The Wangjiazhuang porphyry copper (-molybdenum) deposit is located at the Zouping volcanic basin, which is situated in the southeastern part of North China Craton and the northern margin of the Luxi uplift, adjacent to Jiyang depression in the north (Fig. 1). Regional faults here include the nearly EW- trending Qihe-Guangrao fault in the north, the NS-trending Jinshan-Yaojiayu fault in the east and the NNW-trending Wenzu fault in the west, which play an important role in controlling the formation and development of the volcanic basin (Lan et al., 2018; Shen et al., 2018; Yang et al., 2016; Wang et al., 2015; Liu et al., 2013; Han et al., 2008; Zhang et al., 2008; Li and Yuan, 1991; Yuan and Li, 1988, 1987). The Early Cretaceous volcanic sedimentary strata covers an area of ~400 km2 with the thickness about 5 km in the volcanic basin (Li et al., 2008; Yuan and Li, 1987).
Figure 1. Simplified regional geological map of Zouping area, Shandong Province (after Liu et al., 2013)
The volcanic rocks in the basin can be divided into three major eruption cycles from south to north, forming three central volcanic structural systems (Yuan and Li, 1987). The lithology of the first cycle includes basalt-andesite, andesite brecciated lava, tuff and agglomerate, which are distributed outside the ringdyke. During the middle period of volcanic activities, due to the ejection of massive materials, the crater collapsed, resulting in caldera, as well as annular/radial faults. The lithology of the second cycle includes ignimbrite breccia, tuff, trachyandesite and basalt- trachyandesite, which are distributed in the inner ringdyke. The volcanic rocks of the third cycle include trachytic brecciated lava, volcanic agglomerate, breccia and tuff, which are distributed in the north of volcanic basin.
The intrusive rocks in the basin are spatially controlled by volcanic structures, forming a basic-intermediate-alkaline intrusive complex, which can be divided into three phases from early to late: (1) the early phase includes norite-gabbros, diabase porphyrite and diorite porphyrite which are distributed in the southern margin of the basin; (2) the middle phase includes diorite-quartz monzonite, monzonite (ringdyke), syenodiorite which are distributed in Wangjiazhuang, Beilou, Xidong and Zhanggao areas; and (3) the late phase includes monzonite porphyry, syenite porphyry and calc-alkaline subvolcanic rocks (Han et al., 2008; Yuan and Li, 1988).
The Wangjiazhuang intrusion, a concealed complex, has been emplaced along a volcanic passage into the overlying Cretaceous volcanic sedimentary strata and is composed mainly of diorite, monzonite and quartz monzonite. Among them, the quartz monzonite is the host rock of the Wangjiazhuang Cu (-Mo) deposit that has a medium-grained texture and is mainly composed of plagioclase (45%–55%), K-feldspar (25%–30%), quartz (15%–18%) and biotite (5%–10%) with accessory apatite, zircon, magnetite, monazite and rutile (Li and Yuan, 1991; Yuan and Li, 1987).
Hydrothermal alteration is widely developed, and three wallrock alteration zones, potassic (K) alteration zone, potassic- silicic (K-Si) alteration zone and intensive potassic-silicic (K-Si) alteration zone occur from the margin of the intrusion towards the inner core (Lan et al., 2018; Tang, 1990; Yuan and Li, 1988). In addition, phyllic and chlorite alterations surround these alteration zones discontinuously (Figs. 2, 3).
Figure 2. Simplified geological map of the Wangjiazhuang mineralization field (modified after Yuan and Li, 1988)
Figure 3. Sketch diagram showing the cross-section of No. 15 exploration line (modified after Tang, 1990)
The Wangjiazhuang Cu (-Mo) orebodies exclusively occur in the altered quartz monzonite (Figs. 2, 3). Forty-four large and small orebodies have been found at the depths from -100 to 700 m a.s.l. They can be divided into deep orebodies and shallow ones. (1) The deep orebodies are branch-vein-like ones with steep dipping angle (55°–65°) and are small in scale, large in quantity and low in copper grade, having veinlet-disseminated ores (Fig. 4a); they are mainly distributed in the potassic-silicic alteration zone and intensive potassic-silicic alteration zone. (2) The shallow orebodies are nearly horizontal pegmatitic massive sulfide-quartz veins and lenticules (Fig. 4b) characterized with large scale and high grade; they are distributed above the brecciated quartz monzonite which contains deep orebodies. There is a trend from the lower molybdenum-rich orebodies through quartz core to the upper gold-rich copper orebodies (Tang, 1990).
