Petrochemical evolution of subvolcanic granitoid intrusions within the late Devonian Mount Pleasant Caldera, southwestern New Brunswick, Canada: comparison of Au versus Sn-W-Mo-polymetallic mineralization systems.
Two spatially and temporally related subvolcanic intrusive suites, the McDougall Brook Granitic Suite (MBG) and the Mount Pleasant Granitic Suite (MPG), occur within the Late Devonian Mount Pleasant Caldera, southwestern New Brunswick. Auriferous greisenized quartz breccias and veins are associated with the MBG, whereas Sn-W-Mo-Bi-polymetallic (Cu-Zn-Pb-In) mineralization is generically related to the slightly younger MPG. The low silica (< 70 wt. %) MBG and the high-silica (> 74 wt. %) MPG are calc-alkaline and metaluminous to weakly peraluminous (A/CNK = 0.91 - 1.28), and exhibit some crustal A-type granite affinities. However, compared to the MPG, the MBG has lower amounts of incompatible trace elements, a more pronounced Nb anomaly, enriched REE patterns with smaller negative Eu anomalies, and lower negative Ti, Sr, and Ba anomalies. Trace-element modeling, major-element chemistry, and petrologic evidence indicate that the MBG and MPG may have formed through pulse injection of magmas produced by fractional crystallization from felsic magma chambers at depth, involving an assemblage of clinopyroxene, amphibole, plagioclase, K-feldspar, [+ or -] blotite, [+ or -] zircon, [+ or -] apatite. Supracrustal contamination is evident, resulting in abnormally high alkalies (e.g., K) and high initial [([sup.87]Sr/[sup.86]Sr).sub.i] ratio. The parental magmas may be mainly derived from juvenile materials within the crust. Extremely high F ([less than or equal to] 0.99%), Li ([less than or equal to] 610 ppm), Rb ([less than or equal to] 1210 ppm), Cs ([less than or equal to] 28 ppm), U ([less than or equal to] 43 ppm), Th ([less than or equal to] 50 ppm), Nb ([less than or equal to] 107 ppm), Ta ([less than or equal to] 26 ppm), and heavy REE (e.g., Yb [less than or equal to] 36 ppm) in the MPG rocks, with very low K/Rb ratio ([less than or equal to] 56) and Nh/Ta ratio ([less than or equal to] 7.6), suggest fluid fractionation played an important role in late-stage magmatic differentiation.
Although gold background contents in both the MBG and MPG are typically low, ranging from < 2 to 8 ppb, W, Sn, and Mo contents in the MBG are much lower than those in the MPG. Gold mineralization appears to be related to the early MBG, as it behaves as a compatible element during magmatic evolution. The MBG is broadly comparable 8to intrusion-related gold systems elsewhere. However, the MPG has characteristics of evolved topaz-bearing granites commonly associated with Sn-W Mo-Bi-polymetallic (Cu-Zn-Pb-In) mineralization; the youngest phase of the MPG appears have high potential for generating Sn deposits. Both intrusive suites formed within an epicontinental caldera complex and generally have within-plate geochemical character.
Deux suites intrusives subvolcaniques spatialement et temporellement apparentees, la suite granitique du ruisseau McDougall (MBG) et la suite granitique du mont Pleasant (MPG), sont presentes a l'interieur de la caldeira du Devonien tardif du mont Pleasant, dans le Sud-Ouest du Nouveau-Brunswick. Des filons et des breches de quartz grelsenifies auriferes sont associees a la MBG, tandis qu'une mineralisation de Sn-W-Mo-Bi-polymetallique (Cu-Zn-Pb-In) est genetiquement apparentee a la MPG legerement plus recente. La MBG a faible teneur en silice (< 70% pds) et la MBG a forte teneur en silice (> 74% pds) sont calco-alcalines, metalumineuses a faiblement hyperalumineuses (A/CNK = 0,91 - 1,28), et elles affichent quelques affinites granitiques de type A crustales. La MBG presente toutefois, comparativement a la MPG, des quantities interieures d'elements traces incompatibles, une anomalie plus prononcee en Nb et des configurations enrichies de metaux des terres rares avec des anomalies negatives en Eu plus reduites et des anomalies negatives de Ti, Sr et Ba inferieures. La modelisation des elements traces, la composition chimique en elements majeurs et les observations peteologiques revelent que la MBG et la MPG pourraient s'etre formees par injection intermittente de magma produit par cristallisation fractionnee de reservoirs de magma felsique en profondeur et ayant cumporte un assemblage de clinopyroxene, d'amphibole, de plagioclase, de feldspath potassique, [+ or -] biotite, [+ or -] zircon et [+ or -] apatite. La contamination supracrustale est evidente; elle a entraine une presence anormalement elevee d'alcalins (p. ex. K) et un ratio initial eleve ([sup.87]Sr/[sup.86]Sr). Les magmas parents pourraient principalement provenir de materiaux immatures a l'interieur de l'ecorce. Les teneurs extremement elevees de F ([less than or equal to] 0,99%), de Li ([less than or equal to] 610 parties par million), de Rb ([less than or equal to] 1 210 parties par million), de Cs ([less than or equal to] 28 parties par million), d'U ([less than or equal to] 43 parties par million, de Th ([less than or equal to] 50 parties par million), de Nb ([less than or equal to] 107 parties par million), de Ta ([less than or equal to] 26 parties par million) et d'elements de terres rares lourds (p. ex. Yb [less than or equal to] 36) dans les roches de la MPG, avec de tres faibles ratios de K/Rb ([less than or equal to] 56) et de Nb/Ta ([less than or equal permettent de supposer que la cristallisation fractionnee des fluides a joue un role determinant dans la differentiation magmatique du dernier stade.
Meme si les teneurs en or de reference, tant dans la MBG que la MPG, sont generalement peu elevees, variant entre moins de deux et huit parties par milliard, les teneurs en W, Sn et Mo de la MBG sont beaucoup plus basses que celles de la MPG. La mineralisation de l'or semble reliee au stade precoce de la MBG, car elie s'est comportee comme un element compatible pendant l'evolution magmatique. La MBG est d'une facon generale comparable aux systemes auriferes relies a des intrusions d'ailleurs. La MPG possede toutefois les caracteristiques des granites topaziferes evolues communement associes a une mindralisation de Sn-W-Mo-Bi-polymetallique (Cu-Zn-Pb-In); la phase la plus recente de la MPG semble offrir un potentiel eleve de production de cites de Sn. Les deux suites intrusives se sont formees a l'interieur d'un complexe de caldeiras epicontinentales et possedent generalement un caractere geochimique intraplaque.
[Traduit par la redaction]
Intrusion-related gold systems have recently raised great interest among economic geologists, leading to many new discoveries worldwide (McCoy et al. 1997; Thompson et al. 1999; Lang et al. 2000; Lang and Baker 2001; Fan et al. 2003). In southwestern New Brunswick, part of the Canadian Appalachian orogen, several gold deposits and occurrences (e.g., Clarence Stream, Lake George, and Poplar Mountain) have been found to share some similarities with intrusion-related gold systems elsewhere in the world (McLeod and McCutcheon 2000; Thorne and Lentz 2001 ; Chi 2002; Lentz et al. 2002; McLeod 2002 ; McLeod and Fyffe 2002; Thorne et al. 2002; Yang et al. 2002a, 2002b; Thorne and Lentz 2003). The model of intrusion-related gold systems has stimulated exploration efforts for gold in the area and has highlighted the need for studies to determine the essential controlling factors for formation of these deposits. Clearly, metal behaviour during magmatic evolution is one of the most important factors controlling mineralization.
A fundamental problem with granitoid intrusions is that the precursor magmas have traveled vertically (or laterally) from their source regions to the upper crustal environment, and hence experienced various geological processes that may have greatly modilled their original geochemical signatures; therefore, a comprehensive study involving a multidisciplinary approach is required to deal with the complex problem of their petrogenesis (Clarke 1992). In order to better understand the petrogenesis of a granitoid complex and/or batholith, the temporal and spatial framework of the complex must be established from field relations and geochronological data.
The Late Devonian Mount Pleasant Caldera Complex contains two granitic suites (Kooiman et al. 1986; Sinclair et al. 1988; Sinclair and Kooiman 1990; McCutcheon 1990a, 1990b; McCutcheon et al. 1997, 2001). The older McDougall Brook Porphyritic Microgranite of McCutcheon (1990a) is associated with the early stage of caldera development, whereas the younger Mount Pleasant Porphyry of McCutcheon (1990a) is associated with the final stage of the caldera evolution. These two suites are termed the McDougall Brook Granitic Suite (MBG) and the Mount Pleasant Granitic Suite (MPG) in this study (Fig. 1), because both consist of multiple phases with intrusive contacts. The well-known W-Mo-Bi and Sn-Cu-ZnPb-In deposits at Mount Pleasant are genetically related to various phases of the MPG (Kooiman et al. 1986; Lentz et al. 1988; Sinclair et al. 1988; McCutcheon 1990b; Sinclair and Kooiman 1990; Inverno 1991; McCutcheon et al. 2001). Interestingly, auriferous greisenized hydrothermal quartz breccias and veins (200 ppb up to 1 g/t Au, and up to 27 g/t Ag; see Johnson 2003) cut the MBG and its wall-rock, both of which were intruded by the MPG (Figs. 1 and 2).
[FIGURES 1-2 OMITTED]
Previous studies mainly dealt with the Mount Pleasant Granitic Suite (i.e. GRI, GRII, and GRIII) and related mineralization characteristics (see Kooiman et al. 1986; Sinclair et al. 1988; Sinclair and Kooiman 1990; Inverno 1991). In these papers, few complete geochemical data sets on the granites were available. Taylor (1992) reported geochemical data for one sample taken from the MPG, and compared it with the Pleasant Ridge topaz granite to the southwest. Whalen et al. (1996) reported geochemical data for one sample from each of the MPG and MBG suites, and noted geochemical differences and similarities between them. No gold data were reported in these studies, Inverno (1991) concentrated on tin mineralization in the MPG, but did not investigate the MBG. McCutcheon (1990a) and McCutcheon et al. (1997, 2001) focused on the stratigraphy and volcanic sequence correlation within the caldera. He and his coworkers analyzed samples from different phases of the MBG for major elements and selected trace elements as a part of a study on the chemical evolution of magmas within the Mount Pleasant caldera (McCutcheon 1990a; McCutcheon et al. 1997). Again, no gold data were reported in these studies, and no work was done on the petrogenetic linkages between the MPG and MBG, and their relations to gold mineralization.
