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Sequential trace element analysis of zoned skarn garnet: Implications for multi-stage fluxing and flow of magmatic fluid into a skarn system
Journal Pre-proof Sequential trace element analysis of zoned skarn garnet: Implications for multi-stage fluxing and flow of magmatic fluid into a skarn system Changyun Park, Chaewon Park, Yungoo Song, Seon-Gyu Choi PII:
S0024-4937(19)30372-X
DOI:
https://doi.org/10.1016/j.lithos.2019.105213
Reference:
LITHOS 105213
To appear in:
LITHOS
Received Date: 11 February 2019 Revised Date:
2 October 2019
Accepted Date: 2 October 2019
Please cite this article as: Park, C., Park, C., Song, Y., Choi, S.-G., Sequential trace element analysis of zoned skarn garnet: Implications for multi-stage fluxing and flow of magmatic fluid into a skarn system, LITHOS, https://doi.org/10.1016/j.lithos.2019.105213. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V. All rights reserved.
1
ABSTRACT
2
A high-resolution fluid flux–flow model for a shallow crustal system related to skarn
3
formation was established using oscillatory zoning in garnet. In situ analytical methods
4
were used to determine major and trace element contents of andradite-rich and lower-
5
andradite-content garnet zones. Continuous analysis of pure andradite (And87–98) shows
6
these garnets record first- and second-order fluid fluxes. The first-order fluid flux
7
exhibits a stepwise increase in Sn contents and a decrease in the contents of other
8
elements (Ti, V, W, As, Mo, Y, and rare earth elements), indicating that pure andradite
9
records a large range changes in fluid flux from the magma. The second-order flux is
10
evident from oscillatory variations in the contents of Sn and other elements, reflecting
11
small-scale and pulsed changes in the fluid flux from a degassing magma reservoir.
12
Based on the garnet major and trace element variations and mineral textures, these fluid
13
fluxes in the skarn system were controlled by pulsed degassing of a cooling magma.
14
Continuous analysis of oscillatory zoning in garnet with a lower andradite content
15
(And72–81) showed that Sn concentrations increase gradually and other element
16
concentrations decrease gradually. This garnet crystallized from a fluid that was locally
17
equilibrated with a small-scale, stagnant fluid. Thick and low-andradite garnet bands
18
(And62–76) are observed to have grown between the first-order fluid fluxing events. These
19
bands are Al-rich and have retrograde textures, indicating prolonged magma residence
20
after the first-order fluid flux, suggesting that circulating fluids persisted for a relatively
21
long time.
22
Keywords: Garnet; Skarn; Trace elements; Magma processes; Oscillatory zoning
23
24
1
Sequential trace element analysis of zoned skarn
2
garnet: Implications for multi-stage fluxing and flow of
3
magmatic fluid into a skarn system
4 5
Changyun Park1, 2*, Chaewon Park2, Yungoo Song2, and Seon-Gyu Choi3
6
1
7
Mineral Resources (KIGAM), Daejeon 34132, Korea
8
2
Department of Earth System Sciences, Yonsei University, Seoul 120-749, Korea
9
3
Department of Earth and Environmental Sciences, Korea University, Seoul 136-713,
10
Advanced Geo-Materials Research Department, Korea Institute of Geoscience and
Korea
11 12
*Corresponding author: Changyun Park
13
e-mail: [email protected]
14
Tel: +82 54 245 3744
15
Fax: +82 54 245 3759
16 17 18
Manuscript submitted to Lithos as a letter
19
ABSTRACT
20
A high-resolution fluid flux–flow model for a shallow crustal system related to skarn
21
formation was established using oscillatory zoning in garnet. In situ analytical methods
22
were used to determine major and trace element contents of andradite-rich and lower-
23
andradite-content garnet zones. Continuous analysis of pure andradite (And87–98) shows
24
these garnets record first- and second-order fluid fluxes. The first-order fluid flux
25
exhibits a stepwise increase in Sn contents and a decrease in the contents of other
26
elements (Ti, V, W, As, Mo, Y, and rare earth elements), indicating that pure andradite
27
records a large range changes in fluid flux from the magma. The second-order flux is
28
evident from oscillatory variations in the contents of Sn and other elements, reflecting
29
small-scale and pulsed changes in the fluid flux from a degassing magma reservoir.
30
Based on the garnet major and trace element variations and mineral textures, these fluid
31
fluxes in the skarn system were controlled by pulsed degassing of a cooling magma.
32
Continuous analysis of oscillatory zoning in garnet with a lower andradite content
33
(And72–81) showed that Sn concentrations increase gradually and other element
34
concentrations decrease gradually. This garnet crystallized from a fluid that was locally
35
equilibrated with a small-scale, stagnant fluid. Thick and low-andradite garnet bands
36
(And62–76) are observed to have grown between the first-order fluid fluxing events. These
37
bands are Al-rich and have retrograde textures, indicating prolonged magma residence
38
after the first-order fluid flux, suggesting that circulating fluids persisted for a relatively
39
long time.
40
Keywords: Garnet; Skarn; Trace elements; Magma processes; Oscillatory zoning
41
42 43
1. Introduction Skarn deposits are a primary source of Fe, Pb, Zn, W, Cu, and Au, and are commonly
44
generated through fluid–carbonate rock interaction. Magmatic–hydrothermal fluids at
45
temperatures of 450°C–540°C transport these metals into carbonate rocks, and contact
46
and regional metamorphism causes metasomatism of the host rock (Meinert, 1992).
47
Skarn deposits have been extensively studied. The fluid evolutionary paths, including
48
both magmatic and meteoric fluids, are well constrained (Einaudi and Burt, 1982;
49
Meinert, 1992). Traditionally, the flux of skarn-forming fluids is divided into prograde
50
and retrograde stages. Exsolution of H2O and volatile-rich magmatic fluid is responsible
51
for metasomatism of the host rock and formation of calc-silicate minerals in the prograde
52
stage. Incursions of meteoric fluids contribute to metal precipitation in the retrograde
53
stage (Einaudi and Burt, 1982). Recently, numerous studies have documented a pulsing
54
magmatic system in the ore system, and determined this is caused by a gradually cooling
55
or multiply recharged magmatic intrusion (Chelle-Michou et al., 2017; Li et al., 2018;
56
Williamson et al., 2016). Pulsing magmatic processes could cause several prograde or
57
retrograde stages in a skarn deposit. However, few studies have examined in detail the
58
relationship between magmatic processes and fluid flux–flow in skarn deposits. For this
59
purpose, in situ microanalyses of the gangue or ore minerals are essential (Cook et al.,
60
2016).
61
Here we describe a crystalline grandite garnet with unique trace element features from
62
a Fe-Pb–Zn skarn deposit, which shows oscillatory zoning on both a small- and large-
63
scale. High-resolution analysis by laser ablation–inductively coupled plasma–mass
64
spectrometry (LA–ICP–MS) of this zoning reveals information about not only the
65
behavior of the trace elements in the skarn system, but also the reactivation and residence
66
time of the magma in relation to periodic boiling of the magmatic fluid. We also consider
67
how the multi-scale changes in garnet chemistry are related to the changes in the
68
circulation of the magmatic fluid. The results of this study contribute to our
69
understanding of the evolution of fluid flow in skarn systems.
70 71
2. Background
72
Garnet is a common mineral in skarn deposits and, due to its structural stability and
73
chemical attributes, can provide a continuous record of geological processes (e.g., Baxter
74
et al., 2013). Numerous studies have reported that trace element behavior in skarn garnet
75
is strongly related to changes in fluid composition and growth mechanisms (Gaspar et al.,
76
2008; Ismail et al., 2014; Jamtveit and Hervig, 1994; Jamtveit et al., 1993; Park et al.,
77
2017a, b; Zhai et al., 2014). These studies have reported that andradite (Fe-rich garnet) is
78
mainly crystallized in open systems during hydrofracturing. The trace element contents
79
of such garnet reflect the composition of the magmatic–hydrothermal fluid as a result of
80
surface adsorption, whereas grossular (Al-rich garnet) can be formed by the mixing of
81
meteoric fluids under equilibrium in a closed system (e.g., the fault sealing stage). The
82
oscillatory zoning in skarn garnet records open and closed system cycles in the plumbing
83
system of siliceous fluids (Clechenko and Valley, 2003; Jamtveit and Hervig, 1994).
84
Meanwhile, Crowe et al. (2001) and D'Errico et al. (2012) show that some Fe-enriched
85
garnets reflect high magmatic hydrothermal components, but others may reflect low
86
magmatic hydrothermal components. They reported that variable influxes of meteoric
87
fluid affect oxygen isotope rather than Al or Fe concentrations of garnet.
