Ore-controlling effect of structure and distribution of deep rock mass on Pb-Zn deposit in Qingchengzi ore concentration area,Liaoning

NIU Mingxuan, YU Yunliang, TIAN Liya, NIU Wenqing and XU Haibo

College of Earth Sciences, Jilin University, Changchun 130026, China

Abstract: Qingchengzi ore concentration area in Liao-Ji rift is an important lead-zinc ore area of China, and the deep prospecting in this area has great prospects. Based on the spatial occurrence of ore bodies the Pb-Zn deposit can be divided into three types: layered, vein-shaped and pinnate. The deep geological conditions in this area are deduced by analyzing the tectonic evolution process and rock mass gravity inversion. The tectonic evolution of Liao-Ji rift can be divided into three stages: Paleoproterozoic extension period, Mesoproterozoic compression period and Mesozoic reactivation period. The magmatic activities in the Indosinian epoch led to the distribution pattern of the present deposits. According to the gravity inversion, Shuangdinggou- and Xinling rock masses on the north and south sides of the mining area are connected in the deep. The connected rock body might be distributed in the entire mining area. Xinling rock mass may be a branch extending from Shuangdinggou rock mass along the northeast trending fault, the connected rocks provide magmatic hydrothermal fluid for the final, folds and faults result in different types of ore body shapes.

Keywords: fault structure; lead-zinc deposit; profile inversion; Qingchengzi; Liaoning

0 Introduction

Qingchengzi ore concentration area has a long history of mining and a lot of research works have been undertaken. These deposits can be classified into three types based on the ore spatial occurrence of body. They exhibit zoning in spatial distribution with the characteristics of "upper vein and lower layer" (Dong, 2012). There are 13 large and medium-sized deposits such as Zhenzigou-, Benshan-, Nanshan- and Xiquegou deposits (Shaetal., 2011). Ore-forming fluids and sources of ore-forming materials in this area have been well studied. The mineralization of Qingchengzi lead-zinc deposit is jointly controlled by strata, structures and magma (Chi, 2002). It belongs to sedimentary-metamorphic-magmatic hydrothermal superimposed deposit, according to the geochemical characteristics of the mining area, the evolution process of rock masses, the distribution of ore deposits and the time of mineralization. In recent years, it has been found that Indosinian Shuangdinggou--Xinling intrusive bodies are closely related to the lead-zinc mineralization in the mining area (Maetal., 2013; Zhang, 2009). Indosinian magmatic upwelling activated the ore-forming materials in the formations, which enriched and accumulated in the structurally favorable locations. It has been mentioned that Shuangdinggou pluton and Xinling pluton are connected in the deep, but no detailed evidence provided (Songetal., 2017; Wangetal., 2016; Li, 2005). This paper investigates the process of folds and faults evolution in the study area, and analyzes the influences of tectonic and magmatic activities on lead-zinc mineralization. Furthermore, gravity inversion is undertaken to verify the distribution of deep rock mass by taking geological structure as constraints.

1 Geological background

1.1 Strata

Qingchengzi lead-zinc ore concentration area is located in the eastern section of Liao-Ji rift and between Longgang Block and Langlin Block, which belongs to the northern margin of North China Craton. Paleoproterozoic Liaohe Group exposed in this area consists of Gaojiayu Formation (Pt1g), Dashiqiao Formation (Pt1d) and Gaixian Formation (Pt1gx).

Gaojiayu Formation is the main ore-bearing formation, consisting of mica schist, dolomite marble, leptynite, and tremolite schist, which belongs to marine pyroclastic--carbonate formation. Dashiqiao Formation is the main ore-bearing formation of veined ore bodies, which is mainly composed of calcite marble mixed with dolomitic marble, graphite sericite schist,diopsidite, leptynite, belonging to marine claystone--carbonate formation with a small amount of volcanic rocks. The main lithology of Gaixian Formation includes fibrolite mica schist, mica schist, biotite phyllite, quartz schist, etc. It belongs to marine terrigenous clastic rock formation (Wangetal., 2017).