This study focuses on the petrochemical aspects of these two subvolcanic granitic suites within the Mount Pleasant Caldera in order to determine the essential factors that made one suite produce gold mineralization and the other Sn-W-Mo-polymetallic mineralization. On the basis of field relations, petrography, petrochemistry, and trace element modeling, the MBG and MPG are interpreted to be linked through extreme fractional crystallization and supracrustal contamination; fluid fractionation may have played a role in formation of the late MPG. Metal behaviour during the evolution of these magmatic systems indicates that Au behaved as a compatible element, whereas Sn, W, and Mo were incompatible elements. These results can be used to assess the mineralization potential ofgranitoid intrusions in the area.
The Mount Pleasant Caldera (Fig. 1), southwestern New Brunswick, is a northeast-trending elliptical feature with minimum dimensions of 13 by 34 km (McCutcheon 1990a, 1990b). The caldera is sited on polydeformed Ordovician to Silurian turbiditic metasedimentary rocks of the Digdeguash and Flume Ridge formations that are thought to form part of the Gander Zone (Fyffe and Fricker 1987). Post-tectonic Middle Mississippian and Pennsylvanian strata overlie the northern part of the caldera, whereas Late Silurian to Devonian granitic rocks of the Saint George Batholith (McLeod et al. 1988; McLeod 1990) form part of its boundary along the southern margin. Rocks within the caldera constitute the Upper Devonian Piskahegan Group that is divided into exocaldera, intracaldera, and late caldera-fill sequences (McCutcheon 1990a, 1990b; McCutcheon et al. 1997, 2001). The Piskahegan Group is largely composed of bimodal volcanic rocks that have been interpreted to have been derived from two parental magmas: mafic with a mantle source and felsic with a lower crustal source (McCutcheon 1990a; McCutcheon et al. 1997). The Mount Pleasant Caldera formed during the transition from a compressional to a transtensional tectonic setting, i.e. during initial development of the Maritimes Basin (McCutcheon et al. 1997). Tectonostratigraphic terrane analysis and geochemical studies (Fyffe and Fricker 1987; Fyffe et al. 1999; Barr and White 1999; Barr et al. 2002) indicate that subduction processes have not taken place in the region since Late Silurian.
Granitic intrusions within the Late Devonian Mount Pleasant Caldera include the McDougall Brook (MBG) and the slightly younger Mount Pleasant (MPG) granitic suites, in the order of intrusive sequence. The MBG is a polyphase suite mainly consisting of marginal feldspar porphyry (MBG 1), fine-grained (groundmass) porphyritic monzogranite (MBG2), and minor fine-grained equigranular to subporphyritic quartz monzonite (MBG3); these phases occur with intrusive contacts in the sequence of MBG1 [right arrow] MBG2 [right arrow] MBG3 (McCutcheon 1990a; McCutcheon et al. 1997, 2001). The northern half of the MBG intrudes intracaldera rocks, including Seelys Formation (rhyolite and ash flow turfs) to the south, Scoullar Mountain Formation (sedimentary breccia, andesitic and rhyolitic ash flow ruffs) to the west, and exocaldera Bailey Rock Rhyolite to the east. The southern half of the MBG mostly intrudes the Little Mount Pleasant Formation (rhyolitic ash flow tuffs), although small offshoots intrude into the pre-caldera, Early Devonian Flume Ridge Formation and intracaldera Scoullar Mountain Formation to the west (McCutcheon 1990a; Fig. 1). According to McCutcheon (1990a), the MBG represents the transition between a mafic magma that was contaminated by crystal cumulates from overlying felsic magma in a compositionally zoned, high-level subvolcanic magma chamber.
The polyphase MPG (Figs. 1 and 2) is composed of fine-grained equigranular granite (GRI), aplitic to porphyritic granite (GRII), and fine- to medium-grained equigranular granite (GRIII), although locally porphyritic and pegmatitc varieties of GRIII are also present (Rooiman et al. 1986; Sinclair et al. 1988; McCutcheon et al. 2001; Figs. 2 and 3). These units have intrusive contacts indicating the intrusive sequence GRI [right arrow] GRII [right arrow] GRIII (Fig. 2b). The GRI unit is heavily brecciated and altered, and is genetically related to early W-Mo-Bi mineralization (Kooiman et al. 1986; Sinclair et al. 1988; McCutcheon et al. 2001). The GRI unit is not included in the present study. The MPG intrudes intracaldera rocks (Scoullar Mountain and Little Mount Pleasant formations) and cuts the MBG (Fig. 2) along the southwestern margin of the caldera (Figs. 1 and 2; Sinclair et al. 1988; McCutcheon et al. 1997, 2001). McCutcheon (1990a) interpreted the MPG as the top of a zoned, high-level felsic magma chamber that crystallized inward in successive stages.
[FIGURE 3 OMITTED]
Radiometric ages are consistent with the field relations mentioned above, indicating that the MPG suite is slightly younger than the MBG suite. An [sup.40] Ar/[sup.39] Ar age of ca. 361 Ma was obtained for GRIII (Sinclair and Kooiman 1990) and a U-Pb zircon age of 363.4 [+ or -] 1.8 Ma (Tucker et al. 1998) was obtained for the Bailey Rock rhyolite that is considered to be the extrusive equivalent of the McDougall Brook feldspar porphyry (McCutcheon et al. 1997). Conventional K-Ar data and whole-rock Rb-Sr isotopic data yielded less reliable ages of 340-330 Ma for the MPG (Kooiman et al. 1986). A Rb-Sr isochron age of 368 Ma was obtained for the Piskahegan Group (Anderson 1992). These data imply that volcanism and subvolcanic plutonism in the caldera took place during a relatively narrow time interval.
PETROGRAPHY AND RELATED MINERALIZATION
Based on modal and mesonormative analyses, most of the 27 samples in this study are monzogranitc; only three samples from unit MBG1 and one from unit MGB2 are syenogranite (Fig. 4). All of the samples in this study are from subsolvus granite, based on the presence of both plagioclase and K-feldspar.
[FIGURE 4 OMITTED]
McDougall Brook Granitic Suite
Unit MBG2 mainly consists of porphyritic monzogranite that locally grades to syenogranite. The monzogranite is composed of plagioclase, K-feldspar, quartz, hornblende, and biotite in order of decreasing abundance, with minor pyroxene. The phenocryst assemblage is plagioclase, K-feldspar, and quartz (~ 40% total abundance), although a few highly altered hornblende (to chlorite) phenocrysts are evident, in which pyroxene cores were noted. Phenocrystic plagioclase forms euhedral to subhedral tabular crystals, ranging from 1 to 12 mm long, with rectangular cross sections and polysynthetic albite twinning. Glomeroporphyritic plagioclase clusters are common. Plagioclase grains are highly altered, giving them a mottled appearance under both plane and cross-polarized light. The alteration products are mainly saussurite (i.e., fine-grained aggregate of epidote group minerals, albite, prehnite, sericite, and possibly clays). K-feldspar (1-12 mm in grain size) occurs as subhedral, most commonly rectangular, crystals, but rarely euhedral crystals are observed. Coarse patch perthite is common. K-feldspar is also very altered to sericite and clay minerals. Micrographic texture, in which quartz is intergrown in K-feldspar crystals, is present. The quartz forms small, more or less angular grains that are commonly optically continuous over the entire K-feldspar crystal. Phenocrystic quartz generally is less than 4 mm in size; it commonly occurs as subhedral crystals with partly square to hexagonal cross sections, or as anhedral, equant crystals. Groundmass (generally less than 0.5 mm in grain size) constitutes about 60% of the rock and has a similar mineralogy to the phenocryst assemblage. Accessory minerals are apatite, titanite, allanite, zircon, and opaque minerals (mainly magnetite).
The less voluminous MBG1 phase occurs on the margin or border of the MBG2 unit, and consists of porphyritic monzogranite (Fig. 3d) and minor equigranular to subporphyritic (containing plagioclase phenocrysts) fine-grained monzogranite. The porphyritic monzogranite contains two populations of subhedral to euhedral feldspar phenocrysts (1-12 mm) with plagioclase generally more abundant than K-feldspar, constituting 30 35% of the rock. Quartz phenocrysts (1-4 mm) are minor (< 2%). The groundmass (generally less than 0.1 mm) includes minerals similar to the phenocrysts. Accessory minerals include apatite, zircon, titanite, and magnetite. Most magnetite in the MBG is probably secondary because anhedral magnetite crystals are intimately associated with chlorite that formed by hydrothermal alteration of pyroxene, hornblende, and biotite.
A zone of greisenized quartz breccia and veins about 1500 m long and several hundred meters wide cuts the MBG and its wall-rocks (Seelys Formation ash flow tufts). This steeply dipping NW-trending zone (Fig. 1) is notably mineralized and contains significant gold (200 ppb up to 1 g/t Au) and silver (up to 27 g/t Ag) (see Johnson 2003). Gold is typically associated with arsenopyrite and pyrite, although the sulphide abundance is low (< 1%) and unevenly distributed in the quartz breccias and veins. These features are common in intrusion-related gold systems elsewhere (Lang et al. 2000; Lang and Baker 2001).
Mount Pleasant Granitic Suite
Units GRII and GRIII of the MPG are similar in mineralogy, and consist mainly of quartz, K-feldspar, plagioclase, and biotite. They may represent two pulses of magma injection from an inward-cooling magma chamber, as indicated by intrusive contacts (Fig. 2b). Unit GRII is mainly composed of porphyritic monzogranite, with local aplitic and seriate textural varieties. Phenocrysts (30-40%) consist of subhedral to anhedral quartz (0.5 to 1.5 mm), anhedral K-feldspar (0.5 to 3 mm), subhedral to anhedral plagioclase (0.5 to 1 mm), and biotite (0.5 to 2 mm). The groundmass contains minerals similar to the phenocrysts, but the grain size is commonly less than 0.3 mm.
Unit GRIII mainly consists of fine- to medium-grained equigranular monzogranite, although locally porphyritic and pegmatitc varieties are present. Subhedral to anhedral quartz (30-40%) is dominant, but euhedral quartz is also observed; grain size ranges from 0.5 to 1.5 mm. K-feldspar usually occurs as anhedral to subhedral crystals from 0.5 to 3.5 mm, which constitute 40-50% of the rock. Plagioclase (20 to 25%) forms subhedral crystals, 1 to 1.5 mm long. Subhedral biotite flakes, 1 to 3 mm in size, contain pleochroic halos caused by radiation damage induced by zircon inclusions, form 1-3%. Opaque minerals include ilmenite, pyrite, and pyrrhotite. Magnetite is associated with chlorite, suggesting that it formed by hydrothermal alteration.
Accessory minerals in both GRII and GRIII are monazite, zircon, ilmenite, columbite, and Li mica. Topaz and fluorite in the GRII and GRIII are both interpreted as products of hydrothermal alteration based on textural relations. However, a few topaz crystals that occur as inclusions in euhedral quartz in GRIII and the presence of primary topaz in an aplitic contact phase of GRII, led Sinclair and Kooiman (1990) to conclude that the MPG can be classified as topaz granite (Manning 1988; Manning and Hill 1990). Magmatic topaz also occurs in leucogranite in the vicinity of Pleasant Ridge and Kedron, as reported by Taylor et al. (1985) and Taylor (1992), who termed the Kedron granite the Bonny River granite. These granite bodies are similar in composition and age to the MPG.
