Field relations. The Cheever Mine is located on a steeply sloping hillside above Lake Champlain 3.2 km north of the village of Port Henry (Figure 4
). The ore strikes approximately north-south and dips moderately to the west. The exposed workings include several inclined trenches along more than 0.5 km of strike. Bedrock in the immediate area consists of rocks of the Grenville Supergroup (marble and pelitic gneiss), coronitic gabbro and derived amphibolite, pink to green granitic rocks (Figure 4
), and magnetite-bearing pegmatites. A historical cross-section [43
] shows the ore and host rock layers dipping and shallowing to the west. However, all the structural measurements taken in this study indicate foliation and layering in the host rocks dip to the east towards the lake, while the ore dips to the west cross-cutting the host rocks.
Three samples of host rock (CVE-1, 2, 3) and the ore (CVE-4) were taken from the hanging wall of an actively worked quarry face (Figure 5
A) used for road metal or fill. Another granitic sample (CV-7) was taken from one of the numerous sheets that cross-cut and inter-finger with gabbroic rock in the footwall of the mine (Figure 4
) and another was taken from a greenish granitic rock (CV-2013) cross-cut by the ore (Figure 5
B) about one hundred meters south of the quarry face depicted. The granitic host rocks sampled show variable degrees of alteration including the albitization of original feldspars, possible introduction or loss of K, Si, Al, and/or Fe, and the replacement of primary minerals by secondary phases, which is noted below. The rocks are similar in all respects to altered ferroan leucogranites associated with other iron mines in the area and share many characteristics with unaltered leucogranites of the LMG throughout the region [40
]. For simplicity’s sake, we will refer to them as altered leucogranites by acknowledging both their protolith and subsequent alteration.
Petrography. The five altered leucogranite samples are composed primarily of albite, quartz, and magnetite (Figure 5
C). Minor amounts of clinopyroxene and mesoperthite are present in some samples. Trace phases, which were identified optically, include zircon, titanite, and monazite (Figure 5
C). Late alteration phases include serpentine after clinopyroxene, hematite staining along grain boundaries, minor calcite, and minor alteration of plagioclase to clay. No evidence of deformation was noted. However, banding defined by magnetite abundance or variation in grain-size is seen in a subset of samples. The sample of ore (CVE-4) is composed predominantly of a granular, polygonal intergrowth of magnetite with minor quartz, apatite, and clinopyroxene. Monazite and zircon are trace phases while plagioclase is lacking in the ore.
Major elements. The results are shown in Table 1
. The SiO2
concentration of the host rocks ranges between 65.95% to 80.28%. The rocks are sodic with Na2
O ranging between 4.80% to 9.03% while K2
O and CaO contents are minimal. Iron oxide contents vary widely with FeOT
ranging between 1.41% to 12.21% while Al2
varies from 10.35% to 14.40%. The ore sample (CVE-4) contains 86.97% FeOT
, 4.93% SiO2
, and 2.90% P2
(all values in wt. %).
Geochronology. The location of the samples used for geochronology are shown in Figure 4
. Scanning electron microscope photographs of representative zircons and corresponding analyses are shown in Figure 6
. The U-Pb zircon geochronology results are summarized in Figure 7
while REE concentrations are summarized in Figure 8
and Table 2
. Supplemental Files S1–S6
provide all data collected for each of the geochronology samples.
Any study of U-Pb zircon systematics in high-grade terranes should be approached with caution due to well-documented issues of Pb-loss, resetting/recrystallization, inheritance, and the limitations (spot size, depth, precision, etc.) of the various techniques employed. This is particularly true in the study area where ore bodies occur along faults and hydrothermal/metasomatic alteration is anticipated or, in this case, demonstrated previously [12
]. We utilized optical, CL, and BSE examination and zircon REE concentrations, in concert with field observations, to evaluate the U-Pb results. None of the samples investigated show evidence of high-grade metamorphic mineral assemblages. However, it is likely that the rocks interacted with metasomatic fluids capable of chemical and isotopic changes. Recrystallization, resetting, growth, and several generations of apatite, monazite, and other U-Th trace minerals have been documented previously in the ores e.g., [44
]. In some samples from the mining district, small fractures and areas of alteration contain secondary minerals (chlorite, actinolite, hematite, calcite, etc.) thought to be associated with lower-temperature events, which occurred as much as 60 million years after intrusion [44
Our approach to interpreting the geochronological results presented in this study is outlined in detail below. While four of the six samples have a single, well-defined population of igneous zircons, two are clearly more complicated. Typical complications include Pb-loss especially in zircons with elevated U concentrations, older 207Pb/206Pb ages, and visible core/rim relations. In this discussion, “rim” and “core” designations are used when optically distinct interior and exterior areas are seen within individual zircon crystals and were verified by targeted spot analyses on the same grain cross-section. Inner versus outer and/or the center versus the edge is used when no significant age differences could be verified within an individual grain with multiple analyses. Such grains are considered “homogeneous” with regards to their U-Pb systematics in the cross-section analyzed and at the scale of the ablation pit.