There are 13 small and medium-sized lead-zinc deposits in the ore-centralization area. They belong to Proterozoic sedimentary-metamorphic hydrothermal polymetallic deposit. According to the ore bodies morphology, occurrence and occurrence strata, Qingchengzi lead-zinc deposit can be roughly divided into three types, pinnate, vein-shaped and bedded. Pinnate ore bodies are located in western Qingchengzi mining area, including Xiquegou, Erdaogou, Mapao, Benshan, Beilazi, and Kuangdonggou. Vein-shaped ore bodies are located in the central part, e.g., Nanshan and Dadong. layered ore bodies are located in the eastern mining area, such as Zhenzigou, Diannan, Dadong and Dongyanggou.

By summarizing the geochemical characteristics of S and Pb isotopes, major elements, rare earth elements, and fluid inclusions in these ore bodies (Jiang & Wei, 1989; Dai, 2005; Duanetal., 2014), it is found that ore-forming materials are from the same source although the morphology of the deposits is different. The morphology of ore bodies are resulted from different ore-forming environments and structures.

Fig.1 Structural map of Qingchengzi ore concentration area (Dai, 2005)

1.2 Structure

The structures in the ore concentration area are complicated. The faults can be divided into three ca-tegories according to the strike: northeast trending, approximately north-south trending, and northwest trending faults. The folds are mostly approximately east-west trending synclinorium and anticlinorium. According to the evolution process, tectonic activity of Liao-Ji rift can be divided into three stages: Paleoproterozoic extensional period, Mesoproterozoic compression period and Indosinian activation period (Xieetal., 2018; Zhaietal., 2005).

In the first stage, Liao-Ji rift stretching led to crust thinning, accompanied by magma and ore-bearing hydrothermal solution intrusion along Jianshanzi fault and Xinling--Zhujiapuzi contemporaneous fault. Hydrothermal sediments were deposited with normal marine sediments in interlayer fractures or the contact surface of Gaojiayu Formation with granite. The syncline and anticline controlled the distribution of ore-forming components. These favorable elements formed the source layer or layered ore body.

In the second stage, the Liao-Ji rift shrank and squeezed the formation，which made the east-west trending folds tightly closed. Erdaogou anticline was enlarged into a box shape.Qingchengzi syncline was superimposed another fold and inverted. In addition, the process created NE-trending and NW-trending faults such as Erdaogou--Xiquegou, Xinling--Luoquanbei and Yushanggou faults. These faults constituted the rhombus structure framework in the mining area. The energy was released during magmatic emplacement, which caused the mixing of formation water with the crystal water in the mineral. This process produced hydrothermal fluids of high salinity. This process activated the original source layer, which caused the migration of the metallogenetic element, bedded bodies were formed at the interlayer fault, fracture and fold detachment surface, vein-shaped ore bodies were formed in the fractures cutting the strata.

In the third stage, the compression stress of Qingchengzi mining area activated the platform, forming many secondary northeast trending faults and northwest trending faults. Meanwhile, the fractures formed in the early stage changed from tension to compression. Shuangdinggou and Xinling rock masses intruded along the northeast trending Xinling fault, which activated and enriched the original lead-zinc ore body. The ore-forming fluid formed three kinds of ore bodies at different tectonic positions. Layered ore bodies were located in interlayer fractures. Vein-shaped ore bodies were located in steeply inclined faults. Pinnate ore bodies were located in the crack or dike near the main fault.

1.3 Intrusive rock

Intrusive rocks in the ore-concentrating area mainly comprise Paleoproterozoic Liao-Ji granite, Dadingzi plagioclase granite, Shuangdinggou--Xinling intrusive rocks in Indosinian period and Yaojiagou intrusive rocks in Yanshanian. Indosinian rock masses caused the activation and migration of metallogenic material in Qingchengzi mining area and lead-zinc deposit was formed eventually. Magmatic invasion activity in Indosinian period is very strong, which can be divided into three phases. The first phase was dominated by diorite and the exposed area of the rock mass is small, mainly in the south of Shuangdinggou rock mass. The second phase was the most strenuous intrusive activity, and the rock masses were dominated by porphyritic monzonitic granite. The intrusive rocks appeared as stock and batholith. The third phase magmatic emplacement was dominated by medium-fine grained monzonitic granite. Compared with the granites in the second phase, the granites in the third phase have finer grain size, lower magmatic activity intensity and irregular occurrence (Liu, 1995).