Parts of the aplitic to porphyritic phase of the GRII contain miarolitic cavities, comb quartz layers, and unidirectional solidification textures (UST), suggesting fluid saturation and/ or undercooling of the parental magma (Kirkham and Sinclair 1988). Micrographic intergrowths of quartz and K-feldspar are common in the granite associated with comb quartz layers, both of which reflect a significantly undercooled magma (Fenn 1986; Lentz and Fowler 1992). Detailed drilling and mapping of underground developments indicate that major Sn-Zn-Pb-Cu-In mineralization is closely associated with unit GRII (Kooiman et al. 1986; Sinclair et al. 1988; Sinclair and Kooiman 1990; McCutcheon et al. 2001; Fig. 2b).
In the North Zone at Mount Pleasant, unit GRIII clearly intrudes GRI and GRII (Fig. 2b), which is indicated by sharp contacts that in many places are marked by thin layers of UST (mainly K-feldspar) in the GRIII. Miarolitic cavities (1 mm to 2 cm in size) are locally abundant, in which very fine-grained sericite, chlorite, fluorite, and carbonate are present, but arsenopyrite, molybdenite, and other metallic minerals are relatively rare (Sinclair et al. 1988). Despite well preserved fluid saturation textures in GRIII and evidence of hydrothermal activity, only minor tin mineralization is known to be associated with it (Kooiman et al. 1986; Sinclair et al. 1988; Sinclair and Kooiman 1990; McCutcheon et al. 2001). However, further exploration efforts arc needed to confirm this conclusion.
Fluid inclusion studies showed that magmatic fluids were predominant in the early stage of mineralization, but more meteoric fluids were involved in late-stage ore-forming processes (Davis and Williams-Jones 1985; Samson 1990). High salinity (>30 wt.% NaCl equivalent) and high temperature (350-490[degrees]C) fluid is interpreted to have been derived from resurgent boiling, possibly related to magmatic fluids exsolved from the cooling maginas; dilution of this early fluid by convecting meteoric water resulted in low- to moderate-salinity fluids that dominated the inclusion population (Davis and Williams-Jones 1985). Low temperature (180-250[degrees]C) and high-salinity fluid may represent late residual fluid from a change in the pressure regime from dominantly lithostatic to hydrostatic conditions (Davis and Williams-Jones 1985). Clearly, hydrothermal breccias, related pervasive alteration and mineralization (silicification, chloritization, and greisenization) in GRI and GRII (Kooiman et al. 1986) reflect pressure fluctuations of fluids associated with the granitic magmas in the Mount Pleasant deposits. These processes are similar to those that operated in the adjacent True Hill biotite porphyritic granite intrusions (Lentz et al. 1988; Lentz and McAllister 1990; Lentz 1994; Lentz and Grcgoirc, 1995), suggesting a comparable genesis for all Sn-W-Mo-Bi mineral deposits in the region.
Ten least-altered granite samples collected from seven drill cores at the Mount Pleasant mine and 17 samples from outcrops of the McDougall Brook Granitic Suite (see Figs. 1 and 2 for sample locations) were analyzed for major elements and selected trace elements, including rare earth elements (REE). The methods used in this study include inductively coupled plasma-optical emission spectrometry (ICP-OES), inductively coupled plasma-mass spectrometry (ICP-MS), instrumental neutron activation analysis (INAA), and X-Ray fluorescence spectrometry (XRF).
The ten samples from the MPG were systematically analyzed with ICP-OES (Si[O.sub.2], Ti[O.sub.2], [Al.sub.2][O.sub.3], [Fe.sub.2][O.sub.3] (total), MnO, CaO, MgO, [Na.sub.2]O, [K.sub.2]O, [P.sub.2][O.sub.5], S, As, Ba, Cd, Co, Cr, Co, Ni, Sc, Sr, V, and Zn) and ICP-MS (Rb, Y, Zr, Nb, Cs, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Pb, Th, U, Bi, and Sn) at the GSC-Quebec (Geological Survey of Canada-Quebec), with INAA (Au, Ag, As, Ba, Br, Ca, Co, Cr, Cs, Fe, Hf, Hg, Ir, Mo, Na, Ni, Rb, Sb, Sc, Se, Sn, Sr, Ta, Th, U, W, Zn, La, Ce, Nd, Sm, Eu, Tb, Yb, and Lu) at the Activation Labs, Ancaster, Ontario, and with XRF (Si[O.sub.2], Ti[O.sub.2], [Al.sub.2][O.sub.3], [Fe.sub.2][O.sub.2](total), MnO, CaO, MgO, [Na.sub.2]O, [K.sub.2]O, [P.sub.2][O.sub.5], Rb, Ba, Sr, Ga, Nb, Zr, Y, Th, U, Ce, Cr, Ni, Sc, V, Cu, Pb, Zn, Cl, S, and As) on pressed pellets (Longerich 1995) at the Memorial University of Newfoundland, St. John's. The seventeen samples from the MBG were analyzed with INAA for selected trace elements including REE, also at the Activation Labs; major element data for these samples were taken from the original dataset of McCutcheon (1990a), which were determined by XRF. Ccrtiffed standard samples SY2 and JG1 (Govindaraju 1994) were used to monitor data quality. Analytical precision of major elements is better than 3% relative standard deviation (RSD), except for Si[O.sub.2] (5% RSD); and for most trace elements is better than 10% RSD. Elements for which the analyzed values were beyond 5% of the recommended values in the standards were not used. The analytical results that satisfied the requirements of precision and accuracy are shown in Table 1, and are considered to best represent the chemical compositions of the samples.
Although the MBG has low silica content (Si[O.sub.2] < 70 wt. %) relative to the MPG (Si[O.sub.2] > 74 wt. %), both are weakly peraluminous, with four samples from the MBG being metaluminous (Table 1). Aluminum saturation indices (A/CNK = [Al.sub.2][O.sub.3]/ (CaO+[Na.sub.2]O+[K.sub.2]O), molar) range from 0.91 to 1.28 (Table 1). Harker diagrams (Figs. 5a, e and i) show that Ti[O.sub.2], MgO, and [P.sub.2][O.sub.5] decrease with increasing silica, whereas [Al.sub.2][O.sub.3], CaO, [K.sub.2]O, and [Na.sub.2]O show an erratic distribution; [Fe.sub.2][O.sub.3] and MnO (Figs. 5c and d) are somewhat less erratic. Samples from the MPG are characterized by high Si[O.sub.2], ([Fe.sub.2][O.sub.3] + FeO)/MgO ratio, and alkalis with [K.sub.2]O/[Na.sub.2]O > 1, and low CaO, [P.sub.2][O.sub.5], Ti[O.sub.2], and MgO (Table 1; Figs. 5c, e, g, and h), thus sharing many similarities with topaz rhyolite, ongonite, and topaz granite (Christiansen et al. 1983, 1984; Christiansen et al. 1986; Kontak 1990; Taylor 1992; Dostal and Chatterjee 1995; Zhu et al. 2001). The MBG and MPG seem to have two distinctive groups, but each group displays a similar variation trend (Figs. 5a-i) except for those on the plots of A/CNK versus [K.sub.2]O and A/CNK versus CaO (Figs. 5j and k). These trends may indicate that magmatic evolution was controlled by a similar process such as fractional crystalization, but that minerals involved in the process may have been different in the MBG and MPG.
On the plot of Si[O.sub.2] versus [K.sub.2]O (Le Maitre el al. 1989), samples from the MBG plot in the fields of high-K and shoshonite series, whereas those from the MPG fall into the field of medium K (Fig. 6a). Using the MALI (modified alkali-lime index) against Si[O.sub.2] diagram of Frost et al. (2001), the difference in alkalinity between the MBG and MPG is clear: the former belongs to the alkalic-calcic transitional to the alkalic suite, whereas the latter is calcic-alkalic (Fig. 6b). Interestingly, all samples from the MBG are magnesian granites, defining a trend toward higher Fe[O.sup.total]/(Fe[O.sup.total] + MgO) ratios with increasing silica, but the MPG samples range from magnesian to ferroan granite (Fig. 6c), according to criteria established by Frost et al. (2001).
[FIGURE 6 OMITTED]
In terms of the Gottini Index [tau] (Gottini 1968) and the Rittmann Serial Index [sigma] Rittmann 1957), the lavas of active volcanoes can be divided into two distinctive groups that reflect tectonic setting, i.e. anorogenic and orogenic, as shown in Fig. 6d. The formulations for the parameters [tau] (Gottini 1968) and [sigma] (Rittmann 1957) are as follows:
[tau] = ([Al.sub.2][O.sub.3] - [Na.sub.2]O)/Ti[O.sub.2](wt.%)
[sigma] = [([Na.sub.2]O + [K.sub.2]O).sup.2]/(Si[O.sub.2] - 43)(wt.%)
This discrimination diagram was originally proposed by Rittmann (1973), based on plotting of Log [sigma] versus Log [tau] for 1300 lava samples from active volcanoes worldwide. From mafic through intermediate to felsic igneous rocks, the [sigma] value is a very convenient petrochemical indicator, compared to the Peacock index (Peacock 1931), to portray the alkalinity of igneous rocks. Each whole-rock analysis has a defined alkalinity value, if the Rittmann serial index [sigma] is appropriately applied (X. Yang, unpublished data).
Most of the samples from the MBG and MPG suites fall into field B, except for six samples from the MBG (three are located in field C, and the other three fall on the boundary between the fields B and C) (Fig. 6d). Samples from the MPG have [sigma] values of 1.5 to 2.3 (Table 1), i.e. belonging to the calc-alkaline series, except for one sample (PRL95-2-2019) that has [sigma] value of 0.9, which may be attributed to hydrothermal alteration (i.e. chloritization). Their [tau] values are extremely high, ranging from 182 to 449, consistent with the character of highly differentiated rocks.
Most of the MBG samples are calc-alkaline ([sigma] ranging from 1.8 to 3.3; Table 1), but six samples are weakly alkaline ([sigma] ranging from 3.5 to 4.3), consistent with their high alkali contents (Fig. 6a and b). Their [tau] values (13 to 68; Table 1) are much lower than those of the MPG samples, indicating that they are less evolved. Notably, all apparent fractionation trend is exhibited on Figure 6c, suggesting that the MPG could be linked through differentiation of the MBG.
