Sample Details

10 Samples analyzed in this study were collected from diamond drill core recovered from two closely adjacent drill holes at the Road zone (Fig. 2). The drill cores are archived at the Newfoundland Department of Natural Resources core storage facility in Pasadena, Newfoundland.

11 Sample KC-04-034 was collected from drillhole JA-04-12 (Fig. 2), completed by Kermode Resources Ltd., in 2004. This drillhole intersected a mixture of Precambrian granitoid rocks and fine-grained mafic intervals, both of which are variably altered and mineralized (Kerr 2005). The dated sample was collected at a depth of 34 m, from the centre of a fine-grained mafic interval interpreted as a metamorphosed dyke. Although chilled margins are not visible in the drill core, the same rock type forms dyke-like bodies in surface outcrops, and there is no indication that it was intruded by the surrounding granodiorite. The sample was taken from the least altered portion of the mafic interval, but contains some foliation-parallel sericitic alteration zones. The sampled interval contains only 0.09 ppm Au, but adjacent strongly altered and mineralized material contains up to 6.5 ppm Au.

12 Sample KC-04-036 was collected from drillhole RB-09 (Fig. 2), completed by BP Resources Canada Ltd., in 1985. This drillhole mostly intersected variably altered and mineralized Precambrian granitoid rocks, but it also encountered three intervals of fresh diabase, which exhibit clear chilled contacts against the granite, and show no signs of mineralization or alteration. The sample was collected at a depth of 62.3 m, and consists of fresh, dark grey-green diabase, with scattered plagioclase phenocrysts. The mineralized granodiorite above the diabase contains up to 2.3 ppm Au. Below the lower contact of the diabase dyke, a grey, siliceous rock type described in company logs as "diabase with green pyritic alteration" contains disseminated pyrite, and seems to grade downward into altered and mineralized granite. This altered material, sampled with the underlying granite, contains 1.4 ppm Au (McKenzie 1986). Petrographic studies show that the "diabase with green pyritic alteration" is actually a strongly recrystallized and altered granitoid rock, probably forming part of a "screen" preserved within the dyke (Kerr et al. 2006). The diabase dykes were not assayed by BP Resources as they contained no sulphides; however, subsequent analyses (A. Kerr, unpublished data) show only background levels of gold.

13 The metamorphosed dyke (KC-04-034) has granoblastic, foliated texture, and consists mostly of biotite, K-feldspar, and quartz, with lesser amounts of epidote and sericite. The biotite-rich (potassic) composition suggests either a parental lamprophyre dyke, or perhaps metasomatic effects that introduced potassium prior to or during metamorphism. The fresh dyke (KC-04-036) retains good primary igneous textures, including plagioclase phenocrysts and oikocrystic mafic minerals. Primary orthopyroxene is accompanied by amphibole, most of which appears to be late magmatic. Fine-grained amphibole may be secondary or deuteric in origin, and the main alteration product of amphibole is chlorite. Both epidote and sericite occur locally in veinlets, but there is no sign of any widespread sericitic alteration. Analyzed material from KC-04-034 consisted of biotite-rich aggregates that also contained minor quartz; sericitic material was avoided. Analyzed material from KC-04-036 consisted of poor quality amphibole with opaque inclusions, probably iron oxides.

Results

14 Corrected argon isotopic data are listed in Table 1, and presented as gas-release spectra in Figures 3 and 4. Each gas-release spectrum plotted contains step-heating data from up to two aliquots, normalized to the total volume of ³⁹Ar released for each aliquot. Such plots provide a visual image of replicated heating profiles, evidence for Ar-loss in the low temperature steps, and the error and apparent age of each step.

15 The gas-release spectrum for sample KC-04-034 contains step-heating data from two aliquots (A and B). Both aliquots show well-defined multi-step plateaus representing 72% and 74%, respectively, of the total released ³⁹Ar. The combined weighted average ⁴⁰Ar/³⁹Ar age for 9 of 12 Ar release fractions from aliquot A and 8 of 12 from aliquot B is 412.6 ± 2.3 Ma, corresponding to a mean square of weighted deviates (MSWD) of 0.91. The Ar release fractions from the lowest power steps for each aliquot were excluded, as their younger apparent ages suggest Ar loss.

