Exploration and Mining Geology, Vol. 15, Nos. 1-2, pp. 75-88, 2006
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Alkali-Alumina and MgO-Alumina Molar Ratios of Altered and Unaltered Rhyolites
J.F. DAVIES1 AND R.E. WHITEHEAD1
(Received October 1, 2003; accepted November 15, 2005)
Abstract — Alkali-alumina and MgO-alumina plots are an effective instrument for displaying chemical
and mineralogical compositions of altered and unaltered rhyolites and rhyodacites. They reveal that most
supposedly unaltered rhyolites, apart from those of Cenozoic age, have undergone losses in Na2O and
gains in K2O. The extreme loss of Na2O from highly altered rhyolites immediately adjacent to VMS
deposits is strikingly illustrated by alkali-alumina plots. MgO/Al2O3 vs. K2O/Al2O3 diagrams display a
strong negative correlation between MgO and K2O that results from the conversion of sericite to chlorite.
A similar negative correlation between MgO and whole-rock δ18O is apparent on plots of these two
parameters.
Alkali-alumina plots underscore problems in selecting appropriate precursor compositions required
for mass-balance calculations. Molar ratio plots are a viable alternative to such calculations. © 2006
Canadian Institute of Mining, Metallurgy and Petroleum. All rights reserved.
Key Words: Alkali-alumina molar ratio, MgO-alumina molar ratio, Alteration, Rhyolite,
Volcanogenic massive sulfide deposit.
Sommaire —Les diagrammes alcali-alumine et MgO-alumine constituent un instrument efficace pour
la représentation des compositions chimiques et minéralogiques des rhyolites et rhyodacites altérées et
fraîches.Ces diagrammes révèlent qu’à l'exception de celles d’âge Cénozoïque, la plupart des rhyolites
qualifiées de fraîches ont subi une perte de Na2O et un gain de K2O. L’intense perte en Na2O que l’on
note dans les rhyolites fortement altérées associées aux gîtes de type SMV est particulièrement bien mise
en évidence par le diagramme alcali-alumine. Le diagramme MgO/Al2O3 vs. K2O/Al2O3 montre une
forte corrélation inverse entre le MgO et le K2O due à la conversion de la séricite en chlorite. Une
corrélation inverse analogue entre le MgO et le δ18O de la roche totale s’observe sur un graphe comparant
ces deux paramètres.
L’usage du diagramme alcali-alumine fait ressortir combien il peut être difficile de choisir un
précurseur de composition appropriée pour des calculs de balance de masse. Les diagrammes de rapport
molaire constituent une alternative à de tels calculs. © 2006 Canadian Institute of Mining,
Metallurgy and Petroleum. All rights reserved.
1
Department of Earth Sciences, Laurentian University, Sudbury, Ontario P3E 2C6.
75
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Exploration and Mining Geology, Vol. 15, Nos. 1-2, pp. 75-88, 2006
Introduction
An understanding of the composition of felsic volcanic
rocks is central to the interpretation of alteration associated
with rhyolites hosting volcanogenic massive sulfide (VMS)
deposits. Important contributions in this regard have been
made utilizing rare earth element (REE) and other trace
element data (e.g., Lesher et al., 1986; Gorton and Schandl,
2000; Schandl and Gorton, 2002). Schandl and Gorton (2002)
concluded that Archean VMS deposits formed in within-plate
volcanic zones, as opposed to younger VMS deposits such as
Bathurst (Cambrian) and Kuroko (Miocene), which formed at
active continental margins. They also note that the REE data
of Lesher et al. (1986) suggest that the trace element
geochemical characteristics of rhyolites have not changed
significantly since the Archean, a suggestion consistent with
their data for Archean and younger rhyolites. However, the
same does not apply to the mobile oxides, especially Na2O
and K2O. Extensive alteration of rhyolite around VMS
deposits is well known (e.g., Franklin et al., 1981; Franklin,
1996). Perhaps less well known, but documented in this
paper, are widespread differences in the K2O and Na2O
contents of felsic volcanic rocks of different ages, differences
that may be attributed to regional pervasive alteration
apparently unrelated to VMS-associated hydrothermal
alteration.
Taylor and McLennan (1981) summarized published
data (Table 1) that suggested enrichment in K2O and Na2O of
late- and post-Archean upper crust relative to earlier Archean
upper crust. The data compiled in the present study document
major differences in alkali contents of Cenozoic felsic
volcanic rocks relative to those of Paleozoic, Proterozoic, and
Archean age; these differences cannot be explained by
appealing to temporal changes in the composition of crustal
sources of felsic magmas.