A few samples (30A2, 33A2, 193A; Table 1) from the MBG1 phase of the MBG show very high Si[O.sub.2] (up to 77.5 wt. %) and [K.sub.2]O (up to 8.41 wt.%) contents, but relatively low [Na.sub.2]O (as low as 0.84 wt.%) and [Al.sub.2][O.sub.3] (as low as 11.6 wt.%) contents. These features likely reflect hydrothermal alteration (e.g., silicification, sericitization, and chloritization), as indicated by petrographic observation, and are not the result of magmatic fractionation. Similarly, two samples (PRL95-2-2019 and NMP89-1-1759) from the GRII phase of MPG contain moderately low Si[O.sub.2] and [Na.sub.2]O (Table 1), which also is attributed to hydrothermal alteration (e.g., chloritization). Consequently, interpretations based on alkalis must be considered with caution. As shown in Kontak and Clark (2002), unaltered and altered granitoids are distinguishable on plots of A/CNK versus [K.sub.2]O and A/CNK versus CaO. However, except for these partly altered samples mentioned above, no apparent alteration trends are evident on these plots (Figs. 5j & k).
Trace and rare earth element (REE) composition
Trace elements, including REE, in the MBG are distinct from those in the MPG (Table 1). The MBG samples have relatively low incompatible element contents, enriched chondrite-normalized REE patterns [[(La/Yb).sub.N] = 3.5-15.7] with more pronounced negative Nb anomalies, but small negative Eu anomalies (Eu/Eu * = 0.43 - 1.09), and more pronounced negative Ti, Sr, and Ba anomalies (Figs. 7 and 8). The flat "bird-wing shape" REE patterns (Fig. 7b) of the MPG samples, which are characterized by large negative Eu anomalies (Eu/Eu* = 0.014 - 0.002) and low [(La/Yb).sub.N] (ranging from 1.4 to 2.0) ratios, reflect the highly evolved nature of the MPG. These data are consistent with the major element data (e.g., high Si[O.sub.2] and alkalis, very low CaO, and high [tau] values). From the MBG to MPG, negative Eu anomalies become increasingly pronounced, with fractionation based on Th enrichment. The MPG samples plot in the A-type granite field, but the MBG samples fall into the I- and S-type field (Fig. 9). If they are associated with fractionation, feldspar must be one of major phases in the crystallizing assemblage (Eby 1992; Christiansen and Keith 1996). It has to be pointed out that highly evolved granites such as the MPG that display some of the geochemical characteristics of A-type intrusions are not necessarily true A-type granites (Whalen et al. 1987; Eby 1990), as no fayalite or sodic hedenbergite that typify "classic" A-type granite occurs in the MPG. Furthermore, petrographic evidence shows that the MPG was saturated with water during crystallization, which also contrasts with the relatively "dry" nature of anorogenic A-type granitoids (Collins et al. 1982; Whalen et al. 1987; Eby 1990).
[FIGURES 7-9 OMITTED]
Relative to I-, S-, A-, and M-type granites (Collins et al. 1982; White and Chappell 1983;Whalen et al. 1987; Eby 1990, 1992), both the MBG and MPG are enriched in F ([less than or equal to] 0.99 wt.%), Li ([less than or equal to] 610 ppm) (see Sinclair and Kooiman 1990), Rb ([less than or equal to] 1210 ppm), Cs ([less than or equal to] 28 ppm), U (43 ppm), Th ([less than or equal to] 50 ppm), Nb ([less than or equal to] 107 ppm), Zr ([less than or equal to] 580 ppm), and heavy REE (e.g., Yb [less than or equal to] 36) (Table 1), which is typical of crustal A-type felsic magmas (Fig. 10), topaz rhyolite (Christiansen et al 1983, 1984, 1986), ongonite, and topaz granite (Kontak 1990; Taylor 1992; Zhu et al. 2001). Both the MPG and MBG suites are temporally and spatially related, i.e. part of the Mount Pleasant Caldera, and both intrusive suites formed in a within-plate tectonic setting (McCutcheon et al. 1997), but the petrogenetic relationship between them is less clear. It is noteworthy that Nb and Zr behave differently in the MBG and MPG suites. From early to late in time sequence (GRII [right arrow] GRIII), Nb decreases with decreasing Zr in the MPG, whereas Nb increases with increasing Zr from MBG1 [right arrow] MBG 2 in the MBG (Fig. 5l).
[FIGURE 10 OMITTED]
The distribution of major and trace elements in the MBG and MPG seems to indicate two distinctive groups with an apparent "gap" (Figs. 5, 11a-c and 12). It would be easy to ascribe this difference to diverse petrogenetic processes for formation of these two granitoid suites within the Mount Pleasant Caldera, without consideration of their field relations, detailed petrographic features, systematic chemical variations, and isotopic compositions. Brophy (1991) noted that compositional gaps, like the ones above, are not uncommon in calc-alkaline magmatic systems. He proposed that fractionation-generated compositional gaps may result from (1) simultaneous interior crystallization and vigorous chamber-wide convection which leads to progressive crystal suspension, (2) cessation of convection when the percentage of suspended crystals reaches the critical crystallinity of the magma, and (3) eventual buoyancy-driven crystal-melt segregation producing a discrete body of fractionated magma that is separated from the initial magma by a compositional gap. Marsh (1996) pointed out that in a magma chamber with phenocrysts, differentiation is controlled by a balance between capture of crystals by the solidification front and crystal settling; massive crystal separation results in less viscous residual magma with discontinuous compositions. Solidification front instability causes silicic melt segregation and produces a separate magma (Marsh 1996).
Published isotopic data suggest that the MBG and MPG have a similar source, as they have the same [epsilon]Nd(t) (-0.2) and similar [[delta].sup.18]O (7.1 and 8.2 ~ 8.6 [per thousand]) values (see Taylor 1992; Whalen et al. 1996). The slightly lower [[delta].sup.18]O value of the MBG may be attributed to meteoric water interaction (Whalen et al. 1996). These characteristics suggest that the MBG and MPG either were derived from the same parent magma or from different batches of partial melting of the same protolith. The initial [sup.87]Sr/[sup.86]Sr ratio for the MPG is 0.7126 (Kooiman et al. 1986), which allows for a crustal source dominated by recycled crustal materials or a mantle source strongly contaminated by supracrustal rocks. The initial [sup.87]Sr/[sup.86]Sr ratio for the Piskahegan Group is 0.7061 (Anderson 1992), requiring some input from the mantle, consistent with the bimodal nature of the caldera rocks (McCutcheon 1990a; McCutcheon etal. 1997, 2001). A mantle-derived mafic magma could cause partial melting of crustal rocks and generate the felsic magmas responsible for the MBG and MPG. Given low Sr and high initial [sup.87]Sr/[sup.86]Sr ratio together with [epsilon]Nd(t) and [[delta].sup.18]O values of the MPG rocks, the most likely scenario is that the magma originated from partial melting of juvenile materials and was subsequently contaminated by supracrustal rocks. Based on a systematic study of geochemistry, Taylor (1992) reached the same conclusion for the source material of the Pleasant Ridge topaz granite that has many chemical similarities to the MPG.
Petrogenetic linkage between the MBG and MPG
Two models are commonly used to explain topaz-bearing granitic rocks: very small degrees of partial melting of continental crustal material that was subjected to an earlier episode of melting (Christiansen et al. 1986; Manning and Hill 1990) and extensive fractionation of felsic magma (Dostal and Chatterjee 1995, 2000; Zhu et al. 2001). Generally, the magmas are generated in an extensional tectonic regime (Christiansen et al. 1986; Whalen et al. 1987, 1996). Moreover, extensive liquid fractionation involving thermogravitational diffusion, liquid immiscibility, and vapour-phase transport has also been suggested to explain the petrochemical variations and the substantial enrichment of incompatible trace elements (Christiansen et al. 1983, 1984, 1986; Dostal and Chatterjee 1995, 2000; Zhu et al. 2001).
Large variations in compatible elements, such as Sr, Ba, and Eu in the MBG and MPG (Table 1, Figs. 7 and 8), rule out the possibility of variable degrees of partial melting of the same source, although small degrees of partial melting can generate high contents of incompatible elements (see Rollinson 1993; Christiansen and Keith 1996). Fractional crystallization, however, not only reduces the abundance of compatible elements, but also increases the concentrations of incompatible elements (e.g., Li, Rb, Cs, U, Th, Nb, and Ta), without any variations in the isotopic ratios if the effect of supracrustal contamination is not considered.
The petrochemical data presented in this study (Table 1; Figs. 5-9, 11, 12) are modeled assuming that the MBG and MPG were produced by extensive low-T fractional crystallization of the same parental magma, rather than from two batch-melts that evolved separately. The extreme fractional crystallization is interpreted to have taken place mainly at depth, as indicated by two separate compositional groups, but with the same evolutionary trends for the MBG and MPG (Figs. 5, 6c & d, 9, 11, and 12). However, supracrustal contamination of selective components is also reflected in some major elements (Fig. 5) and high initial [sup.87]Sr/[sup.86]Sr ratio, as well as trace elements such as Rb and Cs (Fig. 12; Table 1). The erratic distribution of the alkali elements is interpreted as the manifestation of contamination or alteration, whereas the tightly constrained distribution of Ti[O.sub.2], [P.sub.2][O.sub.5], and MgO is interpreted as primary features. No alteration trends are apparent on Figures 5j, k (cf. Kontak et al. 2002). Figure 13 illustrates the log-log plot of incompatible (Nb) versus compatible (Sr) elements for the samples from the MBG and MPG, in which the samples are mainly confined in the field between fractional crystallization (FC) and assimilation-fractional crystallization (AFC)(r = 0.5) curves. Assuming a leucogabbro (Thorne and Lentz 2002) magma as the starting point and using upper continental crust (Taylor and McLennan 1985) as contaminant, the FC and AFC paths (Fig. 13) can be constructed with the equations of Rollinson (1993), given the bulk partition coefficients ([D.sub.Sr] and [D.sub.Nb]) estimated by the method of Allegre et al. (1977) (see details below). The parameters used in the modeling are given in the caption of Fig. 13. General vectors produced by Rayleigh fractionation of single phases (e.g., plagioclase, K-feldspar, biotite, hornblende, and clinopyroxene) and their effects on melt composition are also shown for comparison. The compositions of average N-type MORB (Saunders and Tarney 1984), average OIB (Sun 1980), lower continental crust (Weaver and Tarney 1984), and upper continental crust (Taylor and McLennan 1985) are plotted on Fig. 13, which constrains the probable sources for the granitoids from the Mount Pleasant Caldera. All these data suggest that magma evolution was controlled by extreme fractional crystallization plus selective supracrustal contamination. Figure 6c indicates that fractional crystallization may have proceeded under relatively reduced conditions, as high f([O.sub.2]) would have resulted in crystallization of magnetite and depleted the residual magma in iron.
