Download Selleck, B., McLelland, J. and Hamilton, M. (2004)

Geological Society of America
Memoir 197
2004
Magmatic-hydrothermal leaching and origin of late to post-tectonic
quartz-rich rocks, Adirondack Highlands, New York
Bruce Selleck*
James McLelland
Colgate University, Hamilton, New York 13346, USA
Michael A. Hamilton**
Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada
ABSTRACT
Magmatic-hydrothermal processes produced significant metasomatic alteration
of country rock, with resultant metal transport and deposition of low-Ti magnetite
bodies, during the intrusion of late granitic magmas in the Adirondack Highlands of
the Grenville Province. Manifestations of these ore-forming systems occur as sillimanite-bearing quartz-rich rocks within ca. 1040 Ma Lyon Mountain leucogranites in
the southwestern Adirondack Highlands. Field and petrographic relationships
demonstrate that emplacement of quartz-sillimanite and bull quartz veins into granite was accompanied by leaching of feldspar and other labile minerals from the granite to produce aluminum- and silica-rich residua. These relationships, coupled with
fluid inclusion and stable isotope data from quartz in the host granite, in quartz-sillimanite veins, and in massive quartz veins, suggest that crystallization of granite
released acidic magmatic fluids that resulted in high-temperature hydrolytic leaching
of feldspars. These fluids also transported and redeposited silica and, locally, alumina
to form a quartz-rich sillimanite-bearing carapace near the margins of the granite pluton. Later low-temperature hydrothermal processes mobilized silica and altered sillimanite to illite and diaspore. Zircons exhibiting U-Pb ages overlapping those of the
host granite are abundant in the leached quartz-rich rock and are interpreted as residual grains incorporated from the granite. The spatial relationship of both hightemperature and low-temperature hydrothermal features relative to the intrusive
Lyon Mountain leucogranite strongly suggests that emplacement of granitic magmas
drove the hydrothermal system during progressive unroofing and cooling of the
Adirondack orogen.
Keywords: Adirondack, hydrothermal granite
*E-mail: [email protected].
**Current address: Jack Satterly Geochronology Laboratory, Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Candada.
Selleck, B., McLelland, J., and Hamilton, M.A., 2004, Magmatic-hydrothermal leaching and origin of late to post-tectonic quartz-rich rocks, Adirondack Highlands, New York, in Tollo, R.P., Corriveau, L., McLelland, J., and Bartholomew, M.J., eds., Proterozoic tectonic evolution of the Grenville orogen in North America: Boulder, Colorado, Geological Society of America Memoir 197, p. 379–390. For permission to copy, contact [email protected]. © 2004 Geological
Society of America.
379
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B. Selleck, J. McLelland, and M.A. Hamilton
INTRODUCTION
High-temperature hydrothermal processes have long been
recognized within the Adirondack Highlands of the Grenville
Province. In particular, the formation of major low-Ti Kirunatype Fe-oxide deposits (Alling, 1939; Postel, 1952; Buddington
and Leonard, 1962; Leonard and Buddington, 1964; Foose and
McLelland, 1995) is associated with leucogranite of the late- to
post-tectonic Lyon Mountain granite (Fig. 1). In this paper we
describe the development of marginal facies of the Lyon Mountain granite that document quartz and quarz-sillimanite vein
emplacement and leaching of host granite that occurred during
and after granite intrusion. Using U-Pb zircon geochronology
we establish the age relationship between intrusion of the granite and development of its quartz-rich carapace. These distinctive quartz-rich facies represent potential source zones for oreforming fluids related to the formation of Lyon Mountain granite iron ores, and recognition of these facies in other regions
may be important in the exploration and modeling of similar
ore-forming systems. Exposures of Lyon Mountain leucogranite near Port Leyden, New York (Fig. 1), display a range of
metasomatic or hydrothermal features, including the quartzsillimanite veins described by McLelland et al. (2002a). The
quartz-rich rocks that are the subject of this study occur adjacent
to these Lyon Mountain leucogranite exposures.
PREVIOUS STUDIES
The Lyon Mountain granite comprises a distinctive suite of
relatively undeformed granites and associated orthogneiss
emplaced across wide tracts of the Adirondack Highlands during the waning stages of the ca. 1090–1030 Ma Ottawan
orogeny (McLelland et al., 2001). The fluids involved in the
Lyon Mountain granite hydrothermal systems may include both
magmatic and surface-derived hydrothermal components
(McLelland et al., 2002a, 2002b). Fluid inclusion data suggest
Na-Cl fluid systems similar to those described by Battles and
Barton (1995) and by Barton and Johnson (1996) for Fe-Cu
hydrothermal ore deposits in the southwestern United States. In
the Lyon Mountain granite, leaching of country rock and earlycrystallized igneous rock by circulating fluids resulted in the
dissolution of metals and local transport and redeposition of
magnetite within host granites and metasedimentary country
rock. The leaching of leucogranite by acidic fluids removed
cations, especially soda and potash, from feldspars and formed
veins of quartz-sillimanite that were disrupted by later magmatic flow (McLelland et al., 2002a, 2002b). The low-Ti magnetite deposits of the Adirondack Highlands are commonly
associated with the quartz-albite facies of the Lyon Mountain
granite and are interpreted as having resulted from metasomatism by the same Na- and Fe-rich hydrothermal fluids. The Lyon
Mountain granite hydrothermal systems operated during and
after the intrusion of Lyon Mountain granite plutons at temperatures near 700 °C with fluids derived from the crystallizing
magma or from evolved meteoric brines of regional scale. The
emplacement age of the Lyon Mountain granite is well constrained by U-Pb zircon dating, and the early stages of hydrothermal activity were coeval with intrusion of Lyon Mountain
granites and pegmatite at ca. 1060–1030 Ma (McLelland et al.,
2001, 2002a, 2002b).