88 89
3. Geological Setting
90
3.1. Gagok skarn Fe-Pb–Zn deposit
91
Southern Korea was in a subduction-related environment at the border between the
92
Eurasian and Pacific (or Philippine) plates from the Middle Jurassic to Cenozoic
93
(Chough et al., 2000). This tectonic setting caused extensive calc-alkaline magmatism,
94
and the formation of the Taebaeksan ore belt (120 to 40 Ma), which produced various
95
skarn-, Carlin-, and greisen-type deposits (Choi et al., 2005). In the Taebaeksan ore belt,
96
skarn is the most representative ore-forming rock and comprises a wide range of marine
97
sedimentary rocks, although it is mainly limestone. The Gagok deposit is a skarn Fe–Pb–
98
Zn deposit in this belt. The geology of the region comprises Precambrian Hongjesa
99
granite basement and Cambrian–Ordovician sedimentary rocks (Fig. 1A). An associated
100
intrusion in the studied area is a Cretaceous granitic porphyry (A/CNK = 0.98–1.06; I-
101
type) (Fig. 1A) (Koo, 2012), with a K–Ar emplacement age of 72.6 ± 2.2 Ma (Yun and
102
Silberman, 1979).
103
The Gagok deposit is a stratabound and structurally constrained skarn deposit. The
104
deposit has been divided into a proximal skarn with Fe–Zn–Pb ore and a distal Zn–Pb
105
skarn (Table S1) (Choi et al., 2010). The Fe–Zn–Pb skarn occurs closest to the associated
106
intrusion, and consists of magnetite, garnet, pure diopside, and native bismuth from the
107
early prograde stage (Fig. 1B–D). Representative ore minerals, such as sphalerite, galena,
108
and pyrrhotite, formed during the retrograde stage along with fluorite, amphibole, quartz,
109
and calcite. In contrast to the Fe–Zn–Pb skarn, prograde minerals in the distal Zn–Pb
110
skarn zone are spessartine (Al1Ad10Gr10Sp79), grossular (Al1Ad45Gr45Sp9), hedenbergite
111
(Hd51Di29Jo20), in addition, carbonate replacement is common (Table S1) (Choi et al.,
112
2010; Koo, 2012). Retrograde processes are dominant in this distal zone, and most of the
113
Zn–Pb ore is related to the distal skarn (Table S1). Euhedral sphalerite and galena are the
114
ore minerals, and retrograde scheelite, quartz, calcite, epidote, cassiterite, and chlorite are
115
dominant. A study of fluid inclusions in fluorite showed that the Gagok polymetallic
116
deposit formed at shallow depths (~1 km), and salinities of boiling fluid range from 7.4–
117
1.2 wt.%(Yang et al., 2013).
118 119
4. Results
120
4.1. Garnet occurrence and growth texture
121
The garnet analyzed in this study was collected from the fracture zone of a garnet-
122
dominated zone in exoskarn (Fe-skarn) (Figs 1A–B and S1; Table S1). Garnet in the
123
proximal skarn has well-developed oscillatory zoning, coexists with diopside or
124
magnetite, and exhibits pressure induced textures (P) (Fig. 1B, C, E). Thick dark bands
125
(T bands) are present in the garnet (Fig. 1B, D), which often show retrograde textures
126
such as replacement and partial dissolution. Manganese oxide precipitates are present in
127
the dissolution regions (Fig. 1B, D). The garnets are considered to have started growing
128
in the massive skarn zone, and progressively grew and became interconnected with the
129
overgrowth garnet in the vein formed by hydrofracture (Figs 1B and S1).
130 131 132
4.2. Major and trace element data The analytical methods are described in detail in the supplementary materials (S2).
133
Electron microprobe data and element maps (Fe, Al, and Sn) show that the studied garnet
134
is a Fe-dominant garnet (Figs 2A and S1; Table S2). This grandite garnet, which
135
crystallized continuously from a massive skarn to its vein, can be divided into three main
136
zones (MS-I, -II, and -III) according to major element data (Fig. 2A). The studied garnet
137
contains bright zones of pure andradite (And91–98) and dark zones of garnet with a lower
138
andradite component (And72–81). The composition of the T bands (T1 and T2) is And62–76.
139 140
The pure andradite bands in the massive skarn (MS-I stage) have positive Eu anomalies and are light REE-enriched, particularly with respect to La (Fig. 2B). The
141
positive Eu anomalies reflect garnet growth in chlorine-enriched hydrothermal fluids
142
(Gaspar et al., 2008). Bands of less pure andradite garnet in the massive skarn are
143
characterized by Pr- and Nd-enriched REE patterns, and have variable Eu anomalies (Fig.
144
2B). The pure andradite bands in MS-II and -III stages have positive Eu anomalies and
145
are light REE-enriched, particularly with respect to La and Ce (Fig. 2C). Bands with less
146
pure andradite are characterized by garnet with Pr- and Nd-enriched REE patterns, which
147
have slightly positive Eu anomalies (Fig. 2C).
148
Trace element analyses were performed continuously from the garnet cores to rims
149
(Table S3; Fig. 3A). The pure and less pure andradite bands were analyzed separately,
150
due to the different growth mechanisms described above (Table S3). It was then possible
151
to further subdivide the garnet bands into sub-stages (1–2 and s1–s3 in the MS-I massive
152
skarn; 3–11 and s4 in MS-II and 12 in MS-III in garnet vein; T1 and T2 bands in garnet
153
vein; Fig. 3). In the pure andradite bands in all garnet zone, each element shows abrupt
154
changes at the stage boundaries. In overgrowth garnet at vein in massive skarn, a marked
155
increase in Sn contents occurs in MS-II (close to 20,000 ppm; Fig. 3B) as compared with
156
the MS-I massive skarn. In contrast to the Sn enrichment in MS-II, other elements (Ti, V,
157
As, Mo, Y, and REE) are depleted in MS-II (Fig. 3B). In the MS-III, only the REE are
158
enriched as compared with MS-II (Fig. 3B). The sub-stage trends of each element (1 to
159
12) show oscillatory variations, and the Sn contents are negatively correlated with most
160
other elements in the MS stages (Fig. 3B). The U and W concentrations in all MS stages
161
were relatively constant during garnet growth, however, U and W show oscillatory
162
variations in the sub-stages (3–12; Fig. 3B). The elemental trends for the less pure
163
andradite, including the T1 and T2 bands, are different. Tin concentrations increase up to
164
4500 ppm, but the concentrations of most other elements decrease from S1 to T2 (Fig.
165
3C). These trends are not characterized by the stepwise changes observed in pure
166
andradite, but rather by a continuous increase or decrease.
167 168
5. DISCUSSION
169
5.1. Large-scale (first-order) and pulsed (second-order) changes in magmatic fluid flux
170
All parts of the garnets are characterized by light REE-enriched patterns (Fig. 2B–C).
171
Although garnet has high DHREE (D; distribution coefficient), extremely low heavy REE
172
concentrations in the granite-derived fluid are responsible for the light REE-enriched
173
garnet (Gaspar et al., 2008). However, the REE patterns of each bright and dark zone
174
show different features. The dark bands are characterized by enrichment of Pr and Nd,
175
indicating equilibrium crystallization (Van Westernen et al., 2000). Compared with the
176
dark bands, light REE (especially La and Ce) are enriched in the pure andradite (Fig. 2B–
177
C). These REE patterns mimic those of a magmatic–hydrothermal fluid (e.g., the light
178
REE-enriched pattern of granitic porphyry; Koo, 2011). As such, La- or Ce-enriched
179
garnet crystallized in disequilibrium via a surface adsorption mechanism and the REE
180
patterns in the pure andradite reflect that of the magmatic–hydrothermal fluid (Gaspar et
181
al., 2008).
182
Other trace elements in skarn garnet can also reflect their relative abundances in the
183
skarn-forming fluid, and provide information regarding intermittent and cyclical fluid
184
fluxing (Ismail et al., 2014; Jamtveit et al., 1993; Park et al., 2017a, b; Xiao et al., 2018;
185
Zhang et al., 2017). We should first consider changes in the oxygen fugacity during the
186
growth of the zoned garnet. It is possible that incorporation of multi-valence elements
187
(e.g., Sn and W) into garnet structure due to changes in fluid oxidation state was variable,
188
meaning that the stepwise trends in Fig. 3B could have been caused by changes in
189
oxygen fugacity. Previous studies have demonstrated a relationship between the
190
oxidation state of elements such as Sn, W, and their incorporation into garnet (Xu et al.,
191
2016; Zhou et al., 2017). Sn4+ and W6+ are more readily incorporated into the octahedral
192
site of garnet than in their reduced state. However, the behavior of Sn is different to W in
193
the studied garnet (Fig. 3B). This means that the changes in elemental concentrations
194
cannot be explained by variable oxygen fugacity. In fact, magma boiling in a skarn
195
system is closely related to the oxygen fugacity. When the boiling stage occurs during
196
brittle deformation (i.e., the hydrofracturing stage), the oxygen fugacity, water/rock ratio,
197
interface control of mineral/fluid, and temperature can all increase (Keankeo and
198
Hermann, 2002). These conditions can enhance the uptake of trace elements into garnet.
199
However, considering the different trends between Sn and the other elements, oxygen
200
fugacity doesn’t seem to be a primary control factor on garnet trace element chemistry.
