Analysis on the Construction of Huayuan MVT Lead-Zinc Ore Field in Zhangjiajie of the Western Hunan , China

Huayuan lead-zinc ore field in Zhangjiajie of western Hunan, is a large-sized, and the ore bodies algal limestones the margin of the shallow platform in the lower part of the Early Cambrian Qingxudong Formation. The authors expounded the ore-hosting lithology, sedimentary environment and petrographic evolution of Huayuan lead-zinc ore field. According to the metallogenic characteristics of the Huayuan lead-zinc ore field, the formation and evolution of the ore field is divided into the First construction (geological processes), the secondary construction (lithofacies characteristics) and the third-level construction (basic rock unit).


Introduction
The prospective reserves of MVT Lead -Zinc Ore Field in Zhangjiajie of Western Hunan have more than 10 million tons in China.The tectonic position is the junction of the upper Yangtze platform fold belt and the south Tielong two Tertiary tectonic units on the southeastern margin of the Yangtze block (Ren, 1990) (Fig. 1A).The tectonic-sedimentary environment is a kind of self-generated carbonates in the shallow margin of the Middle-Cambrian.The ore field is located in the north-eastern section of the Xiangqian Pb-Zn metallogenic belts.The ore deposits are located on both sides of the Huayuan-Chaodong fault zone, and on the northwest side of the fault zone deposits are less, the southeastern side is more.For this reason, author uniquely based on the analysis of construction to discusses the metallogenic feature of ore field (Lv, 2015;Sakya, 1986;Kubrat, 1989).The predecessors have made deep researches on them in many aspects (Zhou, 2011;Liu, 2000;Yang, 2007;Xia, 2010;Feng, 2001).

Geological Overview of Ore Field
The crustal tectonic moving in the study area have experienced four development stages: Wuling period, Xuefeng-Caledonian period, Hercynian period, Indosinian-Yanshanian period.In study area fold and fault structure were developed, but there was no obvious angular unconformity between formations.The direction of the regional tectonic line is NNE and NE-trending vein, which form the arc-shaped structural belt of Western Hunan (Fig 1B)  distributed in the light gray thick lamellar crystal sand -ooids -Karyotype limestones which overlying the Qingxudong Formation.Generally, the thicker the alginate limestone is, the more favorable is the mineralization, and the ore layer is multilayered (Xia, 2010;Zhou, 2011).The wall rock alteration of the Ore field is less, and the scale is small, mainly silicification, fluorite, barostic, pyrite.

The concept of ore field construction
Ore field construction refers to a combination of rocks associated with (magmatic, sedimentary, and metamorphic) genetic linkage of specific tectonics zone.(geosyncline, platform and tectonic belt).The main task is to analyze low-grade lithofacies, which is equivalent to subphase (Lv, 2012).
Generally, the ore field construction belongs to the tectonic use range, but also refers to the specific and local geological body combination.The primary classification of the construction is geological process, such as deposition, metamorphism, magmatic rock formation and so on.The second-level classification of the construction can be called "lithofacies".The lithofacies refer to middle-scale geological body section and are often referred to as tectonic facies belts because of their zonal distributions, for example, carbonate-hosted reef facies, metamorphosed greenschist facies or exploded volcanic breccia facies.The third-level classification is the basic unit of lithofacies composition and can be called the rock unit (Lv, 2012).

Ore construction analysis
The formation of Huayuan lead-zinc ore field is a complex process of mineralization and ore field construction analysis is of great significance to determine the basic regional background, construction type, tectonic environment and further prospecting exploration in the known ore field.Now we respectively analyze the first, the second and the third construction i the ore field scope: The first stage of the construction, Huayuan lead-zinc ore field geological processes experienced are the deposition of carbonate rock construction.On the early Cambrian period, the Huayuan area is in the shallow continental shelf facies carbonate platform and platform margin, deposited a thick layer of various types of carbonate rock.On the early Paleozoic period, this area is located in the transition zone of southeast edge of the Yangtze and the jiangnan massif north rim.seabed is high in the northwest and low in the southeast.Westward, the rising Kangdian ancient land provides the source of land debris.Eastward, being intermittent Jiangnan islands and deep water area.On the early and middle Cambrian period, the palaeogeographic contours were similar to those of late Sinian.On the basis of the shallow ocean-margins marine sediments of the late Sinian, the area was composed of coastal and shallow continental shelf areas.On early Cambrian to middle-late Cambrian belongs to platform shoal, tidal shoal, tidal bar, platform front slope or platform, mainly carbonate sedimentary area, local reef, Which directly provided the ore-bearing space for the distribution of lead-zinc deposits.On early Ordovician, the area belongs to the platform and platform edge shoal, high energy tidal flat shoal, mainly for the detrital, bioclastic carbonate depositional areas, control the Huayuan lead-zinc ore field distribution.On Late Ordovician, the depositional area shrink into the coastal wetland basin and continental shelf stagnant basin.Occurred in the late period of the Ordovician "Yichang rise", the region witnessed a wide range of oscillation lift.Forming a complete sedimentary cycle of the Early Paleozoic.
Secondary construction, equivalent to the ore field sedimentary facies.Based on the analysis of the cores of the ore deposits in the orefield, the authors established a cylindrical cross section of the Huaguan MVT lead-zinc orefield (Fig. 2).Marine sedimentary facies in western HuNan developed carbonate platform, mesa edge, slope and basin sedimentary lithofacies characteristics in succession from the northwest to the southeast.The study area is mainly distributed in the lithofacies of the platform margin.The sedimentary facies belts in the platform margin can be divided into two types, namely shoal facies and algal limestone facies.Sedimentary lithology of shoal facies is mainly sandstone limestone, oolitic limestone, sandy dolomitic limestone, dolomite and stratified dolostone (Fig. 2.Ⅱ).The size of the rock deposit is large, and it is well sorted, which reflects the repeated fluctuation of the shallow sea water.In the shoal deposits, various sedimentary structures have been developed under high energy conditions, such as trough interlaced bedding, oblique bedding and lamellar staggered bedding.The main lithologies of the algal limestones are algal limestone, sand dolomite limestone, breccia limestone and banded limestone, etc. (Fig. 2.I), the rock layer was thick layer.can be seen l The calcite con % (Fig. 3b).Do ontact between (Fig. 3c). .
Figure 1 Fig