16 For amphibole from sample KC-04-036, an age plateau for 7 power steps represents 98% of the total argon released. The weighted average age for 5 of the 8 Ar release fractions (excluding the lowest power step, and fractions for power steps 6.5 and 7.5, which have large uncertainties) is 412.9 ± 4.3 Ma, corresponding to a MSWD of 0.013. An identical age is obtained using a regression analysis for an isotopic correlation diagram (not shown). Excluding those steps having large uncertainties, the limited variation in Ca/K ratios (Fig. 4) suggests that the analyzed material is homogeneous.

17 The results demonstrate that the foliated, altered, metamorphosed, gold-bearing dyke (KC-04-034) and the fresh, unaltered, unmineralized diabase dyke (KC-04-036) have identical apparent ages within their respective errors. Accounting for errors, the apparent ages lie between 417 and 409 Ma, and correspond to latest Silurian or earliest Devonian time, using the current geological time scale (International Commission on Stratigraphy; Gradstein et al. 2004).

Table 1. Isotopic ⁴⁰Ar/³⁹Ar Data

Display large image of Table 1

Display large image of Figure 3

Display large image of Figure 4

Interpretation of Ages

18 Geological relationships in surface outcrops and drill core indicate that the metamorphosed dyke was emplaced and metamorphosed prior to alteration and gold mineralization, and that the fresh diabase was emplaced following alteration and mineralization. As the best gold grades at the Rattling Brook prospect typically occur in altered mafic rocks, the lack of alteration and mineralization in a dyke that has mineralized wall rocks is unlikely to be coincidental. However, as the closure temperatures for Ar in biotite and hornblende differ significantly, the identical apparent ⁴⁰Ar/³⁹Ar ages do not necessarily demonstrate simultaneous crystallization of the two mafic dykes, as discussed below.

19 The closure temperature for Ar in hornblende is around 550 °C (McDougall and Harrison 1999). Much of the hornblende in the post-mineralization diabase (KC-04-036) appears to be late-magmatic, and the fine grain size suggests rapid cooling and solidification. There is certainly no indication that this sample was subjected to high temperatures during some later event. The hornblende age for KC-04-036 is thus attributed to the time of crystallization, and provides a minimum age for hydrothermal alteration and gold mineralization in its granodioritic host rocks.

20 The closure temperature of Ar in biotite is typically around 300 °C, although it can be significantly higher (McDougall and Harrison 1999). It is therefore unlikely that the ⁴⁰Ar/³⁹Ar age from the metamorphosed dyke (KC-04-034) actually records the time of its emplacement and crystallization, which is suspected to be late Precambrian. The result must instead be a younger cooling age linked to Paleozoic metamorphism. As the upper greenschist- to lowest amphibolite- facies metamorphic mineral assemblage reflects temperatures slightly above the closure temperature for biotite, the timing of peak meta-morphism must be prior to 412.6 ± 2.3 Ma; data from other studies (see below) suggest that metamorphism occurred no earlier than 430 Ma.

