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Ore-forming processes and mechanisms of the Hongshan skarn Cu–Mo deposit, Southwest China: Insights from mineral chemistry, fluid inclusions, and stable isotopes

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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 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.
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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:
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1
Highlights
Ore-forming fluids are of predominating magmatichydrothermal 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
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Ore-forming processes and mechanisms of the Hongshan skarn CuMo 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: LiQiang 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 CuMo 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 CuFeMo sulfides (stage 2), and PbZn sulfides associated
with calcite and quartz (stage 3). Prograde skarns contain mainly andraditic garnet
(two series: And3782Gro1761Spe+Alm+Pyr04 and andradite) and pyroxene (Di64
88Hd1235) 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 CO2Liquid
CO2H2O (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 3.911.5 wt% NaCl eqv. A boiling fluid
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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 100300°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; DSO isotopes; Quartz LAICPMS trace element;
Hongshan CuMo 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
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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 SongpanGarze 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 CuMo mineralization occurred in the Hongshan district (Xu et
al. 2006; Li et al. 2013). Other workers have argued that both the skarn and porphyry
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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. ReOs 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
CDOSPbCu 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 DO 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)
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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 GarzeLitang 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 GarzeLitang
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 QiangtangChangdu 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 NWSE-trending faults paralleling to the
GarzeLitang 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
AgCuPbZn 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 Cupolymetallic 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 AgSnPbZn 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 UPb ages of 8879
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
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The Hongshan skarn CuMo 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). SmNd 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 CuMo 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 400850 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 CuMo 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 (0.52.0 %) 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.
5ac), 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
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skarn alteration (Garnet I: And3782Gro1761Spe+Alm+Pyr04, 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 7080 % 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. 5dh). 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. 6cf). 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). CuFe 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).
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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
JXA8230 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,
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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 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 LAICPMS 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 LAICPMS using
an Agilent 7500cs quadrupole ICPMS 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 4060 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 500600 °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 DO 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 3.8210.99 wt%, 11.1116.32 wt% and 18.79
16
24.95 wt%, respectively. These results show that pyroxenes at Hongshan
predominantly consist of diopside (6488 mol%), with minor hedenbergite (1235
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 13.3215.18 wt%, 12.2514.74 wt% and 9.4411.85
wt%, respectively. The tremolites have higher SiO2 abundances (13.3215.18 wt%),
MgO (19.2020.90 wt%) and CaO (12.8613.24 wt%), and lower Al2O3 (0.782.05
wt%) and FeOT (4.826.64 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 (10.958.6 ppm), Rb
(0.090.28 ppm), Sr (0.010.36 ppm) and Cs (0.020.18 ppm), low contents of Al
(41142 ppm), Li (216 ppm), Ge (0.882.12 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 (0.56.2 ppm),
Rb (0.030.05 ppm) and Sr (0.010.06 ppm), but higher abundances of Al (731423
ppm), Li (9144 ppm), Ge (2.2410.81 ppm) and Sb (0.198.88 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.
9af). Four types of fluid inclusions were recognized from Hongshan: vapor CO2
liquid CO2H2O inclusions (C-type; Fig. 9g); vapor-rich inclusions (V-type; Fig. 9h)
with a vapor volume of 5599 vol. %; liquid-rich inclusions (L-type; Fig. 9i) with
relatively small vapor bubbles (1050 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 3060 vol. % of the total volume at
room temperature. H-type fluid inclusions were observed in quartz of stage 2. They
have diameters of 1020 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 NaClH2O 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 7.37.9 °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 5.16.3 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 31.132.7 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. Fiftytwo 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 InαgarnetH2O = 1.14 × 106T-2 3.70, Lu and Yang 1982; 1000
InαmagnetiteH2O = 2.88 × 106T-2 11.36 × 103T-1 + 2.89, 1000 InαtremoliteH2O = 3.95 ×
106T-2 8.28 × 103T-1 + 2.38, InαsericiteH2O = 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 DO data from the two
paragenetic stages.
5.3.2 S isotopes
Twentyfive 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 andraditediopside
wollastonite assemblage. The retrograde skarn assemblage consists of quartz
amphiboleepidote ± magnetite, and CuFeMo sulfides that are associated with this
hydrous assemblage. The quartzcalcitesphaleritegalena 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 (400550 °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 250450 °C), and CuMoFe sulfides formed under reduced conditions
(H2S-predominate) and moderate sulfur fugacities (sulfides rather than sulfates;
Cooke et al. 2000). Stage 3 PbZn 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
magmatichydrothermal 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 DO 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 (3.911.5 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 CuMoFe 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 (109293 °C) and moderate to low salinities (6.016.3
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 DO 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 PbZn sulfides.
7. Conclusions
Hongshan is a large CuMo 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). DO
isotopic compositions show that the ore-forming fluids at Hongshan are of
30
predominating magmatichydrothermal 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 CuFeMo 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 PbZn sulfides due to the decreased
temperature
Acknowledgements
We are grateful to David Cooke, QiHai Shu, WenYan He and LinNan Guo for
valuable comments on earlier versions of this manuscript. We would like to thank
ZhenYu Chen and XiaoDan Chen for their help with the EPMA analysis, Mu Liu on
stable isotopes analysis, and Jay Thompson on quartz LAICPMS analysis. Special
thanks are extended to Engineer JianXin Li, ZhongHua 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
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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, SongpanGarze 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, XiangchengGeza Fault; XJP:
Xuejiping;.XWC, Xiuwacu.
Figure 2. (a) Geological map of the Hongshan CuMo 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 CuMo 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 CuMo
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 CuMo deposit.
