Available via license: CC BY-NC-ND 4.0
Content may be subject to copyright.
Journal Pre-proof
Ore-forming processes and mechanisms of the Hongshan skarn
Cu–Mo deposit, Southwest China: Insights from mineral chemistry,
fluid inclusions, and stable isotopes
Xue Gao , Li-Qiang Yang , Han Yan , Jian-Yin Meng
PII: S2666-2612(20)30003-1
DOI: https://doi.org/10.1016/j.oreoa.2020.100007
Reference: OREOA 100007
To appear in: Ore and Energy Resource Geology
Received date: 2 March 2020
Revised date: 10 May 2020
Accepted date: 18 June 2020
Please cite this article as: Xue Gao , Li-Qiang Yang , Han Yan , Jian-Yin Meng , Ore-forming pro-
cesses and mechanisms of the Hongshan skarn Cu–Mo deposit, Southwest China: Insights from min-
eral chemistry, fluid inclusions, and stable isotopes, Ore and Energy Resource Geology (2020), doi:
https://doi.org/10.1016/j.oreoa.2020.100007
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.
©2020 Published by Elsevier Ltd.
This is an open access article under the CC BY-NC-ND license.
(http://creativecommons.org/licenses/by-nc-nd/4.0/)
1
Highlights
Ore-forming fluids are of predominating magmatic–hydrothermal origin at Hongshan
Positive and uniform sulfur isotopes suggest the magmatic sulfur with predominant H2S
Metals mainly precipitated due to the increased pH caused by water-rock interaction,
decreasing temperature and decompression boiling
2
Ore-forming processes and mechanisms of the Hongshan skarn Cu–Mo deposit,
Southwest China: Insights from mineral chemistry, fluid inclusions, and stable
isotopes
Xue Gaoa, b, Li-Qiang Yanga, *, Han Yana, Jian-Yin Menga, c
a State Key Laboratory of Geological Processes and Mineral Resources, China
University of Geosciences, Beijing 100083, China
b ARC Research Hub for Transforming the Mining Value Chain & CODES, Centre for
Ore Deposit and Exploration Science, University of Tasmania, TAS 7005, Australia
c Minmetals Exploration & Development Co., Ltd., Beijing 100010, China
*Corresponding author: Li–Qiang Yang
State Key Laboratory of Geological Processes and Mineral Resources
China University of Geosciences
29# Xue-Yuan Road, Haidian District
Beijing 100083, China
Phone: (+86-10) 8232 1937 (O)
Fax: (+86-10) 8232 1006
Email: lqyang@cugb.edu.cn
Submitted to: Ore & Energy Resource Geology
3
Abstract
The Hongshan skarn Cu–Mo deposit is located in the southern Yidun terrane, SE
Tibet Plateau, with more than 78.7 Mt resources (Cu: 0.64 Mt @ 1.23 %, Mo: 5769 t
@ 0.03 %). The ore deposit was spatially and temporally associated with
post-subduction Late Cretaceous monzogranite porphyries. Detailed geological
mapping and deep drill-hole loggings reveal the vertical skarn zonation patterns of
pyroxene skarn – garnet skarn – magnetite skarn – pyrrhotite-chalcopyrite skarn –
garnet skarn – pyroxene skarn away from the marble, which is similar with typical
skarn Cu deposit worldwide. Three hydrothermal stages have been recognized at
Hongshan. They are characterized by assemblages of prograde skarn (stage 1),
retrograde skarn and Cu–Fe–Mo sulfides (stage 2), and Pb–Zn sulfides associated
with calcite and quartz (stage 3). Prograde skarns contain mainly andraditic garnet
(two series: And37–82Gro17–61Spe+Alm+Pyr0–4 and andradite) and pyroxene (Di64–
88Hd12–35) resulted from the interaction between magmatic-hydrothermal fluids and
carbonate wall-rocks. Retrograde skarn mineralogy is controlled by hydrous Mg–
Fe-rich silicate minerals, such as tremolites, actinolites, and epidotes. Petrographical
and microthermomertic studies on fluid inclusions (FIs) in garnet, epidote, quartz and
calcite from the three stages reveal four types of fluid inclusions: vapor CO2–Liquid
CO2–H2O (C-type), vapor-rich two-phase inclusions (V-type), liquid-rich inclusions
(L-type) and halite (sylvite)-bearing hypersaline inclusions (H-type). The C-type,
L-type and V-type FIs within the garnet of stage 1 have homogenization temperatures
between 400 and 550°C, and salinities of 3.9–11.5 wt% NaCl eqv. A boiling fluid
4
inclusion assemblage with coexisting L-type and V-type FIs was defined within the
epidote and quartz of stage 2. The fluids of stage 3 are characterized by lower
homogenization temperatures of 100–300°C, developing a fluid inclusion assemblage
defined solely by L-type FIs. The wide range of calculated δ18OH2O values in garnet
(2.0 to 13.1 ‰), magnetite (10.9 to 26.3 ‰), tremolite (15.9 to 16.4 ‰) and sericite
(10.5 ‰) further indicate the mixing of δ18O-enriched components with magmatic
fluids. Sulfur isotope compositions of sulfides have a narrow range of δ34S values,
ranging from 3.5 to 5.4 ‰, consistent with a magmatic origin and reducing conditions
throughout the process of sulfide precipitation. The increased pH caused by
water-rock interaction and CO2 degassing, decreasing temperatures and
decompression boiling could be crucial for the extensive ore deposition.
Key words: Fluid inclusion; D–S–O isotopes; Quartz LA–ICP–MS trace element;
Hongshan Cu–Mo deposit; Yidun Terrane
1. Introduction
Skarn deposits are one of the most abundant ore types in the Earth’s crust. They
are typically found within or adjacent to hypabyssal intrusions emplaced into
carbonate rocks (Meinert et al. 2005). Studies of skarn deposits elsewhere have
confirmed the magmatic origin of fluids involved in the early prograde anhydrous
stage of skarn-forming metasomatism (e.g., Einaudi et al. 1981; Layne et al. 1991;
Bowman 1998; Meinert et al. 2003), whereas the source of fluids that responsible for
5
the retrograde hydrous alteration is still under debate. Some studies have shown that
the formation of both anhydrous and hydrous assemblages only involved magmatic
fluids (e.g., Meinert et al. 2003; Williams-Jones et al. 2010). By contrast, numerous
studies verified the presence of meteoric water in late-stage of skarn mineralization
(Yücel Öztürk et al. 2008; Palinkaš et al. 2013; He et al. 2015; Zhu et al. 2015).
The Yidun Terrane is located between the Songpan–Garze Fold Belt and the
Qiangtang Block in SE Tibet. Extensive Mesozoic magmatism that are spatially and
genetically associated with large polymetallic mineral deposits occurred in the Yidun
Terrnae (Fig. 1). Late Triassic porphyry- or skarn-type Cu deposits in the southern
Yidun Terrane, including Xuejiping, Pulang and Langdu, have been described by
many workers (e.g., Hou et al. 1993, 2003; Leng et al. 2012), but the Late Cretaceous
Mo-dominant mineralization in this belt has been recognized in recent two decades (Li
et al. 2007; Wang et al. 2015a; Yang et al. 2018). The Hongshan deposit has been
originally described as a typical skarn deposit that developed adjacent to the Late
Triassic quartz diorite porphyries, resembling those other skarn deposits such as
Langdu in the southern Yidun Terrane (Hou et al. 2003; Zeng et al. 2004). However,
further prospecting combined with detailed geochronological studies revealed the
Late Cretaceous intrusions (Xu et al, 2006; Wang et al. 2011), providing evidences for
multiple metallogenic events. It has beensuggested that two-stage metallogenic
events including the Late Triassic skarn Cu-polymetallic mineralization and Late
Cretaceous porphyry Cu–Mo mineralization occurred in the Hongshan district (Xu et
al. 2006; Li et al. 2013). Other workers have argued that both the skarn and porphyry
6
mineralization are genetically related to the Late Cretaceous stocks (Peng et al. 2014;
Yang et al. 2015, 2016a, 2016b). Nevertheless, their conclusions lack some solid
geological evidences, such as typical skarn zonation patterns from intrusions to
marble front and the corresponding change of Cu grade. Re–Os dating of molybdenite
and pyrrhotite from skarn ores at Hongshan yielded isochron ages of 79.7 ± 3.1 Ma
and 79 ± 16 Ma (Zu et al. 2015), indicating that Hongshan formed in the Late
Cretaceous. In addition to the mineralization epochs, several recent studies have
examined the prograde skarn mineralogy (garnet and pyroxene; Gao et al. 2014;
Peng et al. 2015; Zu et al. 2019), fluid inclusions (Li et al. 2013; Peng et al. 2016) and
C–D–O–S–Pb–Cu stable isotopes (Song et al. 2006; Wang et al. 2008; Li et al. 2013;
Zu et al. 2016; Wang et al. 2017a, 2017b). However, studies on retrograde skarn
minerals and different stages of quartzes which are closely related to the evolution of
ore-forming fluids have not been reported.
Mineral chemistry, fluid inclusion and stable isotope of the skarn and
hydrothermal mineral assemblages from the Hongshan district were analyzed in this
study to better understand the genesis of this deposit and this kind of mineralization.
Compared with the previous studies, we reveal the skarn zonation by carrying out
detailed geological mapping and drill-hole loggings. We also firstly present the results
of D–O isotopic compositions of tremolite and sericite, as well as color CL and trace
elements of quartz from different paragenetic stages at Hongshan. These obtained
results allow us to: (1) constrain the physiochemical conditions during skarn and ore
formation; (2) determine the nature, origin and evolution of the mineralizing fluids; (3)
7
reveal the mechanisms of ore mineral precipitation.
2. Geological Setting
2.1 Regional Geology
The Yidun Terrane is the largest and well-preserved arc terrane in the Sanjiang
Tethys Domain, Southwest China (Hou et al., 1993, 2003). It was formed in response
to the westward subduction of Garze–Litang oceanic slab during Late Triassic (Pan et
al., 2012; Deng et al., 2014a, 2014b, 2017), with its widths varying from 50 to 100 km
and length of more than 500 km. The Yidun Terrane is bounded by the Garze–Litang
Suture Zone in the east, the Jinshajiang Suture Zone in the west, and the Lijiang–
Yanyuan Depression of western Yangtze Block in the southeast (Fig. 1). It consists of
the eastern Yidun Terrane and Zhongza Block from east to west (Reid et al., 2005).
The Zhongza Block is mainly composed of Paleozoic carbonates and mafic lavas, and
these rocks have experienced deformation and metamorphism during the Early
Triassic collision between the Yidun Terrane and Qiangtang–Changdu Block along
Jinshajiang Suture Zone (Reid et al., 2007). The eastern Yidun Terrane is almost
entirely overlain by the Triassic flysch sedimentary sequence and Late Triassic
subvolcanic to volcanic rocks. Regional NW–SE-trending faults paralleling to the
Garze–Litang Suture Zone are widely developed within the Yidun Terrane and provide
conduits for the ascent of magmas and mineralized materials.
