J. Cent. South Univ. Technol. (2011) 18: 1217-1225

DOI: 10.1007/s11771-011-0825-0

Petrological characteristics, geochemical feature and metallogenetic relation of alkaline-rich rocks in northwest of Yunan Province, China

ZHANG De-xian(张德贤)^{1, 2}, DAI Ta-gen(戴塔根)¹

1. School of Geosciences and Info-physics, Central South University, Changsha 410083, China;

2. Economic Geology Research Unit, James Cook University, Townsville, 4811, QLD, Australia

? Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: The alkali-rich rocks, spreading along the suture zone of Jingsha River, refer to the alkali-rich porphyry rocks, which emplace during the Himalaya epoch in northwest of Yunnan Province, and consist of syenit, syenit porphyry, monzonite porphyry and granite porphyry. Petrological chemical analysis results suggest that silica is poor and aluminum is rich, and high potassium large ion lithophile elements (LILE), light rare earth element (LREE) and Sr are obviously detracted in these rocks. High field strength elements (HFSE) and heavy rare earth element (HREE) are depleted, especially Nb, Ta, P and Ti. δ_Eu: 0.09-1.64 shows that plagioclase does not appear fractional crystallization during the formation of alkali-rich rocks. δ³⁴S, H and O isotopes and Pb isotopes suggest that ore-forming fluid is derived from the mantle, and Pb is possibly mixed by mantle, wall rock and crust. The age of Pb in alkali-rich rocks is about 250-220 Ma. The age of alkali porphyry rock (dykes) varies from 30 Ma to 50 Ma. Alkali rocks have strong metallogenetic relation. Au mineralization is associated to the alkali magmatic activities with a relatively high temperature, low pressure and high oxygen fugacity. However, copper mineralization is mainly associated with alkali-sub-alkali magmatic activities in a process of relatively low temperature, high pressure and lower oxygen fugacity.

Key words: alkali-rich porphyry rocks; Sanjiang metallogenetic belt; geochemistry; metallogenetic relation

1 Introduction

Sanjiang metallogenetic belt is composed of Jingsha River, Langcan River and Nu River, which flows through Tibet, Sichuan Province and Yunnan Province of southwest of China. Northwest territory of Yunnan Province lies in the south end of Sanjiang metallogenetic domain, and belongs to part of Tethyan tectonic belt (see Fig.1). This belt lies to the east collision orogen belt in the eastern section of Tethyan tectonic domain with striking E-S. A large number of porphyry Cu and gold deposits occur within this domain, such as Yangla Cu deposit in Deqin country [1-3], Baiyanping lead and zinc deposit in Weixi country, Dapingzhan Cu deposit in Simao country, Pulang Cu deposit in Zhongdian country [4-5], and Chang’an Au deposit in Jingping country. Sanjiang Tethyan metallogenetic belt is becoming a crucial part for non-ferrous metal exploration in China.

A suite of volcanic rocks with rich alkali occurs within this domain along the Jingsha Jiang-Ailoashan fault, which is resulted by collision between Indian plate and Eurasian plate owing to uplift of Tibetan plateau [6-12], and extend towards north and west to Panxi and Tibet [12-18]. TU et al [16-17] named it alkali-rich intrusions, and HU and HUANG [19] and LAI et al [20] called it alkali-rich porphyry. Rick alkali and high potassium are the typical characteristics in this suite of rocks. This suite of rocks is composed of orthophyre, ivernite and volcanic rocks sharing approximate chemical petrological composition [15]. This suite of rocks displays dramatically metallogenetic relations regionally and is closely associated with the Cu and Au mineralization, which are constitutes of Cu, Mo, Au, Pb-Zn deposits within west Yunnan and northwest Yunnan Province [2, 5, 8-9, 21-28]. Globally, alkali-rich rocks occur in a wide range of tectonic setting [29-30], and include a variety of petrologic compositions from schoshonites, cac-alkaline volcanic rocks to ulapoassic leucities. The petrological characteristics and geochemistry of alkali-rich rocks and their metallogenetic relations combining some typical deposits throughout this area will be discussed in this work.