49
Figure 5. Photographs of representative metasomatic rocks in the Hongshan skarn
CuMo 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) Pyroxenegarnet skarn with disseminated chalcopyrite; (g) Epidote
skarn cut by veinlets of light-brown garnet (Grt II) ; (h) Quartzpyrrhotitechalcopyrite
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 CO2liquid CO2H2O (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 CO2liquid CO2H2O 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 CuMo deposit, compared
with values from typical skarn Cu deposits, and the Xiuwacu MoW 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) andraditepyroxenewollastonite
represents stage 1; (2) amphibolequartzpyritepyrrhotite ± magnetite represents
stage 2; (3) quartzcalcitepyrite 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, 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
529 to
533
5.6 to
6.7
L
10
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
332 to
412
5.1 to
6.7
L
16
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
255 to
415
8.4 to
15.0
L
52
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
163 to
293
6.7 to
16.3
Calcite
L
16
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 InαgarnetH2O = 1.14 × 106T-2
3.70, Lu and Yang (1982); 1000 InαmagnetiteH2O = 2.88 × 106T-2 11.36 × 103T-1 + 2.89,
1000 InαtremoliteH2O = 3.95 × 106T-2 8.28 × 103T-1 + 2.38, InαsericiteH2O = 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
... Iron was derived by oxidation of hydrothermal iron chloride complexes in the hydrothermal fluids. As shown in Figure 11, the mineral assemblages of quartz-calcite-wollastonite as well as hematitequartz-calcite-andradite in the skarn zones are considered as the oxidized type of prograde calc-silicate skarn mineralization formed at about 390°C-500°C (Gao et al. 2020;Gerdes and Valley 1994;Gustafson 1974) under oxygen fugacity of 10 −33 -10 −15 bar (above the magnetite-hematite buffer equilibrium line). ...
... The chemical composition of the marble is mainly of calcite without dolomite. Although no data are reported on the systematic mine feasibility and economic appraisal of the marble outcrops, geochemical data shows that the marble samples are free from FIGURE 11 | Oxygen fugacity versus temperature diagram (modified after Gao et al. 2020) in skarn mineralization at the Kuh-e-Gabri. The gray lines and gray circles show variations in oxygen fugacity and temperature of skarn mineralization, respectively. ...
Article
Full-text available
The prograde calcic skarn mineralization at Kuh‐e‐Gabri area in Kerman province, Iran, is composed of andradite–grossularite, wollastonite, diopside, hematite, and vesuvianite formed by intrusion of Oligo‐Miocene highly differentiated I‐type granitic stocks into the Cretaceous limestone and conglomerate. The retrograde skarn includes epidote, calcite, hematite, and quartz overprinting on the prograde skarn mineralization. This was followed by impregnations of hydrothermal veins of hematite, quartz, fluorite, and calcite. Supergene oxidation processes led to the occurrence of chrysocolla, dioptase, malachite, and goethite. XRD, DTA, FTIRS, EDXS, XRF, and ICP‐OES data indicate that the garnets are enriched in andradite (22.97%–99.50%)–grossularite (62.50%–75.52%) compositions. The composition of wollastonite is highly depleted in iron, manganese, and hazardous elements and close to pure wollastonite (98.31%–99.79%). The mineral assemblages of quartz–calcite–wollastonite as well as hematite–quartz–calcite–andradite suggest that the prograde skarn formed at about 390°C–500°C under oxygen fugacity of 10 ⁻³³ —10 ⁻¹⁵ bar. The incompatible element patterns in andradite–grossularite and wollastonite display enrichment in Cs, U, Pb, LREE, and negative Eu anomaly. The occurrence of andradite–grossularite skarn at the Kuh‐e‐Gabri may be the most suitable mineral for abrasive without creating environmental hazards. Acicular wollastonite‐rich skarn may be used for production of ceramics and metallurgical applications. Development of colored andradite, transparent and colored quartz in the skarn and veins may be considered as the valuable resource for the semi‐gem cutting industry. The light‐colored marble is a suitable raw material for metallurgical and refractory applications.
... Barker (1995) also has suggested that the iron in the Iron Springs mining district liberated from mafic phenocrysts in the intrusive body which was oxidized in place. High estimated fluid δ 18 O values in equilibrium with prograde and retrograde mineral assemblages from other skarn− type deposits also have been reported (Rose et al., 1985;Catchpole et al., 2015;Peng et al., 2016;Sepidbar et al., 2017;Hong et al., 2020;Gao et al., 2020;Xie et al., 2017Xie et al., , 2021. The isotopic composition of the hydrothermal minerals in whatever type of ore deposit depends on various factors such as the composition of the host rock, the nature of the ore-forming fluids, and that the processes affecting mineral deposition. ...
... Some studies (e.g., Bowman et al., 1985;Valley, 1986;Gerdes and Valley, 1994;Holness, 1997;Buick and Cartwright, 2000;Shin and Lee, 2003;Orhan et al., 2011;Demir and Dişli, 2020) suggested that the lowering of δ 13 C and δ 18 O values in hydrothermal calcites, compared to the original carbonate host rocks, is due to fluid− rock interaction. For the interpretation of the C-O isotopic variation in SAA deposit, we used the equation of Taylor and O'Neil (1977) for calculating the amount of fluid that exchanged with surrounding rock based on mass balance constraints. ...
Article
The southern Abbas Abad iron skarn deposit is located in the western part of the Central Iran zone, SW edge of the Urumieh˗Dokhtar magmatic arc. The skarn alteration occurred mainly at the contact of the Abbas Abad gabbro and Triassic Nayband Formation. The Triassic Nayband Formation in the southern Abbas Abad area consists of limestone, shale, siltstone, and marl with interlayers of tuff. The Abbas Abad gabbro is calc˗alkaline with adakitic signatures and continental arc setting affinity. Four iron ore bodies occur along the NW˗SE trending normal fault along the southern margin of Abbas Abad gabbro within sedimentary host rocks of the Nayband Formation. Three main paragenetic stages of skarnification and ore deposition have been recognized: i) prograde stage, ii) retrograde stage, and iii) supergene stage. Spinel, garnet, and pyroxene formed during the prograde stage. Epidote, tremolite˗actinolite, calcite and ore minerals, such as magnetite, hematite-I, pyrite, and minor chalcopyrite formed the retrograde stage assemblage. Magnetite is the main economic ore mineral, which is diverse in terms of fabric and texture, including massive, disseminated, needle˗like, blade˗like, prismatic, crustification, and cubes. Stable isotope data from Abbas Abad iron skarn deposit indicated that the interaction of non-magmatic brine (connate) fluids with Triassic carbonate host rocks at high temperature and its mixing with meteoric water has led to changes in the isotopic composition of the mineralizing fluid. Subsequently, garnet and magnetite with relatively high estimated fluid δ18O values (average 12.3 and 14.6 ‰, respectively) formed from this modified fluid. The estimated δ18O values of fluid in equilibrium with calcite decreased as a result of the greater influx of the meteoric water into the hydrothermal system in the late stages of ore formation. Fluid inclusion data from this iron skarn deposit are consistent with minor boiling during the prograde stage, although cooling and dilution seem to be principally responsible for Fe precipitation. The petrogenetic model proposed for the Southern Abbas Abad iron skarn deposit suggests that during the Eocene a gabbroic stock intruded the sedimentary rocks of the Nayband Formation and acted as a heat source for the non-magmatic brine (connate and meteoric) fluids convection cells, then reacted with cooler Triassic carbonate rocks. In this model, leaching of the gabbroic rock provided the iron, whereas carbon came from degassing and dissolution of carbonate rocks during contact metamorphismmetasomatism.