The eastern Yidun Terrane is further divided into northern and southern portions
based on their distinctive volcano-plutonic rock types, mineralization styles and
8
lithospheric architecture (Gao et al., 2018). The northern Yidun Terrane (NYT) mainly
develops Late Triassic bimodal volcanism and associated volcanic massive sulfide
Ag–Cu–Pb–Zn deposits (Hou et al., 2003). Whereas, the southern Yidun Terrane
(SYT) is dominated by calc-alkaline andesite and dacite, minor alkaline basalt and
dioritic-monzonitic hypabyssal stocks (Wang et al., 2011; Leng et al., 2012). These
porphyry stocks host several porphyry- or skarn-type Cu–polymetallic deposits, such
as Pulang giant porphyry Cu deposit (Li et al., 2011) and Langdu large skarn Cu
deposit. During the Late Cretaceous, several granitic batholiths intruded into NYT and
part of them were genetically and spatially related to magmatic-hydrothermal
vein-type Ag–Sn–Pb–Zn deposits or skarn-type Sn deposit (Yang et al., 2016a). While,
the SYT hosts four Late Cretaceous hypabyssal porphyry stocks and associated Cu–
Mo deposists (Xiuwacu, Relin, Hongshan and Tongchanggou).
The geochemical signatures of these Late Cretaceous porphyry stocks in SYT
have been studied systematically by previously published literatures (Wang et al.,
2015a; Yang et al., 2016a, 2017a, 2018). These porphyries are dominated by
monzogranite porphyry and granodiorite porphyry, with zircon U–Pb ages of 88–79
Ma (Yang et al., 2017b). They are characterized by relatively high SiO2 contents and
belong to shoshonitic to calc-alkaline series, possessing I-type affinities; they also
have acquired arc-like trace-element signatures, with enrichments of large ion
lithophile elements and depletions of high field strength elements (Yang et al., 2018).
2.2 Hongshan Ore Deposit
9
The Hongshan skarn Cu–Mo deposit is located ~45 km northeast of Shangri-La
city (Fig. 1), with an estimated resource of 78.7 Mt (Cu: 0.64 Mt @ 1.23 %, Mo: 5769 t
@ 0.03 %; Zu et al. 2019). Sedimentary sequence at Hongshan consists of Late
Triassic sandstone, slate, limestone intercalated with andesite and volcano-clastic
rocks (Fig. 2). Intrusive rocks include the NNW-oriented quartz diorite porphyry of ~
216 Ma (Huang et al., 2012), the Late Triassic diorite porphyry dykes paralleling to
NNW-trending faults and the poorly exposed monzogranite porphyry of ~78 Ma (Fig.
2). A deep-seated monzogranite porphyry intrusion had also been identified by recent
drilling and it is genetically related to the genesis of skarn and ore minerals (Peng et al.
2014, 2015, 2016). Sm–Nd dating of garnet (76.48 ± 7.29 Ma; Zu et al. 2019) and Re–
Os dating of molybdenite (79.32 ± 0.87 Ma; Meng et al. 2013) from massive skarn
ores further indicate that the skarn Cu–Mo mineralization occurs in the Late
Cretaceous, consistent with the emplacement ages of monzogranite porphyries.
Orebodies are mainly distributed along the contact zone between the marble and
hornfelsic sandstone-slate. They are strata-bound, lenticular or podiform. Individual
orebodies extend for hundreds of meters, are tens to hundreds of meters wide, and
have been interbedded at depths of 400–850 m (Fig. 2b, c). Ten main Cu ± Mo
orebodies consist of variable amounts of skarn-type (~70%) and hornfels-type ores
(~30 %). Marble-hosted hydrothermal Pb ± Zn orebodies are scattered on the
periphery of the Cu–Mo orebodies (Fig. 2a; Zu et al. 2019). Wall-rock alteration is
extensive and intensive in the mining area, and consists of skarn, hornfels, chlorite,
epidote and carbonate. Hornfels and skarn zones are the most important and
10
well-developed alteration types, and are considered to have appreciable
mineralization potential. Due to the local interfingering of skarn and hornfels, it is very
difficult to show their spatial relationships on the large-scale geological map. Detailed
drill-hole loggings show that massive skarn and skarn orebodies with relatively high
Cu grade (0.5–2.0 %) mainly occurred between the marble and hornfels (Fig. 3). The
skarn types change following the sequences: pyroxene skarn – garnet skarn –
magnetite skarn – pyrrhotite-chalcopyrite skarn – garnet skarn – pyroxene skarn away
from the marble. The skarn mineralogy predominantly consists of garnet, which has
been partially altered to epidote and amphiboles. Typical endoskarn zone has not
been observed at Hongshan, besides narrow pyroxene skarn developed in the
contact between porphyry stocks and marble (Wang et al. 2017a).
3. Skarn Mineralogy and Paragenesis
Based on detailed field investigations and observed cross-cutting relationships,
three stages of skarn formation and ore generation have been identified (stage 1,
stage 2 and stage 3). The detailed paragenesis is presented in Fig. 4.
3.1 Stage 1 (prograde skarn stage)
Anhydrous silicate minerals, including wollastonite, garnet and pyroxene (Fig.
5a–c), defined a prograde skarn assemblage, indicating the earliest stage of skarn
formation at Hongshan. Wollastonite crystals, with parallel extinction and radial
textures, have locally overgrown the marble (Figs. 5a and 6a). Garnet is typically
red-brown to brown, with solid solution series enriched in the early stage of prograde
11
skarn alteration (Garnet I: And37–82Gro17–61Spe+Alm+Pyr0–4, Fig. 6b; Gao et al. 2014),
and andradite dominant later (Garnet II: And94-98, Figs. 5b and 6b; Gao et al. 2014).
Pyroxenes in the Hongshan district are mainly fine- to medium-grained anhedral to
subhedral diopsides, and are generally intergrown with garnet (Figs. 5f and 6b).
Garnet accounts for 70–80 % of the total skarn minerals. Most of the garnets have
been partially altered to hydrous silicate minerals during stage 2 (Fig. 6c, d). No
mineralization took place during prograde skarn alteration.
3.2 Stage 2 (main-ore stage)
Stage 2 mineral assemblage is characterized by hydrous silicate minerals,
Fe-oxides, metallic sulfides and granular quartz (quartz-1), correlated with retrograde
skarn alteration (Fig. 5d–h). A large number of retrograde minerals including epidote,
actinolite, tremolite, chlorite and sericite, have overprinted the prograde garnet-rich
calcic skarns and host intrusive rocks (Fig. 6c–f). Epidote is abundant in the
retrograde skarns and locally comprises 10 to 20 vol. %, occurring between garnet
crystals (Figs. 5g and 6d), and/or as pseudomorphs of garnet (Fig. 5e). Massive
magnetite, together with disseminated pyrrhotite and chalcopyrite, have produced
magnetite skarn (Fig. 5d). Cu–Fe sulfides (pyrite, pyrrhotite and chalcopyrite) could
replace garnets in clusters sporadically (Fig. 5f), and /or entirely (Fig. 5e). Molybdenite
veinlets crosscut the monzogranite porphyry and were then cut by later chalcopyrite
vein (Fig. 5i). Molybdenite also occurs as disseminated grains or aggregates within
the monzogranite porphyry (Fig. 5j).
12
3.3 Stage 3 (late-ore stage)
Stage 3 mineral assemblage is characterized by calcite and quartz (quartz-2),
with abundant sphalerite and galena, and minor pyrite, pyrrhotite and chalcopyrite
(Fig. 5k, l). Sphalerite and galena overprint the Cu-Fe sulfides, contain chalcopyrite
inclusions, and cut pyrrhotite aggregates (Fig. 6e).
4. Analytical methods
4.1 Major element analysis
Compositions of skarn minerals (pyroxene and amphibole) were obtained on a
JXA–8230 electron probe microanalyzer (EPMA) at the institute of Mineral Resources,
Chinese Academy of Geological Sciences, Beijing. Operating conditions used were
accelerating voltage of 15 kV, beam current of 20 nA and beam size of 5 µm. Natural
minerals and synthetic oxides were used as standards.
4.2 Fluid inclusion microthermometry
Samples analyzed in ore fluid inclusion (FI) study include stage 1 garnet, stage 2
epidote and quartz-1, and stage 3 quartz-2 and calcite. Eleven samples from
Hongshan skarn deposit were prepared as doubly-polished thin sections (~100 µm
thick). Two samples of garnet skarn (7ZK16-B37 and 7ZK16-B51) from stage 1 were
analyzed. One epidote (HZK0906-B10), two quartz-1 of stage 2 from skarn ores
(7ZK16-B36 and HS4078-B5), and three from hornfels-hosted ores (HN0ZK10-B5,
13
HN0ZK10-B5-2 and HN0ZK10B15) were also analyzed. The samples of stage 3
included two quartz-2 veins (HZK0901-B36 and HN2012KT6-B1) and one calcite vein
(HZK0906-B4).
Fluid-inclusion microthermometry was carried out on a Linkam THMSG600
heating-freezing system (-198 °C to 600 °C) attached to an Olympus microscope at
the fluid inclusion laboratory, at China University of Geosciences (Beijing). Detailed
procedures for detecting the phase-changing temperature have been summarized in
Li et al. (2012). Thermocouples were calibrated at the triple-point of pure CO2
(−56.6°C), the freezing point of water (0.0°C) and the critical point of water (+374.1°C)
using synthetic FIs supplied by FLUID INC. The precision of temperature
measurements on cooling runs is about ± 1°C at temperatures > 30 °C and ± 0.1°C at
temperatures <30 °C. Fluid inclusions were cooled to −150°C and then heated
progressively until total homogenization, with all observed phase changes recorded.
Sequential freezing was typically required to obtain accurate ice-melting temperatures.
Heating/cooling rates were restricted to <20°C/min, and 1° to 0.1°C/min near phase
transformations.
4.3 Trace element analysis
Two representative samples (quartz-1: 7ZK16-B36; quartz-2: HZK0901-B36)
were selected to prepare for quartz LA–ICP–MS trace element analysis. Quartz
grains soaked from these two doubly-polished thin sections were mounted and gently
polished in epoxy resin. Color SEM-CL images were acquired on a Tescan Vega3
14
SEM equipped with a Chroma2 CL detector at the Central Science Laboratory at the
University of Tasmania (UTAS). Most images were taken with an acceleration voltage
of 15 kV and a probe current of ~10 nA, and a magnification of 600×. The selection of
spots for analysis was guided by the combination of CL images and SEM images to
avoid fluid inclusions, sulfides and cracks.
Trace elements concentrations of quartz were determined by LA–ICP–MS using
an Agilent 7500cs quadrupole ICP–MS coupled with a Resolution S-155 ablation
system at the Arc Centre of Excellence in Ore Deposits (CODES), UTAS. The laser
spot was operated at 5 Hz, with a laser fluency of 10.3 J/cm2 and a spot size of 29 μm.
Analytical time for each spot extends 90 s with the vacant background time of 30 s.
The analyzed elements include Li, Na, Mg, Al, Si, P, K, Ca, Ti, V, Cr, Mn, Fe, Cu, Zn,
Ge, As, Rb, Sr, Cr, Nb, Ag, Sn, Cs, Nd, Gd, Hf, Ta, Au, Pb, Bi and U. Analyses were
calibrated using the NIST 612 glass, GSD-1 and BCR-2 standards, and the NIST 612
measured at 74 μm was used as the primary standard for quantification and drift
correction. Meanwhile, the Spec-pure SiO2 glass and natural quartz (Audétat et al.