The temporal variations in alkali-alumina values in felsic
volcanic rocks unrelated to VMS mineralization have an
important bearing on the interpretation of alteration around
VMS deposits. The primary purpose of this study is to
demonstrate the advantages of presenting and interpreting
alteration data in terms of molar ratios, both for regional
alteration apparently unrelated to VMS mineralization, and
for hydrothermal alteration localized around VMS deposits.
Also discussed is a comparison of molar ratios with other
methods of presenting alteration data, alteration indices, and
mass balance calculations.
Alkali-Al2O3 Molar Ratio Plots
Molar K2O/Al2O3 and Na2O/Al2O3 plots derived from
whole-rock chemistry have facilitated the study of alteration
of heterogeneous graywacke-argillite assemblages in which
K2O content may vary as much as five-fold across a few
millimeters or centimeters (Davies and Whitehead, 1994).
The process involves ratioing the mobile alkalies against an
oxide such as Al2O3, which is considered to be essentially
immobile (e.g., Jenner, 1996). Similar plots are used in the
present study of felsic volcanic rocks, rhyolites, and
rhyodacites, which display compositional variations across
short distances within any particular region and between
regions. The essential features of such diagrams are
illustrated in Figure 1, in which the various mineralogical
assemblages corresponding to different molar K2O/Al2O3 and
Na2O/Al2O3 values are labeled. By definition, rhyolites and
rhyodacites lie within the field outlined by K-feldspar (±
biotite) on the x-axis, and by plagioclase with Na2O/Al2O3
between about 0.6 and 1.0 on the y-axis. Besides the most
abundant alumina-bearing minerals in rhyolites (feldspars
and micas), less-common aluminous phases are epidote and
chlorite, and rarely cordierite. On alkali-alumina diagrams,
these plot at the origin (0,0) as in Figure 1a. Chlorite, which is
a common phase in highly altered rhyolites associated with
VMS deposits, is conveniently displayed on a K2O/Al2O3 vs.
MgO/Al2O3 molar ratio plot (Fig. 1b), which also shows the
relationship between muscovite and chlorite. Cordierite and
chloritoid, which are rare minerals in rhyolites, plot at or
below 1.0 on the MgO/Al2O3 axis.
Alkali Characteristics of Rhyolites
It was the similarity in major element composition of a
particular type of volcanic rock from different tectonic
environments that prompted the use of trace element data in
identifying particular tectonic sources for specific samples
(e.g., early work by Floyd and Winchester, 1975, and more
recently Shandl and Gorton, 2002). Consequently, it is not
expected that alkali-alumina values in rhyolites of the same
age will vary greatly from one tectonic environment to
another. This expectation appears to be borne out by the data
shown in Figure 2, in which plots of Cenozoic felsic volcanic
rocks from four tectonic environments are similar but not
identical (for sources of data, see Appendix). All of these
samples were deposited subaerially and have undergone little
or no alteration. They constitute the standard to which older
rhyolites and rhyodacites are compared. Some samples from
within-continental plates are peralkaline and plot slightly
above the field for rhyolites and rhyodacites, whereas
samples from within-oceanic plates have slightly lower
average K2O/Al2O3 values than samples from the other
environments. The variations displayed in Figure 2 are
relatively minor compared to those between rhyolites of
different ages.
Data for Cenozoic (Tertiary to modern), Paleozoic, and
Archean felsic volcanic rocks from a variety of areas are
shown in Figure 3 (for sources of data, see Appendix). The
Cenozoic data (Fig. 3a) include samples of rhyolites and
rhyodacites from different environments: orogenic zones at
active continental margins, island arcs, within-continental
plates, and within-oceanic plates, which were shown
separately on Figure 2. Only samples of well-documented
unaltered Cenozoic felsic volcanic rocks (rhyolites and
rhyodacites) were selected as a basis for comparison with pre-
Alkali-Alumina and MgO-Alumina Molar Ratios of Altered and Unaltered Rhyolites • J.F. DAVIES AND R.E. WHITEHEAD
77
Fig. 1. Molar ratio plots. (a) Alkali-alumina ratios for plagioclase,
K-feldspar, muscovite, and alumina-bearing ferromagnesian
minerals. (b) MgO/Al2O3 vs. K2O/Al2O3 plot showing the
muscovite-chlorite relationship.