A simplified calculation was performed to test the probability of petrogenetic connection between the MBG and MPG through fractional crystallization. Preliminary trace-element modeling, together with major-element petrochemical evidence, indicates that the mineral assemblage involved in fractional crystallization was clinopyroxene, amphibole, plagioclase, K-feldspar, biotite, zircon, and apatite (Fig. 13). Some minerals rich in light REE (e.g., monazite and allanite) may also occur as separate phases, although only a very small proportion is required. The most evolved sample (NMP89-1-1849) from the MPG is equivalent to 10-15% residual melt (for most incompatible elements, such as Nb, Ta, and HREE) generated by the most primitive sample (295) from the MBG that is assumed to represent the composition of the parental magma in terms of a fractional crystallization model (Christiansen and Keith 1996) and the estimated bulk partition coefficient (D) as described below. Inconsistency with a lower degree of fractionation for incompatible elements (e.g., Rb) is interpreted to imply that the enrichment of the high field strength elements, such as Nb, Ta, and HREE, may require fluid involvement. Dostal and Chatterjee (1995, 2000) showed that fluid fractionation could have resulted in differential enrichment of these elements to the level observed in ca. 370 Ma topaz leucogranite from South Mountain Batholith in Nova Scotia. Compared to their data, the present study indicates that fluid fractionation may have played an important role in the extreme enrichment of these elements (e.g., Nb, Ta, and HREE) in the MPG (see discussion below).
This model of fractional crystallization coupled with fluid fractionation plus supracrustal contamination proposed in this study (Fig. 14) differs from the previous model by McCutcheon (1990a), wherein he suggested that two different magmas (i.e. two stages of partial melting of lower crust) evolved separately in high-level chambers by AFC processes to give rise to the different phases. McLeod (1990) proposed that highly evolved granites like the MPG may be derived from partial melting of a F-rich source (crust) at depth, which subsequently fractionated repeatedly in a higher level subvolcanic chamber. In that model, the origin of the MBG and its relation to the Mount Pleasant granites were not discussed.
Metal behaviour during fractional crystallization
In terms of the Rayleigh fractionation law, the partition coefficient D of a hygromagmatophile element such as U can be assumed to be zero in order to evaluate geochemical behaviour of the other elements in a flactionating magmatic system. An equation for this relation is deduced based on this assumption (Allegre et al. 1977):
ln [C.sub.L] = ln[C.sub.o] + (1 - D)ln([C.sup.*.sub.L]/[C.sup.*.sub.o])
where [C.sub.L] and [C.sub.o] denote the concentration of an element of interest in residual melt and parental melt, respectively, D is the bulk partition coefficient of the element between the crystallizing mineral assemblage and the residual melt, [C.sup.*.sub.L] and [C.sup.*.sub.o] represent the content of a hygromagmatophile element in the residual and parental melts, respectively, whose [D.sup.*] is supposed to equal to zero. Therefore, plotting ln [C.sub.L] against ln[C.sup.*.sub.L] gives a straight line, given D remains constant during fractional crystallization (Allegre et al. 1977; Christiansen et al. 1983; Fowler and Jensen 1989). Using least-square regression, the slope (m) of the regressed line can be obtained (Fig. 12). Hence, the bulk partition coefficient (D) can be determined for that element (i.e. m = 1 - D).
The D calculation results indicate that Au (1.14) together with Sr (1.81), Ba (2.73), Hf(1.27), Zr (1.53), La (1.20), Ce (1.12), Pr (1.31), Eu (3.29), Sc (1.65), Zn (1.21), and Pb (1.24) were compatible elements during magmatic evolution, suggesting that there is less chance to find Au mineralization in the evolved MPG. Thus, it is better to look for gold within and/or around the early MBG. Based on their D values, Rb (0.34), Cs (0.80), Ga (0.70), Ta (0.16), Nb (0.22), Th (0.56), Y (0.22), Sm (0.83), Gd (0.6), Yb (0.21), Lu (0.23), Sn (-0.18), W (0.96), Cu (0.52), and Mo (0.60) behaved as incompatible elements during magmatic evolution.
The technique of Allegre et al. (1977) applied to granitoid systems, such as those in the Mount Pleasant Caldera, may not be interpreted easily. Firstly, the mobility of U in such subvolcanic systems must be considered during late- to post-magmatic hydrothermal alteration processes, although a large proportion of U is hosted in zircon in the MBG and MPG. Incompatibility of Th in a granitoid system is fairly similar to that of U, but Th is considered to be immobile during alteration processes (see Fowler and Jensen 1989; Jenner 1996). Therefore, Th is a better fractionation index than U. However, using Th as the fractionation index to conduct the same modeling as U described above gave the same tendency of compatibility for all elements listed above. Secondly, the fact that some elements such as Sn, Sb, Cu, and Au (see Jenner 1996) would partition into volatile phases during crystallization of the magma, must be also considered in the trace-element simulation. This consideration suggests that a significant fraction of the metals enters volatile phases that reduce their abundance in the degassed magmas. Thus, compatibility of these elements modeled in this empirical fashion (Allegre et al. 1977) may not really reflect their behaviour in the crystal-melt system alone. It is noticeable that the bulk partition coefficient of Sn is negative (-0.18). This number itself is meaningless, although it is smaller than 1. Thirdly, alteration may elevate or decrease some elements in altered granitoids, which also influences the modeling results. All these effects must be carefully evaluated before a conclusive determination of D for a metal during evolution of a granitic magma system as described here can be achieved. Then, these data for estimated metal bulk partition coefficients can be applied to assess the mineralization potential.
It is notable that the MPG has very low K/Rb ratios (26-56), requiring fluid involvement in formation of the rocks (Shaw 1970; Dostal and Chatterjee 1995, 2000). Conversely, the MBG has high K/Rb ratios (172-376), which are typical for magmatic processes. Nb/Ta ratios are lower for the MPG (2.7-7.6) compared to the MBG (7.7-28.6), also requiring fluid involvement in the late-stage granites. Intriguingly, the MPG has relatively constant Zr/Hf ratios (12.8-17.1), whereas the MBG shows remarkably higher Zr/Hf ratios (24.3-48.0), suggesting that Zr/Hf ratios in the residual melts were controlled mainly by fractional crystallization of zircon (Linnen and Keppler 2002), or by fluid fractionation (Dostal and Chatterjee 1995, 2000). The Th/U ratios are lower in the MPG (1.1-1.8) and relatively high in the MBG (3.0-7.1), suggesting that U increases more rapidly than Th, consistent with the above estimation of the bulk partition coefficient (D). In other words, U was more incompatible than Th in the magma system. It is known that Nb and Ta are strongly affected by the presence of volatiles and can be considered with Li, B, F, Rb, and Sn as volatile-associated elements (Manning and Hill 1990). In contrast, Zr and Hf are considered to be more refractory, in view of the low solubility of their accessory minerals in peraluminous granitic melts (Watson 1979; Watson and Harrison 1983). P, Ti, Sr, and Ba mainly behave as compatible elements in the granitic system.
Estimation of liquidus temperatures
Liquidus temperatures of granitic rocks can be estimated from accessory mineral saturation temperatures (Watson and Harrison 1983; Harrison and Watson 1984; Montel 1993). These temperatures are capable of reflecting the mechanism of magmatic evolution. In a fractional crystallization sequence, the liquidus temperatures of successive intrusive phases in a granitoid complex decrease from early to late stage crystallization. On the other hand, the liquidus temperatures of a second stage melt would be higher than a first-stage melt during partial melting, but once formed both evolve by FC related to decreasing temperatures.
Monazite-saturation geothermometry (Montel 1993) yields temperatures as high as 825-950[degrees]C for the MPG, similar to those of the MBG (834-950[degrees]C). Abnormally high estimated temperatures for the MPG, assuming it formed through fractional crystallization discussed above, are attributed to LREE contents (Table 1 and Fig. 7) similar to those of the MBG. Possibly other unidentified minerals host a significant part of the LREE in the bulk rock. If this portion of LREE were removed from the calculation, then the estimated temperatures would have been lowered in terms of the monazite-saturation geothermometry. However, zircon-saturation geothermometry (Watson and Harrison 1983) produces different results for the MPG (725-789[degrees]C) and MBG (769-933[degrees]C). Apatite-saturation geothermometry (Harrison and Watson 1984) gives results similar to those from zircon-saturation geothermometry for the MPG (712-791[degrees]C) and MBG (833-963[degrees]C). Decrease of the estimated liquidus temperatures (Fig. 15) from the MBG to MPG is consistent with dropping temperatures in a fractionating magma sequence.
High F concentration (3500-9900 ppm,) together with high Li (up to 610 ppm) (see Sinclair and Kooiman 1990) and high alkalis in the water-saturated MPG could remarkably reduce solidus temperatures to as low as 525[degrees]C, as shown by experimental work on F-rich (1.2 wt.%) topaz-bearing granitic melts (Webster et al. 1987). The large temperature range for crystallization (950-525[degrees]C) allows protracted extreme fractionation of granitic melts such as the MPG, resulting in unusual enrichment of incompatible elements such as F, Li, Rb, Cs, Y, Nb, Ta, Ga, Sn, and W (Fig. 14).
Discrimination of tectonic setting
The geochemical features of granitoids may reflect their tectonic environment to some extent (Pearce et al. 1984). Various geochemical discrimination diagrams for tectono-magmatic classification have been proposed to distinguish tectonic settings of magmatic rocks in terms of their chemistry (see Rollinson 1993).
Geochemical discrimination diagrams suggest that the MBG could have formed in volcanic-arc to within-plate tectonic settings, whereas the MPG samples plot entirely in the within-plate field (Figs. 11a and 11b). Figure 11d yields a similar result. As shown above, extensive fractionation plus fluid involvement have strongly enriched high field strength elements, which may shift the MPG samples to the within-plate field (Figs. 11b & d). Supracrustal contamination also affects the discriminant result (Fig. 11a). Therefore, caution must be exercised when using these diagrams to discriminate tectonic setting. These classification diagrams are suspect (cf. Barker et al. 1992), because they characterize the tectonomagmatic environment under which the granite protolith formed, not necessarily the environment prevailing during granite emplacement (Whalen et al. 1996; Christiansen and Keith 1996). Thus, the trace element signatures of granitoids directly reflect their protolith, melting, and crystallization histories, rather than their tectonic settings. Granitoids from the Mount Pleasant Caldera appear to be crustal A2-type (Fig. 11c), inferred to have been derived from continental crust or underplated crust that has been through a cycle of continent-continent collision or island-arc magmatism (Eby 1990, 1992). Particularly, the MPG is similar in geochemistry to topaz rhyolite (Christiansen et al. 1983, 1984; Christiansen et al. 1986), ongonite, and topaz granite (Kontak 1990; Taylor 1992; Dostal and Chatterjee 1995; Zhu etal. 2001), and also to the Late Devonian Mount Douglas Granite in the eastern part of the Saint George Batholith (Lentz et al. 1988; McLeod et al. 1988; McLeod 1990).