201
Second, we should consider incorporation energy in garnet crystal. U contents show
202
similar variation with REE contents (Fig. 3B). In this case, it is possible that the
203
incorporation energy of U and REE in the dodecahedral site significantly decreases with
204
the increase in Fe3+ cations within the neighboring tetrahedral site (Rak et al., 2011; Deng
205
et al., 2017). However, as shown the Fig. 3b and table S2, concentrations of Fe3+ in MS
206
stages are not correlated with U and REE contents. Therefore, we considered that
207
incorporation energy of element in garnet crystal doesn’t seem to be a primary control
208
factor on garnet trace element chemistry, also.
209
Third one is about fluid composition. This study suggest that the fluid composition
210
was the primary control on garnet trace element chemistry. The magmatic fluid flux can
211
be divided into first-order (MS-I, -II, and -III) and second-order fluxes (1–12) (Fig. 3A–
212
B). Stepwise variations in the trace element composition of the first-order fluid flux
213
indicate that the garnet recorded release of an evolved fluid (i.e., late-stage) during
214
magma crystallization (Fig. 3B). For example, in the transition from MS-I to -II, the
215
abrupt increase in Sn compared with other elements suggests the influence of an evolved
216
magmatic–hydrothermal fluid, as Sn is diagnostic of late-stage crystallization in a silicic
217
magma (Fig. 3B) (Chen et al., 2000; Groves and McCarthy, 1978; Pirajno, 2009; Zhou et
218
al., 2017). The first-order flux is a large-scale fluid pulse, which reflects a specific
219
magmatic evolutionary stage, whereas the second-order flux reflects small-scale
220
processes superimposed on the first-order fluid flux (red and blue arrows in Fig. 3B).
221
There are two proposed models to explain the first- and second-order fluid fluxes.
222
These are the multiple magma recharge event (i.e., periodic magma injection)
223
(Williamson et al., 2016) and the multiple pulsing fluid models during cooling of the
224
source pluton (i.e., the associated intrusion) (Chelle-Michou et al., 2017; Li et al., 2018).
225
Williamson et al. (2016) demonstrated that excess Al in oscillatory zoned plagioclase is
226
caused by multiple magma injections. This allows incompatible elements, such as Cu, to
227
be periodically concentrated in the late-stage melt. The multiple recharging by less
228
evolved melts fertilizes the granitoid magma body at shallow crustal depths. Uranium
229
concentrations do not change significantly across the studied garnet in relation to the
230
first-order fluid flux (Fig. 3B). U is a highly incompatible element and less abundant in a
231
less evolved melt. In 3, 5, 8, and 11 in Figure 3B, concentrations of Sn increase when
232
concentrations of U decrease. This is additional evidence showing that the fluid flux
233
mechanism does not follow a multi-recharge model. In comparison, Chelle-Michou et al.
234
(2017) and Li et al. (2018) proposed that successive degassing of the magmatic intrusion
235
mostly controls the formation of a skarn deposit. Although the initial degassing is
236
responsible for 50–75 wt.% of the fluid released and has a major role in Cu deposition,
237
the relative flux of each element from each degassing stage is different. The oscillatory
238
zoning, such as in 3, 5, 8, and 11 in Fig. 3B, shows similar concentrations with those of
239
MS-I. This indicates that the degassing event responsible for MS-II was possibly related
240
to chemical diffusion in the magma (Fridrich and Mahood, 1987). As such, the garnet
241
trace element chemistry was affected by fluid injections from the associated intrusion,
242
rather than by magma recharge of primitive melts. Based on the stepwise and oscillatory
243
garnet zoning, and relationship between Sn and other elements (especially U), we
244
propose that this skarn system was controlled by pulsing fluid discharge from a cooling
245
single intrusion.
246
However, in MS-III, most element concentrations decrease. This could be caused by
247
an input of meteoric fluid. Although the garnet composition in MS-III is nearly pure
248
andradite, D’Errico et al. (2012) and Jamtveit et al. (1994) have reported that Fe-rich
249
garnet can form when the magmatic–hydrothermal fluid component is low. Only the REE
250
and U contents increased in MS-III, and the positive Eu anomalies and the nearly pure
251
andradite garnet, indicate that MS-III may be result by mixing of meteoric fluid and
252
highly evolved hydrothermal fluid. As mentioned before, Sn is diagnostic of late-stage
253
crystallization in a silicic magma, however, Sn contents decrease in MS-III. This
254
suggests that cassiterite may have crystallized between MS-II and -III because cassiterite
255
has been frequently observed in the low-temperature skarn zone (Koo, 2012).
256 257 258
5.2. Stagnant fluid related to magma residence Aluminum-rich garnet is related to fluid mixing and circulation in shallow crustal
259
systems (Clechenko and Valley, 2003; Crowe et al., 2001; D’Errico et al., 2012; Page et
260
al., 2010), and equilibrium conditions in skarn systems. Although most of the Al-rich
261
garnet has heavy REE-enriched patterns, the less pure andradite has light REE-enriched
262
patterns. This means the skarn-forming fluid had extremely low concentrations of heavy
263
REE.
264
The Pr- and Nd-enriched REE patterns of the less pure andradite are distinct from the
265
REE patterns of the pure andradite bands (Fig. 2B–C). This indicates that the less pure
266
andradite crystallized in a closed system (Smith et al., 2004). The trace element data for
267
the less pure andradite indicate that it formed in an equilibrium state related to stagnation
268
in magmatic fluid activity. The low magmatic fluid activity formed pressure-induced
269
textures due to backflow (Fig. 1B, E) (Ciobanu and Cook, 2004) and fault sealing in the
270
skarn system. Aluminum-rich garnet might have also formed due to development of a
271
different fluid pathway. However, Keankeo and Hermann (2002) reported that Fe-rich
272
skarn garnet crystallizes at high fluid flow during the fracturing stage, with decreasing
273
pressure and increasing temperature. In such an environment, the fluid is likely to re-
274
infiltrate along an existing fault and the garnet can continuously interact with the fluid.
275
The less pure andradite bands (s1, s2, s3, s4, T1, T2 band) between the oscillatory
276
zoning reflect multiple influxes of meteoric fluid between the multiple influxes of
277
magmatic fluid (Fig. 3B–C) (D’Errico et al., 2012). Under equilibrium conditions,
278
mineral trace element contents can be used to reconstruct the composition of the
279
equilibrated fluid (van Hinsberg et al., 2010). The Sn concentration of the fluid increased
280
or decreased, as recorded by the composition of the pure andradite bands (Fig. 3C). Most
281
other trace element concentrations also decreased in the fluid (Fig. 3C). The geochemical
282
trends of s1, s2, s3, and s4 indicate that the dominant fluid involved in the crystallization
283
of the less pure andradite bands was a stagnant, magmatically exsolved fluid, rather than
284
a meteoric fluid. The T bands, which occur between MS-I, -II, and -III (Fig. 3),
285
correspond to abrupt stepwise changes in the elemental composition of garnet before and
286
after the deposition of the bands (Fig. 3B). These suggest that the magma remained in a
287
long-term, steady-state throughout skarn formation (e.g., MS-I and -II; Fig. 3B). The T1
288
band has retrograde textures (e.g., recrystallization and dissolution fabrics), indicating
289
that the reaction between pre-existing garnet and fluids lasted longer than for the less
290
pure andradite in the MS stages (Fig. 1B). The T bands are also characterized by the
291
highest concentrations of Al (And70–74), meaning that the T bands were more influenced
292
by a steady-state magmatic system than s1–s4 (Crowe et al., 2001). Most elements are
293
depleted in the T2 band, indicating that the skarn-forming fluid was diluted by an
294
increasing amount of meteoric fluid. Based on the mineral textures and garnet chemistry
295
of the T bands at the end of the large-scale evolutionary stages (i.e., MS-I and -II), there
296
was an increased magma residence time, which prolonged the circulation of the skarn-
297
forming fluids.
298 299
5.3. Internal self-organization vs. extrinsic fluctuation
300
(Fowler et al., 2002; Holten et al., 2000). An internal self-organization process
301
produces nonlinearities that may cause chemical oscillatory patterns. These patterns are
302
said to be self-organized that is, they arise from intrinsic crystal growth processes rather
303
than extrinsic or bulk system scale fluctuations (Fowler et al., 2002; Holten et al., 2000).
304
In our results, compared to amount of W contents in the pure andradite bands, W in less
305
pure andradite bands decrease continuously (Fig. 3B–C). Because prograde garnet cannot
306
co-precipitate with scheelite formed during retrograde skarn, this result shows that the
307
internal self-organization and local re-equilibrium of fluids could in fact generate this
308
oscillatory zoning, which means garnet zoning can be grown by a combination of both
309
intrinsic and extrinsic processes. However, geological condition such as fluid fluctuation
310
model in the skarn system and a mineralogical characteristic such as linearity of zoning
311
sequence (Fig. 1SA and Fig 1B) in studied sample show these garnet grains are mainly
312
crystallized by extrinsic or bulk system fluctuation rather than internal self-organization.
