Ore geology and fluid inclusions of the Hucunnan deposit, Tongling, Eastern China: Implications for the separation of copper and molybdenum in skarn deposits
Graphical abstract
Introduction
The Mo–Cu ± Au association is common in magmatic hydrothermal deposits, as for example in polymetallic porphyry–skarn deposits/orefields (Meinert et al., 2005, Sillitoe, 2010, Seo et al., 2012). However, many such deposits/orefields show a clear separation of Mo versus Cu ± Au mineralization, in both space and time (e.g., Meinert et al., 2005, Simon et al., 2006, Xu et al., 2011, Seo et al., 2012, Cao et al., 2015). For example, the metal grades in some porphyry–skarn deposits show a rough correlation with formation depth; i.e., deep deposits tend to be enriched in Mo but are Cu and Au poor, as in Butte, Montana, and Bingham Canyon, Utah, USA (Singer et al., 2005, Rusk et al., 2008, Murakami et al., 2010, Seo et al., 2012). In some deposits, molybdenite precipitation postdates the precipitation of Cu sulfides ± Au, as for example in the Bingham Canyon deposit (Seo et al., 2012), but the opposite is observed in other deposits, such as in the Datuanshan and Fenghuangshan deposits in East China (Lai and Chi, 2007, Li et al., 2014, Cao et al., 2015) and the Bangpu deposit in Tibet (Wang et al., 2015a). Although the separation of Mo and Cu (Au) at deposit/orefield scales is known and is of economic importance, the geological causes for the partitioning and precipitation of the minerals remain poorly known.
The Tongling region in the renowned Middle–Lower Yangtze River metallogenic belt (MLYRB) of eastern China is an important district of Cu–Au–Fe–Mo mineralization (Fig. 1A) containing numerous skarn deposits (Tang et al., 1998, Pan and Dong, 1999, Mao et al., 2006, Mao et al., 2011). The polymetallic district (approximately 40 km long in an E–W direction and 20 km wide, with a total area of ~ 800 km2; Fig. 1B) contains > 50 known mineralized skarn occurrences that are closely associated with Early Cretaceous intermediate–acid intrusions (Wu et al., 2014). Some of these mineralized skarns show porphyry-style alteration (Mao et al., 2011, Yang et al., 2011, Wang et al., 2015b).
The total Cu–Au reserves in the Tongling district have been estimated at 5 Mt Cu and 150 t Au (Wu et al., 2010). The Hucunnan deposit, discovered by Team 321 of the Bureau of Geology and Mineral Exploration of Anhui Province, is a representative skarn deposit in the district, and shows mineralization associated with porphyry-style alteration. In the Hucunnan deposits that contain both Cu and Mo, a paragenetic relationship is observed between Cu sulfides and molybdenite, from vein to orebody scales, with molybdenite precipitation predating the precipitation of Cu–Fe sulfides. Drilling has revealed that Cu mineralization in the deposit is developed mainly within exoskarn, whereas the Mo mineralization occurs mainly in endoskarn, and is located deeper than the Cu mineralization.
The Hucunnan deposit is particularly suitable for comparing the mechanisms of copper sulfide and molybdenite precipitation, because the Cu mineralization is temporally and spatially distinct from the Mo mineralization. As is well known, fluid processes are critical for understanding the transport and concentration of metals in hydrothermal systems. Therefore, in this study we examined the interrelationships between Cu and Mo mineralization at field and petrographic scales, combined with data on H–O isotopes, fluid inclusion (FI) microthermometry, and laser Raman spectrometry. The aim is to reveal the sources and evolution of ore-forming fluids and the factors that lead to the separation of molybdenite from copper sulfides in skarn systems.