2 Regional geology

The north-west of Yunnan Province within Sanjiang Tethyan metallogenetic domain is a significant non-ferrous metallogenetic belt in China (see Fig.1). Sangjiang Tethyan metallogenetic domain lies in the Himalayn-Tibetan orogen of Eastern Tethys, where magma activity predominantly occurs in the Indo-Chinese epoch, Yanshanian, Himalayan orogenic epoch, and consists of a wide range of rocks. Copper and gold mineralization is closely associated with both magma activity and diverse rocks within this domain [5, 7-8, 16-17, 23, 31-36]. Jingsha River orogen in north west of Yunnan Province predominantly extends NNW striking and undergoes many repeated tectonic movement. WANG [37] divided the later Mesozoic regional tectonic field stress into several stages as follows: J-K₁¹ (208-135 Ma) mainly NW-SE compressing; K₁²- E₂¹ (135-52 Ma) nearly SN compressing; E₂²-E³ (52- 23.3 Ma) nearly EW compressing; N-Q₁ (23.3-0.83 Ma) nearly SN compressing and Q₂ (0.73-0 Ma) mainly NE-SW compressing, which act on the Cenozoic geological structure, and then form the vital ore-forming structure within the north west of Yunnan Province [38]. ZENG et al [5] presented that Cu-(Mo)-Au mineralization is closely related to two sets of porphyry systems which are developed dominantly in the eastern part of Himalayan orogen belt, and distribute within calc middle-acid porphyry rocks of Zhongdian island arc during Indo-Chinese epoch (such as Xuejiping, Hongshan and Langdou). Zhongdian island lies in the west of Garzre-Litang ophiolite-mictite, and the intrusive age is around 215 Ma [39-40], of which the west belt is Himalayan orogenic period alkali-rich belt (Such as Yulong, Yaza, Ninlang, Lijian, Yaoan, Beiya and Machangqing), and distributes along the Jinsha River suture. The age of magma forming is 250-220 Ma; however, the intrusive rocks appear before around 38 Ma, and the interval time between magma forming and rocks appear to be 200 Ma, which indicates a kind of “hysteresis effect”. Majority of these typical rocks occur in the upper Himalayan orogenic period, minor in later Yanshanian, and locate in a post-collision intercontinental setting, which is resulted from the crust thickening of western of Yunnan. According to diversity of wall rock and ore-bearing potential, these alkali-rich rocks can be divided into five subclasses: mantle-derived alkali rocks, mantle-derived K-rich lamprophyre, mixed melting of crust and mantle derived alkali-rich granite, ivernite, and orthophyre [41-42]. WANG [37] sub- divided alkali-rich rocks into three groups in spatial based on the forming mechanism and chemical control factor. It is still in a comparatively controversial about the genetic type of alkali-rich rocks. ZHANG et al [43] proposed that it was derived from melting source of part of mantle. XU et al [44], however, believed that it was derived from melting source of a transition region between crust and mantle. WANG and LI [45] argued that it was derived from “island-arc” mantle melting source action by a series of large scale striking-slip extension fault. A large number of porphyry Cu-(Mo)-Au, Pb-Zn deposits exist within this alkali-rich belt, and are intimately associated with this suite of alkali-rich rocks. Furthermore, obvious metallogenetic relations occur in this suite of alkali-rich rocks [8-9, 21, 32, 46-49].

Fig.1 Distribution of porphyry metallogenetic belts in Himalayan-Tibetan orogen, east Tethys: 1—Suture belt (A Garze-Litang Suture belt, B Jinsha Suture belt, C Yarlung Zangbo River Suture belt, D Bangong-Nujiang Suture belt); 2—Thrust fault; 3— Strike-slip fault ((1) Chabu fault, (2) Jiali fault, (3) Geza He fault, (4) Cesuo fault, (5) Ailao Shan-Red River fault); 4—Normal fault; 5—Porphyry deposit: Duobuza, Qulong, Yulong, Pulang, Xifanping; 6-Porphyry metallogenetic belt: Yidun-Zhongdian belt, Yulong-Malasongduo belt, Lijiang-Jinping belt, Gangdise belt, Bangong Nujiang Yanshanian belt

3 Petrological characteristics of alkaline- rich rocks

3.1 Occurrence and distribution

Alkali-rich rocks were divided into four magmatic body formations based on the petrological feature and occurrence: Jiangchuan formation, Beiya-Liuhe formation, Dahuoshan formation and Dayingjie formation [24, 50]. Jiangchuan formation includes six magmatic bodies which consist of trachyte porphyry, amphibole-orthophyre, biotite-ivernite and biotite-granite porphyry with typical porphyritic texture, of which porphyry consists of a majority of idiomorphic orthoclase, minor amphibole and zoning structure augite. Beiya-Liuhe formation includes more than ten magmatic bodies in Matouwang and Liuhe area as stock, pipe and dyke, and consists of quartz syenite porphyry, beschtauite, and amphibole-orthophyre porphyry. Dahuoshan formation consists of schist and phyllite existing from Zhanhe country to Zhirong country, and other intrusive rocks contact with clastic rocks, which is made up of amphibole-orthophyre, granite porphyry and ivernite. Dayingjie formation consists of the Himalayan magmatic body existing in the Dayingjie area and with a very complex rock assemblage package.

This suite of alkali-rich rocks is controlled by EW trend concealing the structure and forming during 50-  30 Ma. Considering the structure, diagenesis mechanism and spatial distribution, alkali-rich rocks develop in three major sub belts, which can be divided into several concentration points according to the occurrence of magmatic body [14-15, 38] as follows:

1) Southern zone distributes from west to east in a line of Yongping, Weishan, Xiangyun, Yaoan Nanhua, and can be subdivided into Yongping-Zhuofang formation, Weishan-Nanjiang formation, Xiangyun (including Bingchuang)-Midu formation, and Yaoan- Nanhua (including Chuxiong) formation.

