East Kemptville Research Articles

Towards a Model-Based Interpretation of Measurements of Mineralogical and Chemical Compositions

We introduce a new methodology for inference of fluid composition from measurements of mineralogical or chemical compositions, expanding upon the use of reactive transport models to understand hydrothermal alteration processes. The reactive transport models are used to impute a latent variable explanatory mechanism in the formation of hydrothermal alteration zones and mineral deposits. An expectation maximisation algorithm is then employed to solve the joint problem of identifying alteration zones in the measured data and estimating the fluid composition, based on the fit between the mineral abundances in the measured and predicted alteration zones. Using the hydrothermal alteration of granite as a test case (greisenisation), a range of synthetic tests is presented to illustrate how the methodology enables objective inference of the mineralising fluid. For field data from the East Kemptville tin deposit in Nova Scotia, the technique generates inferences for the fluid composition which compare favourably with previous independent estimates, demonstrating the feasibility of the proposed calibration methodology.

A petrological and geochronological study of a 360 Ma metallogenic event in Maritime Canada with implications for lithophile-metal mineralization in the Canadian Appalachians

The occurrence of ca. 360 Ma greisen-related molybdenite mineralization in the Clayton Hill biotite–muscovite leucomonzongranite (<1 km2) is unusual for the southwestern Meguma terrane of the Maritime Appalachians, where 380 Ma granite-related Sn mineralization dominates. The presence of extensive hornfelsing and occurrence of mineralized (Sn–W–Cu–Zn) skarns in the area suggests a larger intrusion occurs at depth. The intrusion and mineralization are constrained to 361 Ma based on concordant Re–Os (molybdenite) and 40Ar/39Ar (muscovite) ages, which equates to other ca. 360 Ma granites in the area, hence magmatism of this age is more widespread than previously considered. The intrusion is highly evolved (74 wt.% SiO2), metaluminous (A/CNK = 1–1.05; A/CNK = molar ratio [Al2O3/(CaO + Na2O + K2O)]) and depleted in cafemic and transition elements. Rare-earth elements (∑REE = 106–23 ppm) are strongly fractionated with light rare-earth elements (LREE) enriched (La/LuN = 10), and negative Eu anomalies. Biotite is Fe-rich (Fe/(Fe + Mg) = 0.75), and both biotite and muscovite are F deficient. These geochemical features indicate affinity to the other 360 Ma granites and suggest a similar petrogenesis with some A-type affinity. Mineralization occurs as molybdenite-bearing greisens and rusty miaroles lined with pyrite and enrichment in Cu–As–Bi–Au. Stable isotopes (O, S) indicate greisen formation at 450 °C from fluids with δ18OH2O = 7‰ and δ34SH2S = 2‰–3‰, which suggests an A- or I-type magmatic reservoir rather than a peraluminous magma. These data indicate the presence of a widespread magmatic event that is 20 Ma younger than the 380 Ma Sn-base metal event in southern Nova Scotia (e.g., East Kemptville). That this event is the same age as the world class Mount Pleasant Mo–Sn–W–Zn–In deposit of southern New Brunswick indicates additional potential for significant granite-related mineralization in this part of Atlantic Canada.

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

ABSTRACTPegmatites 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 P2O5 (to 1 wt. %) indicates that melts contained 2–3 wt. % P2O5, 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, H2O) nature of the melt. Stable isotopes demonstrate that a magmatic fluid (δ18OH2O = +5·5 to +10‰, δDH2O = −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).

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

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

Origin and evolution of the greisenizing fluid at the East Kemptville tin deposit, Nova Scotia, Canada