Fig. 3. (a–c) Na2O/Al2O3 vs. K2O/Al2O3 molar ratio plots of felsic
volcanic rocks remote from mineral deposits: (a) unaltered
Cenozoic rocks; (b) and (c) variably altered Paleozoic and Archean
rocks, respectively. (d) MgO/Al2O3 vs. K2O/Al2O3 molar ratio plot
of the Archean rocks. For sources of data see Appendix.
In contrast, Paleozoic rhyolites (Fig. 3b) have a much
wider range in composition than the Cenozoic rocks (Fig. 3a),
reflecting variable degrees of alteration. Most of the samples
of Paleozoic rocks lie below the plagioclase-K-feldspar join
and are depleted in Na2O; many are also enriched in K2O.
Proterozoic rhyolites, which are not particularly abundant and
which are not shown on Figure 3, form a pattern similar to
that of the Paleozoic rhyolites (Fig. 3b).
Fig. 2. Molar ratio plots of unaltered Cenozoic rhyolites and
rhyodacites from different tectonic environments. For sources of
data, see Appendix.
Cenozoic rocks. Of the 149 samples, 51 were fresh glass and
pumice. Peralkaline samples, which would plot above the
albite-K-feldspar line, were not included. In this diagram,
almost all of the samples lie within an area bounded by
stoichiometric and non-stoichiometric K-feldspar (and/or
biotite, probably with some muscovite), and albite and
oligoclase-andesine, corresponding to various proportions of
these minerals. All of the samples plot to the right of the
albite-muscovite join.
The Archean data (Fig. 3c; also variably altered) are the
most distinctive of all the felsic volcanic rocks, differing from
younger rocks in two respects: (1) the samples are, on
average, K2O-poor relative to samples of Paleozoic and
Cenozoic rocks; and (2) the pattern of Archean rocks is
somewhat more compact and linear (along the albite–
muscovite join) than the data for younger volcanic rocks.
Chlorite is a common constituent of highly altered
Archean rhyolites hosting VMS deposits. For comparison
with those Mg-bearing altered rhyolites, Figure 3d presents
the MgO/Al2O3 characteristics of regional rhyolites remote
from VMS deposits. The pattern is primarily random, in
contrast to the linear trends displayed by hydrothermally
altered rhyolites as discussed below.
78
Exploration and Mining Geology, Vol. 15, Nos. 1-2, pp. 75-88, 2006
Table 1. Summary of Average Alkali-alumina Data on Early
Archean, late Archean, and Post-Archean Upper Crust.
Early Archean
Upper Crust
Late Archean
Upper Crust
Post- Archean
Upper Crust
K2O
Na2O
Al2O3
0.0095
0.0430
0.1540
0.028
0.060
0.158
0.035
0.052
0.158
K2O/Na2O
K2O/Al2O3
Na2O/Al2O3
0.2200
0.0680
0.2800
0.470
0.180
0.380
0.670
0.220
0.330
Molar
Notes
After Taylor and McLennan (1981).
For purposes of comparison with the felsic volcanic plots
of Figure 3, Figure 4 presents corresponding data for
Cenozoic, Paleozoic, Proterozoic, and Archean felsic
granitoid rocks. Only the Cenozoic plutons (Fig. 4a)
correspond closely to the Cenozoic felsic volcanic rocks (Fig.
3a). Relative to the Paleozoic plutons (Fig. 4b), many of the
Paleozoic felsic volcanic rocks (Fig. 3b) are somewhat
depleted in Na2O. The Proterozoic plutons (Fig. 4c) display
much less scatter than do coeval felsic volcanic rocks, which
are not shown but whose pattern is similar to that of Paleozoic
rhyolites. Many of the Proterozoic and Paleozoic volcanic
rocks are depleted in Na2O relative to the plutons.
The Archean plutons (Fig. 4d) form two overlapping
clusters, one of which corresponds fairly closely to the
Cenozoic, Paleozoic, and Proterozoic plutons. The other
Archean cluster is characterized by lower K2O/Al2O3 values,
which reflects the dominance of tonalite-trondhjemite
compositions in Archean granitoid rocks (Goodwin, 1972;
Drummond and Defant, 1990). The pattern of the Archean
felsic volcanic rocks (Fig. 3c) corresponds fairly closely to
that of the trondhjemite plutons (Fig. 4d), although the former
extends to lower Na2O/Al2O3 values and lacks the high-K
cluster of the granitoids (Fig. 4d).