Figure 4. Photographs showing (a) the altered quartz monzonite porphyry with quartz veinlet ore and (b) sulfide quartz ores
The ore minerals are mainly pyrite, chalcopyrite, and subordinately magnetite, tennantite, molybdenite with minor bornite, enargite, galena and sphalerite, etc., and gangue minerals include K-feldspar, biotite, quartz, muscovite-sericite, chlorite and calcite. The ore textures include vein-dissemination, stockwork, breccia and massive textures, etc., and the mineral paragenesis is given in Table 1.
Table 1. Mineral paragenetic sequence
Fluid inclusion study has been conducted to reconstruct the evolution of magmatic-hydrothermal fluids that formed the Wangjiazhuang porphyry Cu (-Mo) deposit.
Doubly polished thin sections were prepared for petrographic investigation of fluid inclusions, including the phases present in the inclusions, the shape, the size, the mode of occurrence and the distribution, following the techniques outlined by Roedder (1984) and Lu et al. (2004).
Two types of quartz containing fluid inclusions are distinguished and studied: (1) type Ⅰ quartz in the mineralized quartz monzonite with potassic-silicic alteration (Figs. 5a–5d), and (2) type Ⅱ quartz in the sulfide-quartz veins (Figs. 5e, 5f).
Figure 5. Photomicrographs showing quartz and fluid inclusions. (a) Isolated Bt-bearing and aqueous fluid inclusions in type Ⅰ quartz (sample W33C); (b) Ms-bearing and, to a lesser extent, halite-bearing inclusions occur in growth zones of type Ⅰ quartz crystals (W33G); (c) many type IV and some type Ⅲ inclusions occur in the core and intracrystalline fractures of quartz crystals (W33-2A); (d) type IV inclusions occur in the core and 4 rhythmic growth zones of quartz crystals whereas in the clear quartz between the zones some type Ⅲ and type Ⅰ inclusions occur randomly (W33-3A); (e) type Ⅰ and type Ⅲ fluid inclusions occur in the intracrystalline fractures of type Ⅱ quartz crystals in the Cu (-Mo)-quartz vein where types III and IV inclusions are absent (W22B); (f) isolated types I, II and III fluid inclusions occur in type Ⅱ quartz associated with sulfosalt mineralization where type IV inclusion is absent (W7H). Bt. Biotite; FI. fluid inclusions; Ms. muscovite; Qz. quartz
Four types of primary-pseudosecondary fluid inclusions were recognized in the two kinds of quartz based on the observed phases at room temperature. They are one- or two-phase aqueous inclusions with vapor/liquid ratios less than 30% to 40% (type I); gas-rich inclusions with vapor/liquid ratios more than 50% (type Ⅱ), some of which contain a small opaque mineral, probably chalcopyrite; multiphase fluid inclusions with daughter minerals halite±anhydrite±opaque (chalcopyrite)± sylvite±hematite±unknown crystal (type Ⅲ); and mica-bearing fluid inclusions (type IV) (Fig. 5). It is noted that hematite occurs mainly in matrix quartz of quartz monzonite and in the early quartz veinlet cutting it. The types and abundance of fluid inclusions in type Ⅰ and type Ⅱ quartz of different samples are listed in Table 2.
Sample Type of quartz containing fluid inclusions Lithology Type Ⅰ Type Ⅱ Type Ⅲ Type Ⅳ W33 Ⅰ Quartz monzonite with K-Si alteration +++ ++ +++ +++ W22 Ⅱ Cu (-Mo) Qz vein ++++ +++ ++ － W7 Ⅱ Sulfosalt-Qz vein +++ ++ － － －. Absent; ++. common; +++. many; ++++. abundant; Cu (-Mo); copper (-molybdenum); K-Si. potassic-silicic; Qz. quartz.