The process responsible for greisen-hosted tin mineralization at East Kemptville was investigated using petrographic, chemical, and fluid inclusion analyses of samples from a single deep drill hole (90-1) in the western part of the deposit (Baby zone). Based on their trace element chemistry, the greisens intersected in the hole appear to have all formed from a single leucogranite protolith. Alteration developed as symmetrically zoned halos around central fractures as a result of the interaction of a fluoride-rich orthomagmatic fluid with evolved leucogranite. During alteration, K feldspar was first replaced by albite. With further alteration, albite was replaced by muscovite resulting in the formation of quartz-sericite greisen. Ore minerals are locally present in this zone. Closer to veins, muscovite is replaced by topaz and quartz, producing a quartz-topaz greisen with which the most intense mineralization is associated. This latter greisen is enriched in F, Fe, S, Zn, and Sn, suggesting that these elements were added by the mineralizing fluid. Next to the vein, dissolution of pyrrhotite, sphalerite, and cassiterite characterizes quartz-greisen in which the concentration of ore-forming elements (Sn, Fe, S, F, Zn, Cu) is lower than in the quartz-topaz greisen. Microthermometric measurements of fluid inclusions in quartz from the various alteration zones show that this alteration sequence was formed by only one greisenizing event and that temperature was approximately constant (450 degrees C). The fluid responsible for greisen formation was an NaCl-brine, containing subordinate and variable concentrations of Fe, Mn, and K.Measured eutectic temperatures of fluid inclusions are lowest in quartz-topaz greisen as a result of an increase in the Fe concentration due to pyrrhotite dissolution. Oxygen fugacity, which was calculated from the CO 2 /CH 4 ratio in gases released by crushing fluid inclusion-rich samples, displays a corresponding minimum since pyrrhotite dissolution releases Fe (super 2+) . The distribution of pyrrhotite and the fluid inclusion data indicates that pyrrhotite was precipitated close to the vein during early stages of the alteration and reprecipitated farther out as alteration progressed. This suggests that alteration zones moved away from the vein and widened with time.The salinity of fluid inclusions varies between 27 and 41 wt percent NaCl equiv and increases linearly with increasing distance from the vein, even in the absence of Na-bearing phases. It, therefore, follows that sodium was transported toward the fracture (vein) down a chemical potential gradient. This occurred through compensated infiltration, i.e., a regime in which flow was dominantly parallel to the fracture but individual aliquots of fluid followed complex paths back and forth between the fracture and the rock.Cassiterite precipitated in quartz-topaz greisen in response to a pH increase of the mineralizing fluid due to its interaction with the wall rock. Other components affecting cassiterite solubility were either constant (temperature) or acted against its precipitation (f (sub O 2 ) and a (sub Cl (super -) ) ).

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 role of greisenization in cassiterite precipitation at the East Kemptville tin deposit, Nova Scotia

The role of fluid-rock (leucogranite) interaction in forming the greisen-hosted tin (Cu-Zn-Ag) deposit at East-Kemptville, Nova Scotia was investigated by evaluating compositional and mineralogical changes in a representative zoned greisen, and by using this information to estimate physicochemical changes in the fluid-rock system at each step of the alteration. A large sample comprising one side of a symmetrically zoned greisen was cut into 1.5-cm-wide slabs and each slab was analyzed for its major and trace element content. The results of these analyses were used to calculate the modal mineralogy of each slab which, in conjunction with petrographic observations, revealed two main alteration zones. First, or distal to the vein, the leucogranite was converted to a quartz-sericite greisen through the replacement of albite by muscovite. This alteration, specifically the dissolution of albite, created some porosity. Closer to the vein, the neoformed muscovite was converted to topaz and quartz, creating a quartz-topaz greisen; pyrite, sphalerite and cassiterite precipitated in the earlier created pore space.Mass balance calculations show that during alteration almost all major elements, including aluminum, were mobile. Only silicon was essentially immobile because the fluid was saturated with quartz due to earlier interaction with the leucogranite. The alteration of the leucogranite resulted in a quantitative removal of the alkalies, Mg and Mn, and an addition of Fe, S, F, Zn, and Sn. The difference in the modal mineralogy of each pair of adjacent slabs was used to quantify mineralogical changes in the two main alteration reactions responsible for the observed zonation. Variations in the molar abundances of species in solution were evaluated by balancing the alteration reactions and by estimating the water/rock ratios at each step of the alteration using oxygen isotope data. The study indicates that there was an increase in the pH of the fluid and a decrease in f (sub O 2 ) during alteration, and that these changes were most important close to the vein. Cassiterite deposition occurred contemporaneously with the alteration of muscovite to topaz and quartz and is interpreted to have been caused by the sharp increase in pH that accompanied this reaction. Changes in f (sub O 2 ) and Cl (super -) acted against cassiterite precipitation, but were apparently insufficient to prevent its saturation.