Postpeak metamorphic infiltration of fluids into the granulite-facies rocks of the Adirondack Highlands has been documented (e.g., Morrison and Valley, 1988; Whitney and Davin,
1987), but the timing of these events is unclear. Some fluidrelated alteration is undoubtedly of Paleozoic age, related to
flushing of basinal fluids from Paleozoic sediment cover downward, perhaps by means of tectonically driven processes (Whitney and Davin, 1987) in the Taconic or the Acadian event.
However, unroofing of the orogen in the Late Proterozoic could
have allowed infiltration of surface-derived fluids as soon as
pressure-temperature conditions were permissive of brittle
behavior and fracture-related fluid flow. Given appropriate
regional hydrologic conditions, long-lived hydrothermal systems may have operated in the vicinity of late-stage plutonic
complexes like the Lyon Mountain granite. At Lyonsdale, late
illite-diaspore-hematite veins document low-temperature
hydrothermal alteration in the Lyon Mountain granite. The latest-stage fluids were oxidizing and had relatively high salinity.
These low-temperature assemblages are discussed later along
with the high-temperature processes cited earlier to provide documentation of the range of field and petrologic features that
record magmatic-hydrothermal leaching systems that may be
linked to the formation of important ore bodies in the Adirondacks and elsewhere.
GEOLOGIC SETTING OF THE
QUARTZ-RICH FACIES
The quartz-rich rocks of this investigation are located near
the southwestern margin of the Adirondack Highlands (Fig. 1).
The Adirondacks constitute a domical outlier of the Grenville
Province that has experienced multiple high-grade metamorphic
and intrusive events. The metaigneous and minor metasedimentary rocks that characterize the Adirondack Highlands include
early granodioritic and tonalitic rocks (ca. 1350–1300 Ma) that
were deformed and metamorphosed at ca. 1170 Ma (Wasteneys
et al., 1999). Voluminous magmas of the anorthosite-charnockite-mangerite-granite (AMCG) suite were intruded at ca. 1150
Ma following delamination of the overthickened Elzevirian orogen. After a hiatus of ∼60 m.y. the region was intruded by the
ca. 1100 Ma Hawkeye granite suite (McLelland et al., 1996).
Subsequently, all of the Highlands were subjected to vaporabsent granulite-facies metamorphism and multiple deformations associated with the ca. 1090–1030 Ma Ottawan orogeny
(McLelland et al., 2001). Late- to post-tectonic leucogranites of
the Lyon Mountain granite (ca. 1060–1030 Ma) are interpreted
as related to delamination, rapid uplift, and extensional collapse
of the overthickened Grenville orogen (McLelland et al., 2001).
Figure 1. (A) Generalized geological map of the Adirondack Highlands of New York state showing distribution of Lyon Mountain granite and older igneous rocks. CCZ—CarthageColton zone separating the Adirondack Highlands from the Adirondack Lowlands. MCG— mangerite-charnockite-granite. Inset shows location of the Adirondacks relative to the
Grenville Province. (B) Sketch map of Lyonsdale, New York, and the vicinity showing key localities mentioned in the text.
382
B. Selleck, J. McLelland, and M.A. Hamilton
In the southwestern Adirondack Highlands near Port Leyden, New York, leucogranites of the Lyon Mountain Gneiss (Fig.
1) are found in intrusive contact with older metasedimentary
and metaigneous rocks. Dikes of leucogranite crosscut foliated
metapelites, and Lyon Mountain granite commonly contains
xenoliths of calc-silicate country rock. Large tracts of the leucogranite mass exposed in the bed of the Moose River at Lyonsdale contain quartz-sillimanite segregations. The suite of quartzrich rocks, exposed on the western margin of the leucogranite
mass, are described below.
Quartz-Rich Rock and Related Lithologies
near Port Leyden
The quartz-rich facies vary progressively from leucogranite with a variable concentration of narrow veins to vein-rich
granite and finally to massive sillimanite-bearing quartz-rich
rock containing only a few small, scattered occurrences of
leucogranite. The quartz-sillimanite vein–bearing leucogranite
facies exposed between Lyonsdale Bridge and Ager’s Falls (Fig.