Table 2. Types and abundance of fluid inclusions in quartz
The fluid inclusion microthermometric experiments were conducted on the Linkam THMS600 heating/cooling stage mounted on the microscope at Shandong Institute of Geological Sciences (SIGS), Jinan. Liquid nitrogen was used as a coolant and the stage was routinely calibrated at 0 and 374.1 ℃ based on the ice-melting temperature and homogenization temperature of a pure water synthetic fluid inclusion having the critical density, and at -56.6 ℃ based on the CO2 melting temperature of a pure CO2 fluid inclusion, respectively. The uncertainty is estimated as ±0.2 ℃ below room temperature and 5 ℃ in the temperature range higher than 200 ℃, respectively (Bodnar, 1994). In addition, the melting temperatures of muscovite in type IV fluid inclusions were measured in the hydrothermal diamond anvil cell (HDAC) of the fifth generation Bassett diamond anvil cell (1.0 mm in diameter and 0.25 mm in thickness) at the Laboratory of High Temperature and High Pressure, Institute of Mineral Resources, Chinese Academy of Geological Sciences (CAGS), Beijing (Li et al., 2017; Li and Li, 2014).
The ice melting temperatures (Tm ice) and homogenization temperatures to liquid water (Th V-L) for type Ⅰ inclusions in the early quartz veinlet are in the range of -13– -1.5 and 290–417 ℃, respectively, indicating the moderate-to-low salinity and higher temperature for the early hydrothermal fluid. The Tm ice and Th V-L for type Ⅰ inclusions in the Cu (-Mo) quartz vein are in a wide range from -25 to -2 ℃ and from 380 to 120 ℃. The Tm ice and Th V-L for type Ⅱ inclusions in the late quartz related to sulfosalts deposition are in the range of -4.1– -0.8 ℃ and 110–215 ℃, respectively.
The Tm ice and Th L-V (to vapor phase) for type Ⅱ inclusions in the early quartz veinlet and in the Cu (-Mo) quartz vein are in the range of -4.3–0.3 and 270–417 ℃, and around 0 and 293–386 ℃, respectively.
The Th V-L and halite melting temperatures (Tm halite) for type Ⅲ inclusions in the matrix quartz and the early quartz veinlet are in a wide range of 120–215 and 200– > 400 ℃, and of 150–260 and 346–500 ℃, respectively. Many type Ⅲ inclusions which have been finally homogenized by halite disappearance, may have been re-equilibrated. However, some type Ⅲ inclusions have been finally homogenized with similar temperatures of vapor and halite disappearance. Occasionally, sylvite daughter crystal in type Ⅲ inclusions was observed to melt around 100 ℃ before halite disappearance at 340 ℃. The Th V-L and Tm halite for type Ⅲ inclusions in the Cu (-Mo) quartz vein are in the range of 240–360 and 264–380 ℃. Some of them have been homogenized by halite disappearance whereas others by vapor disappearance or both. Because fluid inclusion petrography has shown that in some quartz crystals adjacent to chalcopyrite and molybdenite type Ⅱ and type Ⅲ inclusions coexist and they have varying but similar homogenization temperature range, the fluid boiling probably occurred during Cu-Mo mineralization.
Type IV inclusions are only found in quartz monzonite with K-Si alteration and the Th V-L of aqueous phase in the type IV inclusions are mainly concentrated in the range of 300–360 ℃ whereas the melting temperatures of muscovite are in the range of 773–790 ℃. The microthermometry data of fluid inclusions are listed in Table 3.
Sample FI type Tm ice/Tm hh* (℃) Tm halite* (℃) Th V-L* (℃) Salinity (wt.% NaCl eq.) Remarks W33 Type I -16.5– -2.0 245–400 19.84–3.39 -5.2– -2.0 153–173 8.14–3.39 Type Ⅲ 170–445 180–360 30.48–52.65 Type Ⅳ -25.1– -2.0 300–360 > 26–3.39 *Tm Ms > 700 ℃ W22 Type Ⅰ -5.6– -3.8 210–260 6.16–8.68 -3.0– -2.2 106–126 3.71–4.96 Type Ⅱ -4.8– -2.0 298–390 7.59–3.39 Type Ⅲ 300–380 298– > 400 38.16–45.33 Tm halite≈Th V-L W7 Type Ⅰ -4.1– -0.8 115–212 1.40–6.59 (-4.0– -3.0) Type Ⅱ Not detected *Tm ice/Tm hh. Last melting temperature of ice/hydrohalite; Tm halite. final melting temperature of halite; Tm Ms. muscovite melting temperature; Th V-L. homogenization temperature of vapor to liquid phase.
Table 3. Microthermometry of fluid inclusions in the Wangjiazhuang Deposit