Interpretation
The following discussion is predicated on two
assumptions: (1) that there has been little change in the
composition of the crust and granitoid plutons since late
Archean-early Proterozoic; and (2) that the composition of
the upper crust and granitoid intrusions approximate the
composition of the magmas from which the volcanic rocks
were formed. The first assumption is supported by the data on
plutons presented here (Fig. 4) and by the crustal averages of
Taylor and McLennan (1981) summarized in Table 1.
According to Taylor (1987), late Archean and younger crust
which is not K-deficient compared to earlier Archean crust is
a result of massive intracrustal melting near the end of the
Archean.
Regarding the second assumption, it is generally
accepted that felsic volcanic rocks are chemically similar to
granitoid intrusive rocks, and it is, therefore, logical to
assume that the original compositions of the felsic volcanic
Fig. 4. Alkali-alumina molar ratio plots of Cenozoic, Paleozoic,
Proterozoic, and Archean granitoid intrusions.
rocks should correspond reasonably closely to the
composition of the granitoid plutons of the same age. The
similarity in alkali-alumina patterns of Archean felsic
volcanic rocks (Fig. 3c) and the low-K trondhjemitic plutons
(Fig. 4d) are consistent with this suggestion. Independent of
the alkali-alumina patterns, Thurston and Fryer (1983) noted
that trace element data indicate that many of the felsic
volcanic rocks of the Uchi-Confederation Lake area were
derived by melting of trondhjemitic crust. Sage et al. (1996a)
suggested a similar derivation for the felsic volcanic rocks of
the Wawa area. Finally, Galley (2003) noted that the felsic
volcanic rocks hosting VMS deposits in the Archean Noranda
and Sturgeon Lake areas, and Paleoproterozoic Snow Lake
area are compositionally similar to the dominant tonalitetrondhjemite phases of synvolcanic intrusions of those areas.
Regardless of whether or not all of the granitoid
intrusions fairly represent the composition of unaltered felsic
volcanic rocks of the same age, it is undeniable that many of
the Archean and Paleozoic felsic extrusive rocks fall outside
the field defined by unaltered Cenozoic volcanic rocks (Fig.
3a). Those samples lying outside the defining field are
unquestionably altered, being depleted in Na2O and
somewhat enriched or depleted in K2O compared to unaltered
rhyolites and rhyodacites. Some samples plotting near the
albite end of the albite-muscovite join (Na2O/Al2O3 > 0.8)
may have been spilitized immediately after deposition and
subsequently subjected to only minor alteration, for example
Alkali-Alumina and MgO-Alumina Molar Ratios of Altered and Unaltered Rhyolites • J.F. DAVIES AND R.E. WHITEHEAD
prehnite-pumpellyite alteration characteristic of part of the
eastern Abitibi belt (Jolly, 1978). According to Munha et al.
(1980), Na-enrichment in spilites occurs at temperatures
above 140°C, whereas K-enrichment is characteristic below
that temperature. Depletion or enrichment in Na2O or K2O
can be effected by alkali and H+ metasomatism with loss or
gain of K+ or Na+ according to the following reactions:
Alteration of K-feldspar to muscovite by addition of H+
and loss of K+:
3KAlSi3O8 + 12H2O + 2H+ = KAl3Si3O10(OH)2 + 2K+ + 6H4SiO4
79
to (early) reactions with seawater and how much to later,
post-VMS regional metamorphism.
The precise mechanism and timing of the alteration of
felsic volcanic rocks not directly associated with VMS
mineralization is not of particular relevance to the central
theme of this paper and will not be discussed further. The fact
is that many, perhaps most, pre-Cenozoic felsic volcanic
rocks have undergone extensive alkali redistribution
unrelated to any form of mineralization; this must be taken
into account when considering and interpreting hydrothermal
alteration associated with those deposits.