313
314 315
6. Conclusions Skarn garnet zoning records not only the intermittent and pulsing fluid flux from an
316
exsolved magmatic fluid, but also multiple stages of magma residence (Fig. 4). At
317
shallow crustal depths (<2 km), fluid flux and flow in the skarn system was controlled by
318
a pulsing magmatic system. Large-scale and small-scale oscillatory changes in fluid flux
319
were identified, which originated by continuous magma crystallization and pulsed fluid
320
discharge during cooling of a single magma body. Between these states, the magma was
321
in a steady-state, and was then followed by a large-scale fluid fluxing event.
322 323 324
Acknowledgements This work was supported by the Korea Meteorological Administration Research and
325
Development Program Grant KMI 2018-01910 and National Research Foundation of
326
Korea Grant No. 2018051418, and the Basic Research Project (19-3214) of the Korea
327
Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of
328
Science and ICT.
329
330 331 332 333
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degassing and the genesis of porphyry copper deposits. Scientific Reports 7, 40566.
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Chen, J., Wang, R., Zhou, J., Ji, J., 2000. Geochemistry of Tin. Nanjing: Nanjing
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University Publishing House 320. Choi, B.-K., Choi, S.-G., Seo, J.-E., Yoo, I.-K., Kang, H.-S., Koo, M.-H., 2010.
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Mineralogical and geochemical characteristics of the Wolgok-Seongok orebodies in the
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Gagok skarn deposit: their genetic implications. Economic and Environmental Geology
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43, 477-490.
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Choi, S.-G., Ryu, I.-C., Pak, S.J., Wee, S.-M., Kim, C.S., Park, M.-E., 2005.
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Cretaceous epithermal gold–silver mineralization and geodynamic environment, Korea.
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Ore Geology Reviews 26, 115-135.
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Chough, S., Kwon, S.-T., Ree, J.-H., Choi, D., 2000. Tectonic and sedimentary
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evolution of the Korean peninsula: a review and new view. Earth-Science Reviews 52,
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Ciobanu, C.L., Cook, N.J., 2004. Skarn textures and a case study: the Ocna de FierDognecea orefield, Banat, Romania. Ore Geology Reviews 24, 315-370.
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Clechenko, C., Valley, J., 2003. Oscillatory zoning in garnet from the Willsboro
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Wollastonite skarn, Adirondack Mts, New York: a record of shallow hydrothermal
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processes preserved in a granulite facies terrane. Journal of Metamorphic Geology 21,
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Cook, N., Ciobanu, C., George, L., Zhu, Z.-Y., Wade, B., Ehrig, K., 2016. Trace element analysis of minerals in magmatic-hydrothermal ores by laser ablation
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inductively-coupled plasma mass spectrometry: Approaches and opportunities. Minerals
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6, 111.
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Crowe, D.E., Riciputi, L.R., Bezenek, S., Ignatiev, A., 2001. Oxygen isotope and trace
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element zoning in hydrothermal garnets: Windows into large-scale fluid-flow behavior.
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Geology 29, 479-482.
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Deng, X. D., Li, J. W., Luo, T., Wang, H. Q., 2017. Dating magmatic and
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hydrothermal processes using andradite-rich garnet U–Pb geochronometry. Contributions
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to Mineralogy and Petrology 172, 71.
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D’Errico, M.E., Lackey, J.S., Surpless, B.E., Loewy, S.L., Wooden, J.L., Barnes, J.D.,
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Strickland, A., Valley, J.W., 2012. A detailed record of shallow hydrothermal fluid flow
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in the Sierra Nevada magmatic arc from low-δ18O skarn garnets. Geology 40, 763-766.
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Einaudi, M.T., Burt, D.M., 1982. Introduction; terminology, classification, and composition of skarn deposits. Economic Geology 77, 745-754. Fridrich, C.J., Mahood, G.A., 1987. Compositional layers in the zoned magma chamber of the Grizzly Peak Tuff. Geology 15, 299-303.
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Fowler, A., Prokoph, A., Stern, R., & Dupuis, C., 2002. Organization of oscillatory
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zoning in zircon: analysis, scaling, geochemistry, and model of a zircon from Kipawa,
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Quebec, Canada. Geochimica et Cosmochimica Acta, 66, 311-328.
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Gaspar, M., Knaack, C., Meinert, L.D., Moretti, R., 2008. REE in skarn systems: A
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LA-ICP-MS study of garnets from the Crown Jewel gold deposit. Geochimica et
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Cosmochimica Acta 72, 185-205.
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Groves, D., McCarthy, T., 1978. Fractional crystallization and the origin of tin deposits in granitoids. Mineralium Deposita 13, 11-26. Holten, T., Jamtveit, B., & Meakin, P., 2000. Noise and oscillatory zoning of minerals. Geochimica et Cosmochimica acta, 64, 1893-1904.
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Ismail, R., Ciobanu, C.L., Cook, N.J., Teale, G.S., Giles, D., Mumm, A.S., Wade, B.,
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2014. Rare earths and other trace elements in minerals from skarn assemblages, Hillside
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iron oxide–copper–gold deposit, Yorke Peninsula, South Australia. Lithos 184, 456-477.
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Jamtveit, B., Hervig, R.L., 1994. Constraints on transport and kinetics in hydrothermal
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Jamtveit, B., Wogelius, R.A., Fraser, D.G., 1993. Zonation patterns of skarn garnets: Records of hydrothermal system evolution. Geology 21, 113-116. Keankeo, W., Hermann, J.R., 2002. The oscillatory intergrowth of feldspars in titanian
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andradite, Little Dromedary, NSW, Australia. European Journal of Mineralogy 14, 379-
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Koo, M.-H., 2012, Skarn Evolution, and Zinc- Lead-Iron Mineralization at the Gagok mine [Ms. Thesis]: Seoul, University of Korea, 80p. Li, Y., Li, X.-H., Selby, D., Li, J.-W., 2018. Pulsed magmatic fluid release for the
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formation of porphyry deposits: Tracing fluid evolution in absolute time from the Tibetan
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Qulong Cu-Mo deposit. Geology 46, 7-10.
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Meinert, L.D., 1992. Skarns and skarn deposits. Geoscience Canada 19, 145-162.
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Page, F.Z., Kita, N.T., Valley, J.W., 2010. Ion microprobe analysis of oxygen isotopes
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in garnets of complex chemistry. Chemical Geology 270, 9-19. Park, C., Choi, W., Kim, H., Park, M.-H., Kang, I.-M., Lee, H.-S., Song, Y., 2017a.
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Oscillatory zoning in skarn garnet: Implications for tungsten ore exploration. Ore
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Park, C., Song, Y., Kang, I.-M., Shim, J., Chung, D., Park, C.-S., 2017b. Metasomatic
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changes during periodic fluid flux recorded in grandite garnet from the Weondong W-
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skarn deposit, South Korea. Chemical Geology 451, 135-153.
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Pirajno, F., 2009. Hydrothermal processes associated with meteorite impacts, hydrothermal processes and mineral systems. Springer, pp. 1097-1130. Rák, Z. S., Ewing, R. C., Becker, U., 2011. Role of iron in the incorporation of uranium in ferric garnet matrices. Physical Review B 84, 155128.
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Smith, M., Henderson, P., Jeffries, T., Long, J., Williams, C., 2004. The rare earth
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elements and uranium in garnets from the Beinn an Dubhaich Aureole, Skye, Scotland,
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UK: constraints on processes in a dynamic hydrothermal system. Journal of Petrology 45,
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457-484.
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van Hinsberg, V.J., Migdisov, A.A., Williams-Jones, A.E., 2010. Reading the mineral record of fluid composition from element partitioning. Geology 38, 847-850. Williamson, B., Herrington, R., Morris, A., 2016. Porphyry copper enrichment linked to excess aluminium in plagioclase. Nature Geoscience 9, 237. Xiao, X., Zhou, T.-f., White, N.C., Zhang, L.-j., Fan, Y., Wang, F.-y., Chen, X.-f.,
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2018. The formation and trace elements of garnet in the skarn zone from the Xinqiao Cu-
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S-Fe-Au deposit, Tongling ore district, Anhui Province, Eastern China. Lithos 302, 467-
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Xu, J., Ciobanu, C.L., Cook, N.J., Zheng, Y., Sun, X., Wade, B.P., 2016. Skarn
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formation and trace elements in garnet and associated minerals from Zhibula copper
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deposit, Gangdese Belt, southern Tibet. Lithos 262, 213-231.
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Yang, K., Heo, S., Hwang, J., Park, S.-G., Koh, S., 2013. Calcite-saturated fluids
425
trapped in fluorite from the Gagok Zn-Pb Skarn Deposit, South Korea. The Canadian
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Mineralogist 51, 613-628.
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Yun, S., Silberman, M., 1979. K-Ar geochronology of igneous rocks in the Yeonhwa-
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Ulchin zinc-lead district and southern margin of the Taebaegsan basin, Korea. Journal of
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Geological Society of Korea 15, 89-99.
430
Zhai, D.-G., Liu, J.-J., Zhang, H.-Y., Wang, J.-P., Su, L., Yang, X.-A., Wu, S.-H.,
431
2014. Origin of oscillatory zoned garnets from the Xieertala Fe–Zn skarn deposit,
432
northern China: In situ LA–ICP-MS evidence. Lithos 190, 279-291.