... However, research on skarn-type Mo deposits has not received sufficient attention (Meinert et al., 2005). Skarntype Mo deposits are either studied as subsidiary mineralization to porphyry-type Mo deposits in the Cordillera Mo metallogenic belts (Pettke et al., 2010;Audétat and Li, 2017) and Qinling Mo metallogenic belts (Zeng et al., 2013;Chen et al., 2022), or as an associated mineralization with skarn-type tungsten deposits (e.g., in the Jiangnan orogen, Lu et al., 2003;Hu et al., 2022) or copper deposits (e.g., in the middle-lower Yangtze metallogenic belt and eastern Tibet metallogenic belt, Gao et al., 2020;Liu et al., 2020;Han et al., 2020a;Wang et al., 2022;Bi et al., 2023). This limitation hampers our understanding of the ore-forming processes of skarn-type Mo deposits and restricts the comprehension of the Mo-associated W mineralization processes. ...
... However, amphiboles that coexist with sulfides (Amp IIc) show a deviation in the Y/Ho ratio (Fig. 13F) and a different REE distribution (Fig. 10B), suggesting the influx of meteoric water (Bau and Dulski 1995). This is consistent with the abundance of sulfides in this assemblage, which is a typical response to enhanced influx of meteoric water into a retrograde skarn system (Gao et al. 2020;Meinert et al. 2003;Öztürk et al. 2008). Later phases (chlorite and cassiterite) probably formed from a mixture of magmatic and meteoric fluids as prominently shown in the Y/Ho systematics of cassiterite (Fig. 11E). ...
Article
Full-text available
The Geyer tin skarn in the Erzgebirge, Germany, comprises an early skarnoid stage (stage I, ~ 320 Ma) and a younger metasomatic stage (stage II, ~ 305 Ma), but yet, the source and distribution of Sn and the physicochemical conditions of skarn alteration were not constrained. Our results illustrate that contact metamorphic skarnoids of stage I contain only little Sn. REE patterns and elevated concentrations of HFSE indicate that garnet, titanite and vesuvianite of stage I formed under rock-buffered conditions (low fluid/rock ratios). Prograde assemblages of stage II, in contrast, contain two generations of stanniferous garnet, titanite-malayaite and vesuvianite. Oscillation between rock-buffered and fluid-buffered conditions are marked by variable concentrations of HFSE, W, In, and Sn in metasomatic garnet. Trace and REE element signatures of minerals formed under high fluid/rock ratios appear to mimic the signature of the magmatic-hydrothermal fluid which gave rise to metasomatic skarn alteration. Concomitantly with lower fluid-rock ratio, tin was remobilized from Sn-rich silicates and re-precipitated as malayaite. Ingress of meteoric water and decreasing temperatures towards the end of stage II led to the formation of cassiterite, low-Sn amphibole, chlorite, and sulfide minerals. Minor and trace element compositions of cassiterite do not show much variation, even if host rock and gangue minerals vary significantly, suggesting a predominance of a magmatic-hydrothermal fluid and high fluid/rock ratios. The mineral chemistry of major skarn-forming minerals, hence, records the change in the fluid/rock ratio, and the arrival, distribution, and remobilization of tin by magmatic fluids in polyphase tin skarn systems.
... The ore metals in these deposits include Zn-Pb-Cu-Ag, and the mineralization ages are concentrated in the Late Triassic, such as the Gacun and Gayiqiong (Sichuan 210-230 Ma) (Table 5.1; Hou et al. 2001Hou et al. , 2003Hou et al. , 2004Wang et al. 2013a;Dang et al. 2014;Xue et al. 2014); (2) in the southern part, a series of porphyry Cu polymetallic deposits was developed, such as the Pulang, Xuejiping, Chundu, Lannitang and Langdu Leng et al. 2010);and (3) in the Yanshanian post-collision extensional setting, the thickened lower crust may have delaminated and partially melting, forming extensive felsic magma emplacement into the Indosinian porphyries, consequently the Indosinian-Yanshanian intrusive complexes. Thus, there are many porphyry-skarn and granite-related quartz vein-type Mo polymetallic deposits distributed in this region, such as the Hongshan Cu-Mo (Li, et al. 2013;Peng et al. 2016;Gao et al. 2020), Xiuwacu W-Mo deposits Zhang et al. 2017Zhang et al. , 2020Zhang et al. , 2021, Tongchanggou, Donglufang and Relin deposits Li et al. 2017;He et al. 2018). ...
Chapter
Full-text available
The metallogenic belts (or regions) of the Sanjiang orogenic domain are divided under the ideas of systematic, hierarchical and correlated metallogenic geological-tectonic unit, based on the spatial–temporal structure of the MABT evolution → intracontinental convergence strike-slip → tectonic transformation orogenic process.