1) consists of sharp-walled, 2–10 cm–thick veins of coarsely
crystalline quartz and sillimanite hosted by equigranular granite
(Fig. 2, A). These veins contain coarsely crystalline sillimanite
as well as minor magnetite, with both concentrated toward the
vein centers. The relative timing of the quartz-sillimanite veins
is established by the observation that they crosscut country rock
calc-silicate schlieren, flow-related layering, and earlier quartzsillimanite veins within the host leucogranite (McLelland et al.,
2002a, 2002b). Within the veins, clusters of tabular sillimanite
crystals up to 5 cm in length are aligned parallel to the vein
walls. Veins bearing quartz and sillimanite may grade along
strike into veins of nearly pure quartz. Sillimanite-bearing veins
were formed within the granite by deposition from high-temperature hydrothermal fluids of probable magmatic origin (McLel-
Figure 2. (A) Quartz-sillimanite veins in equigranular leucogranite at Lyonsdale Bridge. Upper part of scale in cm. (B) Centimeter-scale quartzsillimanite-magnetite veins in weakly foliated quartz-sillimanite bearing nodular granite at Shelby Bridge. Arrows point to quartz-sillimanitemagnetite veins. Hammer is 35 cm in length. (C) Coarsely crystalline “bull quartz” vein in leucogranite of the Lyon Mountain gneiss at the power
line locality near Lyonsdale. Hammer head rests on contact. Hammer is 35 cm in length. (D) Nodular quartzite at Shelby Bridge. Lumps resembling sedimentary clasts are boudin of coarsely crystalline quartz in granular, tectonized quartzite. Hammer is 35 cm in length.
Magmatic-hydrothermal leaching and origin of quartz-rich rocks
383
land et al., 2002b). These fluids were capable of transporting
dissolved silica, and apparently alumina, since the large clusters
of sillimanite crystals appear to have grown within the veins via
precipitation from fluid and thus represent at least local mobilization of alumina.
Coherent quartz-sillimanite veins are often found within
granite that contains isolated, dismembered layers, lenses, knots,
and boudin of quartz-sillimanite segregations formed relatively
early in Lyon Mountain granite magmatic history. These earlier
segregations were then disrupted and deformed by the later
flow of the still partly molten magma. If these earlier segregations were formed as sharp-walled veins similar to the latest
undisrupted veins, the enclosing granite must have crystallized
sufficiently to permit brittle fracture. Alternatively, the highly
viscous, partially crystallized leucogranite magma may have
fractured due to high local strain rates and elevated fluid pressure
(McLelland et al., 2002a).
Thicker decimeter-scale quasi-continuous veins of quartzsillimanite rock that separate isolated masses of quartz-sillimanite nodule bearing leucogranite occur to the west between
Ager’s Falls and Kosterville Dam (Fig. 2, B). Magnetite is a persistent accessory in the veins. The sharp contacts between veins
of quartz-sillimanite rock and nodular granite suggest that this
facies represents successive and continued emplacement of
quartz-sillimanite veins into host granite. Exposures of this
lithology are commonly striped with centimeter-scale red and
green alteration zones related to extensive later low-temperature
processes. Wider “bull” quartz veins 50–100 cm thick are also
present in these exposures and may grade along strike into pegmatitic leucogranite from nearly pure quartz to typical granite
pegmatite over distances of 5–10 m.
Farther to the west at Kosterville Dam (Fig. 1), massive sillimanite-bearing quartz-rich rock, together with minor centimeter-scale quartz-sillimanite veins, occurs as a weakly layered,
coarsely crystalline facies with disseminated low-temperature
alteration zones. At Shelby Bridge, the later mechanical deformation of nearly pure quartz layers in this facies produced boulder-size, rounded boudins of coarsely crystalline quartz in finer,
grain-size reduced granular quartz, forming outcrops that superficially resemble sedimentary conglomerates, but clearly are not
(Fig. 2, D). Overall, this east-to-west sequence of quartz-rich
rocks is interpreted as representing the marginal zone, or carapace, of a Lyon Mountain granite intrusive body.
quartz. Larger segregations of illite-diaspore are also present,
forming irregularly shaped masses of salmon- and green-colored, porcellaneous rock up to 20 cm in overall dimension. Thin
sections suggest that the illite-diaspore material forms pseudomorphs after sillimanite in the quartzite (Fig. 3, B and C). In
addition, the illite-diaspore rock often contains concentrations
of euhedral to subhedral, elongate zircon crystals that strongly
resemble zircon grains in typical Lyon Mountain leucogranite
(Fig. 3, D).
The illite-diaspore veinlets and segregations within sillimanite-bearing granite and quartz-sillimanite rock are interpreted as low-temperature (<200 °C) features related to a later
phase of hydrothermal alteration. This phase included local redistribution of alumina and silica as fluids permeated the fracture network and dissolved and redeposited the low-temperature
mineral phases. Dissolution of magnetite also occurred related
to the formation of hematite and chlorite. Fluid inclusion data
suggest that fluid salinities were high (>20% NaCl equivalent)
and oxidizing, based on the common occurrence of minor
hematite in the veinlets and as spherules within fluid inclusion
trains associated with fractures that continue from veinlets. The
low-temperature event may have further concentrated zircon
grains as labile minerals were removed, leaving only ultrastable
zircon, illite, and diaspore. The zircon-rich illite-diaspore material represents a hydrothermal leachate, resembling the leached,
kaolinitic jasperite materials that form residua in the near-surface portions of hydrothermal vents in active volcanic systems
(Heald et al., 1987).