34 S enrichment in late-stage veins, East Kemptville tin and base metal deposit, Nova Scotia, Canada; evidence for late incursion of metasedimentary processed sulfur in a magmatic system

34 S enrichment in late-stage veins, East Kemptville tin and base metal deposit, Nova Scotia, Canada; evidence for late incursion of metasedimentary processed sulfur in a magmatic system

The East Kemptville tin deposit, Yarmouth County, Nova Scotia: a Pb-isotope study of the leucogranite and mineralized greisens—evidence for a 366 Ma metallogenic event

Pb isotopic data for whole-rock leucogranite and mineral separates, variably mineralized greisen, and galena from the East Kemptville Sn(–Cu–Zn–Ag) deposit, Yarmouth County, southwestern Nova Scotia, Canada, are presented. In contrast with ca. 300–340 Ma ages (40Ar/39Ar, Rb–Sr) previously published, an age of 366 Ma is indicated from (i) a 207Pb/206Pb slope age for whole-rock samples (leucogranite and greisen) and galena; (ii) 207Pb/206Pb slope age for leachate analyses of whole-rock and mineral separates (muscovite, K-feldspar) for a leucogranite sample (EK-161); (iii) 207Pb/206Pb slope age for all samples (leucogranite, greisen, galena) combined; and (iv) a concordia plot for 12 whole-rock samples of leucogranite and greisen. The best estimate for the time of magmatic and hydrothermal events at East Kemptville, derived by using the entire Pb–Pb data set for both mineralized and barren samples, is 366 ± 4 Ma (2σ, N = 34, mean square of weighted deviates (MSWD) = 2.63), comparable to the concordia age of 367 ± 10 Ma (2σ, N = 10, MSWD = 4.62). Isochron plots for the U–Th–Pb data may suggest mild overprinting at ca. 300 Ma, but the data are not as conclusive as results from previous Rb/Sr and 40Ar/39Ar geochronological studies.Leachate analyses of mineral separates indicate that less radiogenic compositions are obtained with each progressive leach; however, initial lead compositions are not provided by the analyses for the residues of either K-feldspar or muscovite separates. This suggests, possibly, that these minerals have reequilibrated (i.e., exchanged lead) during the subsolidus stage with fluids enriched in radiogenic Pb. Instead, the best estimate of the initial lead compositon at East Kemptville (207Pb/204Pb = 15.601, 206Pb/204Pb = 18.141) is provided by the intersection of the 366 Ma Pb–Pb regression line for a leucogranite sample (EK-161) with a second-stage growth curve having μ = 9.75; this value is less radiogenic than the average measured isotopic composition of galena (207Pb/204Pb = 15.629 ± 0.004, 206Pb/204Pb = 18.627 ± 0.005, N = 4). The calculated μ value (9.75) for the source is consistent with an evolved upper crustal reservoir. Using the calculated initial Pb composition, it is calculated that 3–5 Ma would be required to generate the measured Pb isotopic composition of the galena assuming μ values of 980 and 590, respectively. The measured day average μ value for the leucogranite and greisen is 674 (N = 12, 1σ), which is consistent with the aforementioned calculations.The coincidence of independently derived ages for both leucogranite and mineralized greisens indicates that the host muscovite–topaz leucogranite and mineralizing fluids had a common magmatic reservoir, a conclusion supported by δ18O analyses for whole-rock leucogranite (8.2–10.5‰) and greisen (7.9–10.9‰) and previously published δ34S data. The data also indicate that magmatic and hydrothermal processes at East Kemptville are coeval with emplacement of the adjacent Davis Lake pluton. In addition, the results reaffirm the 365–370 Ma period to be a significant time for Sn(–W–Mo) metallogeny within the Meguma Zone of the Canadian Appalachians.