Section snippets
Geologic setting
The Tongling district, located in the central part of the MLYRB (Fig. 1A), is the largest Cu–Au–Fe–Mo ore district in the belt (Tang et al., 1998, Pan and Dong, 1999, Mao et al., 2006, Lai and Chi, 2007). Marine sedimentary rocks in the Tongling district, which include clastic sedimentary rocks, carbonates, and evaporates, were deposited in the Silurian–Middle Triassic, with the exception of the Early–Middle Devonian. The sedimentary rocks are developed on stable Precambrian basement, forming a
Geology of the Hucunnan deposit
The Hucunnan deposit is a recently discovered Cu–Mo skarn deposit located within the southeast limb of the NE-trending Qingshanjiao anticline. The country rocks of the Hucunnan deposit are mainly limestone and dolomitic limestone of Carboniferous–Triassic age (Fig. 2, Fig. 3), which were transformed to marble in a > 100-m-wide thermal metamorphic aureole around a granodiorite porphyry intrusion. The ore-bearing strata of the Hucunnan deposit consist of the lower Permian Qixia Formation and the
Hydrothermal alteration and paragenesis
Hydrothermal alteration associated with both Cu and Mo ores in the Hucunnan deposit is represented chiefly by skarnization, silicification, chloritization, and carbonation, accompanied by potassic alteration (K-feldspathization and biotitization), sericitization, and epidotization. Skarnization is developed in the contact zone between the granodiorite porphyry and country rocks (Fig. 2, Fig. 3). The silicification, chloritization, and carbonation overprint the skarn minerals, forming metallic
Fluid inclusion petrography and microthermometry
Zheng et al. (2015) reported the microthermometry of FIs in the Hucunnan deposit. But the relationship between Cu and Mo mineralization cannot be well constrained by the limited data of Zheng et al. (2015). Therefore, in the present study, we conducted a new detailed petrographic observations and microthermometric analysis of FIs based on the interrelationships between Cu and Mo mineralization at field and petrographic scales. The microthermometric study was conducted at the China University of
Fluid inclusion petrography
Fluid inclusions (FIs) are present in garnet, quartz, and calcite in the Hucunnan deposit. Five types of FIs have been identified, based on their characteristics at room temperature (Fig. 7): type 1 FIs are solid-bearing inclusions containing a vapor bubble, an aqueous liquid, and halite ± sylvite crystals; type 2 FIs are liquid-rich inclusions (10–40 vol% vapor); type 3 FIs are vapor-rich inclusions (60–90 vol% vapor); type 4 FIs are pure vapor inclusions; and type 5 FIs are CO2-bearing inclusions
Sources and evolution of ore-forming fluids
The oxygen and hydrogen isotope compositions of hydrothermal minerals in the ore deposits help to constrain the source and evolution of ore-forming fluids. The δ18O values of the hydrothermal fluids calculated from the oxygen isotope data of garnets (7.60‰–8.91‰; Chu, 2003) are consistent with those of magmatic fluids (5.5‰–10.0‰; Taylor, 1997). On a δD versus δ18Owater diagram (Fig. 11), samples from the skarn stage plot among or close to those of primary magmatic water, indicating that the
Conclusions
An integrated analysis of geological, FI, and isotopic data from the Hucunnan skarn Cu–Mo deposit enabled us to reach the following conclusions.
- (1)
The Hucunnan skarn Cu–Mo deposit shows a clear separation of Mo and Cu mineralization in both space and time. Field evidence and petrographic observations indicate that the ore-forming processes in the deposit can be divided into four stages: the skarn, quartz–molybdenite, quartz–chalcopyrite, and carbonate stages.
- (2)
Five types of FI were identified in the
Conflict of interest
We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us.
We confirm that we
Acknowledgments
This study was supported financially by the National Natural Science Foundation of China (grant 41302062), Fundamental Research Funds for the Central Universities (grant 2652015053), and the China Geological Survey (grant 12120113069900, 12120115034401). This is CUGB petro-geochemical contribution No. PGC-201506. Senior Engineers Linjie Chen and Chenghuo Zhang provided valuable assistance in the field.
References (68)
Synthetic fluid inclusions: XII: the system H2O–NaCl. Experimental determination of the halite liquidus and isochors for a 40 wt.% NaCl solution
Geochem. Cosmochim. Acta.
(1994)Revised equation and table for determining the freezing point depression of H2O–NaCl solutions
Geochim. Cosmochim. Acta.
(1993)- et al.
The use of bromine pentafiuoride in the extraction of oxygen from oxides and silicates for isotopic analysis
Geochim. Cosmochim. Acta
(1963) - et al.
Experimental and simulation study of salt effects and pressure/density effects on oxygen and hydrogen stable isotope liquid-vapor fractionation for 4–5 molal aqueous NaCl and KCl solutions to 400 °C
Geochim. Cosmochim. Acta
(2000) Fluid inclusions in sedimentary and diagenetic systems
Lithos
(2001)- et al.
Distinctive features of Late Palaeozoic massive sulfide deposits in South China
Ore Geol. Rev.
(2007) - et al.
The activity-composition relationship of oxygen and hydrogen isotopes in aqueous salt solutions: III. Vapor-liquid water equilibration of NaCl solutions to 350 °C
Geochim. Cosmochim. Acta
(1995) - et al.
The activity-composition relationship of oxygen and hydrogen isotopes in aqueous salt solutions: I. Vapor-liquid water equilibration of single salt solutions from 50 to 100 °C
Geochim. Cosmochim. Acta
(1993) - et al.
Petrogenesis and mineralization of the Fenghuangshan skarn Cu–Au deposit, Tongling ore cluster field, Lower Yangtze metallogenic belt
Ore Geol. Rev
(2014) - et al.
Fluid inclusion characteristics and geological significance of the Xi’ao copper–tin polymetallic deposit in Gejiu, Yunnan Province
J. Asian Earth Sci
(2014)
A tectono–genetic model for porphyry–skarn–stratabound Cu–Au–Mo–Fe and magnetite–apatite deposits along the Middle–Lower Yangtze River Valley, Eastern China
Ore Geol. Rev
Molybdenite Re–Os and albite 40Ar/39Ar dating of Cu–Au–Mo and magnetite porphyry systems in the Yangtze River valley and metallogenic implications
Ore Geol. Rev.