For each sample, the data was regressed and analyzed in several ways [46
]. These included standard Concordia diagrams displaying all the data collected, weighted average diagrams with data arranged by increasing 207
Pb age, and by increasing concordancy, a weighted average diagram utilizing only those analyses that fall between 97% to 103% concordant and a Concordia age diagram. In addition, individual spots analyzed were plotted on scatter plots showing U/Th versus concordancy, U versus concordancy, U versus U/Th, and concordancy versus 207
Pb. Ablation pits for zircon REE analyses were located directly adjacent to ablation pits used for U-Pb analyses. This approach allowed screening for Pb-loss, alteration, inheritance, concordancy, and homogeneity of the zircon population.
Geochronological constraints suggest that the interval between crystallization of the leucogranite and its alteration is limited, which complicates the interpretation of Pb-loss trajectories that would parallel the Concordia curve [35
]. When they exist, both cores (red) and rims (green) are shown on the plots. To save space, only the Concordia (all data) and final age diagrams (Weighted Average or Concordia Age) are shown in the manuscript. All weighted average diagrams are displayed at the same scale to allow for direct comparison between samples (Figure 7
CVE-1. Sample CVE-1 is a magnetite-rich, altered leucogranite composed of albite, quartz, and magnetite. Trace minerals include zircon and monazite. It is exposed on the face of the quarry where it occurs as a 3-meter thick sheet cut by the ore and in contact with overlying sample CVE-2 and an undated gabbro body. It contains 70.59% SiO2, 10.68% FeOT, 5.77% Na2O, 0.64% K2O, and 0.36% CaO (all in wt. %).
While variable in detail, most zircons from sample CVE-1 are elongated (4:1), have an average length of 300 µm, and have core-rim relations (Figure 6
A–D). The external shape of the zircon ranges from subhedral to slightly rounded or equant. Although quite variable, cores and rims are both enriched in uranium (ca. 1000 pm) and have U/Th ratios averaging ca. 10. Cores appear slightly brighter but patchy in BSE. The rims are slightly darker and more homogeneous in appearance. Zoning and inclusions are more prevalent in core regions.
Thirty-four of 35 analyses passed initial quality screening and 28/34 are 97% to 103% concordant. The analyses are distributed along Concordia between ca. 1030 and 1170 Ma (Figure 7
A). When analyses are separated by their position as “cores” or “rims,” they yield two clusters with the rims centered on 1060 Ma and cores and/or xenocrysts with 207
Pb ages up to 1242 ± 19 Ma (Figure 6
D). A weighted average diagram for all analyses yields an age of 1091 ± 15 (MSWD = 3.4). A weighted average diagram for all rims analyses that are 97% to 103% concordant yields an age of 1062 ± 10 (MSWD = 0.66). A Concordia age from the same subset of zircons is 1059.6 ± 3.4 Ma (MSWD = 0.09, Figure 7
Interpretation. The physical, chemical, and isotopic characteristics of zircons from sample CVE-1 display considerable variability. This variability is likely the result of the incorporation of inherited zircon from the source rock(s). Most, but not all, of the inherited zircon is incorporated into the center of large igneous grains mantled by thick rims while some remain as discrete xenocrysts. The outer portion of many grains (rims) and homogeneous grains, which lack cores, yield an age of 1059.6 ± 3.4 Ma. This is interpreted as the time of crystallization.