The East Kemptville tin deposit, Yarmouth County, southwestern Nova Scotia: a lithogeochemical study of the wallrock metasedimentary rocks

The results of a petrographic and lithogeochemical study of metasedimentary rocks (Lower Paleozoic Meguma Group) surrounding the northern part of the East Kemptville tin deposit, southwestern Nova Scotia are presented. The metasedimentary rocks, including interbedded psammites and pelites, are fine-grained and highly indurated. Pelitic horizons contain homfelsic textures (i.e., <5 mm cordierite grains) near the granite; sparse veining includes some cassiterite and sulphides. Hydrolhermal alteration is manifested by disseminated, fine-grained, secondary muscovite and/or muscovite and chlorite infilling and lining microfractures with bleached haloes and associated cassiterite, sulphides, tourmaline, fluorite and carbonate.
 
 The major and trace element chemistry, including the rare-earth elements, of the metasedimentary rocks are compared to different data bases that comprise: (1) averages for pelites and psammitcs of Phanerozic age, (2) nonmineralized Meguma Group rocks, and (3) fields in discriminant binary clement diagrams. For most of the major and trace elements analyzed an apparently unaltered, normal chemistry is indicated, except for some LILE (e.g., Li, Rb, U) and ore-forming elements (e.g., Sn, Zn, Bi) where enrichment is considered to reflect infiltration of mineralizing fluids from the proximal granite/greisen of the main deposit area. The data indicate that infiltration of fluids was minimal and that, for the most part, the metasedimentary rocks acted as an impermeable cap to the ore-forming solutions.
 
 RÉSUMÉ
 Les résultats d'une étude pétrographique et lithogéochimique des roches métasédimentaires (Groupe de Meguma du Paléozoique inférieur) entourant la partie nord du gisement d'étain de East Kemptville, dans le sud-ouest de la Nouvelle-Écosse, sont présentés. Les roches métasédimentaires, comprenant des grès et des pelites, sont à grain fin et fortement induces. Les horizons pélitiques montrent des textures de corndennes (i.e. des grains de cordidrite <5 mm) près du granite; des veines éparses contiennent de la cassitérite et des sulfures. L'altération hydrothermale se manifeste par de la muscovite secondaire à grain fin dissémindée ou par de la muscovite et de la chlorite en remplissage et en tapis sage de microfractures aux épontes lessivées et associées à de la cassitérite, des sulfures, de la tourmaline, de la fluorite et des carbonates.
 
 La composition en éléments majeurs et traces, incluant les terres rares, des roches métasédimentaires sont companies à diverses bases de données, incluant: (1) les composition moyennes des pélites et des grès d'âge Phanérozoique, (2) les roches non minéralisdes du Groupe de Meguma et (3) les champs des diagrammes de discrimination binaires. Les teneurs de la plupart des éléments majeurs et traces analyses indiquent une composition normale, apparamment inaltérée, à part pour certains éléments à grand rayon ionique (e.g., Li, Rb, U) et certains éléments économiques (e.g., Sn, Zn, Bi) ou l’enrichissement est considéré comme représentant l’inftitration de fluides minéralisateurs provenant du greisen proximal de la région du gisement principal. Les données indiquent que l’infiltration des fluides était minimale et que les roches métasédimentaires ont agi principalement comme une enveloppe imperméable pour les solutions minéralisatrices.
 
 [Traduit par le journal]

Geochronological evidence for multiple tectono-thermal overprinting events in the East Kemptville muscovite–topaz leucogranite, Yarmouth County, Nova Scotia, Canada