433
Zhang, Y., Liu, Q., Shao, Y., Li, H., 2017. Fingerprinting the hydrothermal fluid
434
characteristics from LA-ICP-MS trace element geochemistry of garnet in the Yongping
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Cu Deposit, SE China. Minerals 7, 199.
436 437 438
Zhou, J., Feng, C., Li, D., 2017. Geochemistry of the garnets in the Baiganhu W–Sn orefield, NW China. Ore Geology Reviews 82, 70-92.
439
FIGURE CAPTIONS
440 441
Figure 1. A: Geological map of the Gagok skarn deposit (37°09´N, 129°16´E). The
442
intrusive rocks comprise granitic porphyry and the Hongjesa granite. The granitic
443
porphyry is considered to be an associated intrusion. These intrusive rocks intrude
444
Cambrian–Ordovician sedimentary rocks. The ore deposits are mainly distributed at the
445
contacts of the granitic porphyry. The ore deposit comprises a high-temperature Fe–Pb–
446
Zn ore zone and low-temperature Pb–Zn ore zone (Choi et al., 2010). The sampling site
447
was within the Fe–Pb–Zn ore zone. The map was modified from Yang et al. (2013). B: A
448
garnet from the proximal Fe–Pb–Zn ore zone showing continuous growth from a massive
449
skarn (early) to its vein (late). The change from prograde (e.g., garnet, magnetite, and
450
diopside) to retrograde minerals (e.g., amphibole and pyrrhotite) in the fracture zone
451
indicates that space caused by boiling-induced hydrofracturing was maintained until the
452
late retrograde stage. Internal fractures due to multiple brittle deformation events are
453
observed in all garnets. These fractures were filled by native bismuth, magnetite, or
454
pyrrhotite during or after garnet formation. The dashed line indicates the boundary
455
between the massive and massive skarn’s vein. C: Oscillatory zoned garnet with
456
magnetite and diopside in the massive skarn. D: A thick, dark band of oscillatory zoning
457
(T). The T bands exhibit retrograde textures such as dissolution, recrystallization, and
458
resorption. E = pressure-induced texture, which is common in garnet (P); Amp =
459
amphibole; Bi = native bismuth; Di = diopside; Mt = magnetite; Mng = manganite; Po =
460
pyrrhotite; T = thick dark bands; P = pressure-induced texture.
461 462
Figure 2. A: BSE and X-ray elemental mapping (Fe, Al, and Sn) images. The analyzed
463
garnet was divided into three main stages (MS-I, -II, and -III) and thick dark bands (T1
464
and T2) in the massive skarn and its vein. In the BSE image, the garnet is characterized
465
by oscillatory zoning. B: REE patterns of pure andradite (bright zones; La-enriched) and
466
less pure andradite (dark zones; Pr- and Nd-enriched) in massive skarn. C: REE patterns
467
of oscillatory zoned garnet of vein in massive skarn.
468 469
Figure 3. A: BSE image of a garnet showing the analysis points along a transect through
470
the garnet. The garnet is divided into pure andradite (points 1–12), less pure andradite
471
(points s1–s4), and thick dark bands (less pure andradite; T1 and T2). B: Elemental
472
concentrations of pure andradite along the transect. Tin is a representative element that
473
can be compared with the other elements. C: Elemental concentrations of less pure
474
andradite along the transect.
475 476
Figure 4. Schematic diagram showing the growth history of the garnet on a large-scale
477
and smaller, oscillatory zoned scale, which was related to multi-stage magmatic fluid
478
pulsing. (1) Large-scale fluid flux (first-order flux; MS-I) with a superimposed pulsing
479
flux (second-order flux) is released from the associated intrusion. The first-order flux is
480
related to a specific magmatic stage, which was caused by fractional crystallization. The
481
second-order flux was related to magma heterogeneity, such as chemical diffusion in the
482
magma. Both the first- and second-order fluid fluxes were derived from a single cooling
483
magma reservoir. (2) Between the second-order fluid flux events, short-term fluid
484
circulation or stagnation resulted due to the stable magmatic state. This process was
485
responsible for the crystallization of the less pure andradite. (3) After the first-order fluid
486
flux event, the magma was in a steady-state. In this stage, garnet with the lowest Fe3+
487
concentration was crystallized, and large-scale fluid circulation or stagnation occurred.
488
(4) A more evolved stage (MS-II) begins and Sn-saturated fluid is released. (5–6) As in
489
(2) and (3), a magmatic steady-state occurs between each fluid flux event.
1
Highlights
2
1. Oscillatory zoning in skarn garnet resulted from changing magmatic fluid flux
3
2. Pulsed magma degassing led to variable fluid flux into the skarn deposit
4
3. Stable magmatic stages were followed by magma degassing
No declaration of interest statement.
S0024-4937(19)30372-X
DOI:
https://doi.org/10.1016/j.lithos.2019.105213
Reference:
LITHOS 105213
To appear in:
LITHOS
Received Date: 11 February 2019 Revised Date:
2 October 2019
Accepted Date: 2 October 2019
Please cite this article as: Park, C., Park, C., Song, Y., Choi, S.-G., Sequential trace element analysis of zoned skarn garnet: Implications for multi-stage fluxing and flow of magmatic fluid into a skarn system, LITHOS, https://doi.org/10.1016/j.lithos.2019.105213. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V. All rights reserved.
1
ABSTRACT
2
A high-resolution fluid flux–flow model for a shallow crustal system related to skarn
3
formation was established using oscillatory zoning in garnet. In situ analytical methods
4
were used to determine major and trace element contents of andradite-rich and lower-
5
andradite-content garnet zones. Continuous analysis of pure andradite (And87–98) shows
6
these garnets record first- and second-order fluid fluxes. The first-order fluid flux
7
exhibits a stepwise increase in Sn contents and a decrease in the contents of other
8
elements (Ti, V, W, As, Mo, Y, and rare earth elements), indicating that pure andradite
9
records a large range changes in fluid flux from the magma. The second-order flux is
10
evident from oscillatory variations in the contents of Sn and other elements, reflecting
11
small-scale and pulsed changes in the fluid flux from a degassing magma reservoir.
12
Based on the garnet major and trace element variations and mineral textures, these fluid
13
fluxes in the skarn system were controlled by pulsed degassing of a cooling magma.
14
Continuous analysis of oscillatory zoning in garnet with a lower andradite content
15
(And72–81) showed that Sn concentrations increase gradually and other element
16
concentrations decrease gradually. This garnet crystallized from a fluid that was locally
17
equilibrated with a small-scale, stagnant fluid. Thick and low-andradite garnet bands
18
(And62–76) are observed to have grown between the first-order fluid fluxing events. These
19
bands are Al-rich and have retrograde textures, indicating prolonged magma residence
20
after the first-order fluid flux, suggesting that circulating fluids persisted for a relatively
21
long time.
22
Keywords: Garnet; Skarn; Trace elements; Magma processes; Oscillatory zoning
23
24
1
Sequential trace element analysis of zoned skarn
2
garnet: Implications for multi-stage fluxing and flow of
3
magmatic fluid into a skarn system
4 5
Changyun Park1, 2*, Chaewon Park2, Yungoo Song2, and Seon-Gyu Choi3
6
1
7
Mineral Resources (KIGAM), Daejeon 34132, Korea
8
2
Department of Earth System Sciences, Yonsei University, Seoul 120-749, Korea
9
3
Department of Earth and Environmental Sciences, Korea University, Seoul 136-713,
10
Advanced Geo-Materials Research Department, Korea Institute of Geoscience and
Korea
11 12
*Corresponding author: Changyun Park
13
e-mail: [email protected]
14
Tel: +82 54 245 3744
15
Fax: +82 54 245 3759
16 17 18
Manuscript submitted to Lithos as a letter
19
ABSTRACT
20
A high-resolution fluid flux–flow model for a shallow crustal system related to skarn
21
formation was established using oscillatory zoning in garnet. In situ analytical methods
22
were used to determine major and trace element contents of andradite-rich and lower-
23
andradite-content garnet zones. Continuous analysis of pure andradite (And87–98) shows
24
these garnets record first- and second-order fluid fluxes. The first-order fluid flux
25
exhibits a stepwise increase in Sn contents and a decrease in the contents of other
26
elements (Ti, V, W, As, Mo, Y, and rare earth elements), indicating that pure andradite
27
records a large range changes in fluid flux from the magma. The second-order flux is
28
evident from oscillatory variations in the contents of Sn and other elements, reflecting
29
small-scale and pulsed changes in the fluid flux from a degassing magma reservoir.
30
Based on the garnet major and trace element variations and mineral textures, these fluid
31
fluxes in the skarn system were controlled by pulsed degassing of a cooling magma.
32
Continuous analysis of oscillatory zoning in garnet with a lower andradite content
33
(And72–81) showed that Sn concentrations increase gradually and other element
34
concentrations decrease gradually. This garnet crystallized from a fluid that was locally
35
equilibrated with a small-scale, stagnant fluid. Thick and low-andradite garnet bands
36
(And62–76) are observed to have grown between the first-order fluid fluxing events. These
37
bands are Al-rich and have retrograde textures, indicating prolonged magma residence
38
after the first-order fluid flux, suggesting that circulating fluids persisted for a relatively
39
long time.