CVE-2. Sample CVE-2 is an altered leucogranite composed of albite, quartz, perthite, and magnetite. Trace minerals include zircon, monazite, and titanite. A sharp boundary between a magnetite-rich and magnetite-lean layers, which is apparent in the thin-section, is thought to represent compositional layering. CVE-2 is exposed on the face of the quarry where it occurs as the uppermost sheet in the hanging wall in contact with sample CVE-1. It contains 80.28% SiO2, 1.41% FeOT, 4.80% Na2O, 1.40% K2O, and 0.57% CaO (all in wt. %).
The zircons separated from sample CVE-1 are elongated (4:1), have an average long dimension of 300 µm, and show distinct zoning (Figure 6
E,F). The external shape of the zircon is predominantly euhedral to subhedral. A few small, bright cores are observed in CL, but most grains lack them. In general, interior portions of the zircons are darker than the outermost zones even though variation in lighter and darker zones can be seen on the small-scale. Analyses show consistent U-contents (ca. 190 ppm) and U/Th ratio of ca. 4.0.
Thirty-four of 35 analyses passed initial screening and 28/34 are 97% to 103% concordant. The analyses fall in a tight cluster along Concordia at ca. 1040 Ma (Figure 7
C). A weighted average calculated from all data yields an age of 1041.0 ± 9.5 (MSWD = 0.34). A weighted average for all analyses that are 97% to 103% yields an age of 1041 ± 11 (MSWD = 0.32). A Concordia age from the same subset of zircons is 1036.3 ± 2.9 Ma (MSWD = 0.72, Figure 7
Interpretation. This sample is high in silica but otherwise appears to be one of the least altered by Na-metasomatism based on K2O wt. %. The physical, chemical, and isotopic characteristics of zircons from sample CVE-2 display high similarity and suggest a single, homogeneous population of zircon. The analyses lie along Concordia and form a tight cluster at ca. 1040 Ma. The Concordia age of the zircons yield an age of 1036.3 ± 2.9 Ma, which is interpreted as the time of crystallization of the rock.
CVE-3. Sample CVE-3 is a magnetite-rich, altered leucogranite composed of albite, quartz, clinopyroxene, and magnetite. Trace minerals include monazite, apatite, and zircon. It is exposed on the face of the quarry where it occurs as the lowermost unit in contact with the overlying ore (sample CVE-4). It contains 67.94% SiO2, 12.21% FeOT, 6.38% Na2O, 0.07% K2O, and 0.76% CaO (all in wt. %).
The zircons from sample CVE-3 are homogeneous, elongated (4:1), and have an average long dimension of 300 µm. The external shape of the zircon is euhedral and dipyramidal (Figure 6
G–J). Zircons have relatively high uranium (ca. 1740 pm) concentrations and U/Th ratios of ca. 11.0. Interior regions, some of which contain cores, are generally darker and complex in BSE with brighter outer portions. Some grains show subtle zoning. Interior portions of the grains are often mottled, inclusion-rich, and have rounded to irregular shapes.
Thirty-five of 35 analyses passed initial screening and 7/35 are 97% to 103% concordant. The analyses fall in a loose cluster just below Concordia at ca. 1030 Ma (Figure 7
E). A few analyses have slightly older 207
Pb ages, which are suggestive of inheritance. The weighted average calculated for all the analyses is 1062.1 ± 8.3 Ma (MSWD = 1.5). A weighted average diagram for all zircons between 97% to 103% concordant yields an age of 1040 ± 14 (MSWD = 0.52, Figure 7
F). A Concordia age from the same subset of zircons is 1028.3 ± 4.3 Ma (MSWD = 3.5).
Interpretation. The physical, chemical, and isotopic characteristics of zircons from sample CVE-3 are homogeneous and suggests that the bulk of the zircon population likely formed during a single event. However, the analyses cluster loosely below Concordia. The high U-content likely resulted in Pb-loss, which increases the scatter and complicates interpretation. Inheritance was minimal. The oldest analysis gave a 207Pb/206Pb age of 1120.1 ± 31.7 Ma. The Concordia age of 1028.3 ± 4.3 Ma has a large MSWD (3.5) and the analyses fall below Concordia. The best age for the crystallization of the rock is 1040 ± 14 Ma derived from the weighted average.