2015) were conducted to ensure accurate results for most trace elements. Detailed
analytical procedures and data reduction methods follow Hong et al. (2017).
4.4 Stable isotopes
Analyses of hydrogen, oxygen, and sulfur isotopic compositions were performed
at the Analytical Laboratory of the Beijing Research Institute of Uranium Geology.
Silicate, oxide and sulfide grains were handpicked under a binocular microscope to
15
achieve >99% purity, and crushed to 40–60 mesh.
For analyses of hydrogen isotopes, water was extracted from the liquid phase of
FIs within magnetite, garnet and quartz, by heating to ~1380 °C in the Flash EA
elemental analyzer. The water vapor was then converted to hydrogen by passage
over heated glassy carbon in a stoneware pipe and the hydrogen was analyzed with a
MAT-253 mass spectrometer. For analyses of oxygen isotopes, quartz grains were
crushed into 200 mesh and reacted with BrF5 at 500–600 °C for 14 h. The O2 was
then reacted with graphite to produce CO2 at 700 °C and the catalysis of platinum.
CO2 was measured by MAT-253 mass spectrometer. For sulfur isotopic analyses,
sulfur was extracted with a continuous-flow device, wherein 0.1 to 0.3 mg of sulfide
was converted to SO2 in an EA-IsoPrime isotope ratio mass spectrometer (Euro3000,
GV Instruments). The D–O and S isotope analysis results adopted the Vienna
Standard Mean Ocean Water (V-SMOW) and Vienna Cañon Diablo Troilite (V-CDT)
standards, respectively, with a precision of ± 1‰ for δD, and ± 0.2‰ for δ18O and δ34S
(Liu et al. 2013).
5. Results
5.1 Mineral Chemistry
5.1.1 Pyroxene
Twenty-three pyroxenes were analyzed in this study (Table S1). Pyroxenes have
SiO2 concentrations ranging from 46.69 to 53.94 wt%, Al2O3 from 0.11 to 0.93 wt%,
and FeOT, MgO and CaO contents of 3.82–10.99 wt%, 11.11–16.32 wt% and 18.79–
16
24.95 wt%, respectively. These results show that pyroxenes at Hongshan
predominantly consist of diopside (64–88 mol%), with minor hedenbergite (12–35
mol%; Fig. 7b). The magnesium-rich pyroxene mineralogy is typical for Cu skarns,
and resembles the pyroxene compositions from other Cu skarn deposits (Meinert et al.
2005).
5.1.2 Amphibole
Amphiboles at Hongshan include actinolite and tremolite, which form radiating or
acicular aggregates that have replaced subhedral garnets (Fig. 6c). Field
observations and mineral processing experiments have shown that tremolite is
commonly coexisted with chalcopyrite. Six spot analyses for actinolite and eight spot
analyses for tremolite are shown in Table S2. The actinolites have SiO2
concentrations ranging from 46.69 to 53.35 wt%, Al2O3 from 2.06 to 4.52 wt%, and
FeOT, MgO and CaO contents of 13.32–15.18 wt%, 12.25–14.74 wt% and 9.44–11.85
wt%, respectively. The tremolites have higher SiO2 abundances (13.32–15.18 wt%),
MgO (19.20–20.90 wt%) and CaO (12.86–13.24 wt%), and lower Al2O3 (0.78–2.05
wt%) and FeOT (4.82–6.64 wt%).
5.1.3 Quartz
Different-generations of quartzes (quartz-1 in stage 2 and quartz-2 in stage 3)
show distinctive textural and geochemical signatures (Fig. 8). Early-generation quartz
shows euhedral texture, with the development of abundant fluid inclusions, and are
17
generally intergrown with pyrite (Fig. 8a and b). Eighteen analyses of early-generation
quartzes (quartz-1) show relatively high concentrations of Ti (10.9–58.6 ppm), Rb
(0.09–0.28 ppm), Sr (0.01–0.36 ppm) and Cs (0.02–0.18 ppm), low contents of Al
(41–142 ppm), Li (2–16 ppm), Ge (0.88–2.12 ppm) and Sb (below detection limits).
While, the late-generation quartz displays chaotic texture, with rare occurrence of fluid
inclusion (Fig. 8c and d). Thirteen analyses yield lower contents of Ti (0.5–6.2 ppm),
Rb (0.03–0.05 ppm) and Sr (0.01–0.06 ppm), but higher abundances of Al (73–1423
ppm), Li (9–144 ppm), Ge (2.24–10.81 ppm) and Sb (0.19–8.88 ppm) (Table A3; Fig.
8e and f).
5.2 Fluid inclusion microthermometry
5.2.1 Fluid-inclusion petrography
Fluid inclusions generally occur as fluid inclusion assemblages (FIAs) or isolated
inclusions in garnet, epidote and quartz, with oval and negative crystal shapes (Fig.
9a–f). Four types of fluid inclusions were recognized from Hongshan: vapor CO2–
liquid CO2–H2O inclusions (C-type; Fig. 9g); vapor-rich inclusions (V-type; Fig. 9h)
with a vapor volume of 55–99 vol. %; liquid-rich inclusions (L-type; Fig. 9i) with
relatively small vapor bubbles (10–50 vol. % vapor); hypersaline inclusions (H-type;
Fig. 9 e, f) which contained daughter minerals (halite and sylvite), liquid and vapor
when observed at room temperature.
Liquid-rich fluid inclusions are the most abundant FI type observed at Hongshan.
The size of L-type FIs is typically from 4 to 12 µm, with the largest value being 40 µm
18
in diameter. The volume of vapor in L-type FIs at room temperature varied from 10 to
40 vol. %, mostly between 20 and 35 vol. %. Most of the vapor-rich inclusions were
observed in garnet, epidote and quartz. C-type FIs were observed in garnet, epidote
and quartz. They are typically round- or oval-shaped with diameters varying from 8 to
18 µm. The volume of CO2 vapor accounts for 30–60 vol. % of the total volume at
room temperature. H-type fluid inclusions were observed in quartz of stage 2. They
have diameters of 10–20 µm, and typically contain one or two daughter minerals
including halite and sylvite, when observed at room temperature. Several fluid
inclusion assemblages can be identified at Hongshan. Quartz of stage 2 contains fluid
inclusion assemblages consisting of coexistent H-type and V-type, and V-type and
L-type FIs with similar homogenization temperatures (Fig. 9e, f).
5.2.2 Microthermometric results
The results of fluid inclusion homogenization temperature measurements are
summarized in Table 1 and shown in Figures 10 and 11. Only primary fluid inclusions
and FIAs were analyzed in this study, and they were identified based on criteria given
by Roedder (1984) and Lu et al. (2004). Microthermometric analyses have revealed
that most of the fluid inclusions homogenize to liquid, minor V-type fluid inclusions
homogenize to vapor. Salinities of aqueous FIs were calculated using data from
Bodnar (1993) for the NaCl–H2O system. Salinities of halite daughter mineral-bearing
FIs were calculated using the methodology of Bodnar and Vityk (1994), while salinities
of halite-sylvite-bearing FIs were calculated using the methodology of Hall et al.
19
(1988). Salinities of CO2-rich FIs were calculated using equations from Collins (1979).
Stage 1
Garnet contains C-type, V-type FIs and L-type FIs. Homogenization temperatures
of ten L-type FIs vary from 406 to 522 °C, with ice-melting temperatures from −8.6 to
−2.3 °C corresponding to salinities of 3.9 to 11.5 wt% NaCl eqv (Table 1; Fig. 10a).
Two V-type FIs have higher homogenization temperatures from 529 to 533 °C, along
with ice-melting temperatures of −4.2 to −3.4 °C, corresponding to salinities of 5.6 to
6.7 wt% NaCl eqv (Figs. 10a, b and 11a). Two C-type FIs had first melting
temperatures (Tm, CO2) ranging from −55.0 to −52.1 °C, and CO2 clathrate dissolution
temperatures (Tm, clath) of 7.3–7.9 °C. Homogenization of CO2 (Th, CO2) occurred
between 30.1 and 30.2 °C, and total homogenization to liquid occurred between 402
and 503 °C (Figs. 10a and 11a). Calculated salinities for C-type FIs vary from 4.1 to
5.2 wt% NaCl eqv (Fig. 10b).
Stage 2
Fluid inclusions from epidote and quartz-1 of stage 2 were analyzed. Epidote
contains abundant L-type FIs and subordinate V-type FIs, which define a fluid
inclusion assemblage (Fig. 9f). C-type FIs were also observed in epidote, with
inclusions giving Tm, CO2 values ranging between −54.8 and −52.4 °C, Tm, clath of 6.7 to
9.4 °C, Th, CO2 between 28.4 and 30.6 °C, and total homogenization to liquid from
296.4 to 341.8 °C (Table 1; Fig. 10c). Calculated salinities were 5.1–6.3 wt% NaCl
20
eqv (Fig. 10d). Homogenization temperatures measured from sixteen L-type FIs in
epidote are between 192 and 450 °C (average = 301 °C). Ice-melting temperatures
vary from −9.4 to −3.5 °C (average = −5.7 °C), corresponding to salinities of 5.7 to
13.3 wt% NaCl eqv.
Four types of inclusions have been observed in quartz-1. Upon heating, halite
dissolution is followed by disappearance of vapor bubble in H-type fluid inclusions.
Temperatures of halite-melting are respectively recorded at 184, 196, 199 and 216 °C,
corresponding to the estimated salinities as high as 31.1−32.7 wt% NaCl eqv (Table 1;
Figs. 10c, d). A temperature of sylvite-melting is recorded at 111 °C. Their
homogenization temperatures are between 393 and 451 °C. Fourteen C-type FIs
were analyzed from quartz-1, with Tm, CO2 values between −56.5 and −40.1 °C, Tm, clath
of 4.1 to 8.7 °C, Th, CO2 between 19.2 and 30.9 °C, and total homogenization to liquid
occurs from 327 to 482 °C. Calculated salinities are varying from 2.6 to 10.4 wt%
NaCl eqv. The Tm, CO2 and Th, CO2 values are much higher than the temperature of pure
CO2 inclusions (−56.6 °C, 31°C), indicating that other gas species are presented in
these fluid inclusions. Homogenization temperatures of nine V-type FIs in quartz-1
range between 255 and 415 °C, with ice-melting temperatures of −11.0 to −5.4 °C and
salinities of 8.4 to 15.0 wt% NaCl eqv. Fifty–two L-type FIs are characterized by
homogenization temperatures between 250 and 430 °C, ice-melting temperatures of
−12.1 to −4.1 °C and salinities of 6.6 to 16.1 wt% NaCl eqv.
Stage 3
21
Liquid-rich FIs are the only type recognized in quartz-2 and calcite of stage 3.
Thirty-one L-type FIs in quartz-2 yield homogenization temperatures ranging from 163
to 293 °C (average = 222 °C). Ice-melting temperatures vary from −12.2 to −3.8 °C
(average = −7.4 °C), corresponding to salinities of 6.7 to 16.3 wt% NaCl eqv.
Homogenization temperatures of sixteen L-type FIs in calcite range from 109 to
246 °C (average = 151 °C). Ice-melting temperatures vary from −12.2 to −3.7 °C
(average = −6.9 °C), equaling to salinities of 6.0 to 16.2 wt% NaCl eqv.