40
Keywords: Garnet; Skarn; Trace elements; Magma processes; Oscillatory zoning
41
42 43
1. Introduction Skarn deposits are a primary source of Fe, Pb, Zn, W, Cu, and Au, and are commonly
44
generated through fluid–carbonate rock interaction. Magmatic–hydrothermal fluids at
45
temperatures of 450°C–540°C transport these metals into carbonate rocks, and contact
46
and regional metamorphism causes metasomatism of the host rock (Meinert, 1992).
47
Skarn deposits have been extensively studied. The fluid evolutionary paths, including
48
both magmatic and meteoric fluids, are well constrained (Einaudi and Burt, 1982;
49
Meinert, 1992). Traditionally, the flux of skarn-forming fluids is divided into prograde
50
and retrograde stages. Exsolution of H2O and volatile-rich magmatic fluid is responsible
51
for metasomatism of the host rock and formation of calc-silicate minerals in the prograde
52
stage. Incursions of meteoric fluids contribute to metal precipitation in the retrograde
53
stage (Einaudi and Burt, 1982). Recently, numerous studies have documented a pulsing
54
magmatic system in the ore system, and determined this is caused by a gradually cooling
55
or multiply recharged magmatic intrusion (Chelle-Michou et al., 2017; Li et al., 2018;
56
Williamson et al., 2016). Pulsing magmatic processes could cause several prograde or
57
retrograde stages in a skarn deposit. However, few studies have examined in detail the
58
relationship between magmatic processes and fluid flux–flow in skarn deposits. For this
59
purpose, in situ microanalyses of the gangue or ore minerals are essential (Cook et al.,
60
2016).
61
Here we describe a crystalline grandite garnet with unique trace element features from
62
a Fe-Pb–Zn skarn deposit, which shows oscillatory zoning on both a small- and large-
63
scale. High-resolution analysis by laser ablation–inductively coupled plasma–mass
64
spectrometry (LA–ICP–MS) of this zoning reveals information about not only the
65
behavior of the trace elements in the skarn system, but also the reactivation and residence
66
time of the magma in relation to periodic boiling of the magmatic fluid. We also consider
67
how the multi-scale changes in garnet chemistry are related to the changes in the
68
circulation of the magmatic fluid. The results of this study contribute to our
69
understanding of the evolution of fluid flow in skarn systems.
70 71
2. Background
72
Garnet is a common mineral in skarn deposits and, due to its structural stability and
73
chemical attributes, can provide a continuous record of geological processes (e.g., Baxter
74
et al., 2013). Numerous studies have reported that trace element behavior in skarn garnet
75
is strongly related to changes in fluid composition and growth mechanisms (Gaspar et al.,
76
2008; Ismail et al., 2014; Jamtveit and Hervig, 1994; Jamtveit et al., 1993; Park et al.,
77
2017a, b; Zhai et al., 2014). These studies have reported that andradite (Fe-rich garnet) is
78
mainly crystallized in open systems during hydrofracturing. The trace element contents
79
of such garnet reflect the composition of the magmatic–hydrothermal fluid as a result of
80
surface adsorption, whereas grossular (Al-rich garnet) can be formed by the mixing of
81
meteoric fluids under equilibrium in a closed system (e.g., the fault sealing stage). The
82
oscillatory zoning in skarn garnet records open and closed system cycles in the plumbing
83
system of siliceous fluids (Clechenko and Valley, 2003; Jamtveit and Hervig, 1994).
84
Meanwhile, Crowe et al. (2001) and D'Errico et al. (2012) show that some Fe-enriched
85
garnets reflect high magmatic hydrothermal components, but others may reflect low
86
magmatic hydrothermal components. They reported that variable influxes of meteoric
87
fluid affect oxygen isotope rather than Al or Fe concentrations of garnet.
88 89
3. Geological Setting
90
3.1. Gagok skarn Fe-Pb–Zn deposit
91
Southern Korea was in a subduction-related environment at the border between the
92
Eurasian and Pacific (or Philippine) plates from the Middle Jurassic to Cenozoic
93
(Chough et al., 2000). This tectonic setting caused extensive calc-alkaline magmatism,
94
and the formation of the Taebaeksan ore belt (120 to 40 Ma), which produced various
95
skarn-, Carlin-, and greisen-type deposits (Choi et al., 2005). In the Taebaeksan ore belt,
96
skarn is the most representative ore-forming rock and comprises a wide range of marine
97
sedimentary rocks, although it is mainly limestone. The Gagok deposit is a skarn Fe–Pb–
98
Zn deposit in this belt. The geology of the region comprises Precambrian Hongjesa
99
granite basement and Cambrian–Ordovician sedimentary rocks (Fig. 1A). An associated
100
intrusion in the studied area is a Cretaceous granitic porphyry (A/CNK = 0.98–1.06; I-
101
type) (Fig. 1A) (Koo, 2012), with a K–Ar emplacement age of 72.6 ± 2.2 Ma (Yun and
102
Silberman, 1979).
103
The Gagok deposit is a stratabound and structurally constrained skarn deposit. The
104
deposit has been divided into a proximal skarn with Fe–Zn–Pb ore and a distal Zn–Pb
105
skarn (Table S1) (Choi et al., 2010). The Fe–Zn–Pb skarn occurs closest to the associated
106
intrusion, and consists of magnetite, garnet, pure diopside, and native bismuth from the
107
early prograde stage (Fig. 1B–D). Representative ore minerals, such as sphalerite, galena,
108
and pyrrhotite, formed during the retrograde stage along with fluorite, amphibole, quartz,
109
and calcite. In contrast to the Fe–Zn–Pb skarn, prograde minerals in the distal Zn–Pb
110
skarn zone are spessartine (Al1Ad10Gr10Sp79), grossular (Al1Ad45Gr45Sp9), hedenbergite
111
(Hd51Di29Jo20), in addition, carbonate replacement is common (Table S1) (Choi et al.,
112
2010; Koo, 2012). Retrograde processes are dominant in this distal zone, and most of the
113
Zn–Pb ore is related to the distal skarn (Table S1). Euhedral sphalerite and galena are the
114
ore minerals, and retrograde scheelite, quartz, calcite, epidote, cassiterite, and chlorite are
115
dominant. A study of fluid inclusions in fluorite showed that the Gagok polymetallic
116
deposit formed at shallow depths (~1 km), and salinities of boiling fluid range from 7.4–
117
1.2 wt.%(Yang et al., 2013).
118 119
4. Results
120
4.1. Garnet occurrence and growth texture
121
The garnet analyzed in this study was collected from the fracture zone of a garnet-
122
dominated zone in exoskarn (Fe-skarn) (Figs 1A–B and S1; Table S1). Garnet in the
123
proximal skarn has well-developed oscillatory zoning, coexists with diopside or
124
magnetite, and exhibits pressure induced textures (P) (Fig. 1B, C, E). Thick dark bands
125
(T bands) are present in the garnet (Fig. 1B, D), which often show retrograde textures
126
such as replacement and partial dissolution. Manganese oxide precipitates are present in
127
the dissolution regions (Fig. 1B, D). The garnets are considered to have started growing
128
in the massive skarn zone, and progressively grew and became interconnected with the
129
overgrowth garnet in the vein formed by hydrofracture (Figs 1B and S1).
130 131 132
4.2. Major and trace element data The analytical methods are described in detail in the supplementary materials (S2).
133
Electron microprobe data and element maps (Fe, Al, and Sn) show that the studied garnet
134
is a Fe-dominant garnet (Figs 2A and S1; Table S2). This grandite garnet, which
135
crystallized continuously from a massive skarn to its vein, can be divided into three main
136
zones (MS-I, -II, and -III) according to major element data (Fig. 2A). The studied garnet
137
contains bright zones of pure andradite (And91–98) and dark zones of garnet with a lower
138
andradite component (And72–81). The composition of the T bands (T1 and T2) is And62–76.
139 140
The pure andradite bands in the massive skarn (MS-I stage) have positive Eu anomalies and are light REE-enriched, particularly with respect to La (Fig. 2B). The
141
positive Eu anomalies reflect garnet growth in chlorine-enriched hydrothermal fluids
142
(Gaspar et al., 2008). Bands of less pure andradite garnet in the massive skarn are
143
characterized by Pr- and Nd-enriched REE patterns, and have variable Eu anomalies (Fig.
144
2B). The pure andradite bands in MS-II and -III stages have positive Eu anomalies and
145
are light REE-enriched, particularly with respect to La and Ce (Fig. 2C). Bands with less
146
pure andradite are characterized by garnet with Pr- and Nd-enriched REE patterns, which
147
have slightly positive Eu anomalies (Fig. 2C).