CVE-4. Sample CVE-4 is a sample of the ore composed of magnetite with <15% by volume apatite, quartz, and clinopyroxene. Trace minerals include monazite and zircon. It is exposed on the face of the quarry where it occurs as a ca. 0.5 m thick sheet that cuts or is in fault contact with CVE-1 (hanging wall) and CVE-3 (foot wall). It contains 4.95% SiO2, 86.97% FeOT, 0.29% Na2O, 0.07% K2O, 2.65% CaO, and 2.90% P2O5 (all in wt. %).
A small number of zircons were recovered from sample CVE-4. While variable in detail, most zircons from sample CVE-4 are elongated (3:1) in shape, have an average long dimension of 400 µm, and show several very small and distinct cores (Figure 6
K–N). The external shape of the zircon ranges from subhedral to euhedral. Although quite variable, zircons are enriched in uranium (ca. 1000 pm) and have U/Th ratios of ca. 7. Zircons show slight variation in the BSE response, which is often patchy in appearance and subtle, generally wide, zonation.
Thirty-four of 35 analyses passed initial screening and 27/35 are 97% to 103% concordant. Concordant analyses cluster at ca. 1040 Ma with several discordant analyses falling off Concordia (Figure 7
G). The weighted average calculated for all analyses is 1035.6 ± 6.9 (MSWD = 1.14). The weighted average for all the analyses that are 97% to 103% concordant is 1035.2 ± 6.9 (MSWD = 0.72). A Concordia age from the same subset of analyses is 1036.6 ± 2.3 Ma (MSWD = 0.09, Figure 7
Interpretation. The physical, chemical, and isotopic characteristics of zircons from sample CVE-4 are compatible with an igneous origin. A few, small internal areas, which are bright in BSE mode, are likely cores but were not targeted during analysis. The analyses lie along Concordia and form a cluster with some spread near 1040 Ma. The Concordia age 1036.6 ± 2.3 Ma is interpreted as the time of intrusion and crystallization of the ore.
CV-7. Sample CV-7 is a magnetite-rich, altered leucogranite composed of albite, quartz, and magnetite. Trace minerals include zircon, monazite, and apatite. It is one of numerous sheets intruding and interfingering with a large gabbroic body in the footwall of the mine. It contains 65.93% SiO2, 8.68% FeOT, 9.03% Na2O, 0.20% K2O, and 0.56% CaO (all in wt. %).
Zircons from sample CV-7 are elongated (3:1), have an average length of 450 µm, and are very similar in appearance (Figure 6
O–R). Dark metamict cores are observed in a subset of grains. The external shape of the zircon ranges from subhedral to euhedral. Although variable, zircons are moderately enriched in uranium (ca. 665 pm) and have intermediate U/Th ratios (ca. 6). Most grains lack cores and show fine to diffuse zoning. Metamictization and inclusions are prevalent in core regions.
Thirty-three of 35 analyses passed the initial screening and 16/34 are 97% to 103% concordant. The bulk of the analyses fall in a tight cluster just below Concordia at ca. 1050 Ma (Figure 7
I). A weighted average calculated for all analyses is 1068.8 ± 8.5 Ma (MSWD = 1.4). A weighted average calculation for all analyses that are 97% to 103% yields an age of 1051 ± 10 (MSWD = 0.38). A Concordia age from the same subset of zircons is 1043.9 ± 4.1 Ma (MSWD = 2.6, Figure 7
Interpretation. The physical, chemical, and isotopic characteristics of zircons from sample CVE-7 are homogeneous. The analyses lie below Concordia in a tight cluster with a few grain interiors yielding older ages (oldest 1162.4 ± 29.5 Ma), which is presumably the core material. The outer portion of many grains and some grains lacking cores yield a Concordia age of 1043.9 ± 4.1 Ma, which is interpreted as the time of crystallization of the leucogranite.
CV-2013. Sample CV-2013 is an altered leucogranite composed of albite, quartz, clinopyroxene, and magnetite. Trace minerals include zircon and monazite. In contrast to the other granitic samples, it has a greenish color. It is exposed about 100 m south of the face of the quarry where it is crosscut by the ore (Figure 5
Zircons from sample CV-2013 are elongated (4:1), have an average length of 400 µm, and show well-defined core-rim relations (Figure 6
S,T). The external shape of the zircon ranges from subhedral to euhedral. Cores, which dominate the interior of the zircon grains, are rounded, highly metamict, and mottled with numerous inclusions and appear resorbed. Rims are much more homogeneous in appearance, display zoning, and vary in thickness. Some grains have full rims and others are partially overgrown with rim material. Cores and rims are enriched in uranium (1326 to 1107 ppm).