... The ore metals in these deposits include Zn-Pb-Cu-Ag, and the mineralization ages are concentrated in the Late Triassic, such as the Gacun and Gayiqiong (Sichuan 210-230 Ma) (Table 5.1; Hou et al. 2001Hou et al. , 2003Hou et al. , 2004Wang et al. 2013a;Dang et al. 2014;Xue et al. 2014); (2) in the southern part, a series of porphyry Cu polymetallic deposits was developed, such as the Pulang, Xuejiping, Chundu, Lannitang and Langdu Leng et al. 2010);and (3) in the Yanshanian post-collision extensional setting, the thickened lower crust may have delaminated and partially melting, forming extensive felsic magma emplacement into the Indosinian porphyries, consequently the Indosinian-Yanshanian intrusive complexes. Thus, there are many porphyry-skarn and granite-related quartz vein-type Mo polymetallic deposits distributed in this region, such as the Hongshan Cu-Mo (Li, et al. 2013;Peng et al. 2016;Gao et al. 2020), Xiuwacu W-Mo deposits Zhang et al. 2017Zhang et al. , 2020Zhang et al. , 2021, Tongchanggou, Donglufang and Relin deposits Li et al. 2017;He et al. 2018). ...
... The ore metals in these deposits include Zn-Pb-Cu-Ag, and the mineralization ages are concentrated in the Late Triassic, such as the Gacun and Gayiqiong (Sichuan 210-230 Ma) (Table 5.1; Hou et al. 2001Hou et al. , 2003Hou et al. , 2004Wang et al. 2013a;Dang et al. 2014;Xue et al. 2014); (2) in the southern part, a series of porphyry Cu polymetallic deposits was developed, such as the Pulang, Xuejiping, Chundu, Lannitang and Langdu Leng et al. 2010);and (3) in the Yanshanian post-collision extensional setting, the thickened lower crust may have delaminated and partially melting, forming extensive felsic magma emplacement into the Indosinian porphyries, consequently the Indosinian-Yanshanian intrusive complexes. Thus, there are many porphyry-skarn and granite-related quartz vein-type Mo polymetallic deposits distributed in this region, such as the Hongshan Cu-Mo (Li, et al. 2013;Peng et al. 2016;Gao et al. 2020), Xiuwacu W-Mo deposits Zhang et al. 2017Zhang et al. , 2020Zhang et al. , 2021, Tongchanggou, Donglufang and Relin deposits Li et al. 2017;He et al. 2018). ...
Chapter
Full-text available
As an important tectonic unit of Eastern Tethyan tectonic domain, Sanjiang orogenic belt started from the expansion, subduction, subtraction and closure of the Paleozoic Paleo-Tethys Ocean, went through the opening and closure of the Neo-Tethys Ocean and the subduction and collision process of the Indian continent and ended in the Himalayan comprehensive intracontinental convergence and uplift orogeny, forming a giant composite orogenic belt composed of several island arc collision orogenic belts and several stable blocks. With the wide range of orogeny, huge uplift amplitude, strong magmatic activity and intense mineralization, it ranks first in the global orogenic belt. The Sanjiang orogenic belt is also an important part of Tethys metallogenic domain, which is comparable to the Andean metallogenic belt in South America for its huge metal reserves, concentrated and large ore deposits, diverse ore deposit types, complex mineralization and huge prospective scale. There are not only a number of world-famous large and super-large ore deposits, such as Yulong copper deposit, Gacun polymetallic deposit, Jinding lead–zinc deposit, Ailaoshan gold deposit and Tengchong tin deposit, but also a number of new large and super-large ore deposits, such as Pulang copper deposit, Beiya gold deposit, Baiyangping silver polymetallic deposit, Xiasai silver deposit, Gala gold deposit, Yangla copper deposit, Dapingzhang copper polymetallic deposit, Nongduke gold-silver deposit, Duocaima lead–zinc deposit, Duri lead–zinc deposit, Hetaoping copper polymetallic deposit and Luziyuan lead–zinc polymetallic deposit.
... The ore metals in these deposits include Zn-Pb-Cu-Ag, and the mineralization ages are concentrated in the Late Triassic, such as the Gacun and Gayiqiong (Sichuan 210-230 Ma) (Table 5.1; Hou et al. 2001Hou et al. , 2003Hou et al. , 2004Wang et al. 2013a;Dang et al. 2014;Xue et al. 2014); (2) in the southern part, a series of porphyry Cu polymetallic deposits was developed, such as the Pulang, Xuejiping, Chundu, Lannitang and Langdu Leng et al. 2010);and (3) in the Yanshanian post-collision extensional setting, the thickened lower crust may have delaminated and partially melting, forming extensive felsic magma emplacement into the Indosinian porphyries, consequently the Indosinian-Yanshanian intrusive complexes. Thus, there are many porphyry-skarn and granite-related quartz vein-type Mo polymetallic deposits distributed in this region, such as the Hongshan Cu-Mo (Li, et al. 2013;Peng et al. 2016;Gao et al. 2020), Xiuwacu W-Mo deposits Zhang et al. 2017Zhang et al. , 2020Zhang et al. , 2021, Tongchanggou, Donglufang and Relin deposits Li et al. 2017;He et al. 2018). ...
Chapter
Full-text available
In terms of collision, Suess (Die Entstehung der Alpen. Wien (Braumüller), p 168, 1875) divided orogenic belts into Pacific type and Tethyan type according to collision types of orogenic belts and took Alpine and Himalayan orogenic belts as typical examples of collision orogeny. Sengor (Earth Sci Rev 27:1–201, 1992) divided collision orogenic belts into Alps type, Himalayan type and Altai type according to the collisional type and different internal tectonics of the orogenic belt. Based on the various tectonic units involved in the collision, Li et al. (Chin J Geol 34:129–138, 1999) pointed out that collision may occur in various geological bodies (such as land-land, land-frontal arc, land-residual arc, land-accretionary arc, arc-arc, land-arc-land), which is the most complete description of collision behavior between collision objects and various geological bodies so far.
... Such fluid-rock interactions caused an increase in the pH due to the neutralization of the acidic hydrothermal-magmatic fluids by limestone. The increase in pH of the ore-forming fluids is known to trigger the precipitation of sulfide (Gao et al., 2020;Reed and Palandri, 2006). The solubility of chalcopyrite is lower at high temperatures compared to that of sphalerite (Barrie et al., 1999;Fontboté et al., 2017;Kojima, 1990), explaining the observed metal zoning with abundant chalcopyrite at depths where high temperatures prevailed compared to sphalerite formed at shallower levels. ...
