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

Abstract

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.