The East Kemptville muscovite–topaz leucogranite, located in Yarmouth County, Nova Scotia, Canada, is host rock to the only producing primary tin deposit in North America (56 Mt, 0.165% Sn). Previous geochronological studies include (i) Rb–Sr whole-rock analyses of the quartz–topaz greisens, which indicated a date of 337 ± 5 Ma, and (ii) 40Ar/39Ar analyses of greisen muscovite, which indicated apparent plateau dates of ca. 300 Ma. However, the pervasive development of deformational fabrics at East Kemptville suggests that both the Rb–Sr whole-rock and 40Ar/39Ar muscovite ages are at best minimum estimates for the inferred time of mineralization. In the present study, Rb–Sr whole-rock and mineral (muscovite, plagioclase, K-feldspar) analyses and 40Ar/39Ar incremental-step heating of a muscovite separate indicate the following: (i) diffusion of Sr on the whole-rock scale terminated at 344 ± 5 Ma (11 point isochron date), coincident with closure of muscovite to intracrystalline diffusion of Ar (apparent plateau date of 338 ± 2 Ma) and (ii) internal reequilibration of Sr among muscovite, feldspar, and whole rock varied considerably such that Rb–Sr whole rock – muscovite pairs give dates of 361–311 Ma (mean = 330 Ma, n = 7), whereas whole rock – plagioclase – K-feldspar give dates of 276–240 Ma (mean = 254 Ma, n = 7). This younger thermal event is reflected in apparent dates of 269–286 Ma for the low-temperature steps of the 40Ar/39Ar muscovite age spectrum.Collectively the data indicate that the East Kemptville area either cooled slowly over a protracted period of time (ca. 100 Ma) or experienced episodic tectono-thermal activity at ca. 344, 330, and 254 Ma. Examination of previously published geochronological data for the southern Meguma Terrane indicates that these aforementioned ages broadly coincide with earlier documented magmatic or tectono-thermal events (e.g., intrusion of Wedgeport Pluton at 315 Ma). Inferences by some workers of mid-Carboniferous magmatism in the Meguma Terrane are, however, not supported by the present study.

Metallogenic domains and their reflection in regional lake sediment surveys from the Meguma Zone, southern Nova Scotia, Canada

Abstract Approximately 3000 samples from a regional centre-lake bottom sediment survey of southern Nova Scotia were re-analyzed for sixteen elements of metallogenic significance. Analytical results for Cu, Pb, Zn, Ag, Li, F, Nb, Sn, Ti, Zr, Rb, Th, Sb, As, W and Au confirm that most known mineral deposits and metallogenic domains in the Meguma Zone have detectable geochemical signatures in lake sediments. Lake sediment exploration is most effective for mapping when a catchment basin model is employed. In southern Nova Scotia, lake sediment surveys are especially useful when prospecting and selecting areas of potential lithophile mineralization. Mineralogical expressions of Sn, W, Th and Zr in lake sediment are reflected by the presence of discrete grains of cassiterite, scheelite or wolframite, monazite and zircon. Free gold has also been identified in lake sediment, and may be the cause of some identified Au anomalies. Lake catchment basins contain mixtures of hydromorphically and mechanically dispersed components. After relatively short transport distances from a bedrock or till source heavy minerals can be trapped in lake catchment basins. Meguma Zone lake sediment anomalies may be grouped into those dominated by these hydromorphic and mechanical-weathering processes. Geochemical mapping using lake sediments is undertaken through the application of simple data reduction procedures using single- and multi-element methods. Geochemical maps for individual elements are produced as computer-contoured colour plots at a fixed percentile scale. Maps for Sn, W, Au and Ag outline known mineral occurrences as distinct anomalies for the East Kemptville Sn-Zn-Cu ± W, Millet Brook U-Cu-Ag and numerous Au vein deposits within defined bedrock metallogenic domains. Geochemical mapping of Au and Ag anomalies also indicates large-scale structures such as northwesterly trending shear zones. Large shear zones are outlined by Au anomalies, and W highs define areas of new potential in exo-contact zones of the South Mountain Batholith. Gold potential for granitoid rocks is indicated by numerous Au anomalies. Associations of Cu-Pb-Zn-Ag, F-Li-Nb-Rb + Sn, Zr-Ti-Th and Sb-As-Au-W are plotted with multi-element score maps. Boundaries of the score maps closely follow the bedrock metallogenic domains and outline areas of potential economic mineralization. Regional geochemical mapping can be accomplished successfully, accurately and at reasonable cost using lake sediments. Modern analytical packages offer high-precision techniques for lake sediment analysis. Compared to regional till surveys in this area, lake sediment surveys cost less and have spatial attributes that produce data more suited to geochemical mapping projects.