Article
The Amensif Zn-Cu (Pb-Ag-Au) ore deposit is located in the northern part of the western High Atlas. The deposit hosts 1 million tons (Mt) of Zn-Cu-Pb metal reserves at 3% Zn, 2% Cu, and 1% Pb. The lenticular sulfide ores are structurally controlled open-space fillings (fractures, faults) in the Lower Cambrian carbonates. The mineralogical paragenetic sequence consists of andradite-quartz-epidote-dolomite-calcite-sphalerite-chalcopyrite-pyrite-galena with subordinate amounts of barite. The pre-ore stage and ore stage I dolomite samples have δ13CPDB and δ18OSMOW values in the range of -10.2 to -2.6‰ and 17.4 to 26.8‰, respectively. The lower δ18O value of these dolomites suggests oxygen isotope exchange between the ore-forming fluids and the carbonate host rock whereas negative δ13CPDB values reveal the interaction between these hydrothermal fluids and black shales. The δ34S values of sulfides ore (sphalerite, chalcopyrite, and pyrite) are high, ranging from 6.4 to 14.4‰. These S isotopic values reveal that sulfur derived from the mixture of magmatic sulfur and lower Cambrian carbonate-derived sulfur in almost equal proportions. Sulfide ores precipitated from the mixing between the magmatic-hydrothermal fluids exsolved from a hidden magma or the nearby Azegour granite of Permian age and an external cooler fluid of unknown origin. Fluid-rock interactions with the carbonate host rock contributed in the precipitation of the ore. Mineralogical and geological features allow us to ascribe the Amensif Zn-Cu (Pb-Ag-Au) polymetallic ore as a distal skarn system which is spatially associated with the lower Permian granite intrusion.
Article
Garnet is a common hydrothermal mineral in skarn-type deposits, and is useful for constraining the ore-fluid composition. Based on the mineralogy and geochemical features of different types of garnets from the Yangla Cu deposit, we discuss the factors and mechanism of rare earth elements (REE) incorporation into skarn-type garnet, together with the skarn-forming environment. Garnet from Yangla belongs to the grossular–andradite solid solution series, and can be divided into two types (i.e., zoned and unzoned). Early garnet (Grt Ⅰ) aggregates (avg. And84.96 Gro9.62 Spe+Pyr+Alm5.42) in the proximal skarn is clearly oscillatory-zoned, whilst most of late garnet (Grt Ⅱ) (avg. And96.26Gro0.58Spe+Pyr+Alm3.16) in the diopside skarn is unzoned. Grt Ⅰ is slightly LREE-enriched and HREE-depleted with positive Eu anomalies, whereas Grt Ⅱ is obviously LREE-enriched and HREE-depleted with positive Eu anomalies. Grt Ⅰ has higher total REE, Y and HFSE contents than Grt Ⅱ. We suggest that fluid composition and physicochemical conditions greatly controlled the REE incorporation into garnet. The Grt Ⅰ oscillatory zoning was likely formed by self-organization under relatively low growth rate, leading to Al/Fe fluctuation in the fluid related to the change of external factors (such as fluid boiling). Based on the differences of microscopic texture and geochemical characteristics between Grt Ⅰ and Grt Ⅱ indicate that Grt I was likely formed under a mildly acidic and reducing environment and low W/R ratios, whereas Grt II may have formed in a more acidic and oxidizing environment with higher W/R ratios. The changes in redox, pH and W/R ratios cause the compositional variation from Grt Ⅰ to Ⅱ during the ascent of the early hydrothermal fluid. During the process of the crystal formation, the oxygen fugacity increased, and the mildly acidic to acidic ore-forming solution shifted from a low W/R ratios condition to a high W/R ratios condition, causing the precipitation of metal sulfide. The fissures of Grt Ⅱ are infilled by pyrrhotite, chalcopyrite and pyrite, showing that the garnets form prior to the mineralization of copper and can provide space for the precipitation and enrichment of metals.
Article
Full-text available
Distinctive magmatic-hydrothermal, tourmaline-rich features have developed in the Heemskirk and Pieman Heads granites from western Tasmania, Australia. They are categorized as tourmaline-rich patches, orbicules, cavities, and veins, based on their distinctive morphologies, sizes, mineral assemblages, and contact relationships with host granites. These textural features occur in discrete layers in the roof zone of granitic sills within the Heemskirk and Pieman Heads granites. Tourmaline patches commonly occur below a tourmaline orbicule-rich granitic sill. Tourmaline-filled cavities have typically developed above the tourmaline-quartz orbicules in the upper layer of the white phase of the Heemskirk Granite. Tourmaline-quartz veins penetrate all exposed levels of the granites, locally cutting tourmaline orbicules and cavities. The tourmalines are mostly schorl (Fe-rich) and foitite, with an average end-member component of schorl45 dravite6 tsilaisite1 uvite0 Fe-uvite3 foitite31 Mg-foitite4 olenite10. Element substitutions of the tourmalines are controlled by FeMg–1, YAlXo(R2+Na)–1, and minor YAlO(R2+OH)–1 (where R2+ = Fe2+ + Mg2+ + Mn2+) exchange vectors. Several trace elements in tourmaline have consistent chemical evolutions grouped from tourmaline patches, through orbicules and cavities, to veins. There is a progressive decrease of most transition and large ion lithophile elements, and a gradual increase of most high-field strength elements. These compositional variations in the different tourmaline-rich features probably relate to element partitioning occurring in these phases due to volatile exsolution and fluxing of aqueous boron-rich fluids that separated from the granitic melts during the emplacement of S-type magmas into the shallow crust (4 to 5.5 km). Tourmalines from the Heemskirk Granite are enriched in Fe, Na, Li, Be, Sn, Ta, Nb, Zr, Hf, Th, and rare earth elements relative to the tourmalines from the Pieman Heads Granite, but depleted in Mg, Mn, Sc, V, Co, Ni, Pb, Sr, and most transition elements. These results imply that bulk compositions of the host granites exert a major control on the chemical variations of tourmalines. The trace element compositions of tourmalines from the Sn-mineralized Heemskirk Granite are different from those of the barren Pieman Heads Granite. Trace element ratios (e.g., Zn/Nb, Co/Nb, Sr/Ta, and Co/La) and Sn concentrations in tourmaline can distinguish the productive Heemskirk Granite from the barren Pieman Heads Granite.