(1)
Alteration Related to VMS Mineralization
Alteration of plagioclase to clinozoisite, albite, and
muscovite by addition of K+ and H+, and loss of Na+:
2Na4CaAl6Si14O40 + K+ + H+ + 9H2O = Ca2Al3Si3O12(OH) +
6NaAlSi3O8 + KAl3Si3O10(OH)2 + 2Na+ + 4H4SiO4
(2)
Alteration of albite to muscovite by addition of K+ and
H+, and loss of Na+:
3NaAlSi3O8 + K+ + 2H+ + 12H2O =
KAl3Si3O10(OH)2 + 3Na+ + 6H4SiO4
(3)
Alteration of muscovite to chlorite by addition of Mg2+,
Fe OH-, and H4SiO4, and loss of K+:
2+,
2KAl3Si3O10(OH)2 + 15(Mg,Fe)OH+ + 3H4SiO4 + 3H2O =
3(Mg,Fe)5Al2Si3O10(OH)8 + 2K+ + 13H+
(4)
Alteration of biotite to chlorite by addition of H+ and loss
of (Mg,Fe)2+ and K+:
2K(Mg,Fe)3AlSi3O10(OH)2 + 4H+ + 6H2O =
(Mg,Fe)5Al2Si3O10(OH)8 + (Mg,Fe)2+ + 2K+ + 3H4SiO4
(5)
Alteration of K-feldspar to albite:
KAlSi3O8 + Na+ = NaAlSi3O8 + K+
(6)
Stabilization of albite may be favored by Archean
seawater that was probably lower in K+ and higher in
temperature (e.g., Munha et al., 1981).
Reactions 1 to 6 are of the type representing H+ and
alkali-metasomatism of granitoid rocks hosting porphyry Cu(Mo) deposits, and felsic volcanic rocks immediately adjacent
to VMS deposits. The same reactions, in situations not related
to mineralization, are involved in alkali exchange between
volcanic glasses and heated seawater that results in Kenrichment and Na-depletion (e.g., Stewart, 1979; Munha et
al., 1980; Thorpe et al., 1993).
The role of regional metamorphism may be significant in
that in the Archean, metamorphism occurred over a period of
several hundred million years after eruption of the volcanic
rocks (2.9 Ga to 2.7 Ga), and a question therefore arises as to
how much of the albite-muscovite alteration can be attributed
Molar ratio plots present alteration data in a readily
interpretable manner. Alkali-alumina and MgO/Al2O3 vs.
K2O/Al2O3 values from altered rhyolites at the Kidd Creek,
Inmont, and Brunswick VMS deposits illustrate some of the
advantages of presenting alteration data in terms of molar
ratios.
Kidd Creek Deposit
Figure 5, compiled from published analyses of felsic
volcanic rocks hosting the Archean Kidd Creek VMS deposit,
illustrates how molar ratio plots can be used to portray and
interpret various aspects of hydrothermal alteration
associated with this class of deposit.
Figure 5a is based on analyses of variably altered felsic
rocks at Kidd Creek (Prior, 1996; Muirhead and Hutchinson,
1999). Samples range from those containing abundant albite
and little sericite to those with abundant sericite and little
albite. With one important exception, Figure 5a resembles
Figure 3c, a plot of Archean regional felsic volcanic rocks
lacking VMS deposits. The exception is the cluster of highly
altered Na-depleted samples at the lower end of the albitesericite join. Those samples consist of varying proportions of
sericite and chlorite (compare with Figure 1a). Figure 5b
displays only moderately to highly altered samples analyzed
by Koopman et al. (1999). The same samples are shown on
Figure 5c, which clearly shows the negative correlation that
reflects the conversion of sericite to chlorite (reaction 4). A
line through the data points projects to about 1.3 on the MgO/
Al2O3 axis representing moderately low-Mg (moderately
high-Fe) chlorite. Figure 5d presents electron microprobe
data for sericite and chlorite in three footwall samples and
three hanging-wall samples (Koopman et al., 1999). The
footwall chlorites have MgO/Al2O3 values of about 1.25
similar to that indicated by the whole-rock data on Figure 5c.
The hanging-wall chlorites have MgO/Al2O3 values greater
than the footwall samples, and are therefore less Fe-rich. Note
that the molar K2O/Al2O3 values of the hanging-wall sericite
are less than the stoichiometric ratio of 0.33, due in part to
Ba2+ substitution for K+.
Inmont Deposit
Figure 6 is based on analyses of highly altered rhyolites
80
Exploration and Mining Geology, Vol. 15, Nos. 1-2, pp. 75-88, 2006
data (Fig. 7a,b) of MacDonald (2001) are similar to those of
Lentz and Goodfellow (1993), and Lentz (1999). The random
pattern of regional data contrasts strongly with the pattern of
highly altered samples from the Brunswick #6 deposit where,
like the Archean examples cited above, the altered rocks are
strongly deficient in Na2O (Fig. 7c), and muscovite is altered
to low-Mg chlorite (Fig. 7d).