Later Alteration Features
ZIRCON GEOCHRONOLOGY
Centimeter- to millimeter-scale alteration zones are common in both the quartz-sillimanite veins and the massive quartzrich facies (hereafter called quartzite) associated with the Lyon
Mountain granite in the Lyonsdale area. In outcrop, the alteration zones are characterized by anastamosing networks of veinlets 1–2 mm in thickness, tapering to healed fractures hosting
illite and diaspore. These veins have a halo of red and/or green
coloration related to minor disseminated hematite and/or chlorite. Hematite spherules are found within healed fractures in
Single-grain thermal ionization mass spectrometry (TIMS)
dating of the nodular leucogranite at Lyonsdale was reported by
McLelland et al. (2002a), who obtained an age of 1035 ± 4 Ma
that is interpreted as the age of igneous emplacement. In addition, McLelland et al. (2001) obtained a U-Pb zircon TIMS age
of 1034 ± 10 Ma for an undeformed pegmatite that crosscuts
the leucogranite and quartz-sillimanite veins and the nodules
within it, thus constraining granite emplacement and vein or
nodule formation to the same brief time interval. These out-
Quartz-Calcite Veins near Mckeever
Approximately 12 km east of the Lyonsdale area, exposures
of calc-silicate rock and metasedimentary quartzite host
decimeter-scale block-structure veins bearing coarse, well-terminated quartz crystals and calcite spar with spherulitic
hematite rinds that postdate the quartz and calcite (Darling and
Bassett, 2001). Aluminosilicate minerals such as phlogophite
and plagioclase in the host rock adjacent to the veins are altered
to intergrown chlorite, illite, and diaspore. The relationships of
these veins with the alteration features observed in the Lyonsdale exposures are unclear, but suggest that late-stage hydrothermal processes were not limited to the areas of Lyon Mountain
leucogranite.
384
B. Selleck, J. McLelland, and M.A. Hamilton
Figure 3. (A) Coarsely crystalline quartz
from vein at the Kosterville Dam locality. Note abundant crosscutting sets of
fluid inclusions and late fractures with
sericite. (B) Illite-diaspore rock from
Kosterville Dam. Note apparent pseudomorphs of illite-diaspore after sillimanite crystals. (C) Illite-diaspore segregation
in quartzite, Kosterville Dam. (D) Zircons (indicated by arrows) in illite-diaspore segregation at Kosterville Dam.
Zircon separates were prepared from this
material for sensitive high-resolution ion
microprobe (SHRIMP) analyses.
crops are within 0.5 km, along strike, of the quartz-rich rocks
discussed here.
Zircons within the diaspore concentrations of the quartz-sillimanite veins and massive quartzite subunits are similar to
those in the nearby leucogranite: they are 100–400 microns in
length, with length-to-width ratios averaging 3:1, and consist of
cores surrounded by relatively thick rims or mantles (Fig. 4).
Almost all grains are characterized by a distinctive mottling that
has been noted in other Adirondack zircons exposed to large
quantities of hydrothermal fluids (McLelland et al., 2001). Due
to the presence of inherited cores, the zircons were analyzed in
situ by means of ion probe techniques at the Geological Survey
of Canada’s Sensitive High-Resolution Ion Microprobe
(SHRIMP) II Laboratory in Ottawa. Analytical methodology
generally followed the protocols described in Hamilton et al.
(this volume). Data from these analyses, as well as further
instrumental details, are presented in Table 1. Zircons in the
diaspore concentrations are texturally characterized by mantles
with elevated uranium concentrations ranging from ∼1100 to
1800 ppm, and Th/U ratios falling narrowly between 0.1 and 0.2.
These characteristics are similar to those described for analogous zircons in the host Lyon Mountain granite and associated
pegmatite (McLelland et al., 2001, 2002a). Inherited cores are
commonly present in the diaspore zircons and have U concentrations that range from ∼500 to 4700 ppm; the most concordant
analyses typically range from 500 to 1000 ppm U, while the
most discordant cores have ∼2380–4700 ppm. As expected,
Th/U ratios in the xenocrystic cores span a diverse range, mostly
between ∼0.3 and 1.4, but extend as low as 0.06 (Table 1).
A concordia plot of all SHRIMP data shows a distinct clus-
tering of mantling zircon near 1050 Ma, together with some
strongly discordant points as well as a scattering of older core
ages between 1160 and 1230 Ma (Fig. 5, A). A concordia plot
of U-Pb data from the zircon mantles is shown in Figure 5, B. A
weighted mean calculation of all eight analyses of texturally
unambiguous zircon mantles (unshaded ellipses in Fig. 5) yields
an age of 1041 ± 8 Ma (mean square of weighted deviates =
0.39). Notably, one broken prismatic and doubly terminated
grain (Fig., 4C) gives the same age of ca. 1040–1055 Ma for
both core and mantling zircon. Both cores and mantles exhibit
faint traces of zoning (Fig. 4, A and B).