Article
Most mineralized porphyries associated with large to giant oxidized porphyry Cu deposits show an affinity with relatively high Sr/Y features. The Mesozoic porphyry–skarn Cu–polymetallic systems of the Yidun Terrane hosted by the Late Triassic and Late Cretaceous intrusions provide chances to investigate the spatial and genetic relationships between subduction- and transtension-related magmatism and mineralization, and determine the petrogenesis and fertility of magmatic rocks. Zircon U–Pb ages indicate that the Late Triassic pre-ore quartz diorite porphyries and syn-ore quartz monzonite porphyries were emplaced at ~225 and ~215 Ma, respectively. The Late Cretaceous syn-ore monzogranite porphyries were emplaced at 83–78 Ma. Our new data, combined with previously published geochronological data, show the spatially overlapping distribution of the multiple Mesozoic porphyry systems in the Yidun Terrane. Although all the Late Triassic intrusive rocks share similar geochemical characteristics, the pre-ore quartz diorite porphyries have normal arc-related chemical features with low Sr/Y and (La/Yb)N ratios, high Y and Yb abundances, while the syn-ore quartz monzonite porphyries exhibit high Sr/Y and (La/Yb)N ratios, low Y and Yb abundances. All samples show similar Sr–Nd–Hf isotopic compositions [(⁸⁷Sr/⁸⁶Sr)i = 0.7060–0.7117, εNd(t) = −6.7 to 0.0, zircon εHf(t) = −4.0 to +3.0], suggesting that they were probably derived from partial melting of juvenile lower crust. Trace-element patterns and partial melt modeling indicate that the quartz diorite porphyries were likely formed by partial melting of normal thick lower crust, while the causative quartz monzonite porphyries were probably formed by partial melting of eclogitized, thickened lower crust. We propose that pre-ore quartz diorite porphyries were probably generated earlier via the subduction of Garze–Litang oceanic crust, and syn-ore quartz monzonite porphyries were formed later by partial melting of sulfide-enriched, thickened juvenile lower crust. Thus, these high Sr/Y quartz monzonite porphyries host several economically important porphyry Cu deposits, such as Pualng, Xuejiping and Songnuo. However, the Late Cretaceous syn-ore monzogranite porphyries have lower εHf(t) values (−8.4 to −2.9) and εNd(t) values (−7.5 to −3.8) than the Late Triassic porphyries, indicating that the former mainly originated from ancient crustal materials. New dataset and previous studies suggest that the Late Cretaceous post-collisional transtension triggered the asthenospheric upwelling and the underplating of mafic magmas, induced the partial melting of garnet-bearing amphibolite and thus caused the emplacement of monzogranite porphyries and associated porphyry–skarn Cu–Mo deposits.
Article
A long-standing controversy exists regarding the tectonic division, lithospheric architecture and evolution of the eastern Yidun Terrane in the Late Triassic. A compilation of geochronological, geochemical and isotopic data (91 whole-rock Nd and 413 laser points of zircon Hf) for volcanic and intrusive rocks across the entire eastern Yidun Terrane has allowed for a detailed investigation into its lithospheric architecture. Two subterranes were identified using Hf–Nd isotopic mapping. They include a high ƐHf(t) (>−3.0) and ƐNd(t) (>−3.5) domain constrained in the Southern Yidun Terrane (SYT), and a low ƐHf(t) (<−3.0) and ƐNd(t) (<−3.5) domain constrained in the Northern Yidun Terrane (NYT). The NYT and SYT are characterized by distinctive arc-related volcanic and plutonic rocks (NYT: 235–230 Ma basalt, andesite, dacite and rhyolite, as well as a 225–215 Ma granite batholith; SYT: 228–215 Ma adakite-like andesite, as well as diorite to monzonite porphyry), detrital zircon populations (NYT: ~2.50–2.45 Ga, ~980–880 Ma and ~480–400 Ma; SYT: ~2.50–2.40 Ga, ~1.90–1.75 Ga, ~1000–720 Ma, ~480–400 Ma and ~240–220 Ma) and mineralization styles (NYT: volcanic massive sulfide Ag–Cu–Pb–Zn and epithermal Ag–Hg deposits hosted in the ~230 Ma rhyolites; SYT: porphyry–skarn Cu–Mo–Fe deposits genetically related to the ~216 Ma dioritic to monzonitic porphyries). This dataset collectively shows that the NYT magmas were likely derived from a Paleoproterozoic or older mafic to intermediate lower crust with a variably minor addition of Triassic juvenile mantle melts, whereas the magmas for the SYT magmatic rocks were dominated by the arc juvenile mantle wedge melts with subordinate input of Late Mesoproterozoic or older crustal materials. The different melting processes between the NYT and SYT were attributed to changing subduction dip in space and time, with earlier steeper subduction at ~235–230 Ma, and later shallow-dip subduction from ~228–220 Ma. During the steeper dip subduction phase, it is likely that the NYT experienced a slightly greater degree of extension than the SYT, raising the possibility of a slab segmented by a major transform fault resulting in slightly steeper subduction beneath the NYT.