During Late Devonian, the studied region was in the early stages of basin extension (McCutcheon et al. 1990a) followed the Acadian Orogeny, consistent with the discriminant results in terms of Fig. 6d and the muhicationic diagram of Batchelor and Bowden (1985) (not shown).
The highly evolved MPG is associated with granophile element mineralization and characterized by high fluorine, lithium (see Sinclair and Kooiman 1990), and alkali concentrations, which enhance the low-T fractionation. The presence of Li, B, and F have a significant effect oil phase equilibria in the water-saturated granite system (Q-Ab-Or-[H.sub.2]O), reducing liquidus and solidus temperatures and affecting minimum melt compositions (Manning 1981; Webster et al. 1987; Manning and Hill 1990) and decreasing viscosity of the melt (Baker and Vaillancourt 1995), thus promoting the duration of crystallization to low temperatures. On the basis of F alone, the shift in minimum position toward the Ab apex in the Q-Ab-Or system has been taken as evidence for origin of topaz granite, and other F-rich granitic rocks, through fractional crystallization of biotite granite magma (Manning 1982). F enrichment generally leads to a concomitant enrichment in Sn, Nb, and Ta, which may possibly be directly complexed with F in the melt (Manning and Hill 1990; Dostal and Chatterjee 1995). A strongly depolymerized F-rich melt is more capable of hosting incompatible elements relative to polymerized volatile-poor granite (Watson 1979; Watson and Harrison 1983; Linnen and Keppler 1997), irrespective of complexing with F (Manning and Hill 1990). Based on the estimated D value of Sn, the late GRIII appears to have high potential for producing Sn mineralization.
Geochemically, the less differentiated MBG is broadly comparable to the granitoids associated with intrusion-related gold systems elsewhere (McCoy et al. 1997; Thompson et al. 1999; Lang et al. 2000; Lang and Baker 2001; Yang et al. 2002a, b). Granitoid intrusions related to gold mineralization are mostly metaluminous, subalkalic (calc-alkalic) to weakly alkalic with intermediate to felsic compositions, forming simple to multiphase complexes (Lang et al. 2000). Locally, peraluminous intrusions are present, but they are ascribed to late-stage fractionation rather than different magmas with distinctive sources (Duncan et al. 1998). According to the summary of Lang et al. (2000), the main-stage, subalkalic (calc-alkalic) intrusions have silica contents between 62 and 72 wt. % Si[O.sub.2], and late differentiates reach 78 wt. % Si[O.sub.2]. REE patterns display LREE enrichment relative to HREE, with La and Yb concentrations about 100 and 10 times chondrite values, respectively. Eu anomalies are absent to slightly negative. Most intrusions are characterized as reduced I-type granitoids.
Most of the analyzed samples from the MGB and MPG (Table 1) have low Au contents, although the MBG samples appear to contain higher gold (up to 8 ppb). Compared to continental crust (2.5 ppb) and normal sedimentary rocks, such as greywacke (4.8 ppb) and shale (3 ppb) (see Lentz 2003), Au does not appear to be elevated significantly in the granitoids from the Mount Pleasant Caldera. Regionally, the granitoids in the area also contain relatively low Au compared to continental crust (McLeod 1990; Whalen 1993). Available studies of other intrusion-related Au deposits have shown that the content of Au in unaltered granitoids is not significantly different from those of barren granitoid rocks (McCoy et al. 1997). These data suggest that background Au abundance in the granitoids is not an essential parameter controlling associated gold mineralization. This observation is in accord with the conclusion by Tilling et al. (1973) that it is impossible to relate gold abundance in igneous rocks and gold mineralization. The geochemical behaviour of gold during the magmatic-hydro-thermal evolution of granitoid systems may be a key factor responsible for gold mineralization, which is controlled by the physiochemical conditions and composition of the magmas. Local geological setting (e.g. structure and lithology) and interaction between the intrusions and country rocks affect the magmatic-hydrothermal systems, which ultimately places controls on the emplacement of gold deposits.
Gold is not expected to concentrate in the residual melt during protracted fractionation processes, because it occurs as a compatible metal. Gold is preferentially partitioned into sulphides in equilibrium with a reduced magma, based on empirical observation (McCoy et al. 1997) and experimental studies (Jugo et al. 1999; Kesler et al. 2002). However, early crystallization of magnetite did not appear to severely influence the ability of a calc-alkaline granitic magma to yield a gold-rich ore fluid, according to experimental study of Au partitioning between magnetite and granitic magma (Simon et al. 2003). Consequently, any sulphide saturation in magma systems must greatly deplete Au in the silicate melts. Inverno (1991) reported much higher Au values in his granite samples from the Mount Pleasant Mine, e.g., Au in the GRII ranging from 8 to 85 ppb (average = 28.7 [+ or -] 2.6 ppb, n = 6), and the GRIII ranging from < 5 to 27 ppb (average = 13.6 [+ or -] 2.5 ppb, n = 6) (see his appendix 7). These high Au contents may be ascribed to hydrothermal alteration, although the Au concentrations decrease from the GRII to GRIII. Our greisenized granite samples taken from True Hill also show higher Au contents (up to 100 ppb), suggesting that late-stage hydrothermal fluids could scavenge gold that is incorporated in early immiscible sulphides in granitoids (Yang and Lentz 2003). At the present stage, it is not easy to evaluate timing of sulphide saturation in felsic magmatic systems, in particular in granite intrusions that were usually influenced by subsolidus processes. These processes may alter the primary sulphides on one hand, and form hydrothermal sulphides on the other. To date, few convincing methods exist for distinguishing primary and hydrothermal sulphides in granitoids, although studies on petrography, chemical, and sulphur isotopic compositions of the sulphides could provide some clues as to their origin. More work needs to be done to better understand the origin of intrusion-related Au ore deposits.
Based on the present study, it is suggested that subvolcanic granitoid rocks in the Late Devonian Mount Pleasant Caldera were formed by extreme fractional crystallization of magmas that may have been derived from recycled crustal materials, although the early MBG may have been influenced by local contamination of wall-rocks through selective assimilation. The fractional crystallization is interpreted to have taken place mainly at depth, as indicated by two separate compositional trends for the MBG and MPG. Extensive liquid fractionation may have played an important role in magmatic-hydrothermal evolution, especially facilitating enrichment of high field strength elements, such as Nb, Ta, Y, HREE, Sn, and W, as well as large ion lithophile elements (e.g. Li, Cs, Rb) in late-stage melts (i.e. the MPG). High volatile (i.e. F, Li and H20) concentrations in the residual melts enhanced the protracted low-T fractional crystallization. Metal elements behaved distinctively during magmatic evolution. Gold occurred as a compatible element, suggesting that gold and metal mineralization are associated with the early granitoid phases such as the MBG, whereas tin behaved as a very incompatible element so that highly evolved granite phases such as the MPG are more likely to generate tin and related mineralization. Hence, it is suggested that exploration for gold related to granitoid complexes such as those in the Mount Pleasant Caldera should be concentrated around the early phase granitoids. In contrast, the late, highly evolved granite may be a good target for exploration for Sn-W-Mo-Bi deposits. It is noteworthy that the youngest phase, GRIII, of the MPG seems to have highest potential for forming Sn deposits.