Extreme variations in strontium initial ratios in ore-related fluids

Magmato-hydrothermal cassiterite-topaz ore at the Carboniferous East Kemptville (EK) greisenhosted tin deposit (Nova Scotia, Canada) is cross-cut by veins containing apatite, triplite, vivianite and fluorite. Initial 87Sr/86Sr ratios of these minerals have an extreme range (0.7135 to 0.8284). The initial ratios of the host rocks, EK quartz-topaz rock and Davis Lake biotite monzogranite (0.729±0.001, 0.727±0.004), are also high. The adjacent Meguma Group metasedimentary rocks are more typical of crustal material (0.712–0.719 at 330 Ma). Rb and Sr contents of EK fluorite (max.: 13.0, 1420 ppm) and apatite (max.: 88.1, 6660 ppm) are unusually high and variable. Unexpectedly, high Sr contents correlate positively with the high initial 87Sr/86Sr ratios. Fluorite and phosphate minerals from the first set of post-greisen veins at East Kemptville reflect the unusual chemistry of a high 87Sr/86Sr fluid present in the deposit after ore formation. The most extreme composition of this fluid was characterized by 87Sr/86Sr>0.8284, high Rb/Sr, high P, Rb, Cu, Zn and Fe contents, but low abundances of Ca, Pb and Sn with respect to the Davis Lake monzogranite. Such a fluid could have been derived from the greisen fluid and modified by reaction with the overlying Meguma metawacke. A second alternative, which cannot be well constrained at present, is that an extremely radiogenic fluid entered the deposit after ore formation and mixed with the postore fluid. In either case, the modified fluid subsequently mixed with meteoric water and precipitated the minerals with much lower 87Sr/86Sr ratios and Rb and Sr contents.

A sulfur isotope study of main-stage tin and base metal mineralization at the East Kemptville tin deposit, Yarmouth County, Nova Scotia, Canada; evidence for magmatic orgin of metals and sulfur

A sulfur isotope study of main-stage tin and base metal mineralization at the East Kemptville tin deposit, Yarmouth County, Nova Scotia, Canada; evidence for magmatic orgin of metals and sulfur

Paragenesis of veins of the Duck Pond tin prospect, Meguma Group, East Kemptville, Nova Scotia

The Duck Pond tin prospect is a vein- and strata-bound cassiterite prospect that is located 2 km west of the East Kemptville open-pit tin mine in southwestern Nova Scotia. The host rocks of the Duck Pond prospect are interbedded metawacke and meta-argillite that belong to the transition unit of the Meguma Group. These rocks contain quartz, sericite, chlorite, hematite, rutile, manganese oxides, feldspar, and porphyroblastic garnet, but not detrital cassiterite. The prospect is structurally controlled and contains several cross-cutting vein sets that have alkalic, chloritic, or argillic alteration assemblages. Muscovite is the main indicator mineral for alkalic alteration and occurs in veins that contain anorthoclase or quartz. Cassiterite is associated with chloritic alteration and occurs as subhedral to euhedral grains, acicular needles, and colloform layers in veins in meta-argillite and as strata-bound disseminations in metawacke. Most cassiterite precipitated under externally buffered conditions with respect to oxygen. Fe, Cu, Zn, and As sulphide minerals and quartz were deposited during argillic alteration. Late-stage processes such as recrystallization, sulphidation, and oxidation also occurred. Chalcopyrite is replaced by bornite and covellite; pyrite is replaced by marcasite.Unlike the F-rich East Kemptville deposit, fluorine-rich and tin-sulphide minerals are not present in the Duck Pond prospect. Trace tourmaline, absent at East Kemptville, is found at Duck Pond. However, the source of tin-mineralizing fluids at Duck Pond and East Kemptville was likely the granitic magma of the Davis Lake complex, which also hosts the East Kemptville deposit. From the mineral assemblages and textural relationships, it appears that as the temperature dropped from 425–405 °C to less than 200 °C at Duck Pond, the pH dropped from 5.2 to no lower than 3. Log [Formula: see text] dropped from at least −19 to −43. Log [Formula: see text] rose from < −15 to > −10. Cassiterite precipitated at the higher ends of the temperature and pH ranges and the lower end of the log [Formula: see text] range.