Article
The Hongshan-Hongniu super-large skarn copper deposit is located in the southern Zhongdian island arc. In this paper, to decipher the ore-forming fluid source and ore-forming material, genetic mineralogy characterization of garnet, pyroxene, pyrite and chalcopyrite were carried out and elemental analyses of C, H, O, S and Pb were performed. The results show that the garnet belongs to grossular-andradite, and the andradite was formed prior to grossular; whereas the pyroxene is mainly consisted of sahlite and diopside, indicating that the early acid ore-forming stage had high temperature and high oxygen fugacity. In the Hongshan-Hongniu deposit, the pyrite is enriched in Fe and depleted in S, with Co/Ni>1, suggesting a magmatic hydrothermal source. The garnet yielded δ¹⁸O and δD values of 6‰-8.8‰ and -93.1‰-149‰, respectively, and the calcite from massive sulfide ranged in δ¹³CV-PDB and δ¹⁸OV-SMOW values of -2.7‰ -2.5‰ and 11.4‰-23.4‰, respectively, suggesting that the magmatic hydrothermal fluid was dominant whilst meteoric water was negligible during mineralization in the Hongshan-Hongniu deposit. δ³⁴S content of the sulfides and quartz monzonite porphyry spanned small ranges from 3.8‰ to 5.6‰, and from 4.7‰ to 7.8‰, respectively, demonstrating that the ore-forming materials were originated from the quartz monzonite porphyry which had a single crustal source with material contribution from the mantle. Lead isotopes exhibited the features of the lower crust and a minor derivation from the upper crust and orogenic lead. © 2017, Editorial Office of Earth Science Frontiers. All right reserved.
Article
The Dabaoshan polymetallic deposits in the Nanling Range, South China, consist of porphyry and skarn-Type Mo mineralization genetically related to Jurassic porphyritic intrusions and adjacent strata-bound Cu-Pb-Zn mineralization hosted in mid-Devonian limestone. Porphyry Mo mineralization is characterized by the superposition of multiple generations of crosscutting quartz-bearing veins including: barren quartz veins (V1), quartz-molybdenite veins with K-feldspar alteration halos that host the bulk of the Mo mineralization (V2), quartz-pyrite veins with muscovite alteration (V3), and late base metal mineralization with argillic alteration (V4), as well as limestone-hosted strata-bound Cu-Pb-Zn mineralization (VS). Fluid inclusion petrography and microthermometry combined with cathodoluminescent textures and trace elements in quartz reveal changes in pressure and temperature of the hydrothermal system that formed the deposit. V1 and V2 veins are dominated by low-salinity (1-6 wt % NaCl equiv), CO2-bearing (4-10 mol %) two-phase inclusions with about 35 vol % bubble trapped in the one-phase field above the solvus of the fluid. V1 veins are dominated by CL-bright granular quartz mosaics with higher CL intensity than any other vein type. This quartz also contains more Ti than any other vein generation (24-89 ppm), while Al, Ge, and Li concentrations overlap with other vein generations. Molybdenum ore-hosting V2 veins are also dominated by CL-bright granular quartz mosaics, but V2 veins display slightly less CL intensity and correspondingly lower Ti concentrations (10-65 ppm). V3 veins have a broader spatial distribution than V1 and V2 veins, extending from inside the porphyries out into the adjacent limestone. These veins are also dominated by low-salinity (2-6 wt % NaCl equiv), CO2-bearing (4-7 mol %) two-phase inclusions, these containing about 45 vol % bubble trapped above their solvus. V3 veins are dominated by CL-dark quartz with euhedral growth zones of oscillating CL intensity and systematically lower Ti concentrations than previous vein generations (1.5-12 ppm). Minor V4 veins cut all the above vein generations and also occur as late infill in V3 veins. Low-salinity (4-7 wt % NaCl equiv), CO2-bearing (4-5 mol %) two-phase inclusions with about 20 vol % bubble prevail in V4 veins. V4 veins have the lowest CL-intensity quartz of all vein types, with euhedral growth zones of oscillating CL intensity and the lowest Ti concentration of all vein types (0.54-5.3 ppm). Intersections of fluid inclusion isochores with Ti-in-quartz isopleths indicate that the hydrothermal system evolved from near-magmatic pressures and temperatures of 2.7 ± 0.2 kbars and 650° ± 40°C for V1 veins to 1.9 ± 0.2 kbars and 530° ± 40°C for V2 veins to 0.65 ± 0.2 kbars and 400° ± 40°C for V3 veins. V4 veins, which lack rutile, formed as the system cooled to 250° to 300°C at maximum pressures of 0.40 to 0.65 kbar. Unlike nearly all other reported porphyry-Type ore deposits, quartz-bearing veins from the Dabaoshan porphyry Mo deposit contain few halite-bearing or vapor-dominated fluid inclusions in any vein type. The dearth of such fluid inclusions, coupled with the abundance of two-And three-phase CO2-bearing low-salinity fluid inclusions in all vein types is evidence that the formation conditions of V1 to V4 veins remained above the V-L surface in the H2O-NaCl-CO2 system, such that fluid unmixing rarely occurred in the Dabaoshan hydrothermal system. This is further supported by only rare evidence for quartz dissolution textures in all vein types, implying that pressures were dominantly higher than zone of retrograde quartz solubility. Taken together, the fluid inclusions, CL textures, and quartz trace element data indicate that the Dabaoshan porphyry Mo deposit is one of the deepest formed porphyry-Type ore deposits, having formed at depths of 6 to 7 km below surface. The extreme depth and lack of fluid unmixing inhibited Cu precipitation in the porphyry system, and instead allowed Cu to remain in solution to precipitate with Pb and Zn upon interaction with the surrounding Devonian limestone.