Interpretation of the zircon ages from these quartz-rich
facies is not straightforward; however, reasonable alternatives
can be ascertained. Clearly the sillimanite-bearing veins cannot
have a sedimentary origin, since they crosscut the host
leucogranite as well as one another. This leaves only the possibility that the veins are of hydrothermal origin. Moreover, they
must be younger than the leucogranite (1035 ± 4 Ma) and older
than the pegmatite (1034 ± 10 Ma) that crosscuts them (McLelland et al., 2001, 2002a). Note that the age of the leucogranite is
within error of the weighted average of the 1041 ± 7 Ma zircons
from the illite-diaspore segregations. It is possible that the zircons within the quartz-rich facies are hydrothermal, but rigorous published studies of hydrothermal zircons are scarce, and
much of what appears in the literature has been debated (ClaouéLong et al., 1990, 1992; Corfu and Davis, 1991; Kerrich and
King, 1993; Nesbitt et al., 1999). In cases where a hydrothermal
origin appears certain, the zircons tend to be small and irregular
in shape (Rubin et al., 1989). In contrast, the zircons under consideration here exhibit morphologies that are consistent with an
Magmatic-hydrothermal leaching and origin of quartz-rich rocks
385
Figure 4. Cathodoluminescence images
of zircons from illite-diaspore segregations in quartz-sillimanite vein complex.
(A) Grain with inherited core of Elzeverian-age (ca. 1190 Ma) zircon surrounded
by mantle of Lyon Mountain granite-age
(ca. 1040 Ma) zircon. Very weak zoning
in both core and mantle indicates a likely
magmatic origin for both. (B) Large grain
with inherited anorthosite-mangeritecharnockite-granite- or Elzeverian-age
core and thick mantle of Lyon Mountain
granite age. The anomalous 777 Ma age
occurs in a mottled, highly altered portion of the grain. (C) Broken grain with
weakly zoned mantle concentric with the
euhedral core. Both core and mantle yield
Lyon Mountain granite ages.
igneous origin and are identical in morphology, uranium content, and, within error, U-Pb age to zircons within the host
leucogranite (McLelland et al., 2002a), and we propose that this
was their source. There are two mechanisms by which zircons
could have been transferred from the leucogranite to the quartzrich facies. The first is by plucking of zircons from the granite
wallrock of the veins. The second, and preferred, mechanism is
by high-temperature (500–650 °C, McLelland et al., 2002b)
hydrothermal leaching of alkalis from leucogranite feldspars,
leaving behind quartz, sillimanite, and zircon. This mechanism
does not rule out some degree of transport for these “lag” deposit
constituents, nor does it require all quartz veins in the region to
have resulted from alkali leaching. In fact, many of the mutually
crosscutting, sharply defined quartz veins are relatively poor in
sillimanite and zircon and probably represent redeposited silica.
The exceptionally high zircon concentrations in the illite-diaspore rock is, as discussed earlier, the result of an additional
phase of low-temperature hydrothermal leaching that removed
most of the silica, leaving these unique aluminous and zirconenriched lag deposits in the massive quartz-rich rock.
quartz and quartz-diaspore rocks have quartz values in the range
from 6.5 to 8.3 per mil. These values suggest that primary
igneous quartz within host granite was characteristically isotopically heavier, and that the lighter values associated with
quartz-sillimanite nodules and leached quartzites are related to
interaction with fluids that circulated within the crystallizing
granite and adjacent leached carapace.
McLelland et al. (2002b) concluded that the oxygen isotope
composition of water in equilibrium with quartz in the Lyon
Mountain granite generally ranged from 9.0 to 13.1 per mil. Fluids with these values are consistent with either (1) magmatic
and/or metamorphic fluids or (2) evolved surface-derived basinal brines. Fluids with somewhat lower δ18O, in the range of 7
to 8 per mil, were calculated for some isotopically lighter quartz,
and were interpreted as magmatic in origin. The data reported
here are consistent with high-temperature (500–650 °C) equilibrium between quartz and magmatic fluids derived from the
adjacent granite pluton.