Alteration Indices and Mass Balance Calculations
Comparison of molar ratios with other methods of
investigating alteration associated with VMS mineralization
is desirable. Ishikawa et al. (1976) proposed the following
alteration index:
AI = 100(K2O + MgO)/((K2O + MgO) + Na2O + CaO).
Fig. 5. (a-b) Alkali-alumina molar ratio plots of altered rhyolites
from the Kidd Creek mine: (a) Whole-rock analytical data for
variably altered samples, from Prior (1996) and Muirhead and
Hutchinson (1999); (b) Whole-rock analytical data for moderately
to highly altered samples, from Koopman et al. (1999). (c-d) MgO/
Al2O3 vs. K2O/Al2O3 plots: (c) Whole-rock analytical data for
moderately to highly altered samples, from Koopman et al. (1999);
(d) Electron microprobe data from footwall (FW) and hanging-wall
(HW) muscovite and chlorite; data from Koopman et al. (1999).
from the Archean Inmont deposit, Noranda area, Quebec
(Barrett and MacLean, 1991). Some of the samples are
enriched in SiO2 (solid diamonds; Fig. 6a,b,d) and contain
mainly quartz, muscovite, and chlorite. The samples
containing muscovite-chlorite (Fig. 6b) form a band that
broadens toward the MgO/Al2O3 axis, reflecting chlorites
with variable MgO/Al2O3 values. This is also seen in Figure
6c, where data form a line from δ18O ≈ +3‰ at MgO/Al2O3 =
0, to MgO/Al2O3 ≈ 1.75 at δ18O = 0‰. The difference
between the end-member δ18O values is consistent with
reaction 4 for which δ18Omusc-chl. is approximately –3.0‰ at
about 300°C (Taylor, 1974). Figure 6c excludes SiO2-rich
samples, which have high-δ18O values unrelated to the
muscovite-chlorite reaction (reaction 4). These samples are
shown in Figure 6d, which also illustrates the lack of
correlation between SiO2/Al2O3 and δ18O for low-silica
samples.
Brunswick #6 Deposit
The Ordovician Brunswick #6 deposit is hosted by felsic
porphyroidal rocks variously labeled tuffs, quartz-feldspar
porphyries, and/or quartz-augen schists. Regional chemical
(7)
With complete alteration AI = 100, whereas AI ranges
from 20 to 60 in unaltered rhyolite. The Ishikawa index (eq.
7) combines K2O and MgO in the numerator, and does not
distinguish between sericite and chlorite alteration. Indeed,
the Ishikawa index provides no information about the
mineralogy of the altered rock. To overcome this problem,
and also the fact that it does not address carbonate alteration
(note that neither does the present molar ratio method), Large
et al. (2001) introduced a second index, the chloritecarbonate-pyrite index:
CCPI = 100(MgO + FeO)/((MgO + FeO) + Na2O + K2O).
(8)
Large et al. (2001) plotted AI vs. CCPI to construct an
“alteration box” plot that is somewhat complex. The
complexity of the box plot obscures the correlation between
chemical and mineralogical changes.
A common method of dealing with hydrothermal
alteration involves calculating gains and losses of mobile
elements in altered samples relative to precursor samples.
Mass changes occurring during alteration are taken into
account by comparing the concentration of an immobile
element, such as Zr, in the precursor and altered samples. A
variety of methods have been applied in identifying
precursors. In some cases, the precursor is simply a leastaltered sample thought to represent or approximate the prealteration composition of the altered samples. In other cases,
an average of several least-altered samples is taken as the
precursor. In still other cases, precursors are calculated from
fractionation curves which, in turn, are compiled from a
series of least-altered samples. Precursor composition
probably is the most poorly constrained factor in mass
balance calculations. Leitch and Lentz (1994) refer to this
problem in their review of mass-balance constraints. The
uncertainty involved in selecting an appropriate precursor is
further exemplified on molar ratio plots (Fig. 8).