Shuangdinggou and Xinling rock masses are Indosinian granites. They belong to Dabao super unit. Shuangdinggou rock mass is distributed in the southwest of the mining area, with an exposure area of about 200 km2. The intrusive body appeared as batholith with assimilation and contamination. Its geochemical characteristics are similar to adakites. The emplacement time measured by zircon dating is 223.5--227.1 Ma (Yu, 2005). The lithology of Xinling rock mass is mainly porphyritic biotite granite, which is located in the northern mining area, with an exposed area of about 2 km2. The rock mass intrudes along the axis of northeast Xinling anticline and appeared stock, with the dip angle of 60° to 70° and diagenetic time of 225.3 Ma (Yuetal., 2009).

2 Gravity characteristics of the study area

The earlier Liaohe Group is deposited, the lower the density and the narrower the range of density. The density of Gaixian-, Dashiqiao- and Gaojiayu formations are 2.78, 2.74 and 2.71 g/cm3, respectively. The average density of Proterozoic Liao-Ji granite, Dadingzi granite and Indosinian granite are 2.59, 2.57 and 2.60 g/cm3, respectively. Compared with sedimentary rocks of Liaohe Group, the density of intrusive rocks in the ore concentration area varied greatly and the density is relatively low. In order to understand the deep geological characters of lead-zinc concentration area in Qingchengzi, and to explore the deep distribution pattern of Shuangdinggou--Xinling rock mass, we implement three gravity inversion profiles in the mining area in this study: A1(Fig.2a), A2 (Fig.2b) and A3 (Fig.2c). Based on the geological data, we infer deep geological conditions and then perform inversion of gravity profile. Fig.1 shows the locations of the profiles. The gravity profile results are described as follows:

(1) Profile A1

The profile strikes northeast, and the base of the inversion profile is Gaojiayu Formation, with a thickness of about 800 m. Dashiqiao Formation is relatively thicker, with a thickness of about 1 900 m. The thickness of Gaixian Formation is 400 m, thinning from northeast to southwest. There are Nan-shan- and Dadong deposits near the section. They are controlled by northwest trending main faults, secon-dary fractures or ineterlayer fractures. The section shows that the Shuangdinggou--Xinling rock mass intruded into Liaohe Group in Indosinian. Low density of Indosinian rock mass causes low gravity anomaly on the profile. Xinling rock mass is a stock and it intrudes onto the surface. Xinling rock mass connects with Shuangdinggou rock mass in the deep and it is a branch of Shuangdinggou rock mass in the northern mining area. Liao-Ji granite is the crystalline basement in this area. The intrusive contact surface between Liao-Ji granite and Shuangdinggou--Xinling rock mass is relatively smooth.

(2) Profile A2

Section A2 mainly intersects Gaixian Formation (Pt1gx) and Dashiqiao Formation (Pt1d). According to the geological data, the thickness of Liaohe Group is about 3 km. The occurrence of the strata is relatively gradual and thinning towards northeast. The thickness of Gaojiayu Formation (Pt1g) is about 560 m, and Dashiqiao Formation (Pt1d) is about 2 000 m. Gaixian Formation is severely weathered, which is relatively thin with a thickness of about 500 m. From north to south, geological structure mainly includes Zhenzigou anticline, Sikeyangshu syncline and overturn anticline, and Huangdianzi--Xinling--Zhujiapuzi fault. The profile intersects Nanshan and Dongyanggou lead-zinc deposits, the former is controlled by Xiquegou--Erdaogou and secondary faults. Section intersects Nanshan and Dongyanggou lead-zinc deposits. Nanshan deposit controlled by the Xiquegou-Erdaogou fault and its secondary faults, and most ore bodies are vein-shaped. Those ore bodies mainly exist in the faults of the third marble section at the top of Dashiqiao Formation. Dongyanggou deposit is type of bedded lead-zinc deposit, which is controlled by interlayer faults mostly, and the ore bodies occurs in the contact zone between Dashiqiao Formation, Gaojiayu Formation and Liao-Ji granite.