Isotopic Results from the Mckeever
Quartz-Calcite Veins
OXYGEN ISOTOPES
Oxygen isotope values from the quartz-rich and related
rocks in the Lyonsdale area are presented in Table 2. Quartz in
host granite and in some quartz-sillimanite nodules shows a
range of values from 6.7 to 12.9 per mil, whereas massive vein
Samples of coexisting crystalline quartz and calcite from
two block veins at the McKeever locality were analyzed as
shown in Table 2. Darling and Bassett (2001) have shown that
the McKeever quartz-calcite veins were emplaced at temperatures of 185–232 °C and pressures of 863–1870 bars, based
c
m
c-a
c-a
c
m
c
c
m
m
c
c-a
m
x
c
c-a
m
c
c
1.1
2.2
3.1
4.1
5.1
5.2
6.1
7.1
8.1
9.1
10.1
10.2
10.3
11.1
13.1
13.2
14.1
14.2
14.3
1377
1608
4717
2013
827
1788
916
514
1359
1149
579
2664
1071
1348
2380
1426
753
651
1118
U
(ppm)
422
159
2960
803
690
261
892
724
237
225
326
153
205
309
363
205
238
235
179
Th
(ppm)
0.306
0.099
0.627
0.399
0.834
0.146
0.974
1.408
0.174
0.196
0.564
0.057
0.192
0.230
0.153
0.144
0.316
0.361
0.160
Th/U
295
282
426
338
186
302
205
109
240
190
115
265
183
236
227
237
114
100
195
Pb*
(ppm)
221729
102145
2905
29586
161290
54113
17615
28580
46816
100000
282486
5821
154321
26021
14667
6696
100000
35778
584795
204
Pb/
Pb
206
Pb/
U
2.3829
1.9003
0.7098
1.7276
2.2549
1.8012
2.1089
2.2148
1.8637
1.7267
2.0917
0.9637
1.8031
1.8772
0.9259
1.7660
1.5777
1.5881
1.8433
235
207
0.0451
0.0252
0.0099
0.0217
0.0319
0.0234
0.0328
0.0356
0.0273
0.0245
0.0366
0.0132
0.0268
0.0266
0.0139
0.0245
0.0275
0.0284
0.0276
±1σ§
Pb/
U
0.2123
0.1848
0.0852
0.1648
0.2048
0.1768
0.1948
0.2004
0.1823
0.1700
0.1912
0.1074
0.1759
0.1808
0.1018
0.1730
0.1537
0.1546
0.1807
238
206
0.0033
0.0023
0.0010
0.0019
0.0026
0.0021
0.0025
0.0026
0.0023
0.0022
0.0028
0.0012
0.0022
0.0023
0.0012
0.0021
0.0021
0.0023
0.0022
±1σ§
Pb/
Pb
0.0814
0.0746
0.0604
0.0760
0.0799
0.0739
0.0785
0.0802
0.0741
0.0737
0.0793
0.0651
0.0743
0.0753
0.0660
0.0740
0.0744
0.0745
0.0740
206
207
0.0007
0.0003
0.0004
0.0003
0.0004
0.0003
0.0006
0.0007
0.0004
0.0003
0.0006
0.0004
0.0005
0.0004
0.0005
0.0004
0.0007
0.0006
0.0005
±1σ§
1231.1
1057.0
617.5
1096.3
1193.4
1038.8
1159.7
1201.3
1045.3
1031.9
1180.5
776.9
1050.8
1076.2
806.3
1042.6
1053.5
1054.9
1041.1
Pb/206Pb
Age (Ma)
207
Note: Corrections for common Pb made using the measured 204Pb. Data were acquired using a 5–7 nA primary O- beam with a spot diameter of ∼17 × 23 microns.
Calibration of Pb/U ratios was referenced to the GSC Kipawa zircon standard (993 Ma).
#
Locations of spot analyses: c—core; c-a—strongly altered core; m—mantle; x—possible mixed mantle and core.
Pb* — radiogenic Pb.
§
All errors on ratios and ages reported at 1σ level of uncertainty.
†
Percent concordant.
Location#
Spot
TABLE 1. SHRIMP II U-TH-PB DATA FOR ZIRCONS IN DIASPORE LENSES IN QUARTZ VEINS
18.0
7.1
14.4
7.7
9.9
7.9
14.6
16.0
12.2
9.3
15.5
12.6
14.2
10.6
15.8
11.1
19.3
16.7
14.1
±1σ
(Ma)§
100.8
103.4
85.4
89.7
100.6
101.0
98.9
98.0
103.3
98.1
95.6
84.7
99.4
99.6
77.5
98.7
87.5
87.9
102.8
%
Concentration†
Magmatic-hydrothermal leaching and origin of quartz-rich rocks
387
Figure 5. Concordia plots of sensitive
high-resolution ion microprobe (SHRIMP)
results on zircon from diaspore segregations in quartz-sillimanite vein complex.
All error ellipses are shown at the 1σ level
of uncertainty. (A) Plot of all U-Pb zircon
data from sample KVD-1. Older zircon
ages (>1150 Ma) are strictly from grain
cores; strongly discordant analyses represent grain cores that are U-rich and
pervasively altered. All data from cores
are shaded. (B) Expanded view of data
for concordant and near-concordant
spot analyses from zircon mantles (unshaded). Results for the seven analyses
yield a weighted mean 207Pb/206Pb age of
1041 ± 7 Ma. Shaded ellipses excluded
from age calculation: spots 4.1 (zoned,
altered, discordant core) and 11.1 (possibly mixed mantle and slightly overlapped
core). MSWD—mean square of weighted
deviates.
upon carefully constrained diamond-anvil press studies of CO2H2O inclusions. The results of Darling and Bassett (2001) are
consistent with fluid inclusion data from quartz-rich facies in the
Lyonsdale area (Table 3).
