion composition. The multiple-pass fluid flow model quantified changes in fluid composition as a function of its interaction with the wall rock and thus enabled us to assess better the factors that controlled cassiterite precipitation. Important conclusions of the model are that the fluid and the rock equilibrated at each stage of the alteration and the water/rock ratio remained constant across the alteration halo. We further concluded that phases precipitated close to the vein in an earlier stage of the alteration were re-dissolved with evolution of the mineralizing system. In particular, re-dissolution of cassiterite caused an increase in the concentration of SnCl+ in the fluid. As a result, cassiterite saturated at a lower pH, and the chemical potential of SnCl+ increased away from the vein. This could, in turn, explain the low tin grades at East Kemptville.

The nature and origin of pegmatites in a fluorine-rich leucogranite, East Kemptville Tin Deposit, Nova Scotia, Canada

Pegmatites at the East Kemptville Sn-Zn-Cu-Ag deposit occur within a F- and P-rich, 370 Ma topaz-muscovite leucogranite, the most evolved phase of the chemically zoned Davis Lake Pluton. Structural observations and geochronology indicate that the leucogranite intruded into an active shear/fault zone environment. Pegmatites are preferentially located near the roof zone of the intrusion and include: (1) quartz-feldspar pods (≤ 1–2 m) aligned parallel to a foliation in the leucogranite. Such pegmatites have marginal aplites or may be cored by aplites; (2) aplite-pegmatite zones with well-developed crenulate and unidirectional solidification textures; and (3) muscovite-bordered miarolitic cavities lined with quartz-feldspar ± cassiterite. Associated with areas of pegmatites are irregular- to cuspate-shaped zones enriched in muscovite. Mineralogically the pegmatites are simple, consisting of clear to black quartz, albite, microcline perthite and muscovite; topaz may be enriched in aplites coring pegmatites and coarse euhedral cassiterite is rare. Chemically the leucogranites marginal to pegmatites are similar to the host leucogranite, but some aphanitic felsic dykes indicate extreme differentiation, as reflected by REE depletion and P-enrichment. Enrichment of pegmatitic feldspar in P 2 O 5 (to 1 wt. %) indicates that melts contained 2–3 wt. % P 2 O 5 , whereas muscovite chemistry (to 5 wt. % F) reflects the F-rich nature of the melt. Trace and REE contents of pegmatitic feldspars are consistent with local segregation of volatile-rich melts to form pegmatites rather than extreme crystal-chemical fractionation. Fluid inclusion studies of pegmatitic quartz and cassiterite indicate the presence of a highly saline brine (40 wt. % eq. NaCl; Na > K > Fe > Mn > Ca > Sr) of magmatic origin. Isochores modelled for a 40 wt. % eq. NaCl fluid constrain pegmatite formation at 550–600°C, thus the depressed solidus is consistent with the volatile-rich (F, P, H 2 O) nature of the melt. Stable isotopes demonstrate that a magmatic fluid (δ 18 O H 2 O = +5·5 to +10‰, δD H 2 O = −33 to −41‰ for 450–500°C) equilibrated with the pegmatites and the system cooled abruptly at c . 425°C. The low volume of pegmatite at East Kemptville suggests that the melt was not near volatile saturation, instead pegmatite generation is interpreted to have resulted from rapid decompression related to the active shear zone setting of the granite. Although feldspar chemistry reflects a local segregation model, the chemistry of aphanitic dyke rocks indicates that the leucogranite did evolve into a more fractionated melt. The local presence of cassiterite in pegmatites and miarolitic cavities indicates that locally, saturation of Sn occurred, but not throughout the EKL where elevated Sn contents are attributed to infiltration of mineralising fluids (i.e. deuteric alteration).

Dating Hydrothermal Alteration Attending IOCG Mineralization Along a Terrane Bounding Fault Zone: The Copper Lake Deposit, Nova Scotia