21 The combined age data can be interpreted in two ways (Fig. 5). The first interpretation is that the age from the meta-morphosed dyke records cooling through ~300 °C soon after regional metamorphism. Cooling was quickly followed by alteration and mineralization, and then by emplacement of the fresh post-mineralization dyke. In this scenario, regional metamorphism occurred prior to 412.3 ± 2.3 Ma (Fig. 5a), but the relative timing of cooling through ~300 °C and gold mineralization is less clear, as the latter could have occurred above the closure temperature for Ar isotopic systems in biotite (see discussion below). The second interpretation is that the age from the metamorphosed dyke actually records resetting of metamorphic biotite during alteration and gold mineralization, which was then followed quickly by emplacement of the fresh post-mineralization dyke. If the first interpretation is correct, the maximum age limit for mineralization could be greater than 412.6 ± 2.3 Ma, if the ambient temperature during the mineralization event was significantly above 300 °C (Fig. 5a). If the second interpretation is correct, the average age of 412.6 ± 2.3 Ma from the metamorphosed dyke directly records the timing of mineralization, during which ambient temperatures must have exceeded 300 °C (Fig. 5b). The temperature régime during gold mineralization is not directly constrained by fluid-inclusion studies, but Saunders and Tuach (1991) proposed 300 to 350 °C, based on alteration types, and suggest 400 °C as an upper limit, based on the presence of secondary microcline in altered granitoid rocks. This temperature estimate is in accordance with the inferred temperature range for most "mesothermal" gold mineralization, i.e., 250 to 400 °C (e.g., Groves et al. 1998; Poulsen et al. 2000). Kerr (2005) presented evidence that mineralization at Rattling Brook has closer affinities to other gold deposit types that likely form at temperatures significantly below 300 °C. From an observational perspective, the brittle fracturing of quartzitic rock types to accommodate mineralized veinlets corroborates this view. On the basis of these data, it seems unlikely that the ambient temperature during mineralization was higher than the closure temperature for Ar isotopic systems in biotite, and the first interpretation (Fig. 5a) is preferred. The identical ages from the metamorphosed dyke and the fresh, post-mineralization diabase are thus interpreted to closely reflect the timing of gold mineralization at the Rattling Brook deposit, and suggest that it formed in earliest Devonian times, between 415 Ma and 409 Ma. The age range is based on the slightly smaller errors for the pre-mineralization metamorphosed dyke, whereas use of the less precise age from the post-mineralization dyke would suggest 417 Ma as the older limit.

Display large image of Figure 5

22 However, the possibility that mineralization occurred prior to this time, at temperatures above 300 °C, cannot be completely excluded without direct constraints on the thermal régime. In this context, it is important to note that U-Pb and ⁴⁰Ar-³⁹Ar isotopic data from adjacent areas (see discussion below) suggest that regional metamorphism occurred ca. 430 Ma ago, and this provides an absolute older limit for the timing of gold mineralization, even if our assumptions concerning the temperature of mineralization ultimately prove invalid.

Local Correlations

23 Several conclusions relevant to local geology may be drawn from the ⁴⁰Ar/³⁹Ar ages determined in this study (Fig. 6). The biotite cooling age for the metamorphosed dyke suggests that peak metamorphism occurred during the Silurian. This timing is consistent with inferences from U-Pb and ⁴⁰Ar/³⁹Ar geochronological studies on the Baie Verte Peninsula and the Corner Brook area, which indicate peak metamorphism (amphibolite facies) at ca. 430 Ma, and cooling through 500 °C at ca. 430 to 420 Ma. (Cawood and Dunning 1993; Cawood et al. 1994). Slightly younger biotite cooling ages akin to those determined for the metamorphosed dyke would thus be expected, and Cawood et al. (1994) report a similar muscovite cooling age of 413 ± 3 Ma. The result is consistent with the importance of the mid-Silurian Salinic Orogeny throughout the Newfoundland Appalachians (e.g., Dunning et al., 1990).

Display large image of Figure 6

24 The fresh, post-mineralization diabase dyke may be part of a more extensive earliest Devonian dyke swarm in western Newfoundland. Kerr and Knight (2004) reported that diabase dykes cut all of the Cambrian-Ordovician formations in the White Bay area, and they were emplaced after the rocks were folded, into zones of local dextral displacement. Similar mafic dykes occur in the Canada Bay area, north of the study area (Fig. 1, inset), where they cut time-equivalent Cambrian and lower Ordovician carbonate strata (Knight 1987). ⁴⁰Ar/³⁹Ar whole-rock plateau ages from several dykes in the Canada Bay area range from 419 ± 3 Ma to 405 ± 4 Ma (I. Knight and D. Lux, unpublished data), overlapping the age determined in this study.

25 Previous workers (e.g., Saunders and Tuach 1991) suggested a link between gold mineralization at Rattling Brook and local plutonic rocks. The age for gold mineralization suggested here is within the wide error envelopes of the Devils Room granite (425 ± 10 Ma; Heaman et al. 2003) and the Gull Lake Intrusive Suite (398 + 27 / -7 Ma; Erdmer 1986). A genetic link to these plutonic rocks thus remains possible, but it is far from proven. There is a striking absence of minor felsic intrusions in the area of the Rattling Brook gold deposit, aside from a narrow (< 3 m) felsic dyke in hole RB-12 that cuts mineralized granodiorite and appears to be of post-mineralization timing (McKenzie 1986).