5.3 Stable isotope geochemistry
5.3.1 D-O isotopes
Seven garnet samples from stage 1, five magnetite, two tremolite and one
sericite samples from stage 2 were analyzed for their oxygen and hydrogen isotopic
compositions. The results are summarized in Table 2 and depicted in Figure 12. The
δDSMOW values of FIs in garnet vary from −115.1 to −93.7 ‰, with an average value of
−108.6 ‰. The FIs in magnetite have δDSMOW values between −117.6 and −83.2 ‰,
with an average value of −96.4 ‰. The hydrous tremolites have δDSMOW values
between −112.3 and −111.8 ‰, and sericite has a δDSMOW value of −95.5 ‰.
Equilibrium fluid isotopic compositions were calculated using assuming
depositional temperatures equaled to homogenization temperatures of FIs for the
different paragenetic stages. When fluid immiscibility occurred, the equilibrium
temperature is considered to be the best estimated trapping temperature (Bodnar and
Vityk 1994; Yang et al, 2016b). Based on fluid inclusion assemblages and
22
homogenization temperatures (Table 1), trapping temperatures of garnet, quartz-1,
and quartz-2 are estimated to be 533, 430 and 294 °C, respectively.
Using the appropriate oxygen isotope exchange coefficients between host
minerals and water (1000 Inαgarnet–H2O = 1.14 × 106T-2 − 3.70, Lu and Yang 1982; 1000
Inαmagnetite–H2O = 2.88 × 106T-2 – 11.36 × 103T-1 + 2.89, 1000 Inαtremolite–H2O = 3.95 ×
106T-2 – 8.28 × 103T-1 + 2.38, Inαsericite–H2O = 4.10 × 106T-2 – 7.61 × 103T-1 + 2.25, Zheng
et al. 2000), the δ18OH2O values of garnets are calculated between 2.0 and 13.1 ‰
(average = 6.0 ‰), magnetites are between 10.9 and 26.3 ‰ (average = 18.3 ‰),
tremolites are between 15.9 and 16.4 ‰, and sericite is 10.5 ‰. These relatively high
δ18OH2O values are interpreted to be the interaction between magmatic fluids and
carbonate wall-rocks that are enriched in 18O (24.0 to 24.7 ‰; Wang et al., 2008). No
significant compositional differences were detected for D–O data from the two
paragenetic stages.
5.3.2 S isotopes
Twenty–five sulfide samples, including pyrite, chalcopyrite, pyrrhotite, galena and
sphalerite, were analyzed for their sulfur isotopic compositions. The δ34S values were
obtained from seven pyrite separates (4.5 to 5.2 ‰; average = 4.9 ‰), five
chalcopyrite samples (3.6 to 4.7 ‰; average = 4.6 ‰), three galena samples (3.7 to
5.4 ‰; average = 4.3 ‰), three sphalerite samples (4.7 to 5.4 ‰; average = 5.0 ‰)
and seven pyrrhotite samples (3.6 to 4.7 ‰; average = 4.3 ‰) (Table 3, Fig. 13). The
sort of average δ34S value of different sulfide is sphalerite > pyrite > chalcopyrite >
23
pyrrhotite = galena, different from the empirical sequence (pyrite > sphalerite
(pyrrhotite) > chalcopyrite > galena; Zheng and Chen 2000). In addition, the majority
of these sulfides are not in equilibrium according to sulfur isotope thermometry,
indicating that these sulfides were not entirely in equilibrium during ore deposition
(Ohmoto and Rye 1979).
6. Discussion
.
6.1 Physicochemical environment of skarn and ore formation
The types of skarn deposits that form in magmatic-hydrothermal environments
are influenced by a number of factors, including magma type, depth of emplacement,
composition and reducing capacity of host rocks, distance of carbonate horizon from
the magmatic source, and degree of meteoric water involvement (Einaudi et al. 1981;
Nakano et al. 1994; Meinert et al. 2005).
Cu skarns are commonly dominated by oxidized skarn mineralogy, including
andradite, diopside, wollastonite, actinolite and epidote (Meinert et al. 1997, 2005).
The Hongshan prograde skarns are characterized by an andradite–diopside–
wollastonite assemblage. The retrograde skarn assemblage consists of quartz–
amphibole–epidote ± magnetite, and Cu–Fe–Mo sulfides that are associated with this
hydrous assemblage. The quartz–calcite–sphalerite–galena assemblage of stage 3 is
not a skarn, but is a common retrograde assemblage observed in many skarn
deposits (e.g., Yücel Öztürk et al. 2008; Palinkaš et al. 2013). According to the
24
mineral equilibria of relevant calcium-silicate minerals, oxides and sulfides, as well as
fluid inclusion assemblages in the different paragenetic stages (Figs 14 and 15), we
conclude that stage 1 formed under relatively high temperature (400–550 °C), high
pressure (~600 bars), and oxidized conditions (andradite-rich garnet in stage 1 and
abundant magnetite in stage 2). Stage 2 is characterized by lower temperature
(around 250–450 °C), and Cu–Mo–Fe sulfides formed under reduced conditions
(H2S-predominate) and moderate sulfur fugacities (sulfides rather than sulfates;
Cooke et al. 2000). Stage 3 Pb–Zn mineralization is precipitated from fluids with
temperatures below 250 °C (Fig. 14).
This is also consistent with the CL textures and trace elements signatures of
quartzes (Fig. 8). The quartz-1 from stage 2 displays homogenous granular mosaics
texture with bright CL signature, and high Ti concentrations ranging from tens of ppm
to ~60 ppm. However, the quartz-2 from stage 3 shows chaotic texture with darker CL
signature, containing lower Ti concentrations of less than 6 ppm. High Ti
concentrations in quartz are consistent with higher temperatures (Huang and Audétat,
2012), and homogeneous CL textures may result from relatively rapid diffusion of Ti at
high temperatures, or from an initial lack of texture due to rapid nucleation and
precipitation (Mao et al., 2017).
6.2 Source of ore-forming fluid
Prograde garnet, pyroxene and quartz in many skarn deposits have calculated
δ18O values for ore-forming fluids (δ18OH2O) between 4 and 9 ‰, consistent with
25
derivation from magmatic waters (Bowman 1998; Meinert et al. 2005). Garnets at
Hongshan are characterized by a wider range of δ18OH2O values, varying from 2.0 to
13.1 ‰. Magnetite has relatively high δ18OH2O values of 10.9 to 26.3 ‰, suggesting
the mixing of δ18O-enriched components with magmatic fluids. The δ18OSMOW values of
marble from the Hongshan district are in the range of 24.0 to 24.7 ‰ (Wang et al.
2008). It is therefore concluded that the continued interaction between fluids exsolved
from felsic magma and carbonates could have caused increased δ18OH2O values in
prograde and retrograde skarn minerals.
The δD values of fluid inclusions trapped in garnet (–93.7 to –116.1 ‰) are lower
than δD values for typical magmatic water area (δDH2O = –80‰ to –40‰; Taylor 1974).
Suzuoki and Epstein (1976) suggest that δD compositions of residual magmatic
waters after crystallization will evolve to lower values (Fig. 12). It is possible for
residual magmatic water to be fixed by primary hydrous minerals in the water-rich
magma, resulting in much lower δD values (< –100 ‰; Taylor 1996). During the stage
2, the δD values of tremolite and sericite are generally much higher than those of the
anhydrous skarn minerals. This could be due to a number of reasons, including
mixing meteoric water with residual magmatic water, interference of secondary fluid
inclusions that contain meteoric water, or other factors.
Sulfur isotopes of coexisting sulfides have a narrow range of δ34S values. The
sulfides of stage 2 are varying from 3.6 to 5.4 ‰ (chalcopyrite, pyrite and pyrrhotite).
The sphalerite and galena of stage 3 are ranging between 3.5 and 5.4 ‰. The δ34S
values of sulfides from Hongshan are similar to other magmatic-hydrothermal
26
deposits in the southern Yidun Terrrane, and other skarn Cu deposits worldwide (Fig.
13; Chen et al. 2011; Kamvong and Zaw 2009; She et al. 2005; Wang et al. 2015b).
The positive and uniform δ34S values (sulfides: 3.5 to 5.4 ‰, this study; ore-related
monzogranite porphyry: 4.7 to 7.5 ‰, Wang et al. 2017b) suggest a magmatic source
of sulfur with H2S being the predominant aqueous sulfur species. This could be
explained by the hydrolysis reaction (4H2O + 4SO2 = H2S + 3H+ + 3HSO4-) that
generates aqueous sulfate below 400°C (Rye et al. 1993). Under highly oxidizing
conditions, cooling of a magmatic fluid will result in a wide range of δ34Ssulfide values
that become increasingly depleted in heavy sulfur isotopes. A narrow range of
δ34Ssulfate will be produced under oxidizing conditions. In contrast, the range of
δ34Ssulfide values produced under reducing conditions will be small, and the range of
δ34Ssulfate will be large (Wilson et al., 2007). There is no evidence for contamination or
assimilation of wall-rock sulfur at Hongshan (Ohmoto and Goldhaber 1997;
Chaussidon and Lorand 1990).
The ore-forming fluids at Hongshan are concluded to be of predominantly
magmatic–hydrothermal origin, possibly modified by the water-rock interaction
between carbonate wall-rock and felsic magma. It is possible that some mixing with
meteoric water caused divergence of D–O isotopic compositions away from primary
magmatic water (Fig. 12).
6.3 Evolution of ore-forming fluid
Fluid evolution at Hongshan is illustrated in Figure 16. Salinities of FIs in garnets
27
from Hongshan are low to moderate (3.9–11.5 wt% NaCl eqv), and homogenization
temperatures are around 530 °C. Fluids during stage 1 prograde alteration contain
some CO2, possibly caused by carbonate interaction or due to dissolution of
carbonate during skarn formation. When Cu–Mo–Fe mineralization of stage 2
occurred, fluids ascended to the site of ore curve followed a cooling path different
from stage 1, possibly due to a lower advective rate (e.g., Meinert et al. 1997, 2003).
The much cooler fluid (trapped at 430 °C) originated from the same magmatic source
but cooled more slowly and sufficiently during ascent, so that it did not intersect its
solvus curve until lower temperatures and pressures (~200 bars; ~2 km under
hydrostatic pressure conditions). When the temperatures cooled below approximately
400 °C, the hydrothermal system evolved from ductile to brittle conditions, and
pressures evolved from lithostatic to hydrostatic (e.g., Fournier 1991). The relatively
high δ18OH2O values (10.9 to 26.3 ‰) of fluid inclusions in magnetite point to
metasomatic reactions between ore-forming fluids and carbonate rocks, which
caused CO2 degassing and the formation of fractures within the prograde skarns. As a
result, meteoric water could have mixed with the magmatic-hydrothermal fluid,
decreasing the temperature and salinity of the ore-forming fluid. Calcite and quartz
veinlets in stage 3 contain a non-boiling fluid inclusion assemblage, with lower
homogenization temperatures (109–293 °C) and moderate to low salinities (6.0–16.3
wt % NaCl eqv). The fluids of stage 3 are interpreted to be simply cooled equivalents
after the stage 2 fluids underwent phase seperation or fluid immiscibility.