Several studies of the Kidd Creek VMS deposit have
resulted in the selection of a variety of least-altered samples
and precursors. Prior et al. (1999) used several criteria to
identify least-altered samples (open squares, Fig. 8a), the
Alkali-Alumina and MgO-Alumina Molar Ratios of Altered and Unaltered Rhyolites • J.F. DAVIES AND R.E. WHITEHEAD
Fig. 6. Whole-rock compositions of altered rhyolites from the
Inmont deposit; data from Barrett and MacLean (1991); solid
diamonds represent SiO2-rich samples: (a) Na2O/Al2O3 vs. K2O/
Al2O3 plot showing highly altered Na-depleted rhyolites; (b) MgO/
Al2O3 vs. K2O/Al2O3 plot showing relationship between muscovite
and chlorite; (c) MgO/Al2O3 vs. whole-rock δ18O; (d) SiO2/Al2O3 vs.
whole-rock δ18O.
average of which (solid square, Fig. 8a) was selected as the
protolith (precursor). These criteria included: Na2O > 0.9
wt.%, L.O.I. < 0.6 wt.%, CO2 < 1.3 wt.%, H2O < 1.8 wt.%, as
well as several alteration indices and low Cu-Zn content.
Shandl et al. (1999) found a high correlation between Th
and Hf in Kidd Creek samples, and identified least-altered
rhyolites (open diamonds in Fig. 8a) as those with
intermediate Th and Hf values, assuming that samples with
low concentrations had undergone dilution and those with
high concentrations had undergone bulk mass loss. However,
the least-altered samples of Shandl et al. (1999) display an
even greater spread on a plot of Na2O/Al2O3 vs. K2O/Al2O3
than those of Prior et al. (1999). It should be noted that
Shandl et al. (1999) did not use these least-altered samples to
make mass balance calculations.
Koopman et al. (1999) used a precursor composition
which was the average of 5 samples selected by criteria
similar to those of Prior et al. (1999), as well as comparison
of REE profiles and δ18O content of rhyolites of similar
composition in the Abitibi belt. The Koopman et al. (1999)
precursor (solid black dot in Fig. 8a) lies well below the field
81
Fig. 7. (a, b) Plots of regional “unaltered” New Brunswick felsic
volcanic rocks (data from MacDonald, 2001); (c, d) Highly altered
equivalents from Brunswick #6 VMS deposit (data from
MacDonald, 2001).
for unaltered rhyolites, but within the range of Archean felsic
volcanic rocks unrelated to VMS mineralization (Fig. 3c).
Muirhead and Hutchinson (1999) based their mass
changes on a “least altered rock composition estimated from
published data and a comparison with least altered rhyolites
in stratigraphically correlatable positions away from the
deposit…” (Muirhead and Hutchinson, 1999, p. 300). Their
precursor (× on Fig. 8a) differs considerably from those of
Prior et al. (1999) and Koopman et al. (1999), but does lie
within the field for unaltered rhyolites (cf. Fig. 3a). The
selection of least-altered samples at Kidd Creek has yielded
highly variable results, the consequences of which are quite
different precursor compositions. This prompts the question
of just how applicable these precursor values are in making
mass balance calculations.
At the Horne VMS deposit, MacLean and Hoy (1991)
determined a separate precursor composition for each altered
sample using a TiO2-Zr fractionation curve constructed from
a series of least-altered samples. The TiO2-Zr composition of
the precursor is calculated from the intersection of an
alteration line from the origin through the altered sample to
the fractionation curve. As MacLean (1990) appropriately
points out, the accuracy of mass-balance calculations depends
82
Exploration and Mining Geology, Vol. 15, Nos. 1-2, pp. 75-88, 2006
altered volcanic rocks at Kidd Creek.
Fresh or least-altered rhyolites from several other VMS
deposits in the Noranda area are plotted on Figure 8c:
Mobrun deposit (data from Barrett and Cattalani, 1992),
Inmont (Barrett and MacLean, 1991), and Millenbach
(Riverin and Hodgson, 1980). They display a small range in
Na2O/Al2O3 and have somewhat higher K2O/Al2O3 values
than the precursor samples at the Horne mine. The Ansil
least-altered samples (Barrett et al., 1991) have low K2O/
Al2O3 similar to the Horne, and Na2O/Al2O3 values between
those of the Horne least-altered and precursor samples.
The variation in the alkali-alumina composition of the
Noranda area least-altered rhyolites (Fig. 8b–c) prompts the
question: does this range reflect variable compositions of the
magma from which the rhyolites were formed, or is it a result
of post-depositional alteration, either related or unrelated to
mineralization processes? The range in composition of
Archean rhyolites in general, many of which lie outside the
field for unaltered felsic volcanic rocks (Fig. 3a), suggests the
latter interpretation may be correct.
Finally, it is suggested that molar ratio plots serve
essentially the same purpose as mass balance calculations in
interpreting mobility of Na2O, K2O, and MgO; a further
advantage of molar ratio plots is that they illustrate the
relationship between both chemical and mineralogical
changes resulting from alteration.