26 Auriferous quartz-carbonate vein systems of more typical mesothermal aspect (e.g., the Browning deposit) are hosted by Silurian volcanic and sedimentary rocks of the Sops Arm Group (Saunders 1991; Kerr 2006; Fig. 1). Thin carbonate units in the host rocks contain mid-Silurian faunas, and the youngest sedimentary rocks in the Sops Arm Group contain uppermost Silurian (Pridoli Stage) faunas (Lock 1969). At face value, the latest Silurian – earliest Devonian gold mineralization event identified at Rattling Brook could also be responsible for the vein-style gold mineralization in the Silurian rocks. However, as discussed above, the characteristics and geochemical associations of the latter differ from those of Rattling Brook, and it remains possible that they are part of a discrete, younger gold mineralizing event (see discussion below).

Regional Correlations

27 Paleozoic epigenetic gold mineralization in Newfoundland occurs in rocks of Precambrian to Silurian age, but the most abundant host rocks are Cambrian and Ordovician. Most gold deposits in Newfoundland occur within the Dunnage Zone of the Central Mobile Belt (Fig. 1, inset). As discussed in the introduction, the difficulty of dating such deposits means that few reliable age constraints exist for comparative purposes (Fig. 6). At the Hammerdown gold deposit near Springdale (Fig. 1, inset) auriferous veins cut felsite dykes that share their structural trend. A U-Pb zircon age of 437 ± 4 Ma from the felsite provides an older limit for the timing of mineralization, which is inferred to be Upper Silurian (Ritcey et al. 1995). At the Stog’ er Tight gold deposit on the Baie Verte Peninsula (Fig. 1, inset), Ramezani et al. (2002) obtained a U-Pb zircon age of 420 ± 5 Ma from "hydrothermal" zircon in skarn-like, gold-bearing alteration. In contrast, a U-Pb xenotime age of 374 ± 8 Ma for pegmatitic quartz- feldspar-carbonate alteration associated with gold at the nearby Nugget Pond deposit (Fig. 1, inset) indicates late Devonian mineralization (Sangster and Pollard 2001; Sangster et al. 2007). More recently, McNicoll et al. (2006) obtained a U-Pb zircon (SHRIMP) age of 381 ± 5 Ma from a mafic dyke that hosts gold-bearing veins at the Titan Prospect, north of Gander (Fig. 1, inset). This age provides an older limit for the timing of mineralization, which must be post-Middle Devonian. Other constraints on the timing of gold mineralization are less definitive. For example, gold mineralization at the Cape Ray deposit in southwestern Newfoundland (Fig. 1, inset) is constrained only between 415 and 386 Ma (Dubé and Lauzière 1997).

28 Although the available dataset (Fig. 6) leaves much to be desired, it hints at two discrete periods of gold mineralization, one corresponding to the Silurian-Devonian boundary and the other to the middle to late Devonian. The data reported in this paper support this overall pattern, and suggest that the Rattling Brook deposit belongs to the earlier of these two mineralization events.

29 Outside Newfoundland, few data exist that reliably constrain the timing of gold mineralization in the northeastern Appalachian Orogen, but some information is available from New Brunswick and Nova Scotia.

30 New Brunswick is host to diverse styles of gold mineralization but information on the ages of the mineralized veins is mostly indirect. The best-known deposit is the Clarence Stream deposit, which is hosted by early Silurian sedimentary rocks of the Gander Zone. Mineralization here is classified as "intrusion-related", and interpreted to be linked to the nearby Magaguadavic Granite, which gave a U-Pb zircon age of 396 ± 1 Ma (Bevier 1990). Muscovite from auriferous quartz veins yielded ⁴⁰Ar/³⁹Ar ages of ca. 389 Ma (Davis et al. 2004), and an upper limit is provided by associated and locally mineralized aplitic dykes that yielded a U-Pb monazite (microprobe) age of 390 ± 8 Ma (Thorne et al. 2002). The Lake George deposit, also within the Gander Zone of New Brunswick, is best known for its antimony mineralization, but also contains significant gold enrichment associated with scheelite and molybdenite (Lentz et al. 2002; Yang et al. 2003). Muscovite from auriferous veins gave ⁴⁰Ar/³⁹Ar ages of 411 ± 8 Ma and 413 ± 8 Ma, and Sb-bearing veins gave a ⁴⁰Ar/³⁹Ar muscovite age of 417 ± 8 Ma (Seal II et al. 1988). The Lake George Granite, believed to be genetically linked to the mineralization, gave a U-Pb age of 412 ± 2 Ma. The inferred age for this deposit is thus similar to that presented here from Rattling Brook, despite their markedly different metallogenic associations.