28
6.4 Mechanisms of mineral deposition
A variety of ore deposition processes, including changes in oxygen fugacity and
pH, cooling, decompression boiling and fluid mixing could have influenced ore
formation at Hongshan.
In stage 1, the primary magmatic fluids, which are characterized by high oxygen
fugacity, high temperature and CO2-rich, carried Cu and Mo derived from fertile
subduction-modified arc cumulates during upward migration (Yang et al., 2016a). Due
to the high oxygen fugacity and CO2 content, the sulfur activity in the fluids is
constrained, and thus deposition of sulfides in this stage is absent. Prograde skarn
minerals dominated by garnet and pyroxene in this stage were formed by the
interaction between magmatic fluid and carbonate wall-rock. This skarnization
process provides spaces for the later mineralized events (Meinert et al., 2005).
Following the decreasing temperature and pressure of fluid system, a series of
physico-chemical changes have taken place in stage 2: (1) precipitation of magnetite
at this stage resulted in a decrease of oxygen fugacity, which promotes the transitions
from SO42− to S2−; (2) decreasing temperature of fluid system would reduce Cu
solubility and facilitate Cu2+ combining with S2−; (3) formation of abundant retrograde
minerals such as epidote and actinolite consumed the H+ and thus induced the
increased pH of ore-forming fluid; (4) boiling FIAs observed in stage 2 suggest that
phase separation or fluid immiscibility could have occurred and induced the
degassing of acidic volatiles, finally ending up with an increase in pH; (5) water-rock
interaction between magmatic-hydrothermal fluid and carbonate also induce the CO2
29
degassing and an increase of pH; (6) The influx of meteoric water and fluid mixing
recorded by D–O isotopes of stage 2 minerals would also decrease the temperature
and increase the pH of fluid (Reed and Palandri, 2006). These changes mentioned
above would cause the decomposition of metal complexes and the decrease of
sulfide solubility (e.g., Simmons and Christenson 1994; Lowenstern 2001; Peng et al.
2016). Overall, the increase in pH could be the main factor controlling the massive Cu
ore deposition at Hongshan (Peng et al., 2016).
Calcite and quartz in stage 3 contain a non-boiling fluid inclusion assemblage
with lower homogenization temperatures and moderate to low salinities, indicating
that lower temperature and increased pH caused by further water-rock interaction
caused the precipitation of Pb–Zn sulfides.
7. Conclusions
Hongshan is a large Cu–Mo skarn deposit hosted in Late Triassic
carbonate-bearing strata. Mineralization was temporally and genetically related to the
Late Cretaceous monzogranite porphyry. Similar to most Cu skarn deposits,
Hongshan is characterized by an early anhydrous skarn mineral assemblage
dominated by andraditic garnet and diopside. However, endoskarn is rare at
Hongshan, with orebodies located between individual marble and hornfels lens.
Hongshan formed from fluids with lower temperatures (around 250 to 450 °C) than
proximal Cu skarn deposits (commonly > 450 °C and can exceed 700 °C). D–O
isotopic compositions show that the ore-forming fluids at Hongshan are of
30
predominating magmatic–hydrothermal origin, possibly modified by assimilation of
carbonate wall-rock. The positive and uniform δ34S values of sulfides suggest a
magmatic source of sulfur with H2S being the predominant sulfur species in the
ore-forming fluids. Boiling fluid inclusion assemblages (coexisting L-type and V-type
FIs, and H-type and L-type FIs) observed in stage 2 suggest that phase separation or
fluid immiscibility could have occurred during ore formation at Hongshan. The
presence of C-type FIs in stage 2 and stage 3 presented a CO2-bearing condition. It is
concluded that Cu–Fe–Mo sulfides mainly precipitated due to the increased pH
caused by water-rock interaction and CO2 degassing, decreasing temperature and
decompression boiling, followed by deposition of Pb–Zn sulfides due to the decreased
temperature
Acknowledgements
We are grateful to David Cooke, Qi–Hai Shu, Wen–Yan He and Lin–Nan Guo for
valuable comments on earlier versions of this manuscript. We would like to thank
Zhen–Yu Chen and Xiao–Dan Chen for their help with the EPMA analysis, Mu Liu on
stable isotopes analysis, and Jay Thompson on quartz LA–ICP–MS analysis. Special
thanks are extended to Engineer Jian–Xin Li, Zhong–Hua He, Zhu Deng and Fei Long
for their help in the field. Insightful comments from editor and two anonymous
reviewers are also highly appreciated. This study was financially supported by the
China Postdoctoral Science Foundation (Grant No. 2019T120121 and 2018M640161),
the National Basic Research Program of China (Grant No. 2015CB452605), and 111
31
Project of the Ministry of Science and Technology, China (Grant No. BP0719021). The
first author X. Gao thanks the China Scholarship Council (201606400011) and highly
appreciates Lejun Zhang, Wei Hong, Jing Chen and Mike Baker for their support and
help during her stay at the CODES and TMVC, University of Tasmania, Australia.
Conflict of Interest
No conflict of interest exits in the submission of this manuscript, and manuscript is approved
by all authors for publication. I would like to declare on behalf of my co-authors that the work
described was original research that has not been published previously, and not under
consideration for publication elsewhere, in whole or in part. All the authors listed have
approved the manuscript that is enclosed.
32
References
Audétat, A., Garbe-Schönberg, D., Kronz, A., Pettke, T., Rusk, B., Donovan, J.,
Lowers, H.A., 2015. Characterisation of a Natural Quartz Crystal as a Reference
Material for Microanalytical Determination of Ti, Al, Li, Fe, Mn, Ga and Ge.
Geostandards and Geoanalytical Research 39, 171–184.
Bodnar, R.J., 1993. Revised equation and table for determining the freezing point
depression of H2O-NaCl solutions. Geochim Cosmochim Acta 57, 683–684.
Bodnar, R.J., Vityk, M.O., 1994. Interpretation of microthermometric data for
H2O-NaCl fluid inclusions, In: De Vivo B, Frezzotti ML (eds) Fluid inclusions in
minerals, methods and applications. Virginia Tech, Blacksburg, 117–130.
Bowman, J.R., 1998. Stable-isotope systematic of skarns, In: Lentz DR (eds)
Mineralized intrusion-related skarn systems: Mineralogical Association of Canada,
Québec. Short Course Series, 99–145.
Burnham, C.W., 1979. Magmas and hydrothermal fluids, In: Barnes HL (eds)
Geochemistry of hydrothermal ore deposits, 2nd edition. John Wiley and Sons,
New York, 71–136.
Chaussidon, M., Lorand, J.P, 1990. Sulphur isotope composition of orogenic spinel
lherzolite massifs from Ariege (N.E. Pyrenees, France). An ion microprobe study.
Geochim Cosmochim Acta 54, 2835–2846.
Chen, L., Qin, K.Z., Li, J.X., Xiao, B., Li, G.M., Zhao, J.X., Fan, X., 2011. Fluid
inclusions and hydrogen, oxygen, sulfur isotopes of Nuri Cu-W-Mo deposit in the
southern Gangdese, Tibet. Resource Geology 62, 42–62.
33
Collins, L.F., 1979. Gas hydrates in CO2-bearing fluid inclusions and the use of
freezing data for estimation of salinity. Economic Geology 74, 1435–1444.
Cooke, D.R., Bull, S.W., Large, R.R., Mcgoldrick, P.J., 2000. The importance of
oxidized brines for the formation of Australian Proterozoic stratiform
sediment-hosted Pb-Zn (Sedex) deposits. Economic Geology 95, 1–18.
Deng, J., Wang, Q.F., Li, G.J., Li, C.S., Wang, C.M., 2014a. Tethys tectonic evolution
and its bearing on the distribution of important mineral deposits in the Sanjiang
region, SW China. Gondwana Research 26, 419–437.
Deng, J., Wang, Q.F., Li, G.J., Santosh, M., 2014b. Cenozoic tectono–magmatic and
metallogenic processes in the Sanjiang region, southwestern China. Earth
Science Reviews 138, 268–299.
Deng, J., Wang, Q.F., 2016. Gold mineralization in China: Metallogenic provinces,
deposit types and tectonic framework. Gondwana Research 36, 219–274.
Deng, J., Wang, Q.F., Li, G.J., 2017. Tectonic evolution, superimposed orogeny,
and composite metallogenic system in China. Gondwana Research 50, 216–
266.
Driesner, T., Heinrich, C.A., 2007. The system H2O-NaCl Part I: Correlation formulae
for phase relations in temperature-pressure-composition space from 0 to 1000°C,
0 to 5000bar, and 0 to 1 XNaCl. Geochim Cosmochim Acta 71, 4880–4901.
Einaudi, M.T., Meinert, L.D., Newberry, R.J., 1981. Skarn deposits: Economic
Geology 75th anniversary volume, 317–391.
Gao, X., Deng, J., Meng, J.Y., Yan, H., Li, J.X., Yang, C.H., Sun, N., Wei, C., 2014.
34
Characteristics of garnet in the Hongniu skarn copper deposit, western Yunnan.
Acta Petrologica Sinica 30, 2695–2708 (in Chinese with English abstract).
Gao, X., Yang, L.Q., Orovan, E.A., 2018. The lithospheric architecture of two
subterranes in the eastern Yidun Terrane, East Tethys: Insights from Hf–Nd
isotopic mapping. Gondwana Research 62, 127–143.
Fournier, R.O., 1987. Conceptual models of brine evolution in magmatic-hydrothermal
systems. US Geol Surv Prof 1350, 1487–1506.
Hall, D.L., Sterner, S.M., and Bodner, R.J., 1988. Freezing point depression of
NaCl-KCl-H2O solution. Economic Geology 83, 197–202.
He, W.Y., Mo, X.X., He, Z.H., White, N.C., Chen, J.B., Yang, K.H., Wang, R., Yu, X.H.,
Dong, G.C., Huang, X.F., 2015. The geology and mineralogy of the Beiya skarn
gold deposit in Yunnan, southwest China. Economic Geology 110, 1625–1641.
Hedenquist, J.W., Lowenstern, J.B., 1994. The role of magmas in the formation of
hydrothermal ore deposits. Nature 370, 519–527.
Hong, W., Cooke, D.R., Zhang, L.J., Fox, N., Thompson, J., 2017. Tourmaline-rich
features in the Heemskirk and Pieman Heads granites from western Tasmania,
Australia: Characteristics, origins, and implications for tin mineralization.
American Mineralogist 102, 876–899.
Hou, Z.Q., Mo, X.X., Tan, J., Hu, S., Luo, Z., 1993. The eruption sequences of basalts
in the Yidun island arc, Sanjiang region and evolution of rift to island arc. Bulletin
of the Chinese Academy of Geological Sciences 26, 49–67 (in Chinese).
Hou, Z.Q., Yang, Y.Q., Wang, H.P., Qu, X.M., Lü, Q.T., Huang, D.H., Wu, X.Z., Yu, J.J.,
35
Tang, S.H., Zhao, J.H., 2003. The collisional orogeny and mineralization systems
of the Yidun arc orogen in Sanjiang region, China. Geological Publishing House,
Beijing, 154–187 (in Chinese with English abstract).