The δ18O of water in equilibrium with calcite and quartz in
the McKeever locality is relatively light (∼–2 to –6 per mil
SMOW, or standard mean ocean water) compared with those of
other Adirondack rock-water systems, including that of the
Lyonsdale-area quartzites reported earlier (Fig. 6). For comparative purposes, the McKeever calcite isotope data are plotted
with reference to isotopic data from Morrison and Valley’s
(1988) study of retrograde calcite in Adirondack anorthosite. We
suggest that the McKeever veins represent the infiltration of
meteoric waters of low δ18O into rock that had cooled sufficiently from peak metamorphic temperatures to allow brittle
fractures to remain open within a zone of hydrothermal circulation driven by nearby Lyon Mountain granitic plutons. This
model suggests that other low-temperature hydrothermal alteration features should be common in the vicinity of Lyon Mountain granite rock. Not surprisingly, the low-Ti magnetite deposits
388
B. Selleck, J. McLelland, and M.A. Hamilton
TABLE 2. OXYGEN ISOTOPE ANALYSES OF QUARTZ
Sample
number
δ 18OSMOW
Description
10.11
10.36
7.99
7.91
10.00
7.19
12.95
7.32
6.70
11.45
12.09
11.71
12.33
11.74
7.92
8.49
8.33
8.19
8.1
Lyon Mountain granite at Lyonsdale
Lyon Mountain granite at Lyonsdale
Lyon Mountain granite at Lyonsdale
Lyon Mountain granite at Lyonsdale
Lyon Mountain granite at Lyonsdale
Lyon Mountain granite at Lyonsdale
Calc-silicate skarn at Lyonsdale
Lyon Mountain granite at Lyonsdale
Lyon Mountain granite at Lyonsdale
Quartz-sillimanite nodule at Ager’s Falls
Quartz-sillimanite nodule at Ager’s Falls
Quartz-sillimanite nodule at Ager’s Falls
Quartz-sillimanite nodule at Ager’s Falls
Quartz-sillimanite nodule at Ager’s Falls
Quartz-sillimanite nodule at Ager’s Falls
Quartz-sillimanite nodule at Ager’s Falls
Quartz-sillimanite nodule at Ager’s Falls
Quartz-sillimanite nodule at Ager’s Falls
“Bull quartz” vein in Lyon Mountain
granite at power line locality*
“Bull quartz” vein with minor sillimanite
at power line locality*
Massive nodular quartzite at Shelby
Bridge*
Quartzite hosting illite-diaspore veins at
Kosterville Dam*
Quartz vein at McKeever*
Quartz vein at McKeever*
11-5-96-1
11-5-96-2
11-5-96-3A
11-5-96-3B
11-5-96-4
11-5-95-5b
11-5-96-7
11-5-96-8
11-5-96-8A
AF-a-1
AF-a-1a
AF-a-2
AF-b-1
AF-b-2
AF-6-12-96a
AF-6-12-96b
AFb-12-96-4a
AFb-12-96-4b
HTR-01-2
HTR-01-3
8.0
SB-01-1
8.3
KVD-01-2
7.7
MRQL-1-Q
MRQL-3-Q
6.5
7.3
Note: SMOW—standard mean ocean water.
*Analyses performed at Geochron Labs, Inc. See McLelland et al. (2001)
for analytical details on other samples.
found within Lyon Mountain granite elsewhere in the Adirondacks commonly have a low-temperature hydrothermal overprint, including the development of hematite-after-magnetite
replacement fabrics in iron ore zones (Buddington, 1965). However, these low-temperature events are not readily distinguished
from more widespread alteration of basement rocks throughout
the Appalachians that is linked to Paleozoic or younger tectonism (Whitney and Davin, 1987).
TABLE 3. MCKEEVER VEIN ISOTOPIC DATA
Sample
number
Mineral
δ18O
MRV-1
MRV-1
MRV-2
MRV-2
Quartz
Calcite
Quartz
Calcite
6.81
3.58
7.33
4.05
δ13C
–2.80
–2.67
Temperature
δ18O water*
185–232
185–232
185–232
185–232
–5.9 to –3.1
–6.3 to –3.9
–5.4 to –2.6
–5.8 to –2.6
*δ18O water calculated using the constants of Clayton et al. (1972),
assuming an equilibrium range of temperatures based on the fluid inclusion work of Darling and Bassett (2001).
Figure 6. Isotopic data showing range of variation of Adirondack rocks
and waters. Marble, retrograde calcite, igneous, and Adirondack
(ADK.) thermal waters fields from Morrison and Valley (1988). LMG—
Lyon Mountain granite.
FLUID INCLUSIONS
Fluid inclusion studies on Lyon Mountain leucogranites
and quartz-sillimanite rock from the Lyonsdale area have previously been reported in McLelland et al. (2002b) and are summarized here. Key results are as follows: (a) The early, hightemperature fluid history of the Lyon Mountain granites is not
preserved by primary fluid inclusions. Halite-saturated mixed
H2O + CO2 inclusions with homogenization temperatures in the
range from 275 to 300 °C represent fluids that reequilibrated
during cooling of the host quartz, perhaps related to the preservation of fractures during uplift and unroofing of the orogen. (b)
Lower-temperature (125–200 °C), two-phase, water-only and
CO2-only inclusions were derived from the unmixing of highertemperature mixed inclusions. (c) Aqueous fluids are Na-Ca-Cl
brines at or near halite saturation. It is important that samples of
quartz associated with the late, low-temperature alteration illitediaspore veinlets at Kosterville Dam contain secondary halitesaturated inclusions that also contain spherulitic hematite. This
suggests that fluids that invaded the rock at this stage were oxidizing, perhaps due to communication with near-surface hydrologic systems.