Rb–Sr age and geochemical distinctions between the Carboniferous tin-bearing Davis Lake complex and the Devonian South Mountain batholith, Meguma Terrane, Nova Scotia

The Davis Lake complex (DLC), composed of biotite monzogranite, leucomonzogranite, and cassiterite–topaz greisen, hosts the East Kemptville tin mine in southwestern Nova Scotia. The DLC monzogranite contains glomeroporphyritic biotite with ilmenite and many rare-earth-element (REE) bearing accessory minerals, zircon-bearing quartz phenocrysts, and xenoliths of biotite granite. Primary muscovite is rare. Major- and trace-element geochemical trends indicate well-defined, but limited, magmatic differentiation trends. REE patterns of the least-evolved granites are flat and show a Ce/Yb ratio of 10.The DLC was previously considered cogenetic with the Devonian South Mountain batholith (SMB) on the basis of its location, lithologies, and similarities in major- and trace-element geochemistry. However, new Rb–Sr whole-rock isotopic data indicate an Rb–Sr date of 330 ± 7 Ma (mean square of weighted deviates (MSWD) = 2.8) for the DLC, implying that it is at least 35 Ma younger than the SMB. The initial 87Sr/86Sr ratio of 0.727 ± 0.004 is significantly higher than those for other Meguma Terrane granites and is the highest yet reported from Appalachian granitoid rocks. Rb–Sr data from biotite indicate open-system behaviour between 260 and 240 Ma and provide more evidence for previously documented tectonothermal events after 300 Ma in the Meguma Terrane.The peraluminous nature of the DLC, its high Rb/Sr and high 87Sr/86Sr ratios, high P, F, and Sn contents, low Ca and B contents, and high differentiation indices indicate that the complex was derived from a highly evolved felsic source. Geochemical distinctions indicate that the DLC is neither derived from nor cogenetic with the SMB. A more probable source for the DLC magma is a dehydrated felsic granulite from which a previous H2O-, B-, Cl-, and Zn-rich granitic magma (perhaps the SMB) had been extracted. Such a source is analogous to that postulated for A-type granites and topaz rhyolites.The DLC shows more similarities to the "stitching" Carboniferous Appalachian volatile- and metal-rich granites than to Devonian Meguma granites. Unlike most of these Appalachian plutons, which occur marginal to terrane boundaries and were probably crystallized from locally generated, anatectic magmas, the DLC was emplaced in the centre of the most-outboard Meguma Terrane, adjacent to the Tobiatic shear zone.

Dissolution of fluorite (CaF 2) for geochemistry

Due to the chemical similarities among Ca, Sr and the rare-earth elements, fluorite (CaF 2), a accessory mineral that occurs in ore deposits, igneous, hydrothermal and metamorphic rocks, can be used to monitor genetic processes in these settings. Using a quantitative, low blank, H 2SO 4-based dissolution technique, we analysed fluorite from the East Kemptville tin mine, Nova Scotia, Canada. An exceptionally wide variation in initial 87Sr/ 86Sr ratio (0.71342 - 0.77495), Sr (30 - 1370 ppm) and Rb (1 - 13 ppm) contents resulted, probably due to fluid interaction after ore formation.

Extreme variations (0.729–0.828) in initial 87Sr/86Sr ratios in magmato-hydrothermal fluids derived from the East Kemptville greisen tin deposit, Nova Scotia, Canada

Extreme variations (0.729–0.828) in initial 87Sr/86Sr ratios in magmato-hydrothermal fluids derived from the East Kemptville greisen tin deposit, Nova Scotia, Canada

U–Pb and Rb–Sr geochronology of the Wedgeport granitoid pluton, southwestern Nova Scotia