Huang, R.F., Audétat, A, 2012. The titanium-in-quartz (TitaniQ) thermobarometer: A
critical examination and re-calibration. Geochimica et Cosmochimica Acta 84,
75–89.
Huang, X.X., Xu, J.F., Chen, J.L., Ren, J.B., 2012. Geochronology, geochemistry and
petrogenesis of two periods of intermediate-acid intrusive rocks from Hongshan
area in Zhongdian arc. Acta Petrologica Sinica 28, 1493-1506 (in Chinese with
English abstract).
Kamvong, T., Zaw, K., 2009. The origin and evolution of skarn-forming fluids from the
Phu Lon deposit, northern Loei Fold Belt, Thailand: Evidence from fluid inclusion
and sulfur isotope studies. Journal of Asian Earth Sciences 34, 624–633.
Layne, G.D., Longstaffe, F.J., Spooner, E.T.C., 1991. The JC tin skarn deposit,
southern Yukon Territory: II. A carbon, oxygen, hydrogen, and sulfur stable
isotope study. Economic Geology 86, 48−65.
Leng, C.B., Zhang, X.C., Hu, R.Z., Wang, S.X., Zhong, H., Wang, W.Q., Bi, X.W.,
2012. Zircon U–Pb and molybdenite Re–Os geochronology and Sr–Nd–Pb–Hf
isotopic constraints on the genesis of the Xuejiping porphyry copper deposit in
Zhongdian, Northwest Yunnan, China. Journal of Asian Earth Sciences 60, 31–
48.
Li, J.K., Li, W.C., Wang, D.H., Lu, Y.X., Yin, G.H., Xue, S.R., 2007. Re–Os dating for
36
ore forming event in the late of Yanshan Epoch and research of ore–forming
regularity in Zhongdian Arc. Acta Petrologica Sinica 23, 2415–2422 (in Chinese
with English abstract).
Li. N., Chen, Y.J., Ulrich, T., Lai, Y., 2012. Fluid inclusion study of the Wunugetu Cu–
Mo deposit, Inner Mongolia, China. Miner Deposita 47, 467–482.
Li, W.C., Zeng, P.S., Hou, Z.Q., White, N.C., 2011. The Pulang porphyry copper
deposit and associated felsic intrusions in Yunnan Province, southwest China.
Economic Geology 106, 79–92.
Li, W.C., Wang, K.Y., Yin, G.H., Qin, H.D., Yu, H.J., Xue, S.L., Wan, D., 2013.
Geochemical characteristics of ore-forming fluids and genesis of Hongshan
copper deposit in northwestern Yunnan Province. Acta Petrologica Sinica 29,
270–282 (in Chinese with English abstract).
Liu, H.B., Jin, G.S., Li, J.J., Han, J., Zhang, J.F., Zhang, J., Zhong, F.W., Guo, D.Q.,
2013. Determination of stable isotope composition in uranium geological samples:
World Nucl Geosci 3, 174–179.
Lowenstern, J.B., 2001. Carbon dioxide in magmas and implications for hydrothermal
systems: Mineralium Deposita 36, 490–502.
Lu, H.Z., Fan, H.R., Ni, P., Ou, G.X., Shen, K., Zhang, W.H., 2004. Fluid inclusions.
Science Publiublishing House, Beijing, 1–300 (in Chinese with English abstract).
Lu, W.C., Yang, S.Q., 1982. Use of oxygen bonds to calculate fractionation on
equations of oxygen isotope for minerals containing kyanite and staurolite.
Journal of Mineralogy and Petrology 2, 106–112 (in Chinese with English
37
abstract).
Mao, W., Rusk, B., Yang, F.C., Zhang, M.J., 2017. Physical and Chemical Evolution of
the Dabaoshan Porphyry Mo Deposit, South China: Insights from Fluid Inclusions,
Cathodoluminescence, and Trace Elements in Quartz. Economic Geology 112,
889–918.
Meinert, L.D., Hefton, K.K., Mayes, D., Tasiran, I., 1997. Geology, zonation, and fluid
evolution of the Big Gossan Cu-Au skarn deposit, Ertsberg district, Irian Jaya.
Economic Geology 92, 509–534.
Meinert, L.D., Hedenquist, J., Satoh, H., Matsuhisa, Y., 2003. Formation of anhydrous
and hydrous skarn in Cu-Au ore deposits by magmatic fluids. Economic Geology
98, 147–156.
Meinert, L.D., Dipple, G., Nicolescu, S., 2005. World skarn deposits: Economic
Geology 100th anniversary volume, 299–336.
Meng, J.Y., Yang, L.Q., Lv, L., Gao, X., Li, J.X., Luo, Y.Z., 2013. Re–Os dating of the
molybdenite from the Hongshan Cu–Mo deposit in the Northwest Yunnan and its
implication for mineralization. Acta Petrologica Sinica 29, 1214–1222 (in Chinese
with English abstract).
Nakano, T., Yoshino, T., Shimazaki, H., Shimizu, M., 1994. Pyroxene compositions as
an indicator in the classification of skarn deposits. Economic Geology 89, 1567–
1580.
Ohmoto, H., Goldhaber, M.B., 1997. Sulfur and carbon isotopes, In: Barnes HL (eds)
Geochemistry of Hydrothermal Ore Deposits. 3rd edition Wiley, New York, 517–
38
611.
Pan, G.T., Wang, L.Q., Li, R.S., Yuan, S.H., Ji, W.H., Yin, F.G., Zhang. W.P., Wang,
B.D., 2012. Tectonic evolution of the Qinghai–Tibet Plateau. Journal of Asian
Earth Sciences 53, 3–14.
Palinkaš, S.S., Palinkaš, L.A., Renac, C., Spangenberg, J.E., Lüders, V., Molnar, F.,
Maliqi, G., 2013. Metallogenic model of the Trepča Pb-Zn-Ag skarn deposit,
Kosovo: Evidence from fluid inclusions, rare earth elements and stable isotope
data. Economic Geology 108, 135–162.
Peng, H.J., Mao, J.W., Pei, R.F., Zhang, C.Q., Tian, G., Zhou, Y.M., Li, J.X., Hou, L.,
2014. Geochronology of the Hongniu-Hongshan porphyry and skarn Cu deposit,
northwestern Yunnan Province, China: Implications for mineralization of the
Zhongdian arc. Journal of Asian Earth Sciences 79, 682–695.
Peng, H.J., Zhang, C.Q., Mao, J.W., Santosh, M., Zhou, Y.M., Hou, L., 2015. Garnets
in porphyry–skarn systems: A LA–ICP–MS, fluid inclusion, and stable isotope
study of garnets from the Hongniu–Hongshan copper deposit, Zhongdian area,
NW Yunnan Province, China. Journal of Asian Earth Sciences 103, 229–251.
Peng, H.J., Mao, J.W., Hou, L., Shu, Q.H., Zhang, C.Q., Liu, H., Zhou, Y.M., 2016.
Stable isotope and fluid inclusion constraints on the source and evolution of ore
fluids in the Hongniu-Hongshan Cu skarn deposit, Yunnan Province, China.
Economic Geology 111, 1369–1396.
Ramboz, C., Pichavant, M., Weisbrod, A., 1982. Fluid immiscibility in natural
processes: use and misuse of fluid inclusion data II. Interpretation of fluid
39
inclusion data in terms of immiscibility. Chemical Geology 37, 29–48.
Reed, M.H., Palandri J., 2006. Sulfide mineral precipitation from hydrothermal fluids.
Sulfide Mineralolgy and Geochemistry 61, 609–631.
Reid, A.J., Wilson, C.J.L., Liu, S., 2005. Structural evidence for the Permo–Triassic
tectonic evolution of the Yidun Arc, eastern Tibetan plateau. Journal of Structural
Geology 27, 119–137.
Reid, A.J., Wilson, C.J.L., Liu, S., Pearson, N., Belousova, E., 2007. Mesozoic plutons
of the Yidun Arc, SW China: U/Pb geochronology and Hf isotopic signature. Ore
Geology Reviews 31, 88–106.
Roedder, E., 1984. Fluid inclusions. Rev Mineral 12, 1–664.
Rye, R.O., 1993. The evolution of magmatic fluids in the epithermal environment: the
stable isotope perspective. Economic Geology 88, 733–752.
Samson, I.M., Williams-Jones, A.E., Ault, K.M., Gagnon, J.E., Fryer, B.J., 2008.
Source of fluids forming distal Zn-Pb-Ag skarns: Evidence from laser ablation–
inductively coupled plasma–mass spectrometry analysis of fluid inclusions from
El Mochito, Honduras. Geology 36, 947–950.
She, H.Q., Feng, C.Y., Zhang, D.Q., Pan, G.T., Li, G.M., 2005. Characteristics and
metallogenic potential of skarn copper-lead-zinc polymetallic deposits in central
eastern Gangdese. Mineral Deposit 24, 508–520 (in Chinese with English
abstract).
Shepherd, T.J., Rankin, A.H., Alderton, D.H., 1985. A practical guide to fluid inclusion
studies. Blackie & Son Ltd, Glasgow, 1–239.
40
Simmons, S.F., Christenson, B.W., 1994. Origin of calcite in a boiling geothermal
system. American Journal of Science 294, 361–400.
Suzuoki, T., Epstein, S., 1976. Hydrogen isotope fractionation between OH-bearing
minerals and water. Geochim Cosmochim Acta 40, 1229–1240.
Taylor, B.E., 1986. Magmatic volatiles: Isotopic variation of C, H, and S. Rev Mineral
16, 185–226.
Taylor, H.P., 1974. The application of oxygen and hydrogen isotope studies to
problems of hydrothermal alteration and ore deposition. Economic Geology 69,
843–883.
Wang, P., Dong, G.C., Santosh, M., Liu, K.R., Li, X.F., 2017a. Copper isotopes trace
the evolution of skarn ores: A case study from the Hongshan–Hongniu Cu deposit,
southwest China. Ore Geology Reviews 88, 822–831.
Wang, P., Dong, G.C., Li, X.F., Chen, W., Li, J.X., Tao, X.X., 2017b. Metallogenic
sources of the Hongshan–Hongniu copper deposit in northwestern Yunnan:
Constraints from mineralogy and stable isotopes. Earth Science Frontiers 24,
176–1193 (in Chinese with English abstract).
Wang, S.X., Zhang, X.C., Leng, C.B., Qin, C.J., Wang, W.Q., Zhao, M.C., 2008.
Stable isotopic compositions of the Hongshan skarn copper deposit in the
Zhongdian area and its implication for the copper mineralization process. Acta
Petrologica Sinica 24, 480–488 (in Chinese with English abstract).
Wang, X.S., Bi, X.W., Leng, C.B., Tang, Y.Y., Lan, J.B., Qi, Y.Q., Shen, N.P., 2011. LA–
ICP–MS Zircon U–Pb Dating of Granite Porphyry in the Hongshan
41
Cu-polymetallic Deposit, Zhongdian, Northwest Yunnan, China and its Geological
Implication. Acta Mineralogica Sinica 31, 315–321 (in Chinese with English
abstract).
Wang, X.S., Bi, X.W., Hu, R.Z., Leng, C.B., Yu, H.J., Yin, G.H., 2015a. S-Pb isotopic
geochemistry of Xiuwacu magmatic hydrothermal Mo–W deposit in Zhongdian
area, NW Yunnan: Constrains on the sources of metal. Acta Petrologica Sinica
31, 3171–3188 (in Chinese with English abstract).