Article
Porphyry Cu ± Mo ± Au deposits typically formed in volcanoplutonic arcs above subduction zones. However, there is increasing evidence for the occurrence of porphyry deposits related to magmas generated after the underplating arc has ceased. Post-subduction lithospheric thickening, lithospheric extension, or mantle lithosphere delamination could trigger the remelting of subduction-modified arc lithosphere and lead to the formation of post-subduction porphyry deposits. The NNW-trending Yidun Terrane, located in the eastern Tethys, experienced subduction of Garze–Litang oceanic plate (a branch of the Paleotethys) in the Late Triassic and witnessed two mineralization events respectively associated with the ca. 215 Ma arc-related intermediate–felsic porphyries and the 88–79 Ma mildly-alkaline granitic porphyries. It is, therefore, an ideal place to investigate the genetic linkage between the subduction-related porphyry deposits and post-subduction porphyry deposits. Our new in situ zircon U–Pb dating of the two granitic intrusions (biotite granite, 213.4 ± 0.9 Ma; monzogranite porphyry, 86.0 ± 0.4 Ma) in the Xiuwacu district, the molybdenite Re–Os age (84.7 ± 0.6 Ma) of the mineralization, and previously published geochronological data, together show the spatially overlapping distribution of the multiple Mesozoic porphyry systems in the Late Triassic Yidun arc system. Furthermore, the arc-like elemental signatures and the mixed Sr–Nd–Hf isotopic signatures of the Late Cretaceous ore-related porphyries (i.e., originating from a mixed components between the ∼215 Ma juvenile arc crust and the Mesoproterozoic mafic lower crust) indicate a genetic linkage between the Late Triassic and Late Cretaceous porphyry systems. This suggests that the remelting of underplated arc-related mafic rocks formed during the subduction of the Garze–Litang Ocean could be responsible for the mixing between the mantle-derived components and the Mesoproterozoic lower crustal materials, when post-subduction transextension occurred in the Late Cretaceous. The formation of the Late Cretaceous porphyry–skarn Cu–Mo–W deposits could most likely be related to the remelting of Late Triassic residual sulfide-bearing Cu-rich cumulates in the subduction-modified lower crust that triggered by the Late Cretaceous transtension.
Article
Transition metal isotopes are sensitive geochemical tracers of ore genesis. Here we present MC-ICP-MS analytical data of Cu isotope compositions from the Hongshan-Hongniu Cu deposit in Yunnan province. The δ⁶⁵Cu values (δ⁶⁵Cu = [(⁶⁵Cu/⁶³Cu)sample/(⁶⁵Cu/ ⁶³Cu)NIST976-1] × 1000) of seven whole-rock quartz monzonite porphyries and twenty-two chalcopyrite samples from the skarn ore-bodies display relatively narrow ranges from -0.15‰ to 0.38‰ and from -0.02‰ to 0.77‰, respectively. The overlap of δ⁶⁵Cu values indicates a genetic relationship between the quartz monzonite porphyry and skarn ore-bodies. We also evaluate the spatial and temporal variations of Cu isotope compositions in the skarn ore-bodies by comparison with some well-documented porphyry deposits in the world. The quartz monzonite porphyry shows compositional zoning with the inner domain enriched in heavy Cu isotope and the skarn related to the porphyry depleted in heavy Cu isotope. The chalcopyrites that formed during the late stage of mineralization tend to be enriched in heavy Cu isotope, and this feature is analogous to porphyry deposits. The δ⁶⁵Cu values of the quartz monzonite porphyry show typical features of hypogene mineralization, suggesting a potential scope for deep exploration and development in this deposit.
Article
The Hongniu-Hongshan Cu skarn deposit (77.8 Mt at 1.8% Cu) is located in the central part of the Zhongdian porphyry and skarn Cu belt in southwestern China. Skarn and orebodies occur mainly between the different units of the Upper Triassic Qugasi Formation or within altered limestone adjacent to Late Cretaceous intrusions (78-76 Ma). Three main paragenetic stages of skarn formation and ore deposition have been recognized on the basis of petrographic observations: (1) pre-ore-stage hornfels with diopside (Di87-72Hd12-7), small-scale endoskarn with reddish grossular (Adr22-57Gr78-43), diopside (Di83-92Hd7-15), vesuvianite, and abundant exoskarn with red-brown andradite (Adr75-98Gr2-22), sahlite (Di28-41Hd58-71), and wollastonite; (2) syn-ore-stage retrograde minerals, sulfides (pyrite, chalcopyrite, pyrrhotite, molybdenite, galena, and sphalerite), quartz, and calcite; and (3) post-ore-stage calcite veins. Sulfur isotope values of sulfides are relatively high, with an average δ34S = 4.9% (n = 40), suggesting that the ore-forming fluid was magmatic and that the sulfides precipitated from a relatively reducing ore fluid. The coexistence of silicate melt and primary fluid inclusions in quartz phenocrysts of the mineralizationrelated quartz monzonite porphyry indicates the simultaneous entrapment of fluid and melt, and records the process of the aqueous fluid exsolving from the crystallizing melt. The initial single-phase fluid has a salinity of 8.8 to 12.7 wt % NaCl equiv and homogenization temperatures of 566° to 650°C, corresponding to pressures of 680 to 940 bar and lithostatic depth of 2.5 to 3.5 km. The primary fluid inclusions in the pre-ore-stage garnet and pyroxene composed of coeval vapor-rich (V type) and halite-bearing (S-I and S-II types containing sylvite) inclusions (32->79 total wt % salts) share similar homogenization temperatures (450°-550°C), indicative of the occurrence of fluid unmixing under lithostatic pressures of ∼550 to 780 bar (>2.0-km depth). Primary fluid inclusions trapped in syn-ore quartz, calcite, and epidote show the common development of S-type inclusions (∼37.3 wt % NaCl equiv) with coexisting V-type, liquid-rich (L type), and CO2-bearing (C-I type) inclusions, all of which have homogenization temperatures of 300° to 400°C and trapping pressures of 100 to 400 bar (∼1.5-km depth). Brine inclusions homogenized by halite dissolution after vapor disappearance in both the pre- and syn-ore stages are interpreted to have been trapped under overpressured conditions (>1,520 bar). Oxygen isotope analyses were conducted on garnet, wollastonite, epidote, quartz, and calcite. The pre-orestage garnet and wollastonite have δ18Ofluid values of 5.6 to 8.1%, whereas the syn-ore-stage epidote, quartz, and calcite have more variable δ18Ofluid values in the range of 3.9 to 17.5%. The δ18Ofluid values of the postore-stage vein calcite (15.2-21.3%) are much higher than both the pre- and syn-ore stages. The vapor phase of inclusions contains H2S, CH4, and C2H6 in the syn-ore stages. All these observations reveal that (1) the formation of the Cu skarn deposit was dominated by a magmatic hydrothermal system, (2) multiple fluid pulses contributed to the formation of the pre- and syn-ore-stage skarn minerals and sulfides, and (3) the increase in pH due to the neutralization of the acidic fluid could be the main factor controlling the large-scale ore deposition in Hongniu-Hongshan.