Fig. 8. Least-altered and precursor rock compositions. (a) Kidd
Creek: open squares are least-altered rhyolites; black square is
average of least-altered rhyolites (Prior et al., 1999); open
diamonds, least-altered rhyolites (Shandl et al., 1999); solid black
dot, precursor rhyolite (Koopman et al., 1999); × symbol, precursor
rhyolite (Muirhead and Hutchinson, 1999). (b) Horne: diamonds are
least altered rhyolites and open circles multiple precursors
(MacLean and Hoy, 1991); × symbols are regional rhyolites (Ujike
and Goodwin, 1987). (c) Various VMS deposits, Noranda area: ×
symbol represents Mobrun hanging wall, and open square, Mobrun
footwall (Barrett and Cattalani, 1992); open diamond represents
Inmont (Barrett and MacLean, 1991); open triangle represents
Millenbach (Riverin and Hodgson, 1980); open circle represents
Ansil (Barrett et al., 1991).
on the estimated TiO2-Zr values of the precursors, which in
turn are a function of the precision of the fractionation curve.
The precision of the fractionation curve, of course, depends
on selecting appropriate least-altered samples. Again, as in
the case of single precursors, there remains a considerable
uncertainty regarding the validity of the least-altered samples.
Analyses of only three of the freshest rhyolites used in
construction of the fractionation curve are published
(MacLean and Hoy, 1991); the Na2O/Al2O3 values (diamond
symbols, Fig. 8b) of these samples are considerably different
to the calculated precursor values (open circles, Fig. 8b).
Both the freshest samples and the estimated precursors do not
differ greatly from the regional Noranda samples of Ujike and
Goodwin (1987), shown as crosses on Figure 8b. Despite
uncertainties regarding the accuracy of the multiple precursor
method, it does yield a tight cluster of values, and appears to
be superior to the averages that were calculated for the least-
Summary and Conclusions
Molar ratio diagrams, on which the mobile oxides,
Na2O, K2O, and MgO are the numerators, and the relatively
immobile Al2O3 is the denominator, conveniently portray
both the main chemical and mineralogical characteristics of
felsic volcanic rocks. Apart from quartz, the most abundant
minerals present in rhyolites and rhyodacites (plagioclase, Kfeldspar, muscovite) are readily identified on K2O/Al2O3 and
Na2O/Al2O3 plots; MgO/Al2O3 vs. K2O/Al2O3 plots illustrate
the relationship between chlorite and muscovite in highly
altered rhyolites associated with VMS deposits.
Alkali-alumina plots of regional Paleozoic and Archean
felsic volcanic rocks that have not been subjected to
hydrothermal alteration related to VMS mineralization reveal
widespread mobility of the alkalis, generally modest
depletions in Na2O, and enrichment in K2O. These depletions
and additions are similar to those found in the peripheral zone
of hydrothermally altered rhyolites hosting VMS deposits.
Alkali-alumina molar ratio plots of highly altered
rhyolites immediately adjacent to VMS mineralization show
distinctive arrays of extreme depletions in Na2O, and partial
to almost complete loss of K2O. The assemblage represented
by such samples, chlorite-sericite, is also impressively
portrayed on MgO/Al2O3 vs. K2O/Al2O3 plots, which display
a strong negative correlation between K2O and MgO,
resulting from the conversion of sericite to chlorite. A similar
negative correlation between MgO and whole-rock δ18O is
portrayed on MgO/Al2O3 vs. δ18O diagrams.
Alkali-Alumina and MgO-Alumina Molar Ratios of Altered and Unaltered Rhyolites • J.F. DAVIES AND R.E. WHITEHEAD
Alkali-alumina molar ratio plots graphically demonstrate
the difficulty in selecting appropriate and meaningful leastaltered or precursor compositions necessary for making massbalance calculations. Molar ratio plots serve essentially the
same purpose in revealing mobility of Na2O, K2O, and MgO
as do mass-balance calculations, but have the further
advantage that they illustrate the relationship between
chemical and mineralogical changes related to alteration.
Acknowledgements
G.J. Prior and D.R. Lentz reviewed an earlier version of
this paper and offered many helpful suggestions, some of
which have been incorporated in the present manuscript. The
advice of our colleges in the Department of Earth Sciences,
Laurentian University, is gratefully acknowledged.
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