Wang, Z.L., Yang, L.Q., Guo, L.N., Marsh, E., Wang, J.P., Liu, Y., Zhang, C., Li, R.H.,
Zhang, L., Zheng, X.L., Zhao, R.X., 2015b. Fluid immiscibility and gold deposition
in the Xincheng deposit, Jiaodong Peninsula, China: a fluid inclusion study. Ore
Geology Reviews 65, 701–717.
Whitney, D.L., Evans, B.W., 2010. Abbreviations for names of rock-forming minerals:
American Mineralogist 95, 185–187.
Williams-Jones, A.E., Samson, I.M., Ault, K.M., Gagnon, J,E., Fryer, B.J., 2010. The
genesis of distal zinc skarns: Evidence from the Mochito deposit, Honduras.
Economic Geology 105, 1411–1440.
Wilson, A.J., Cooke, D.R., Harper, B.J., Deyell, C.L., 2007. Sulfur isotopic zonation in
the Cadia district, southeastern Australia: exploration significance and
implications for the genesis of alkalic porphyry gold–copper deposits. Miner
Deposita, 42, 465–487.
Xu, X.W., Cai, X.P., Song, B.C., Zhang, B.L., Ying, H.L., Xiao, Q.B., Wang, J., 2006.
Petrologic, chronological and geochemistry characteristics and formation
42
mechanism of alkaline porphyries in the Beiya gold district, western Yunnan. Acta
Petrologica Sinica 22, 631–642 (in Chinese with English abstract).
Yang, L.Q., Gao, X., He, W.Y., 2015. Late Cretaceous porphyry metallogenic system
of the Yidun arc, SW China. Acta Petrologica Sinica 31, 3155–3170 (in Chinese
with English abstract).
Yang, L.Q., Deng, J., Dilek, Y., Meng, J.Y., Gao, X., Santosh, M., Wang, D., Yan, H.,
2016a. Melt source and evolution of I-type granitoids in the SE Tibetan Plateau:
Late Cretaceous magmatism and mineralization driven by collision-induced
transtensional tectonics. Lithos 245, 258–273.
Yang, L.Q., Deng, J., Guo, L.N., Wang, Z.L., Li, X.Z., Li, J.L., 2016b. Origin and
evolution of ore fluid, and gold deposition processes at the giant Taishang gold
deposit, Jiaodong Peninsula, eastern China. Ore Geology Reviews 72, 585–602.
Yang, L.Q., Deng, J., Gao, X., He, W.Y., Meng, J.Y., Santosh, M., Yu, H.J., Yang, Z.,
Wang, D., 2017a. Timing of formation and origin of the Tongchanggou porphyry–
skarn deposit: Implications for Late Cretaceous Mo–Cu metallogenes in the
southern Yidun Terrane, SE Tibetan Plateau. Ore Geology Reviews 81, 1015–
1032.
Yang, L.Q., Gao, X., Shu, Q.H., 2017b. Multiple Mesozoic porphyry-skarn Cu (Mo–W)
systems in Yidun Terrane, east Tethys: constraints from zircon U–Pb and
molybdenite Re–Os geochronology. Ore Geology Reviews 90, 813–826.
Yang, L.Q., He, W.Y., Gao, X., Xie, S.X., Yang, Z., 2018. Mesozoic multiple
magmatism and porphyry–skarn Cu-polymetallic systems of the Yidun Terrane,
43
Eastern Tethys: Implications for subduction- and transtension-related
metallogeny. Gondwana Research 62, 112–126.
Yücel, Öztürk. Y., Helvaci, C., 2008. Skarn alteration and Au-Cu mineralization
associated with Tertiary granitoids in northwestern Turkey: Evidence from the
Evciler deposit, Kazdag Massif, Turkey. Economic Geology 103, 1665–1682.
Zeng, P.S., Wang, H.P., Mo, X.X., Yu, X.H., Li, W.C., Li, T.G., Li, H., Yang, Z.Z., 2004.
Tectonic setting and prospects of porphyry copper deposits in Zhongdian island
arc belt. Acta Geoscientia Sinica 25, 535–540 (in Chinese with English abstract).
Zheng, Y.F., 1993. Calculation of oxygen isotope fractionation in anhydrous silicate
minerals. Geochimica et Cosmochimica Acta 57, 1079−1091.
Zheng, Y.F., Chen, J.F., 2000. Stable isotope geochemistry. Science Press, Beijing,
1−316 (in Chinese).
Zhu, J.J., Hu, R.Z., Richards, J.R., Bi, X.W., Zhong, H., 2015. Genesis and
magmatic-hydrothermal evolution of the Yangla skarn Cu deposit, Southwest
China. Economic Geology 110, 631–652.
Zu, B., Xue, C.J., Zhao, Y., Qu, W.J., Li, C., Symons, D.T.A., Du, A.D., 2015. Late
Cretaceous metallogeny in the Zhongdian area: Constraints from Re–Os dating
of molybdenite and pyrrhotite from the Hongshan Cu deposit, Yunnan, China.
Ore Geology Reviews 64, 1–12.
Zu, B., Xue, C.J., Chi, G.X., Zhao, X.B., Li, C., Zhao, Y., Yaxiaer, Y., Zhang, G.Z.,
Zhao, Y., 2016. Geology, geochronology and geochemistry of granitic intrusions
and the related ores at the Hongshan Cu-polymetallic deposit: insights into the
44
late cretaceous post-collisional porphyry-related mineralization systems in the
Southern Yidun arc, SW China. Ore Geology Reviews 77, 25–42.
Zu, B., Xue, C.J., Dong, C, Zhao Y., 2019. Mineralogy and Garnet Sm-Nd Dating for
the Hongshan Skarn Deposit in the Zhongdian Area, SW China. Minerals 9,
243.
45
Figure captions
Figure 1. (a) Tectonic outline of the India and Eurasia collision zone, showing the
study area. (b) Simplified tectonic map of the SE part of the Tibetan Plateau, showing
the Yidun Terrane, Yangtze Block, Songpan–Garze Fold Belt, and the Qiangtang–
Changdu Block, together with the major fault systems, suture zones, and granitoid
intrusions in the region (modified after Yang et al., 2017a, 2017b). GLSZ, Garze–
Litang Suture Zone; HS, Hongshan; JSJSZ, Jinshajiang Suture Zone; L. Triassic, Late
46
Triassic; L. Jurassic, Late Jurassic; L. Cretaceous, Late Cretaceous; LD, Langdu; PL,
Pulang; RL, Relin; TCG, Tongchanggou; XGF, Xiangcheng–Geza Fault; XJP:
Xuejiping;.XWC, Xiuwacu.
Figure 2. (a) Geological map of the Hongshan Cu–Mo deposit in the southern Yidun
Terrane, showing the distribution of various intrusive rock types and related orebodies
in the metamorphosed sedimentary and volcanic host rocks. Age of quartz diorite
porphyry is from Huang et al. (2012), and age of monzogranite porphyry from Yang et
al. (2016a). (b), (c) Exploration line sections through the Hongshan Cu–Mo deposit,
showing the spatial relationship different strata, intrusions and orebodies (modified
after Wang et al. 2014). See Figure 2a for locations.
47
Figure 3. Column map of drill hole HZK17-12 from the Hongshan skarn Cu–Mo
deposit and detailed logs of skarn types among the contact zone between Late
Cretaceous monzogranite porphyry and marble. The roof strata of skarn orebody is
marble, while the bottom strata is dominated by hornfels. From the bottom to the roof,
the skarn types change following the sequences: pyroxene > garnet skarn – garnet >
pyroxene skarn – magnetite skarn – pyrrhotite-chalcopyrite skarn – garnet > pyroxene
48
skarn –pyroxene > garnet skarn.
Figure 4. Mineral paragenesis for the Hongshan skarn Cu–Mo deposit.
49
Figure 5. Photographs of representative metasomatic rocks in the Hongshan skarn
Cu–Mo deposit. (a) Wollastonite skarn formed by isochemical metamorphism; (b)
Early-stage red-brown garnet bands (Grt I) replacing recrystallized limestone (marble);
(c) Zoned garnets with red-brown cores and later light-brown rims (Grt II), surrounding
residual calcite; (d) Magnetite skarn with disseminated chalcopyrite and pyrrhotite; (e)
Epidote replaced the garnet forming metasomatic pseudomorph texture, with massive
pyrrhotite; (f) Pyroxene–garnet skarn with disseminated chalcopyrite; (g) Epidote
skarn cut by veinlets of light-brown garnet (Grt II) ; (h) Quartz–pyrrhotite–chalcopyrite
vein in the garnet-pyroxene skarn; (i) Quartz-chalcopyrite-molybdenite vein in the
monzogranite porphyry; (j) Disseminated molybdenite aggregates wihin the
monzogranite porphyry; (k) Stage 3 quartz (Qtz-2)-galena vein cut the massive
pyrite-chalcopyrite ore; (l) Quartz (Qtz-2)–sphalerite vein within the skarnized marble.
50
Mineral Abbreviations: Cc = Clacite; Ccp = Chalcopyrite; Ep = Epidote; Gn = Galena;
Grt = Garnet; Mol = Molybdenite; Po = Pyrrhotite; Py = Pyrite; Pyx = Pyroxene; Qtz =
Quartz; Sph = Sphalerite; Wo = Wollasonite.
Figure 6. Representative photomicrographs showing mineral assemblages and
paragenesis at Hongshan. (a) Radial wollastonite; cross-polarized light. (b) Zoned
garnet (Grt I); cross-polarized light. (c) Subhedral andradite (Grt II) intergrowth with
actinolite and pyroxene; plane-polarized light. (d) Euhedral andradite (Grt II) with
interstitial epidote and quartz; cross-polarized light. (e) Sphalerite and galena crosscut
chalcopyrite and pyrrhotite; reflected light. (f) Coexisting quartz phenocryst and
altered sericite within the monzogranite porphyry; plane-polarized light. Mineral
abbreviations: Act = Actinolite; Cc = Calcite; Ccp = Chalcopyrite; Ep = Epodite; Gn =
51
Galena; Grt = Garnet; Po = Phrrhotite; Py = Pyrite; Pyx= Pyroxene; Qtz = Quartz; Ser
= Sericite; Sp = Sphalerite; Tr = Tremolite; Wo = Wollastonite.
Figure 7. (a) (Spessartine + almandine + pyrope) – grossularite - andradite ternary
diagram showing the compositions of both generations of garnets at Hongshan. The
data of garnets were collected from Gao et al. (2014). Also show the garnet
compositional ranges for major types of skarn deposits as summarized by Meinert et
al. (2005). (b) Johannsenite – diopside - hedenbergite ternary diagram for the
compositions of pyroxenes from Hongshan. Also show the pyroxene compositional
ranges for major types of skarn deposits as summarized by Meinert et al (2005).
Mineral abbreviations: Alm = almandine; And = andradite; Di = diopside; Gro =
grossularite; Grt = Garnet; Hd = hedenbergite; Jhn = Johannsenite; Pyr = pyrope; Pyx
= Pyroxene; Spe = Spessartine.