Zircons from biotite monzogranite of the Wedgeport Pluton, intrusive into deformed metasediments of the Cambrian(?) Goldenville Formation in the southwestern Meguma Terrane of Nova Scotia, yield concordant U–Pb ages of 316 ± 5 Ma. This is interpreted as the time of intrusion and crystallization. Within the error limits, the 323 ± 12 Ma Rb–Sr whole-rock isochron age is identical and gives an initial 87Sr/86Sr ratio of 0.7137 ± 0.0056. Rb–Sr analyses of mineral separates of biotite, potassium feldspar, and quartz–plagioclase from several samples yield subparallel, internal isochrons with an average age of 257 ± 8 Ma. Initial ratios of the internal isochrons range from 0.716 to 0.759. A slow-cooling model for these latter data is discarded because the mineral data fall on straight lines. Instead, a reheating event related to plutonism ca. 257 Ma ago, which was sufficient to cause local grain-to-grain migration and reequilibration of strontium and rubidium but not large-scale redistribution, is invoked. This reheating is also inferred to be responsible for the hydrothermal alteration and Sn–U mineralization concentrated along the northwestern margin of the pluton. A dextral northeast–southeast shear zone cutting the pluton is also inferred to be ca. 257 Ma old. It may be related to the last stages of westward obduction of the Meguma Terrane.These results provide a clear example of Permo-Carboniferous plutonism in the southwestern Meguma Terrane and suggest a similar interpretation may apply to other anomalously young ages recorded in this area. In light of these results, the Permo-Carboniferous age of the large East Kemptville tin deposit and its location in a dextral shear zone suggest that the association of younger plutonism and shear zones may be a significant factor for economic mineralization.

Lithophile elements and exploration using centre-lake bottom sediments from the East Kemptville area, Southern Nova Scotia, Canada

The East Kemptville Sn deposit in southwestern Nova Scotia was discovered in 1978 by Shell Canada Resources, Limited using regional till exploration procedures. In 1977 and 1978, southern Nova Scotia was covered by a government-funded regional centre-lake bottom sediment survey (1 site per 5 km 2) for Cu-Pb-Zn-Ag-Ni-Co-Fe-Mn-Ca-Mg-Mo-Hg-As-U-LOI. Detailed lithogeochemical studies on drill core from East Kemptville have demonstrated significant roles for the elements Zn-Cu-Th-Li-F-Sn-Cl-Rb in the hydrothermal mineralization event. Most of these significant mineralizing elements were not analyzed as part of the original lake sediment survey. Later, in 1985, a subset of 55 lake sediment samples from the deposit area was analyzed for Sn, Rb, F and Cl. The pilot study was aimed to test the applicability of these lithophile elements to regional centre-lake bottom sediment geochemical programs. Anomalous values of F, Rb and Sn form a distinct halo of elevated levels southwest, southeast and northeast of the East Kemptville deposit, which appears to lie in an area of background values. The detailed till sampling undertaken by Shell Canada at East Kemptville provides a check, especially for Sn as to probable sources for these elements found in the centre-lake bottom sediments. Cassiterite is the main economic mineral at East Kemptville. Trace wolframite is found along with subsidiary chalcopyrite and sphalerite. Zn and Cu are recovered as by-products at the mine. Heavy minerals were separated from a bulk lake sediment sample from Moosefly Lake (the site of the highest Sn value from the regional survey subset). Spot analysis by scanning electron microscope indicates the presence of angular and discrete grains of cassiterite in the centre-lake bottom sediment, in addition to numerous grains of zircon, monazite and magnetite. The Sn anomaly in the centre-lake bottom sediment is due, at least in part, to the presence of discrete cassiterite grains. Their lack of abrasion would seem to indicate derivation from the nearby known cassiterite-rich tills. The lack of correlation of Sn data with organic material (as LOI) and the other elements probably indicates the dominance of mechanical over chemical dispersion processes in Sn distribution from the tills to the lakes. A common dispersion model for centre-bottom lake sediment surveys is one of initial glacial clastic dispersion, followed by subsequent hydromorphic dispersion into the lake basins. Although the dominance of hydromorphic processes is recognized, it is clear that clastic dispersion models must also be considered. Centre-lake bottom surveys appear to have greater application than previously presumed in the search for lithophile elements commonly dispersed as refractory grains.