Twenty-three of 25 analyses passed initial screening and 13/23 are 97% to 103% concordant. When analyses are separated by their position within the crystal, they yield two linear groups on the Concordia diagram, which shows a considerable scatter (Figure 7
K). The weighted average age calculated from all rim analyses is 1060.3 ± 6.3 (MSWD = 9.2). A weighted average for all rims that are 97% to 103% concordant yields an age of 1063.5 ± 6.5 (MSWD = 6.0). A Concordia age from the same subset of zircons is 1059.6 ± 3.4 Ma (MSWD = 0.09, Figure 7
Interpretation. The physical, chemical, and isotopic characteristics of zircons from sample CV-2013 display considerable variability between interiors and rims. This variability results from the incorporation of considerable amounts of inherited zircon in the magmatic precursor to this rock. Most, but not all, of the inherited zircon was incorporated into the center of large grains partially mantled by rims of variable thickness. Rims display zoning and appear to overgrow partially resorbed cores. The Concordia age of zircon rims, 1059.6 ± 3.4 Ma, is interpreted as the time of crystallization of the leucogranite.
Rare Earth Elements. The results for zircon REE analysis are shown in Figure 8
and Table 3
. The REE from each analysis are plotted normalized to chondritic values [47
] for comparison purposes. Aside from a few analyses of the lighter REEs in certain samples, all the REEs are found at concentrations exceeding detection limits. In general, patterns are similar with large positive cerium and negative europium anomalies and an upward arching increase from Gd to Lu ranging from 10 to 10,000× chondritic values. A few samples show strong enrichment in the lighter rare earth elements and these are generally samples with high U (>1000 ppm) content in zircon. Only slight differences between rims and cores in individual zircon populations are observed. The grey field on Figure 8
shows the values for zircons from albitites from the Big Sky Orogen [48
Interpretation. The REE patterns and concentrations for individual zircon populations show considerable variability. In zircon with “rim” and “core” targets, patterns are overlapping (CVE-1) or very similar but have differing concentrations (CV-2013). Among the samples analyzed, that of the ore (CVE-4) shows a large range in concentration but nearly identical patterns. Larger concentrations of LREE are seen in discordant zircons and strong correlations exist between U content and REE concentrations of the spot analyses in the ore zircons. Taken together, these characteristics are compatible with an igneous origin that may be slightly modified by metamictization or metasomatic processes.
Zircon Characteristics. Zircons grow in a variety of metamorphic, hydrothermal, and igneous environments and often retain characteristics indicative of the conditions under which they formed. While not always unequivocal, zircon shape, size, internal structure, trace element composition, and included minerals can be used to assess their origin [48
]. Zircon grown during hydrothermal or metasomatic events can have features like those of igneous origin [12
]. However, unless the original zircons in the altered rock are completely replaced, isotopic and optical evidence of the chronology of growth should be retained. Albitites associated with the Big Sky Orogeny show zircon dissolution and precipitation associated with albitization occurring nearly 1.5 billion years after crystallization of the granodioritic host rock [48
]. Rare earth element analyses, isotopic ages, shape, size, relative position, and variation in internal zonation where all used to differentiate between original zircons and metasomatic zircons. Of interest are the glomerocrystic zircon clusters noted during the in-situ analyses of the albitized rock [48
Zircons from albitites associated with a Moroccan gold deposit are generally small (<50 µm) with solid and fluid inclusion-rich zonesand have dissolution-reprecipitation textures and xenotime inclusions or overgrowths [50
]. In addition, they have subdued Ce and Eu anomalies when compared to magmatic zircons. Separation of hydrothermal and magmatic zircons has been proposed using REE scatter plots [51
]. In this study, REE patterns are most similar to those shown for magmatic zircons in other studies [48
]. On scatter plots (La versus (Sm/La)N
versus Ce/Ce*) zircon populations studied here plot in linear arrays below the magmatic, and to the left of the hydrothermal, fields [51
The physical characteristics of zircon in all the samples analyzed in this study are compatible with an igneous origin (Figure 6
). Their size (avg. 300 µm length), shape (dipyramidal, sub- to euhedral), and U-Pb systematics strongly support an igneous origin. Inclusions, metamict cores, core-rim relations, REE patterns, and zoning are also supportive. Several samples show clear evidence of inherited zircon cores and/or xenocrysts upon which younger zircon crystallized. The amount of inherited zircon varies widely between samples from nearly none in the sample CVE-4 from the ore and CVE-2 from a leucogranite, to thin and sometimes discontinuous rims with large cores in samples CV-2013 and CVE-1. Both core-rich samples yielded ages significantly older than the samples, which have few to no inherited zircon. These older ages may reflect the characteristics of early magmatism or, in part, arise from internal complexities in the zircon grains when targeting seemingly homogeneous areas (i.e., cores or rims).