Tectonic framework and Phanerozoic evolution of Sundaland
Gondwana Res.
The Lower Changjiang (Yangzi/Yangtze River) metallogenic belt, east central China: intrusion– and wall rock–hosted Cu–Fe–Au, Mo, Zn, Pb, Ag deposits
Ore Geol. Rev.
Microanalysis of S, Cl, and Br in fluid inclusions by LA-ICP-MS
Chem. Geol.
Stable isotope fractionation between liquid and vapour in water-salt systems up to 600 °C
Chem. Geol.
Copper partitioning in a melt-vapor-brine-magnetite-pyrrhotite assemblage
Geochim. Cosmochim. Acta
Synthetic fluid inclusions. V. Solubility relations in the system NaCl–KCl–H2O under vapor–saturated conditions
Geochim. Cosmochim. Acta
Origin of the ore-forming fluids and metals of the Bangpu porphyry Mo–Cu deposit of Tibet, China: constraints from He–Ar, H–O, S and Pb isotopes
J. Asian Earth Sci.
Petrogenesis of Dongguashan skarn-porphyry Cu-Au deposit related intrusion in the Tongling district, eastern China: geochronological, mineralogical, geochemical and Hf isotopic evidence
Ore Geol. Rev.
Early Cretaceous dioritic rocks in the Tongling region, eastern China: implications for the tectonic settings
Lithos
Porphyry and skarn Au–Cu deposits in the Shizishan orefield, Tongling, East China: U–Pb dating and in-situ Hf isotope analysis of zircons and petrogenesis of associated granitoids
Ore Geol. Rev.
Geology, geochronology, fluid inclusion and H–O isotope geochemistry of the Luoboling Porphyry Cu–Mo deposit, Zijinshan Orefield, Fujian Province, China
Ore Geol. Rev.
Geological Map of the Hucunnan Cu-Mo Ore Deposit in Jiguanshan-Changlongshan Region
The composition of magmatic-hydrothermal fluids in barren and mineralized intrusions
Econ. Geol.
Geochemistry of Hydrothemal Ore Deposits (third edition)
Geochemistry of Hydrothemal Ore Deposits
Geologic, fluid inclusion and stable isotope constrains on mechanisms of ore deposition at the Datuanshan copper deposit, Middle-Lower Yangtz Valley, Eastern China
Acta Geol. Sin-Engl.
Origin and evolution of hydrothermal fluids in the Taochong iron deposit, Middle–Lower Yangtze Valley, Eastern China: evidence from microthermometric and stable isotope analyses of fluid inclusions
Ore Geol. Rev.
Underplating and assimilation–fractional crystallization of Mesozoic intrusions in the Tongling area, Anhui Province, East China: evidence from xenoliths and host plutons
Int. Geol. Rev.
The Copper–iron Belt of the Lower and Middle Reaches of the Changjiang River
Geology and genesis of Kafang Cu–Sn deposit, Gejiu district, SW China
Ore Geol. Rev.
Formation of the Campbell-Red Lake gold deposit by H2O-poor, CO2-dominated fluids
Mineral. Deposita
Metallogenic System of Shizishan Cu-Au ore-field in Tongling Area and Its Prospectiong Significances
Cited by (29)
Genesis of Chaoshan skarn Au deposit, Tongling, eastern China: Insights from mineral geochemistry
2022, Journal of Geochemical ExplorationMolybdenum isotopic fractionation in the Tibetan Yulong Cu-Mo deposit and its implications for mechanisms of molybdenite precipitation in porphyry ore systems
2020, Ore Geology ReviewsCitation Excerpt :The transport and precipitation of Cu in ore-forming fluids of porphyry Cu deposits has been extensively studied and relatively well constrained (Redmond et al., 2004; Landtwing et al., 2010; Lerchbaumer and Audétat, 2012; Chang et al., 2018). In contrast, the behavior of Mo in porphyry Cu-Mo and Mo deposits has received much less attention and remains highly controversial (Klemm et al., 2008; Seo et al., 2012; Spencer et al., 2015; Cao et al., 2017; Ni et al., 2017; Chang et al., 2018). White et al. (1981) and Sinclair (2007) reported the occurrence of abundant molybdenite in unidirectional solidification textures and vein dikes from porphyry Mo deposits that precipitated at magmatic-hydrothermal transitional conditions with a temperature of 710 ± 25 °C (Audétat and Li, 2017).
GIS-based mineral prospectivity mapping using machine learning methods: A case study from Tongling ore district, eastern China
2019, Ore Geology ReviewsCitation Excerpt :In a skarn Cu mineralization system, interactions between metalliferous fluids and carbonate wall rocks play a major role in the metal deposition. Fluid boiling, which is commonly observed in the studies of fluid inclusions (Deng et al., 2011; Cao et al., 2017; Liu et al., 2019), is an important mechanism for mineral precipitation in the Tongling ore district. The fluid boiling may result from the abrupt pressure release and rapid temperature drop as metalliferous fluids were focused towards physical traps (i.e., multi-layered fracture zones).