Some questions remain to be answered, in particular the relationship of the granitoids in the Mount Pleasant Caldera to those in the Saint George Batholith and the Pomeroy Intrusive Suite (McLeod 1990). More detailed petrochemical investigations and reliable age determinations (e.g. U-Pb zircon or monazite) are required to characterize the petrogenetic linkages among these granitoids, although they generally have been thought to be related temporally, spatially, and thus genetically (Lentz et al. 1988; Lentz and McAllister 1990; McLeod 1990).
Table 1. Analyses of rocks from the Mount Pleasant Granitic Suite, Mount Pleasant Caldera, southwestern New Brunswick Analytical PRL95-1- PRL95-2- Sample method 1771 2019 Rocks GRII GRII Major elements (wt.%) Si[O.sub.2] 1 74.7 72.9 Ti[O.sub.2] 1 0.03 0.03 [Al.sub.2][O.sub.3] 1 12.60 12.69 [Fe.sub.2][O.sub.3.sup.*] 1 1.45 1.61 MnO 1 0.04 0.18 MgO 1 0.08 0.39 CaO 1 0.79 1.88 [Na.sub.2]O 1 3.46 1.96 [K.sub.2]O 1 4.66 3.27 [P.sub.2][O.sub.5] 1 < 0.025 < 0.026 LOI 0.91 3.15 Total 98.7 98.2 A/CNK 1.04 1.25 [sigma] 2.1 0.9 [tau] 316 352 Trace elements (ppm), except for Au (ppb) S 2 98 171 Cl 2 241 112 Rb 3 681.5 964.2 Cs 3 7.8 27.8 Ba 1 16.2 10.3 Sr 1 16.5 29.5 Ga 2 33 31 Ta 3 22.2 13.7 Nb 3 106.5 77.7 Hf 3 10.2 8.7 Zr 3 139.4 121.9 Y 3 178.6 153.9 Th 3 49.9 48.2 U 3 31.5 30.3 La 3 41.2 40.0 Ce 3 111.7 108.6 Pr 3 13.2 12.8 Nd 3 44.5 45.1 Sm 3 11.9 11.6 Eu 3 0.02 0.02 Gd 3 12.1 11.0 Tb 3 2.4 2.1 Dy 3 19.0 16.4 Ho 3 4.1 3.7 Er 3 14.2 13.5 Tm 3 2.6 2.5 Yb 3 18.8 19.1 Lu 3 2.9 3.1 Cr 4 < 5 < 5 Ni 2 8 11 Co 4 < 1 < 1 Sc 1 2.76 2.71 V 1 < 10.1 < 10.5 Cu 1 12.2 15.7 Pb 3 21.8 47.1 Zn 1 134.2 114.6 Bi 3 2.8 2.3 Sn 3 27.6 60.8 W 4 5 24 Mo 4 8 < 1 Au 4 < 2 < 2 Ag 4 < 5 < 5 As 4 2.7 9.7 Se 4 < 3 < 3 Sb 4 0.3 0.5 PRL95-2- E8D1- NMP89-1- Sample 1962 1182 1759 SY87-15 Rocks GRII GRII GRII GRII Major elements (wt.%) Si[O.sub.2] 72.4 76.5 73.1 74.9 Ti[O.sub.2] 0.03 0.02 0.03 0.05 [Al.sub.2][O.sub.3] 12.06 11.77 12.33 13.10 [Fe.sub.2][O.sub.3.sup.*] 2.56 2.13 2.07 1.23 MnO 0.17 0.09 0.05 0.06 MgO 0.26 0.18 0.13 < 0.01 CaO 1.47 1.20 0.78 0.43 [Na.sub.2]O 2.22 2.48 2.72 4.00 [K.sub.2]O 3.72 4.60 4.97 4.63 [P.sub.2][O.sub.5] < 0.027 < 0.026 < 0.026 < 0.01 LOI 2.12 1.23 1.03 0.40 Total 97.1 100.3 97.3 98.7 A/CNK 1.17 1.05 1.09 1.06 [sigma] 1.2 1.5 2.0 2.3 [tau] 337 404 324 182 Trace elements (ppm), except for Au (ppb) S 284 131 202 Cl 44 57 78 Rb 761.7 733.5 688.0 1054.0 Cs 11.3 5.8 5.7 27.9 Ba 17.1 81.3 42.4 17.0 Sr 29.9 22.9 31.5 7.0 Ga 35 30 33 31 Ta 14.6 26.3 12.9 4.7 Nb 97.2 96.8 76.9 74.0 Hf 8.8 8.0 9.0 5.9 Zr 127.5 105.4 122.9 108.0 Y 205.1 265.2 160.6 125.0 Th 53.3 45.3 46.8 30.1 U 31.4 29.6 28.4 40.1 La 44.4 107.2 40.3 39.5 Ce 116.0 294.7 104.9 95.7 Pr 13.9 33.6 12.8 12.1 Nd 49.8 112.7 45.8 40.6 Sm 13.4 27.0 12.2 10.4 Eu 0.02 0.1 0.02 0.0 Gd 14.2 22.9 12.6 10.0 Tb 2.9 4.5 2.5 2.2 Dy 23.0 34.4 19.4 16.2 Ho 5.1 7.4 4.2 3.7 Er 17.5 25.3 14.6 13.3 Tm 3.0 4.8 2.6 2.5 Yb 20.9 35.7 18.9 19.2 Lu 3.2 5.5 2.9 3 Cr 11 < 5 < 5 19.6 Ni 8 10 10 7.3 Co < 1 < 1 < 1 Sc 2.82 3.04 2.96 V < 10.7 < 10.3 < 10.3 < 1 Cu 19.4 23.1 10.9 < 1 Pb 27.1 59.2 43.8 49.0 Zn 121.5 104.0 396.0 22.0 Bi 15.5 21.7 3.1 Sn 20.0 15.1 84.4 W 5 31 13 Mo 8 35 37 Au 4 7 < 2 Ag < 5 < 5 < 5 As 12.9 42.9 8.7 Se < 3 < 3 < 3 Sb 0.8 0.3 < 0.1 AM96-3 PRL95- NMP89-1- NMP90-1- Sample 1223 1-1914 1849 1861 Rocks GRIII GRIII GRIII GRIII Major elements (wt.%) Si[O.sub.2] 74.9 7.5 74.5 74.7 Ti[O.sub.2] 0.02 0.02 0.02 0.05 [Al.sub.2][O.sub.3] 12.15 13.28 12.60 12.49 [Fe.sub.2][O.sub.3.sup.*] 1.31 1.55 1.85 1.33 MnO 0.08 0.10 0.06 0.07 MgO 0.04 0.22 0.05 0.11 CaO 0.36 1.08 0.74 0.79 [Na.sub.2]O 5.76 3.17 3.81 3.40 [K.sub.2]O 4.60 4.26 3.48 4.91 [P.sub.2][O.sub.5] < 0.028 < 0.026 < 0.026 < 0.027 LOI 0.58 1.57 1.05 0.92 Total 97.8 100.8 98.2 98.8 A/CNK 1.03 1.13 1.11 1.01 [sigma] 2.2 1.7 1.7 2.2 [tau] 371 449 388 198 Trace elements (ppm), except for Au (ppb) S 51 85 52 69 Cl 258 65 84 75 Rb 932.6 813.7 10.5 785.4 Cs 20.5 10.0 17.4 12.5 Ba 22.0 58.5 13.8 24.2 Sr 8.3 30.9 33.1 25.8 Ga 33 36 36 29 Ta 1.5 24.1 16.5 12.5 Nb 64.5 84.4 86.3 94.6 Hf 6.1 9.4 7.7 8.8 Zr 85.0 122.1 102.9 176.4 Y 132.2 139.8 159.8 164.8 Th 32.7 40.5 45.9 54.1 U 20.3 28.7 43.4 30.2 La 34.2 41.4 48.8 38.5 Ce 91.1 111.2 131.1 100.5 Pr 10.8 13.4 15.5 11.7 Nd 37.2 44.8 52.6 41.0 Sm 9.7 11.5 13.0 10.9 Eu 0.01 0.01 0.01 0.05 Gd 9.5 10.9 12.3 11.3 Tb 1.9 2.2 2.5 2.3 Dy 14.8 17.6 20.2 17.9 Ho 3.2 3.8 4.4 3.9 Er 11.2 13.4 15.3 14.1 Tm 2.1 2.5 2.8 2.5 Yb 15.2 18.9 20.6 17.9 Lu 2.4 2.9 3.1 2.8 Cr < 5 < 5 < 5 < 5 Ni 16 8 9 6 Co < 1 < 1 < 1 < 1 Sc 1.99 2.45 2.32 1.81 V < 11.2 < 10.3 < 10.6 < 10.7 Cu < 5.6 24.8 55.8 10.2 Pb 24.3 23.5 29.5 56.6 Zn 70.3 88.4 87.8 99.5 Bi 0.1 0.7 0.9 0.4 Sn 43.1 18.8 145.2 31.7 W 13 11 7 10 Mo < 1 9 14 10 Au < 2 < 2 < 2 < 2 Ag < 5 < 5 < 5 < 5 As 8.7 3.5 1.0 2.5 Se < 3 < 3 < 3 < 3 Sb 0.8 0.6 0.4 0.2 LNZ18- Analy. 30A2 Sample 1843 MP-05 meth. MBGI Rocks GRIII GRIII Major elements (wt.%) Si[O.sub.2] 74.3 76.0 2 77.5 Ti[O.sub.2] 0.05 0.03 2 0.15 [Al.sub.2][O.sub.3] 12.11 13.12 2 12.0 [Fe.sub.2][O.sub.3.sup.*] 2.40 1.44 2 1.022 MnO 0.06 0.07 2 0.02 MgO 0.02 0.05 2 0.28 CaO 0.65 0.36 2 0.13 [Na.sub.2]O 3.59 4.08 2 1.77 [K.sub.2]O 4.34 4.38 2 6.10 [P.sub.2][O.sub.5] < 0.027 0.00 2 0.02 LOI 0.57 1.16 Total 98.4 99.5 100.2 A/CNK 1.03 1.08 1.23 [sigma] 2.0 2.2 1.8 [tau] 183 301 68 Trace elements (ppm), except for Au (ppb) S 25 Cl 58 Rb 733.0 1210.0 Cs 5.7 21.0 4 276 Ba 16.5 124.0 4 7 Sr 19.9 < 5 4 < 50 Ga 26 2 70 Ta 16.4 19.0 Nb 85.5 51.0 4 < 0.5 Hf 7.6 8.9 2 10 Zr 129.2 114.0 4 7 Y 161.6 157.0 2 170 Th 47.4 47.0 2 30 U 29.1 30.0 4 27.6 La 40.1 50.0 4 3.9 Ce 105.8 126.0 4 39.4 Pr 12.6 17.0 4 173 Nd 44.0 53.0 Sm 11.2 15.0 4 32 Eu 0.08 < 0.05 4 6 Gd 11.4 15.0 4 < 0.2 Tb 23.0 3.1 Dy 18.5 23.0 4 0.8 Ho 4.1 5.2 Er 14.5 18.0 Tm 2.6 3.4 Yb 19.0 25.0 Lu 3.0 3.7 4 4.9 Cr 21.6 4 0.75 Ni 9 4 < 5 Co < 1 2 32 Sc 2.74 2.60 4 2 V < 10.6 4 5.3 Cu 326.9 3.0 2 10 Pb 41.7 Zn 104.8 101.0 Bi 3.5 4 < 50 Sn 115.7 0.2 W 7 7 4 < 0.02 Mo 16 10 4 < 1 Au < 2 4 5 Ag < 5 4 < 2 As 9.1 4 < 5 Se < 3 4 39.7 Sb < 0.1 0.2 4 < 3 4 1 Sample 33A2 82C3 121A 121C Rocks MBGI MBG1 MBG1 MBG1 Major elements (wt.%) Si[O.sub.2] 74.4 68.7 69.9 68.0 Ti[O.sub.2] 0.23 0.53 0.43 0.54 [Al.sub.2][O.sub.3] 12.9 15.9 14.0 14.7 [Fe.sub.2][O.sub.3.sup.