shows is a plot of concordancy versus 207
Pb age for each of the zircon samples analyzed. There is a negative correlation between concordancy versus 207
Pb age in several of the samples investigated including CVE-2, CVE-3, and CVE-7. In addition, the rims in samples CVE-1 and CV-2013 also show this pattern while the cores do not. Zircons from the ore (CVE-4) lack this pattern. While the mechanism for this is not fully known, it explains why the weighted averages based on 207
Pb ages are often slightly older than the calculated Concordia age or weighted averages based on a more restricted data set (97% to 103% concordant data). The effect is slight since the ages calculated by different methods are often, but not always, within analytical error of each other.
One possible explanation for this observation is the incorporation of inherited material in zircon ablation pit cross-sections or volumes that: (1) are either too small or indistinct to recognize optically and/or (2) sample older domains in the third-dimension. We note that zircons from the ore sample and those samples with abundant and recognizable older cores do not show the correlation between age and concordancy. The incorporation of xenocrystic zircon may be less likely in the ore due to the undersaturation of zircon in the magma at high temperatures [52
]. Older cores in the altered leucogranites are often smaller, rounded, and resorbed with the apparent truncation of zoning in response to magmatic conditions.
One way to produce the negative relationship between 207
Pb and concordancy involves a two-stage Pb-loss history where relict cores or nanoscale areas of inherited Pb in zircon [53
] experience Pb-loss during anatexis and incorporation in the leucogranitic magma. This initial event would facilitate migration of the U-Pb systematics on a discordia line parallel to the Concordia curve due to its nearly straight path over the relatively short interval between crystallization and anatexis (ca. 1150 to 1050 Ma). A second more recent Pb-loss event such as that related to the opening of the Atlantic or tectonic activity in the nearby Taconics/Appalachians to the east would cause discordance at the time of disturbance (i.e., discordant array). Presumably zircon with the least amount of inherited material (or most amount of new zircon formed during the crystallization event) would be the most resistant to this second Pb-loss event due to the lack of accumulated radiation damage and, thus, remain on or near Concordia while other points become slightly discordant.
We suggest that small amounts of inherited zircon impact the U-Pb systematics observed, which results in weighted averages that give slightly older ages than those obtained from concordant analyses in the same population. These isotopically anomalous domains may be exceedingly small, optically indistinguishable, and/or intersected during ablation.
Age and origin of the iron ore. Valley et al. [12
] directly dated the iron-oxide apatite ore from the Eastern Adirondacks by zircon U-Pb Secondary Ion Mass Spectrometry. Four ages were obtained ranging between 1000.9 to 1038.9 Ma and were interpreted to represent ore mineralization that was episodic and younger than the widespread and spatial associated LMG suite [12
]. While the origin of the ores themselves may be in part magmatic, the physical, chemical, and isotopic properties of the zircons recovered from the ore were suggested to be compatible with a hydrothermal origin. Independent evidence for hydrothermal alteration occurs within adjacent LMG with periods of potassic and sodic alteration proposed. The later producing quartz-albite assemblages are widely associated with, and enveloping, some ore bodies [12
]. The LMG host rocks analyzed from the Cheever Mine show a similar assemblage, enrichment in sodium, and, in one case, a relatively high silica content >80% by weight (Table 1
), which suggests variable amounts of alteration.