(3) Profile A3

The profile strikes northeast and intersects Gaixian Formation (Pt1gx), Dashiqiao Formation (Pt1d), and Gaojiayu Formation (Pt1g). Liaohe Group is gradually thinning towards the southeast direction. A nappe structure appeared in the southeast part of the strata, which made Liao-Ji granite onto the surface. The thickness of Gaixian Formation is about 400 m, Dashiqiao Formation is 1 800 m, and Gaojiayu Formation is 500 m. The profile passes through Xique--Erdaogou fault, Zhenzigou anticline, and Nanshan syncline. The intrusive rocks include Liao-Ji granite and Indochinese granite. Liao-Ji granite was pushed up to the terrain surface and intruded into Liaohe Group in the deep. The middle section of the profile shows a low gravity anomaly. According to this fact, we can infer that a geological body with low gravity lies in the deep. Through gravity inversion, we can conclude that a concealed granitic rock of Indosinian period underlies Nanshan deposit. Profile A3, A1 and A2 verify the connectivity of Shuangdinggou--Xinling rock mass. Indosinian magma intruded along the fault, forming the Shuangdinggou- and Xinling rock masses. The profile passes through Mapao-, Nanshan- and Dongyanggou deposits. Mapao deposit is vein-shaped, which is controlled by the northeast trending Erdaogou secondary faults and mostly occurs in Dashiqiao Formation. Dongyanggou deposit is layered and controlled by a northwest trending main fault.

Fig.2 Profiles of inversion results

3 Controlling factors on mineralization and ore prospecting

Qingchengzi layered lead-zinc deposits are mainly controlled by interformational cracks. Vein-shaped deposits are controlled by major fault of the study area. Pinnate deposits are controlled by major fault and interformational cracks (Yu, 2005). The magmatic intrusions of Indosinian orogeny reactivated and migrated the original source beds in Paleoproterozoic strata, forming the current distribution pattern of the depo-sits. This process leads to the current pattern of deposit distribution.

During Indosinian period, the entire ore concentration area successively suffered compression. A large number of northeast trending faults were formed，and the original faults were activated as well. Indosinian magma intruded along the fault, forming the Shuangdinggou- and Xinling rock masses. This fact means Shuangdinggou- and Xinling rock masses are connected at a certain depth. Combination fluids of magmatic differentiation and the meteoric water permeating underground formed high-temperature weak acidity fluids. The fluid continuously leached and dissolved ore-forming materials in the strata along the fault during the process of intrusion upward. The physical and chemical environment changed during the process of invasion make pH and S concentration increasing. At the same time, hydrothermal temperature decreased and lead-zinc complexes became unstable. A part of ore-bearing hydrothermal solution was superimposed on the original lean ore layer. It enriched and formed layered lead-zinc ore bodies. Another part of ore-bearing hydrothermal solution caused the original ore body to be activated and migrated. The vein-shaped and pinnate of lead-zinc ore are formed in various types of structural spaces. The scale of vein-shaped ore bodies controlled by tensile fracture is small. While the vein-shaped ore bodies controlled by the tensional and torsional faults is larger and more regular.

Based on the statistics of the main ore-controlling structures of Qingchengzi and the deep connection between Xinling rock mass and Shuangdinggou rock mass, the favorable prospecting site can be inferred. The first favorable location for prospecting is the intersection of faults. Cutting faults are conducive to intrusion of magma, especially those faults that cutting through the Dashiqiao Formation. And magmatic activity could cause activation, migration and mineralization of the metallogenic material. The second is that superposition of folds with different trending, forming dome-basin structure that is favorable for mineralization. The third is the interlayer detachment surface around the core of anticline, where layered lead-zinc ore bodies are formed.

4 Conclusion

According to the tectonic evolution and the gravity inversion in the study area, the deep geological conditions can be inferred in this area. The inversion profiles further confirm that Shuangdinggou- and Xinling plutons are connected in the deep. The Xinling pluton is an outcrop of Shuangdinggou--Xinling rock mass, and it extends along the fault to the north of the ore concentration area. The formations of lead-zinc deposits are closely related to the tectonic movement in the study area. All deposits are controlled by Indosinian magmatism and fault activities. Most faults and ore layers in the study area were formed during the period of rift-extension. While the north-east faults and folds mainly formed during the period of rift-contraction, and these structures were beneficial for magmatic intrusion and mineralization of layered ore bodies. When the platform was activated, various homologous faults were formed in succession. At the same time, Indosinian rock mass invaded and underlay the entire mining area, and this event eventually led to the formation of vein-shaped and pinnate ore bodies.