*] 1.556 2.666 2.409 3.045 MnO 0.02 0.02 0.07 0.06 MgO 0.26 0.59 0.64 0.92 CaO 0.19 0.69 1.59 0.55 [Na.sub.2]O 2.66 3.05 3.63 3.78 [K.sub.2]O 5.97 5.81 4.98 5.82 [P.sub.2][O.sub.5] 0.03 0.13 0.11 0.15 LOI 1.31 1.85 1.31 1.62 Total 99.5 99.9 99.1 99.2 A/CNK 1.15 1.27 0.98 1.09 [sigma] 2.4 3.1 2.8 3.7 [tau] 45 24 24 20 Trace elements (ppm), except for Au (ppb) S Cl Rb 233 281 198 200 Cs 10 17 6 7 Ba 340 1400 1300 1600 Sr 50 130 190 140 Ga Ta < 0.5 < 0.5 < 0.5 < 0.5 Nb 30 20 10 10 Hf 11 12 10 11 Zr 350 480 390 460 Y 10 60 10 50 Th 21.9 16.9 15 16.5 U 3.1 3.7 4.5 3.2 La 50.8 112 61.9 70.8 Ce 162 167 130 156 Pr Nd 42 60 48 57 Sm 7.1 11.7 9.2 10.6 Eu 0.9 2.6 2.3 2.5 Gd Tb 0.7 1.3 1 < 0.5 Dy Ho Er Tm Yb 3.4 4.8 3.6 4.1 Lu 0.52 0.73 0.55 0.6 Cr < 5 < 5 < 5 < 5 Ni 7 15 7 7 Co 2 < 1 5 5 Sc 6.5 8.4 7.8 9.4 V 12 20 < 10 < 10 Cu Pb Zn < 50 < 50 82 < 50 Bi Sn < 0.02 < 0.02 < 0.01 < 0.02 W < 1 < 1 < 1 < 1 Mo 18 < 1 < 1 < 1 Au < 2 < 2 < 2 < 2 Ag < 5 < 5 < 5 < 5 As 20 22.1 7.4 6.8 Se < 3 < 3 < 3 < 3 Sb 1.9 1 1.3 0.5 Sample 193A 193B 246B 270 Rocks MBG1 MBG1 MBG1 MBG1 Major elements (wt.%) Si[O.sub.2] 76.2 66.4 69.7 68.6 Ti[O.sub.2] 0.15 0.77 0.51 0.46 [Al.sub.2][O.sub.3] 11.6 13.8 15.5 14.7 [Fe.sub.2][O.sub.3.sup.*] 1.471 5.126 1.521 2.678 MnO 0.01 0.14 0.01 0.06 MgO 0.39 0.86 0.14 0.67 CaO 0.13 0.84 0.34 2.23 [Na.sub.2]O 0.59 1.44 3.86 3.79 [K.sub.2]O 8.41 7.56 5.30 5.30 [P.sub.2][O.sub.5] 0.03 0.24 0.11 0.12 LOI 0.85 2.00 1.70 1.08 Total 99.8 99.2 98.7 99.7 A/CNK 1.13 1.14 1.22 0.92 [sigma] 2.4 3.5 3.1 3.2 [tau] 73 16 23 24 Trace elements (ppm), except for Au (ppb) S Cl Rb 313 334 117 172 Cs 12 15 7 5 Ba 460 1100 1400 1400 Sr 10 50 180 250 Ga Ta < 0.5 < 0.5 < 0.5 2.5 Nb 20 20 40 30 Hf 7 12 12 11 Zr 220 0.50 460 480 Y 30 40 50 30 Th 18 14.4 14.3 14.2 U 4.8 5.1 5.4 3.4 La 70.1 29.9 38.7 70.3 Ce 136 165 120 147 Pr Nd 64 22 31 62 Sm 10.3 6.9 7.6 10.3 Eu 0.7 1.7 2.1 2.6 Gd Tb 0.7 < 0.5 < 0.5 1 Dy Ho Er Tm Yb 3.9 5.8 3.8 3.9 Lu 0.59 0.86 0.56 0.6 Cr < 5 78 120 5 Ni 3 4 3 5 Co < 1 5 3 2 Sc 3.7 14.9 13 8.2 V < 10 30 34 < 10 Cu Pb Zn 56 113 < 50 < 50 Bi Sn < 0.01 < 0.02 < 0.01 < 0.01 W 9 16 < 1 < 1 Mo < 1 7 4 4 Au < 2 < 2 < 2 < 2 Ag < 5 < 5 < 5 < 5 As 22.7 23.1 10.1 4.4 Se < 3 < 3 < 3 < 3 Sb 4.3 5.9 0.6 0.6 Sample 289 301 29A1 29A2 Rocks MBG1 MBG1 MBG2 MBG2 Major elements (wt.%) Si[O.sub.2] 67.8 66.7 70.9 64.6 Ti[O.sub.2] 0.51 0.67 0.36 0.78 [Al.sub.2][O.sub.3] 14.6 15.6 14.4 14.1 [Fe.sub.2][O.sub.3.sup.*] 3.442 3.95 2.894 5.859 MnO 0.19 0.11 0.05 0.11 MgO 0.85 0.73 0.46 0.95 CaO 0.72 1.35 0.33 2.73 [Na.sub.2]O 2.50 4.08 2.55 3.39 [K.sub.2]O 6.84 5.06 5.99 4.17 [P.sub.2][O.sub.5] 0.13 0.19 0.08 0.26 LOI 1.54 1.39 1.93 2.31 Total 99.1 99.8 99.9 99.0 A/CNK 1.14 1.07 1.28 0.94 [sigma] 3.5 3.5 2.6 2.6 [tau] 24 17 33 14 Trace elements (ppm), except for Au (ppb) S Cl Rb 323 184 145 100 Cs 10 6 8 9 Ba 1950 1700 900 900 Sr 160 220 90 200 Ga Ta 2.5 < 0.5 < 0.5 < 0.5 Nb 5 5 20 30 Hf 12 11 10 12 Zr 520 500 480 520 Y 50 40 30 80 Th 14.8 15.2 12 15.7 U 4.6 3.9 2.5 4.7 La 62.1 80.2 62.8 77.2 Ce 142 169 143 171 Pr Nd 44 62 54 67 Sm 9.5 11.8 9.8 13.8 Eu 2.6 3.2 1.7 3.4 Gd Tb 0.9 < 0.5 1.3 1.4 Dy Ho Er Tm Yb 4.2 4.6 4.3 6.4 Lu 0.65 0.7 0.65 96 Cr < 5 < 5 < 5 < 5 Ni 5 6 5 6 Co 4 5 4 9 Sc 9.4 11.0 9.2 18.2 V < 10 < 11 < 10 < 10 Cu Pb Zn 352 165 121 215 Bi Sn < 0.01 < 0.01 < 0.01 < 0.02 W < 1 < 1 < 1 < 1 Mo 7 < 1 < 1 < 1 Au < 2 < 2 < 2 < 2 Ag < 5 < 5 < 5 < 5 As 22.3 8.2 6.7 10.5 Se < 3 < 3 < 3 < 3 Sb 1.0 0.5 1.3 1.4 Sample 29B2 62A 110A1 295 Rocks MBG2 MBG2 MBG2 MBG2 Major elements (wt.%) Si[O.sub.2] 68.3 67.4 65.8 64.8 Ti[O.sub.2] 0.37 0.71 0.8 0.73 [Al.sub.2][O.sub.3] 14.2 14.0 14.3 15.7 [Fe.sub.2][O.sub.3.sup.*] 3.255 4.904 5.792 4.152 MnO 0.1 0.11 0.09 0.13 MgO 0.58 0.65 0.76 0.77 CaO 1.37 1.74 2.48 0.78 [Na.sub.2]O 2.61 2.96 3.78 2.26 [K.sub.2]O 6.23 4.93 4.62 7.40 [P.sub.2][O.sub.5] 0.09 0.21 0.24 0.22 LOI 2.23 2.39 1.16 1.70 Total 99.3 100.0 99.8 98.6 A/CNK 1.05 1.05 0.91 1.19 [sigma] 3.1 2.6 3.1 4.3 [tau] 31 16 13 18 Trace elements (ppm), except for Au (ppb) S Cl Rb 207 165 164 525 Cs 10 9 5 9 Ba 1200 990 910 1700 Sr 80 190 210 140 Ga Ta < 0.5 2.6 < 0.5 2.4 Nb 5 20 30 20 Hf 13 13 14 11 Zr 480 540 580 480 Y 50 70 411 30 Th 15.9 18.8 19.6 13 U 4 4 3.9 4.3 La 106 80.9 71.8 73.2 Ce 178 150 150 168 Pr Nd 75 60 56 56 Sm 13.0 12.7 12.1 12.2 Eu 2.9 3.4 3.3 3.3 Gd Tb < 0.5 1.9 1.7 1.6 Dy Ho Er Tm Yb 5.2 6.4 6.2 4.8 Lu 0.8 0.67 0.63 0.73 Cr < 5 < 5 < 5 11 Ni 6 11 2 8 Co 5 6 7 7 Sc 11.5 15.1 16.6 12.4 V < 10 < 10 < 10 < 10 Cu Pb Zn < 50 219 202 106 Bi Sn < 0.03 < 0.01 < 0.01 < 0.01 W < 1 < 1 < 1 5 Mo 3 < 1 < 1 < 1 Au 6 8 < 2 < 2 Ag < 5 < 5 < 5 < 5 As 8.3 7.5 15.5 18.8 Se < 3 < 3 < 3 < 3 Sb 1.9 0.8 0.5 1.3 Sample SY88-55 Rocks Major elements (wt.%) Si[O.sub.2] 64.8 Ti[O.sub.2] 0.59 [Al.sub.2][O.sub.3] 16.2 [Fe.sub.2][O.sub.3.sup.*] 3.879 MnO 0.15 MgO 1.07 CaO 1.88 [Na.sub.2]O 2.40 [K.sub.2]O 6.76 [P.sub.2][O.sub.5] 0.17 LOI 0.24 Total 97.8 A/CNK 1.10 [sigma] 3.8 [tau] 23 Trace elements (ppm), except for Au (ppb) S Cl Rb 320 Cs Ba 1663 Sr 174 Ga 18 Ta 0.5 Nb 14.3 Hf 9.4 Zr 387 Y 25 Th 8.6 U 5.4 La 47.8 Ce 96.9 Pr 11.2 Nd 42.8 Sm 7.29 Eu 2.17 Gd 5.95 Tb 0.8 Dy 4.76 Ho 0.91 Er 2.64 Tm 0.37 Yb 2.42 Lu 0.38 Cr 12.4 Ni 6.8 Co Sc V 37 Cu 10 Pb 63 Zn 187 Bi Sn W Mo Au Ag As Se Sb Notes: Analytical method: 1 = ICP-OES, 2 =XRF, 3 = ICP-MS, 4 = INAA. [Fe.sub.2][O.sub.3.sup.*] total iron oxide; Blank denotes that element was not analyzed. Trace elements are in ppm, except for Au that is in ppb. Symbol "<" indicates element below detection limits. Sample MP-5 is taken from Taylor (1992), containing 5900 ppm F and 499 ppm Li; Sample SY87-15 is taken from Whalen et al. (1996), containing 6182 ppm F and 580 ppm Li; Sample SY88-55 is taken from Whalen et al. (1996), containing 863 ppm F.
The authors thank G. Chi (University of Regina), M.J. McLeod, K. G. Thorne, and L.R. Fyffe (NBDNR) for their thoughtful discussions on the geology and intrusion-related gold systems in southwestern New Brunswick. Thanks are also due to Cliff Shaw for constructive discussion about the physical processes operating in a felsic magma chamber. Reviews of the first draft of the manuscript by M.J. McLeod and Cliff Shaw are greatly acknowledged. We are grateful to G.J.A. Kooiman (Geological Consultant; St. George) for discussion of local geology and help with sampling drill core at the Mount Pleasant Mine. R. Young (Manager of the Mount Pleasant Mine) is thanked for allowing us access to the drill core and geological information on site. S.H. McClenaghan and S.E. Watters are thanked for their help with fieldwork. Critical reviews and constructive suggestions by the journal reviewers Drs. Ken Currie and Dan Kontak significantly improved the manuscript. We are grateful to Prof. Sandra M. Barr for editorial handling of the manuscript. This research is funded by the Geological Survey of Canada--Targeted Geoscience Initiative program (GSC-TGI, 010008) and Natural Sciences and Engineering Research Council of Canada grants to D.R. Lentz, with partial support from the New Brunswick Department of Natural Resources and Energy, New Brunswick Innovation Fund, and the University of New Brunswick.
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Editorial responsibility: Sandra M. Barr
XUE-MING YANG (1), DAVID R. LENTZ (1), STEVE R. MCCUTCHEON (2)
(1.) Department of Geology, University of New Brunswick, P.O. Box 4400, Fredericton, New Brunswick, Canada E3B 5A3 (e-mail: email@example.com)
(2.) New Brunswick Department of Natural Resources, Geological Survey Branch, P.O. Box 50, Bathurst, New Brunswick, Canada E2A 3Z1 (e-mail:firstname.lastname@example.org)
Date received: September 2, 2003 Date accepted: January 20, 2004
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