31 In Nova Scotia, a recent study by Morelli et al. (2005) in the Meguma terrane involved direct dating of sulphide minerals from two important gold deposits (Kontak et al. 1990). Two Re-Os arsenopyrite ages from the Ovens deposit gave identical results of 407 ± 4 Ma and 409 ± 5 Ma, whereas equivalent data from the Dufferin deposit indicate a significantly younger age of 380 ± 3 Ma (Morelli et al. 2005). Previous studies using ⁴⁰Ar/³⁹Ar methods on muscovite in the wall-rocks to veins had indicated a younger age of ca. 370 Ma, suggesting that there may be problems with the latter approach.

32 The discrete periods of gold mineralization indicated by the data from the Meguma terrane, and perhaps also New Brunswick, correspond well with those defined by this study and other information from Newfoundland summarized above (Fig. 6). Given the great distance between the areas in question, varied geological settings and the fact that they are separated by major faults representing terrane boundaries, the coincidence of this temporal pattern is intriguing. In terms of regional orogenic events recognized through the northern Appalachians, these periods of gold mineralization broadly correspond with definitions of the Acadian and so-called "Neoacadian" orogenies, rather than to the Silurian Salinic Orogeny (Robinson et al. 1998; van Staal 2007). Such correspondence raises the possibility that there may be orogen-scale mineralizing episodes linked to these late-stage thermotectonic events, as suggested by the distribution of so-called orogenic gold deposits through geological time (e.g., Goldfarb et al. 2001).

Sample Processing and Analytical Techniques

Selected samples were crushed to granules in the range 0.25 to 0.50 mm. Several grains were loaded into aluminum foil packets along with a single grain of Fish Canyon Tuff Sanidine (FCT-SAN) to act as flux monitor (apparent age = 28.03 Ma; Renne et al. 1998). The sample packets were arranged radially inside an aluminum can. The samples were then irradiated for 12 hours at the research reactor of McMaster University in a fast neutron flux of approximately 3x10¹⁶ neutrons/cm². Laser ⁴⁰Ar/³⁹Ar step-heating analysis was carried out at the Geological Survey of Canada laboratories in Ottawa, Ontario. Upon return from the reactor, samples were split into several aliquots and loaded into individual 1.5 mm-diameter holes in a copper planchet. The planchet was then placed in the extraction line and the system evacuated. Heating of individual sample aliquots in steps of increasing temperature was achieved using a Merchantek MIR10 10W CO₂ laser equipped with a 2 mm x 2 mm flat-field lens. Heating was continued until the grains disintegrated or fused, releasing all gas. The released Ar gas was cleaned over getters for ten minutes, and then analyzed isotopically using the secondary electron multiplier system of a VG3600 gas source mass spectrometer. Details of data collection protocols can be found in Villeneuve and MacIntyre (1997) and Villeneuve et al. (2000). Error analysis on individual steps follows numerical error analysis routines outlined in Scaillet (2000); error analysis on grouped data follows algebraic methods of Roddick (1988).

Neutron flux gradients throughout the sample canister were evaluated by analyzing the sanidine flux monitors included with each sample packet. A linear fit was interpolated against calculated J-factor and sample position. The error on individual J-factor values is conservatively estimated at ± 0.6% (2 sigma). Because the error associated with the J-factor is systematic and not related to individual analyses, correction for this uncertainty is not applied until calculation of dates from isotopic correlation diagrams (Roddick, 1988). The well-defined plateaus (Figs. 3 and 4) from different minerals indicate that excess ⁴⁰Ar was not present in the samples and all regressions are assumed to pass through the ⁴⁰Ar/³⁶Ar value for atmospheric air (295.5). All errors are quoted at the 2 sigma level of uncertainty.