148
Trace element analyses were performed continuously from the garnet cores to rims
149
(Table S3; Fig. 3A). The pure and less pure andradite bands were analyzed separately,
150
due to the different growth mechanisms described above (Table S3). It was then possible
151
to further subdivide the garnet bands into sub-stages (1–2 and s1–s3 in the MS-I massive
152
skarn; 3–11 and s4 in MS-II and 12 in MS-III in garnet vein; T1 and T2 bands in garnet
153
vein; Fig. 3). In the pure andradite bands in all garnet zone, each element shows abrupt
154
changes at the stage boundaries. In overgrowth garnet at vein in massive skarn, a marked
155
increase in Sn contents occurs in MS-II (close to 20,000 ppm; Fig. 3B) as compared with
156
the MS-I massive skarn. In contrast to the Sn enrichment in MS-II, other elements (Ti, V,
157
As, Mo, Y, and REE) are depleted in MS-II (Fig. 3B). In the MS-III, only the REE are
158
enriched as compared with MS-II (Fig. 3B). The sub-stage trends of each element (1 to
159
12) show oscillatory variations, and the Sn contents are negatively correlated with most
160
other elements in the MS stages (Fig. 3B). The U and W concentrations in all MS stages
161
were relatively constant during garnet growth, however, U and W show oscillatory
162
variations in the sub-stages (3–12; Fig. 3B). The elemental trends for the less pure
163
andradite, including the T1 and T2 bands, are different. Tin concentrations increase up to
164
4500 ppm, but the concentrations of most other elements decrease from S1 to T2 (Fig.
165
3C). These trends are not characterized by the stepwise changes observed in pure
166
andradite, but rather by a continuous increase or decrease.
167 168
5. DISCUSSION
169
5.1. Large-scale (first-order) and pulsed (second-order) changes in magmatic fluid flux
170
All parts of the garnets are characterized by light REE-enriched patterns (Fig. 2B–C).
171
Although garnet has high DHREE (D; distribution coefficient), extremely low heavy REE
172
concentrations in the granite-derived fluid are responsible for the light REE-enriched
173
garnet (Gaspar et al., 2008). However, the REE patterns of each bright and dark zone
174
show different features. The dark bands are characterized by enrichment of Pr and Nd,
175
indicating equilibrium crystallization (Van Westernen et al., 2000). Compared with the
176
dark bands, light REE (especially La and Ce) are enriched in the pure andradite (Fig. 2B–
177
C). These REE patterns mimic those of a magmatic–hydrothermal fluid (e.g., the light
178
REE-enriched pattern of granitic porphyry; Koo, 2011). As such, La- or Ce-enriched
179
garnet crystallized in disequilibrium via a surface adsorption mechanism and the REE
180
patterns in the pure andradite reflect that of the magmatic–hydrothermal fluid (Gaspar et
181
al., 2008).
182
Other trace elements in skarn garnet can also reflect their relative abundances in the
183
skarn-forming fluid, and provide information regarding intermittent and cyclical fluid
184
fluxing (Ismail et al., 2014; Jamtveit et al., 1993; Park et al., 2017a, b; Xiao et al., 2018;
185
Zhang et al., 2017). We should first consider changes in the oxygen fugacity during the
186
growth of the zoned garnet. It is possible that incorporation of multi-valence elements
187
(e.g., Sn and W) into garnet structure due to changes in fluid oxidation state was variable,
188
meaning that the stepwise trends in Fig. 3B could have been caused by changes in
189
oxygen fugacity. Previous studies have demonstrated a relationship between the
190
oxidation state of elements such as Sn, W, and their incorporation into garnet (Xu et al.,
191
2016; Zhou et al., 2017). Sn4+ and W6+ are more readily incorporated into the octahedral
192
site of garnet than in their reduced state. However, the behavior of Sn is different to W in
193
the studied garnet (Fig. 3B). This means that the changes in elemental concentrations
194
cannot be explained by variable oxygen fugacity. In fact, magma boiling in a skarn
195
system is closely related to the oxygen fugacity. When the boiling stage occurs during
196
brittle deformation (i.e., the hydrofracturing stage), the oxygen fugacity, water/rock ratio,
197
interface control of mineral/fluid, and temperature can all increase (Keankeo and
198
Hermann, 2002). These conditions can enhance the uptake of trace elements into garnet.
199
However, considering the different trends between Sn and the other elements, oxygen
200
fugacity doesn’t seem to be a primary control factor on garnet trace element chemistry.
201
Second, we should consider incorporation energy in garnet crystal. U contents show
202
similar variation with REE contents (Fig. 3B). In this case, it is possible that the
203
incorporation energy of U and REE in the dodecahedral site significantly decreases with
204
the increase in Fe3+ cations within the neighboring tetrahedral site (Rak et al., 2011; Deng
205
et al., 2017). However, as shown the Fig. 3b and table S2, concentrations of Fe3+ in MS
206
stages are not correlated with U and REE contents. Therefore, we considered that
207
incorporation energy of element in garnet crystal doesn’t seem to be a primary control
208
factor on garnet trace element chemistry, also.
209
Third one is about fluid composition. This study suggest that the fluid composition
210
was the primary control on garnet trace element chemistry. The magmatic fluid flux can
211
be divided into first-order (MS-I, -II, and -III) and second-order fluxes (1–12) (Fig. 3A–
212
B). Stepwise variations in the trace element composition of the first-order fluid flux
213
indicate that the garnet recorded release of an evolved fluid (i.e., late-stage) during
214
magma crystallization (Fig. 3B). For example, in the transition from MS-I to -II, the
215
abrupt increase in Sn compared with other elements suggests the influence of an evolved
216
magmatic–hydrothermal fluid, as Sn is diagnostic of late-stage crystallization in a silicic
217
magma (Fig. 3B) (Chen et al., 2000; Groves and McCarthy, 1978; Pirajno, 2009; Zhou et
218
al., 2017). The first-order flux is a large-scale fluid pulse, which reflects a specific
219
magmatic evolutionary stage, whereas the second-order flux reflects small-scale
220
processes superimposed on the first-order fluid flux (red and blue arrows in Fig. 3B).
221
There are two proposed models to explain the first- and second-order fluid fluxes.
222
These are the multiple magma recharge event (i.e., periodic magma injection)
223
(Williamson et al., 2016) and the multiple pulsing fluid models during cooling of the
224
source pluton (i.e., the associated intrusion) (Chelle-Michou et al., 2017; Li et al., 2018).
225
Williamson et al. (2016) demonstrated that excess Al in oscillatory zoned plagioclase is
226
caused by multiple magma injections. This allows incompatible elements, such as Cu, to
227
be periodically concentrated in the late-stage melt. The multiple recharging by less
228
evolved melts fertilizes the granitoid magma body at shallow crustal depths. Uranium
229
concentrations do not change significantly across the studied garnet in relation to the
230
first-order fluid flux (Fig. 3B). U is a highly incompatible element and less abundant in a
231
less evolved melt. In 3, 5, 8, and 11 in Figure 3B, concentrations of Sn increase when
232
concentrations of U decrease. This is additional evidence showing that the fluid flux
233
mechanism does not follow a multi-recharge model. In comparison, Chelle-Michou et al.
234
(2017) and Li et al. (2018) proposed that successive degassing of the magmatic intrusion
235
mostly controls the formation of a skarn deposit. Although the initial degassing is
236
responsible for 50–75 wt.% of the fluid released and has a major role in Cu deposition,
237
the relative flux of each element from each degassing stage is different. The oscillatory
238
zoning, such as in 3, 5, 8, and 11 in Fig. 3B, shows similar concentrations with those of
239
MS-I. This indicates that the degassing event responsible for MS-II was possibly related
240
to chemical diffusion in the magma (Fridrich and Mahood, 1987). As such, the garnet
241
trace element chemistry was affected by fluid injections from the associated intrusion,
242
rather than by magma recharge of primitive melts. Based on the stepwise and oscillatory
243
garnet zoning, and relationship between Sn and other elements (especially U), we
244
propose that this skarn system was controlled by pulsing fluid discharge from a cooling
245
single intrusion.
246
However, in MS-III, most element concentrations decrease. This could be caused by
247
an input of meteoric fluid. Although the garnet composition in MS-III is nearly pure
248
andradite, D’Errico et al. (2012) and Jamtveit et al. (1994) have reported that Fe-rich
249
garnet can form when the magmatic–hydrothermal fluid component is low. Only the REE
250
and U contents increased in MS-III, and the positive Eu anomalies and the nearly pure
251
andradite garnet, indicate that MS-III may be result by mixing of meteoric fluid and
252
highly evolved hydrothermal fluid. As mentioned before, Sn is diagnostic of late-stage
253
crystallization in a silicic magma, however, Sn contents decrease in MS-III. This
254
suggests that cassiterite may have crystallized between MS-II and -III because cassiterite
255
has been frequently observed in the low-temperature skarn zone (Koo, 2012).
256 257 258
5.2. Stagnant fluid related to magma residence Aluminum-rich garnet is related to fluid mixing and circulation in shallow crustal
259
systems (Clechenko and Valley, 2003; Crowe et al., 2001; D’Errico et al., 2012; Page et
260
al., 2010), and equilibrium conditions in skarn systems. Although most of the Al-rich
261
garnet has heavy REE-enriched patterns, the less pure andradite has light REE-enriched
262
patterns. This means the skarn-forming fluid had extremely low concentrations of heavy
263
REE.