DISCUSSION AND CONCLUSIONS
Field relationships and petrographic evidence indicate that
quartz-rich rocks associated with Lyon Mountain leucogranite in
Magmatic-hydrothermal leaching and origin of quartz-rich rocks
the southwestern Adirondacks resulted from emplacement of
hydrothermal quartz and quartz-sillimanite veins at the margin of
a Lyon Mountain granite pluton. This vein emplacement was
associated with high-temperature hydrothermal leaching of host
granite that removed alkali cations from feldspars and other minerals to produce massive quartz-rich rock resembling typical
quartzite along with quartz-sillimanite veins. Illite-diaspore and
hematite-chlorite assemblages that resulted from further leaching
of quartz and alteration of sillimanite document later low-temperature alteration of the quartz-rich facies. Field relationships in the
vicinity of Lyonsdale, New York, suggest that the intruding pluton, comprising the main mass of leucogranite, lay to the east of
the leached carapace, which is now represented by the quartz-sillimanite rocks near Kosterville and Shelby Bridge.
Illite-diaspore veins and segregations contain abundant zircons similar to those in the nearby Lyon Mountain leucogranite.
While overall morphology and zoning characteristics cannot be
used to unequivocally correlate the source of the zircons in the
quartz-rich rocks, the close similarity between the quartz-rich
facies and granite zircons is striking. The ca. 1034 Ma pegmatite
at Lyonsdale Bridge crosscuts quartz-sillimanite veins in
leucogranite dated at ca. 1035 Ma, demonstrating that at least
some high-temperature vein emplacement occurred nearly synchronously with granite intrusion. Later, low-temperature
extraction of quartz and alteration of sillimanite to diaspore and
illite further concentrated the granitic zircons by volume reduction of the host rock.
Oxygen isotope data from high-temperature quartz in the
Lyon Mountain granite and from quartz-rich facies in the Lyonsdale area indicate that most quartz from unaltered granite is isotopically heavier than quartz in quartz-rich facies. The heavier
quartz formed in equilibrium with the granite at temperatures of
∼650 °C (McLelland et al., 2002b), whereas the somewhat
lighter values reflect interaction with an isotopically lighter fluid
of either meteoric or residual magmatic origin. This is consistent with our interpretation of a hydrothermal origin for the
quartz-sillimanite veins within the main mass of granite at
Lyonsdale and of hydrothermal leaching of the granite to produce the associated carapace of quartz-rich rock. While surfacederived meteoric or basinal brine fluids might provide the
appropriate isotopic signature for the quartz in sillimanitequartz veins, it is unlikely that such fluids would have the acidic,
corrosive character documented by the leaching of granite.
Therefore, a residual magmatic origin is favored for the
hydrothermal fluid in this system.
The isotopic values of quartz and calcite in the ∼230 °C
McKeever vein are significantly lighter than those of the granite and sillimanite-quartz rocks in the Lyonsdale area, and were
in equilibrium with fluids in the range –6 to –2.0 per mil,
strongly suggesting that the fluid was of meteoric origin. Fluid
inclusions in the McKeever vein are relatively lower in salinity
than fluids in the Lyonsdale rocks (Darling and Bassett, 2001),
also supporting involvement of surface-derived waters. Latestage, high-salinity, hematite-bearing fluid inclusions are asso-
389
ciated with the illite-diaspore rock at Kosterville, suggesting
further involvement of oxidizing surface-derived brines.
Zircon ages in the quartz-rich facies and the age relationships established by McLelland et al. (2001, 2002a) for the granite and associated quartz-sillimanite rocks at Lyonsdale Bridge
constrain the earliest phases of hydrothermal activity as coeval
with granite emplacement between ca. 1045 and 1030 Ma. The
timing of later stages of hydrothermal activity documented by
the McKeever vein and lower-temperature leaching that produced the illite-diaspore assemblages is more difficult to determine. The location of these features in the vicinity of the ca.
1035 Ma Lyon Mountain granite strongly suggests a cogenetic
relationship. These later features were generated in brittle fractures and thus provide insight into the unroofing history of the
Adirondack orogen (Darling and Bassett, 2001). It is likely that
some late-stage hydrothermal features within the Adirondacks
are of Paleozoic age and were caused by penetration of saline
brines into the basement beneath the younger sedimentary
cover. Preliminary monazite age dating in the southern Adirondacks (Storm and Spear, 2002) has shown that Paleozoic events
at ca. 500 and 390 Ma were recorded by overgrowths on ca. 1040
Ma metamorphic monazite crystals. Monazite geochronological
studies in the McKeever-Lyonsdale area could shed further light
on the timing of the low-temperature events and their relationships to granite emplacement and high-temperature hydrothermal activity.
ACKNOWLEDGMENTS
National Science Foundation funding (EAR-9103756) is gratefully acknowledged by JMcL. Both JMcL and BS acknowledge
support from the Colgate Research Council. William Willis and
Chapin Brackett were of great help during the summers of 1995
and 1996. The authors also appreciate the helpful reviews of the
manuscript by Louise Corriveau, Nathalie Marchildon, and one
anonymous reviewer.
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