52
Figure 8. (a) SEM-CL images of quartz-1 from stage 2. (b) BSE images of quartz-1
from stage 2. (c) SEM-CL images of quartz-2 from stage 3. (d) BSE images of
quartz-2 from stage 3. (e) Al vs Ge/Ti and (f) Ge vs Ti diagrams of quartz-1 and
quartz-2.
53
Figure 9. Photomicrographs of typical fluid inclusions in the Hongshan deposit. (a)
Liquid-rich (L-type) fluid inclusions developed along the zoning parallel bands in
garnet. (b) L-type fluid inclusion within the garnet crystal. (c) L-type fluid inclusion in
epidote. (d) Vapor CO2–liquid CO2–H2O (C-type) fluid inclusions within the quartz-1. (e)
Fluid inclusions assemblages with L-type, V-type and C-type inclusions in quartz-1. (f)
Hypersaline fluid inclusions (H-type) that contain two daughter minerals, coexist with
L-type fluid inclusions in quartz-1. (g) Vapor CO2–liquid CO2–H2O inclusions (C-type)
in quartz-1. (h) Vapor-rich fluid inclusions (V-type) with a vapor volume exceeding 50
vol. % in quartz-1. (i) Liquid-rich fluid inclusions (L-type) with relatively small vapor
bubbles (no more than 50 vol. %) in quartz-1. LCO2 = Liquid CO2; VCO2 = Vapor CO2;
LH2O = Liquid; VH2O = Vapor. Mineral abbreviations: Ep = Epidote; Grt = Garnet; Qtz =
Quartz.
54
Figure 10. Frequency histograms of homogenization temperature and salinities for all
fluid inclusions types from Hongshan.
55
Figure 11. (a) Salinities versus homogenization temperatures for C-type, L-type and
V-type fluid inclusions hosted in garnet of stage 1. (b) Salinity versus homogenization
temperature diagram showing C-type, L-type and V-type fluid inclusions in epidote of
stage 2, and C-type, L-type, V-type and H-type fluid inclusions in quartz of stage 2. (c)
Salinity versus homogenization temperature diagram showing L-type fluid inclusions
from quartz and calcite of stage 3.
56
Figure 12. Plot of δ18O vs. δD values of water hosted by prograde garnet and
retrograde magnetite, tremolite, sericite and quartz from Hongshan (fields from
Hedenquist and Lowenstern, 1994).
Figure 13. Sulfur isotope compositions of pyrite (Py), pyrrhotite (Po), chalcopyrite
(Ccp), galena (gn), sphalerite (sph) from the Hongshan Cu–Mo deposit, compared
with values from typical skarn Cu deposits, and the Xiuwacu Mo–W porphyry deposit
(modified from Kamvong and Zaw, 2009). Data sources: Chen et al. (2011)–Niru; She
et al. (2005)–Jiama and Zhibula; Wang et al. (2015)–Xiuwacu..
57
Figure 14. Temperature vs. log oxygen fugacity (fO2) diagram, showing the stability
fields of major skarn silicate, oxide and sulfide minerals (modified from Meinert, 1998).
The number of the mineral assemblage in the figure represents the stable minerals in
different paragenetic stages from Hongshan: (1) andradite–pyroxene–wollastonite
represents stage 1; (2) amphibole–quartz–pyrite–pyrrhotite ± magnetite represents
stage 2; (3) quartz–calcite–pyrite represents stage 3. Temperatures are taken from
homogenization temperatures of fluid inclusions for the different paragenetic stages at
Hongshan. The grey line shows how the different paregenetic stages are interpreted
to have evolved.
58
Figure 15. Pressure estimation for primary fluid inclusions from stage 1 to 3. (a) V-
and L-type inclusions in stages 1, 2 and 3 were trapped under two-phase conditions;
thus, estimated pressures can represent the actual trapping pressures for these fluid
inclusions. Isobars were calculated from the equations of Driesner and Heinrich
(2007). (b) H-type inclusions from stage 2 were homogenized via halite dissolution
within quartz-1, which exhibits liquid-vapor homogenization at 211° to 330°C and
undergoes final homogenization at temperatures of 260° to 422°C
59
Figure 16. Possible fluid evolution paths for the Hongshan deposit (after Fournier,
1987; Meinert et al., 1997, 2003). Isopleths of NaCl in liquid (dark lines) and vapor
(dotted lines) are shown for compositions of interest. The initial fluids for stage 1 and
stage 2 are inferred to have been supercritical magmatic fluids with an initial salinity of
6 to 8 wt % NaCl eqv (e.g., Burnham, 1979; Hedenquist and Lowenstern, 1994).
These fluids followed two different paths. In stage 1, the fluid ascended along the
skarn trajectory until it encountered its solvus at about 500 °C and a pressure of ~500
bar (lithostatic pressure at depth of ~2 km). In stage 2, the trajectory intersects the
solvus below 400 °C and a pressure of ~200 bar (hydrostatic pressure at depth of
~2km).
60
Table captions
Table 1. Summary of microthermometric data of fluid inclusions from
Hongshan.
Host
mineral
FI
typ
e
Num
ber
Tm, ice
(°C)
Tm, clath
(°C)
Tm, CO2
(°C)
Th, CO2
(°C)
Th
(°C)
Salinit
y
(wt %
NaCl.
eqv )
Tm, h
(°C)
Stage 1
Garnet
C
2
7.3 to
7.9
-55.0 to
-52.1
30.1 to
30.2
402 to
534
4.1 to
5.2
V
2
-4.2 to
-3.4
529 to
533
5.6 to
6.7
L
10
-8.6 to
-2.3
406 to
522
3.9 to
11.5
Stage 2
Epidot
e
C
2
6.7 to
9.4
-54.8 to
-52.4
28.4 to
30.6
296 to
342
5.1 to
6.3
V
2
-4.2 to
-0.3
332 to
412
5.1 to
6.7
L
16
-9.4 to
-3.5
192 to
450
5.7 to
13.3
Quartz-
1
C
14
4.1 to
8.7
-56.5 to
-40.1
19.2 to
30.9
327 to
482
2.6 to
10.4
V
9
-11.0
to -5.4
255 to
415
8.4 to
15.0
L
52
-12.1
to -4.1
250 to
430
6.6 to
16.1
H
5
393 to
451
28.3 to
32.7
184 to
216
111
(KCl)
Stage 3
Quartz-
2
L
31
-12.2
to -3.8
163 to
293
6.7 to
16.3
Calcite
L
16
-12.2
to -3.7
109 to
246
6.0 to
16.2
Abbreviations: Tm, ice = ice melting temperature; Tm, cla = temperature of CO2-clathrate
dissolution; Tm, CO2 = melting temperature of solid CO2; Th, CO2 = homogenization
61
temperature of CO2; Th = total homogenization temperature; Tm, h = temperature of
halite melting; L = liquid-rich fluid inclusions; V = vapor-rich fluid inclusions; H =
Hypersaline fluid inclusions with daughter minerals; C = carbon-dioxide three-phase
(VCO2 + LCO2 + LH2O) fluid inclusions.
Table 2. Hydrogen-oxygen isotope data of garnet, magnetite, tremolite and sericite from
Hongshan.
Sample No.
Mineral
Stage
δDV-SMOW (‰)
δ18OV-SMOW (‰)
T (°C)
δ18OH2O (‰)
Data source
HZK17-12-B5
Garnet
1
-115.1
4.6
533
6.2
This study
HZK0901-06
Garnet
1
-109.6
6.1
7.6
This study
HZK0901-19
Garnet
1
-104.9
1.4
3.0
This study
HZK17-12-B21
Garnet
1
-116.1
5.8
7.3
This study
HZK17-12-B23
Garnet
1
-112.2
1.5
3.0
This study
HZK0906-B3
Garnet
1
-93.7
11.6
13.1
This study
HN3ZK11-B8
Garnet
1
-108.9
0.5
2.0
This study
HZK17-12-B20
Magnetite
2
-83.2
6.4
430
14.3
This study
HZK17-12-B21
Magnetite
2
-104.3
15.1
23.0
This study
HZK17-12-B22
Magnetite
2
-78.9
18.4
26.3
This study
HZK17-12-B23
Magnetite
2
-117.6
3.0
10.9
This study
HZK17-12-B24
Magnetite
2
-97.8
9.4
17.3
This study
HZK0906-B10
Tremolite
2
-112.3
15.6
430
16.4
This study
HZK0906-B11
Tremolite
2
-111.8
15.1
15.9
This study
7ZK16-B36
Sericite
2
-95.5
11.0
10.5
This study
LWC-1
Quartz-1
2
-81.0
11.7
350
5.6
Li et al. (2013)
LWC-2
Quartz-1
2
-89.0
11.9
5.8
Li et al. (2013)
LWC-3
Quartz-1
2
-117.0
15.3
8.6
Li et al. (2013)
LWC-4
Quartz-1
2
-75.0
12.2
4.8
Li et al. (2013)
LWC-5
Quartz-1
2
-107.0
13.0
4.8
Li et al. (2013)
Notes: δ18OH2O (‰) is calculated by δDV-SMOW (‰) according to the oxygen isotope
exchange coefficients between host minerals and water (1000 Inαgarnet–H2O = 1.14 × 106T-2
− 3.70, Lu and Yang (1982); 1000 Inαmagnetite–H2O = 2.88 × 106T-2 –11.36 × 103T-1 + 2.89,
1000 Inαtremolite–H2O = 3.95 × 106T-2 – 8.28 × 103T-1 + 2.38, Inαsericite–H2O = 4.10 × 106T-2 –
7.61 × 103T-1 + 2.25, Zheng et al. (2000)), V-SMOW = Vienna standard mean ocean
water composition.
62
Table 4. Sulfur isotope data for sulfides from Hongshan.
Sample No.
Mineral
Host rock
δ34S
(‰,V-CDT)
HZK0901-41
Pyrite
Vein
5.1
HZK0901-41
Galena
5.4
HZK0906-B3
Chalcopyrite
Skarn
4.9
HZK0906-B3
Pyrite
4.7
HZK0906-B3
Pyrrhotite
4.5
HZK0906-B3-m
Chalcopyrite
Skarn
5.0
HZK0906-B3-m
Pyrite
4.5
HZK0906-B3-m
Pyrrhotite
4.3
HZK17-12-B21
Pyrite
Skarn
5.1
HZK17-12-B14
Pyrrhotite
Skarn
4.7
HZK17-12-B14
Chalcopyrite
5.4
HN1ZK09-B10
Pyrrhotite
Hornfels
3.7
HN1ZK09-B10
Chalcopyrite
3.7
HN3ZK11-B08
Pyrrhotite
Hornfels
3.6
HN3ZK11-B08
Chalcopyrite
4.0
HN11D1B1
Pyrrhotite
Skarn
4.6
HN11D1B1
Pyrite
5.2
HN11D1B1
Galena
3.9
HN11D1B1
Sphalerite
4.7
HN11D1B4
Sphalerite
Hornfels
4.8
HN11D3B5
Pyrite
Vein
4.8
HN11D3B5
Sphalerite
5.4
HN11D3B4
Galena
Vein
3.5
HN11D3B14
Pyrite
Hornfels
5.1
HN11D3B14
Pyrrhotite
4.7
Notes: V-CDT represents Vienna cañon diablo troilite standards.
63
Garphical abstract