264
The Pr- and Nd-enriched REE patterns of the less pure andradite are distinct from the
265
REE patterns of the pure andradite bands (Fig. 2B–C). This indicates that the less pure
266
andradite crystallized in a closed system (Smith et al., 2004). The trace element data for
267
the less pure andradite indicate that it formed in an equilibrium state related to stagnation
268
in magmatic fluid activity. The low magmatic fluid activity formed pressure-induced
269
textures due to backflow (Fig. 1B, E) (Ciobanu and Cook, 2004) and fault sealing in the
270
skarn system. Aluminum-rich garnet might have also formed due to development of a
271
different fluid pathway. However, Keankeo and Hermann (2002) reported that Fe-rich
272
skarn garnet crystallizes at high fluid flow during the fracturing stage, with decreasing
273
pressure and increasing temperature. In such an environment, the fluid is likely to re-
274
infiltrate along an existing fault and the garnet can continuously interact with the fluid.
275
The less pure andradite bands (s1, s2, s3, s4, T1, T2 band) between the oscillatory
276
zoning reflect multiple influxes of meteoric fluid between the multiple influxes of
277
magmatic fluid (Fig. 3B–C) (D’Errico et al., 2012). Under equilibrium conditions,
278
mineral trace element contents can be used to reconstruct the composition of the
279
equilibrated fluid (van Hinsberg et al., 2010). The Sn concentration of the fluid increased
280
or decreased, as recorded by the composition of the pure andradite bands (Fig. 3C). Most
281
other trace element concentrations also decreased in the fluid (Fig. 3C). The geochemical
282
trends of s1, s2, s3, and s4 indicate that the dominant fluid involved in the crystallization
283
of the less pure andradite bands was a stagnant, magmatically exsolved fluid, rather than
284
a meteoric fluid. The T bands, which occur between MS-I, -II, and -III (Fig. 3),
285
correspond to abrupt stepwise changes in the elemental composition of garnet before and
286
after the deposition of the bands (Fig. 3B). These suggest that the magma remained in a
287
long-term, steady-state throughout skarn formation (e.g., MS-I and -II; Fig. 3B). The T1
288
band has retrograde textures (e.g., recrystallization and dissolution fabrics), indicating
289
that the reaction between pre-existing garnet and fluids lasted longer than for the less
290
pure andradite in the MS stages (Fig. 1B). The T bands are also characterized by the
291
highest concentrations of Al (And70–74), meaning that the T bands were more influenced
292
by a steady-state magmatic system than s1–s4 (Crowe et al., 2001). Most elements are
293
depleted in the T2 band, indicating that the skarn-forming fluid was diluted by an
294
increasing amount of meteoric fluid. Based on the mineral textures and garnet chemistry
295
of the T bands at the end of the large-scale evolutionary stages (i.e., MS-I and -II), there
296
was an increased magma residence time, which prolonged the circulation of the skarn-
297
forming fluids.
298 299
5.3. Internal self-organization vs. extrinsic fluctuation
300
(Fowler et al., 2002; Holten et al., 2000). An internal self-organization process
301
produces nonlinearities that may cause chemical oscillatory patterns. These patterns are
302
said to be self-organized that is, they arise from intrinsic crystal growth processes rather
303
than extrinsic or bulk system scale fluctuations (Fowler et al., 2002; Holten et al., 2000).
304
In our results, compared to amount of W contents in the pure andradite bands, W in less
305
pure andradite bands decrease continuously (Fig. 3B–C). Because prograde garnet cannot
306
co-precipitate with scheelite formed during retrograde skarn, this result shows that the
307
internal self-organization and local re-equilibrium of fluids could in fact generate this
308
oscillatory zoning, which means garnet zoning can be grown by a combination of both
309
intrinsic and extrinsic processes. However, geological condition such as fluid fluctuation
310
model in the skarn system and a mineralogical characteristic such as linearity of zoning
311
sequence (Fig. 1SA and Fig 1B) in studied sample show these garnet grains are mainly
312
crystallized by extrinsic or bulk system fluctuation rather than internal self-organization.
313
314 315
6. Conclusions Skarn garnet zoning records not only the intermittent and pulsing fluid flux from an
316
exsolved magmatic fluid, but also multiple stages of magma residence (Fig. 4). At
317
shallow crustal depths (<2 km), fluid flux and flow in the skarn system was controlled by
318
a pulsing magmatic system. Large-scale and small-scale oscillatory changes in fluid flux
319
were identified, which originated by continuous magma crystallization and pulsed fluid
320
discharge during cooling of a single magma body. Between these states, the magma was
321
in a steady-state, and was then followed by a large-scale fluid fluxing event.
322 323 324
Acknowledgements This work was supported by the Korea Meteorological Administration Research and
325
Development Program Grant KMI 2018-01910 and National Research Foundation of
326
Korea Grant No. 2018051418, and the Basic Research Project (19-3214) of the Korea
327
Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of
328
Science and ICT.
329
330 331 332 333
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432
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433
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434
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439
FIGURE CAPTIONS
440 441
Figure 1. A: Geological map of the Gagok skarn deposit (37°09´N, 129°16´E). The
442
intrusive rocks comprise granitic porphyry and the Hongjesa granite. The granitic
443
porphyry is considered to be an associated intrusion. These intrusive rocks intrude
444
Cambrian–Ordovician sedimentary rocks. The ore deposits are mainly distributed at the
445
contacts of the granitic porphyry. The ore deposit comprises a high-temperature Fe–Pb–
446
Zn ore zone and low-temperature Pb–Zn ore zone (Choi et al., 2010). The sampling site
447
was within the Fe–Pb–Zn ore zone. The map was modified from Yang et al. (2013). B: A
448
garnet from the proximal Fe–Pb–Zn ore zone showing continuous growth from a massive
449
skarn (early) to its vein (late). The change from prograde (e.g., garnet, magnetite, and
450
diopside) to retrograde minerals (e.g., amphibole and pyrrhotite) in the fracture zone
451
indicates that space caused by boiling-induced hydrofracturing was maintained until the
452
late retrograde stage. Internal fractures due to multiple brittle deformation events are
453
observed in all garnets. These fractures were filled by native bismuth, magnetite, or
454
pyrrhotite during or after garnet formation. The dashed line indicates the boundary
455
between the massive and massive skarn’s vein. C: Oscillatory zoned garnet with
456
magnetite and diopside in the massive skarn. D: A thick, dark band of oscillatory zoning
457
(T). The T bands exhibit retrograde textures such as dissolution, recrystallization, and
458
resorption. E = pressure-induced texture, which is common in garnet (P); Amp =
459
amphibole; Bi = native bismuth; Di = diopside; Mt = magnetite; Mng = manganite; Po =
460
pyrrhotite; T = thick dark bands; P = pressure-induced texture.
461 462
Figure 2. A: BSE and X-ray elemental mapping (Fe, Al, and Sn) images. The analyzed
463
garnet was divided into three main stages (MS-I, -II, and -III) and thick dark bands (T1
464
and T2) in the massive skarn and its vein. In the BSE image, the garnet is characterized
465
by oscillatory zoning. B: REE patterns of pure andradite (bright zones; La-enriched) and
466
less pure andradite (dark zones; Pr- and Nd-enriched) in massive skarn. C: REE patterns
467
of oscillatory zoned garnet of vein in massive skarn.
468 469
Figure 3. A: BSE image of a garnet showing the analysis points along a transect through
470
the garnet. The garnet is divided into pure andradite (points 1–12), less pure andradite
471
(points s1–s4), and thick dark bands (less pure andradite; T1 and T2). B: Elemental
472
concentrations of pure andradite along the transect. Tin is a representative element that
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can be compared with the other elements. C: Elemental concentrations of less pure
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andradite along the transect.
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Figure 4. Schematic diagram showing the growth history of the garnet on a large-scale
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and smaller, oscillatory zoned scale, which was related to multi-stage magmatic fluid
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pulsing. (1) Large-scale fluid flux (first-order flux; MS-I) with a superimposed pulsing
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flux (second-order flux) is released from the associated intrusion. The first-order flux is
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related to a specific magmatic stage, which was caused by fractional crystallization. The
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second-order flux was related to magma heterogeneity, such as chemical diffusion in the
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magma. Both the first- and second-order fluid fluxes were derived from a single cooling
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magma reservoir. (2) Between the second-order fluid flux events, short-term fluid
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circulation or stagnation resulted due to the stable magmatic state. This process was
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responsible for the crystallization of the less pure andradite. (3) After the first-order fluid
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flux event, the magma was in a steady-state. In this stage, garnet with the lowest Fe3+
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concentration was crystallized, and large-scale fluid circulation or stagnation occurred.
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(4) A more evolved stage (MS-II) begins and Sn-saturated fluid is released. (5–6) As in
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(2) and (3), a magmatic steady-state occurs between each fluid flux event.
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Highlights
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1. Oscillatory zoning in skarn garnet resulted from changing magmatic fluid flux
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2. Pulsed magma degassing led to variable fluid flux into the skarn deposit
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3. Stable magmatic stages were followed by magma degassing
No declaration of interest statement.