The Copper Lake area of mainland Nova Scotia is one of several vein-controlled mineralized (Cu-Au-Co) systems associated with widespread carbonate and iron-oxide alteration proximal to the east-trending Cobequid-Chedabucto Fault System. Although this mineralization has been known for decades, its metallogenic affinity remains poorly defined, and in recent years an IOCG (iron oxide-copper-gold) model has been suggested. In order to determine the age of mineralization and provide an important time constraint for developing a metallogenic model, direct dating of the mineralization and associated alteration was undertaken. At Copper Lake, mineralization occurs in a set of sulphide-carbonate fissure veins hosted by fine-grained metasedimentary rocks of the Middle Devonian Guysborough Group. Dating of the sulphide-alteration (pyrite) and phyllic-alteration (muscovite) stages of the ore system utilized the Re-Os and 40 Ar/ 39 Ar methods, respectively. The two different chronometers yield ages of about 320 Ma and provide an absolute age for the mineralization. As part of this study additional geochronological data were obtained for detrital zircon (U-Pb age of 1634 ± 11.2 Ma) from the host sedimentary rocks, as well as timing of thermal events at ca. 370–380 Ma, 350 Ma and < 300 Ma based on whole rock 40 Ar/ 39 Ar and chemical Th-Pb dating of host rocks and monazite. The Th-Pb dating of monazite indicates that rare-earth element mobility accompanied mineralization. Collectively, the data indicate that the area experienced multiple thermal events, but hydrothermal activity related to mineralization is constrained to about 320 Ma and is tentatively interpreted to relate to structural focusing of fluids that may have been driven by a mid-crustal level mafic heat source. The mineralizing event coincides with regional Alleghanian deformation in this part of the Appalachian orogen and thus reflects larger-scale tectonothermal processes. RESUME Le secteur du lac Copper, dans la partie continentale de la Nouvelle-Ecosse, constitue l’un de plusieurs systemes mineralises (Cu-Au-Co) regis par des filons associes a une alteration etendue en oxyde de fer et en carbonates proximale du systeme de failles oriente vers l’est de Cobequid-Chedabucto. Meme si l’on connait cette mineralisation depuis des decennies, son affinite metallogenique demeure mal definie et des chercheurs ont avance ces dernieres annees un modele OFCO (oxyde de fer-cuivre-or). On a realise une datation directe de la mineralisation et de l’alteration connexe pour determiner l’âge de la mineralisation et etablir une delimitation chronologique importante pour l’etablissement d’un modele metallogenique. Dans le secteur du lac Copper, la mineralisation est presente dans un ensemble de filons de fissures remplies de sulfures-carbonates a l’interieur de roches metasedimentaires a grains fins du groupe du Devonien moyen de Guysborough. Les methodes Re-Os et 40 Ar/ 39 Ar, respectivement, ont permis la datation des stades de la sulfuration (pyrite) et de l’alteration phylliteuse (muscovite). Les deux differentes methodes chronometriques ont fourni des âges d’environ 320 Ma et conferent un âge absolu a la mineralisation. On a obtenu, dans le cadre de cette etude, des donnees geochronologiques supplementaires de zircon detritique (datation U-Pb de 1634 ± 11,2 Ma) des roches sedimentaires hotes ainsi que le moment des evenements thermiques, situes a environ 370-380 Ma, 350 Ma et < 300 Ma d’apres une datation de la roche totale 40 Ar/ 39 Ar et une datation chimique Th-Pb des roches hotes et de la monazite. La datation Th-Pb de la monazite revele qu’une mobilite des metaux des terres rares a accompagne la mineralisation. Les donnees signalent collectivement que le secteur a connu plusieurs evenements thermiques, mais que l’activite hydrothermale apparentee a la mineralisation est limitee a environ 320 Ma; on l’interprete provisoirement comme une activite liee a une concentration structurale des fluides qui pourrait avoir ete provoquee par une source de chaleur mafique mi-crustale. L’episode de mineralisation coincide avec la deformation alleghanienne regionale dans cette partie de l’orogene des Appalaches et correspond en consequence a des processus tectonothermaux a plus grande echelle. [Traduit par la redaction]

Granite-hosted mineral deposits of the New Ross area, South Mountain Batholith, Nova Scotia, Canada: P, T and X constraints of fluids using fluid inclusion thermometry and decrepitate analysis

The most evolved rocks of the South Mountain Batholith host polymetallic (Sn-W-U-Mo-Cu-Mn) mineral deposits in the New Ross area (NRA). This project presents new data (fluid inclusions, stable isotopes, mineral chemistry, and 40Ar/39Ar and 187Re/187Os dating) to define the hydrothermal fluids related to the mineralisation of the NRA, and to create an integrated model for the origin of the mineral deposits. Single-grain Ar/Ar laserprobe ages for white micas from unmineralised and mineralised samples range from 382 to 320 Ma. Molybdenite Re/Os ages on two pegmatite samples are 377 +/- 3 Ma and 371 +/- 3 Ma. The main phases of hydrothermal activity leading to polymetallic mineralisation occurred in close temporal relationship to granite crystallisation at ∼380 Ma, and episodic reheating events occurred at ∼371 Ma and ∼320 Ma, partially resetting the Ar/Ar system. Three distinct fluids in the mineral deposits of the NRA are: (i) a Na/K-rich magmatic fluid; (ii) a Ca-rich metamorphic fluid; and (iii) a meteoric fluid. Mixing of these fluids occurred, implying that the three fluids were contemporaneous. The isotopic compositions (delta 18O, deltaD) of white mica from unmineralised and mineralised samples of the NRA, and deltaD values for fluid inclusion extracts from coexisting quartz, record a transition from a magma-dominated, low water/rock ratio system to a fluid-dominated, higher water/rock ratio system, into which meteoric water infiltrated at the time of SMB crystallisation and greisen/vein formation. Fluctuating pH conditions of hydrothermal fluids at the contact with host granitoid rocks and variations in redox conditions, triggered by incursion of increasing amounts of meteoric fluid along deep-penetrating faults as the system evolved from magmatic to hydrothermal, caused metal deposition. Over ∼10 My, the concurrence of highly evolved granitic melts rich in incompatible elements in the late-stage of granite emplacement with intense fluid circulation, particularly those of meteoric origin, formed the NRA mineral deposits.

Graphic-textured inclusions in apatite: Evidence for pegmatitic growth in a REE-enriched carbonatitic system

Graphic texture, most common in felsic pegmatites, is documented here in an unusual setting, within apatite of the Hoidas Lake light rare earth element (LREE) deposit, an inferred distal pegmatitic-hydrothermal counterpart of a carbonatite in northern Saskatchewan, Canada. The graphic texture is displayed by vermicular and cuneiform LREE-rich inclusions which define primary growth zones in apatite. Integrated electron microprobe analysis and scanning electron microscope–energy dispersive X-ray spectroscopy imaging document the unusual and heterogeneous compositions of the inclusions, which vary between Ce2O3 + SiO2 (+ ThO2)–dominant and La2O3 + Nd2O3 (+ F)–dominant end members. Here we discuss the formation of these graphic-textured inclusions in the context of models suggested for the formation of graphic quartz–K-feldspar intergrowths in granitic pegmatites, based on the similarity of these features.