Younger ages associated with late mineralization and fluids are widespread in radiogenic trace minerals within the Eastern Adirondack IOA deposits. Apatite from the IOA ore at Mineville was dated by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry yielding an age of 1009 ± 13 Ma [44
]. The study of Lupulescu et al. [5
] indicates numerous generations of apatite within the IOA ores with complex relationships with monazite, stillwellite-(Ce), allanite-(Ce), and thorite. Regan et al. [44
] described complex symplectitic intergrowths between monazite-allanite-fluorapatite while compositional distinct monazite not incorporated in the symplectites yielded an age of 980 ± 5.8 Ma.
These observations support the interpretation of Valley et al. [12
] suggesting hydrothermal zircon in the ore samples they analyzed and brings to the forefront questions on the event(s) reflected by the ages obtained from various radiogenic minerals [44
]. One thing that remains unknown is the extent and effects of the Rigolet pulse (980–1010 Ma) in the Adirondack region. While well documented in structures along and southwest of the Grenville Front [54
], it is not yet recognized in the Adirondack Highlands or Lowlands and its effects are generally presumed, correctly or not, to be absent.
In this study, a sample of IOA ore from the Cheever Mine yields a Concordia age of 1033.6 ± 2.3 Ma by LA-MC-ICP-MS analysis of zircon U-Pb systematics. This age overlaps the age determined by Valley et al. [12
] for the Palmer Hill ore body (1038.9 ± 4.4 Ma). The zircon has physical, chemical, and isotopic properties compatible with a magmatic origin. In addition, it crosscuts LMG samples within the error of the ore’s age. The relationships at the Cheever Mine provide a geochronological as well as direct spatial link to the LMG. It should be noted that the LMG has been dated throughout the northern and eastern Adirondacks numerous times by a variety of techniques and the ages form a near symmetric histogram whose peak is 1047 Ma (Figure 10
), which is coincident with the widespread growth of metamorphic zircon in the AH [35
]. This suggests a significant amount of plutonism and heating occurred shortly before and/or during albitization of the leucogranitic host rocks and intrusion of the Cheever IOA ore. This time is also coincident with rapid fault movement and uplift and exhumation of the region [32
]. Since the ore sample (CVE-4) lacks evidence for Na or K-alteration and is only slightly younger than the host rocks themselves, it places a lower limit on the timing of metasomatism (ca. 1036.6 ± 2.3 Ma) at the Cheever Mine.
As noted above, the magnetite ore at Cheever occurs as relatively thin (several meters wide), inclined, planar sheets that crosscut host rocks and can be followed along strike for as much as a kilometer or more. Where mined, the iron ore can be seen cross-cutting a variety of rock types including granitic, intermediate, mafic meta-igneous lithologies, amphibolite, and rocks of the Grenville Supergroup. The ore is intruded by late magnetite-bearing pegmatites that have been previously dated by U-Pb zircon analyses including the Old Bed (1022.4 ± 13 Ma) and Barton Hill (1039 ± 11 Ma) mines [39
]. Other late pegmatites in the region yield ages of 1025 to 1048 Ma [39
] and collectively provide a minimum age range for the placement of the iron ores cut by them.
shows the chronology of post-1200 Ma events in the Eastern Adirondacks and the Adirondack Highlands. While not directly dated in this study, rocks of the AMCG suite undergoing Shawinigan orogenesis, occur within a few kilometers of the Cheever Mine (Figure 3
). In this study, five samples of the LMG at the Cheever Mine yield ages between 1036 to 1066 Ma in excellent agreement with the range of ages reported from elsewhere in the AH for this rock unit [40
]. Although the Cheever ore does not show evidence of disturbance of the U-Pb system of zircon, monazite from the Cheever Mine has yielded ages as young as ca. 980 Ma [44
]. At the Cheever Mine, we suggest that the ore is magmatic in origin, does not record the metasomatic event in the LMG host rocks it crosscuts, and younger ages from other radiogenic minerals reflect fault-related fluids and/or Rigolet metamorphic effects. Documenting the duration of ore emplacement and reconciling the information from various radiogenic minerals and documenting the timing and extent of hydrothermal alteration along with possible ore remobilization are vexing problems that remain.