2) Central zone distributes from west to east in a line of Jiangchuang-Eryuuan, Lijiang, Yong reng- Huaping, consisting of Laojunshan-Jiangchuang formation, Heqin-Beiya Formation (including Lijiang), Yongshen (Zhanhe)-Huaping formation.

3) Northern zone distributes from Bengge, Sanba of Zhongdian to Ninglang, consisting of Zhongdian- Bengge formation, Sanba-Kemailuo formation and Bainiuchang-Luobodi formation.

3.2 Petrological characteristics

Alkali-rich rocks in the east Tethys are divided into four types [14-15] according to their diversity of chemical components. Within this belt, an alkali-rich rock consists of four groups: syenite group, orthophyre group, ivernite porphyry and granite porphyry. Their petrological characteristics will be discussed as follows:

1) Syenite groups exist as largish magmatic body or batholith, and intrude very early. This group consists of alkali-syenite, pyroxene syenite, biotite syenite and quartz syenite.

2) Orthophyre group consists of orthophyre, biotite orthophyre, quartz orthophyre, pyroxene orthophyre and amphibole orthophyre.

3) Ivernite consists of quartz ivernite, ivernite, amphibole-quartz ivernite and biotite-quartz ivernite.

4) Granite porphyry consists of granite porphyry, granodiorite-porphyry, alkali-feldspar granite porphyry and xlonzonitic grantite porphyry.

3.3 Lithogeochemistry

Chemical petrology analyzing data on 321 samples from the northwest of Yunnan Province and their extension area-eastern Tibet in the eastern Tethys since 1987 [12, 14-15, 19, 22, 24, 44, 46, 50] closely related to the alkali-rich rocks, have been included in this research. By using statistic methods, average value and corresponding error, median value, maximum and minimum of the chemical petrology data are displayed in Table 1.

It can be clearly seen from Table 1 and all the petrochemical data that:

1) SiO₂ is poor and Al₂O₃ is rich. w(Al₂O₃) ranges from 12.80% to 21.68%, the average value is 15.22%, and the median value is 15.06%. w(SiO₂) ranges from 55.04% to 73.68%, the average value is 67.74%, and the median value is 69.07%. By comparing these features with those of the alkali granite in south China and Australia A-type granite, w(SiO₂) value of alkali-rich rocks is evidently lower than that of alkali granite in south China. w(Al₂O₃) value of alkali-rich rocks is higher than that of both alkali granite in south China and Australia A-type granite [16-17].

2) Both rich-alkali and rich-potassium are the typical characteristics of alkali-rich rocks within the northwest of Yunnan Province. Alkali index w(K₂O+ Na₂O) ranges from 7.52% to 13.28%, the average value is 3.05%, and the medium value is 1.90%. w(K₂O) value ranges from 3.75% to 11.32%, and the average value is 6.44%. Both of them are higher than those of alkali granite in south China. w(K₂O/Na₂O) ranges from 0.68% to 20.21%, and the average value is 3.05%, which is higher than that of alkali granite both in south China and Australia A-type granite.

3) Rittman index varies from 2.10 to 14.25, and the average value is 4.22, which indicates that this suite of rocks belongs to alkali-rich rocks.

4) w(Al₂O₃)>w(CaO+Na₂O+K₂O), suggesting that this suite of rocks belongs to peraluminous rocks.

3.4 Geochemistry of alkali-rich rocks

3.4.1 Trace element

Researches on the quartz orthophyre in Hongnitang, Wangtongshan and Bijiashan and amphibole-orthophyre in Matouwang of Beiya district by XU et al [44] suggest that LILE, LREE and Sr in alkali-rich rocks within this district are rich, but HFSE and HREE are poor, especially Nb, Ta, P and Ti. Alkali-rich rocks in Beiya district are coincided with the distribution curve of lower crust, but incompatible elements (such as Ba, Rb, Th and K) are more concentrated. GE et al [14-15] proposed that obvious positive anomaly of Rb, Ba, U, Th, Pb and Sr and distinct negative anomaly of Ni and Ta within alkali-rich rocks reveal that all the rocks are from same magmatic sources and derived from mantle rocks in subduct zone [52-53]. MORB normalized incompatible elements of various district rocks form a nearly same “hump” and normalized element values of respective magmatic body are more than one, indicating that LILE is concentrated within these rocks[15].

3.4.2 Rare earth element

Rare earth element (REE) compositions (Table 2) of 68 samples from Yaoan, Machangqing, Beiya and Xiaolongtang deposit show some feature as follows [1, 12, 19, 21-22, 44, 54-56].

1) The total REE ranges from 8.75 to 7240.3 μg/g, the average value is 550.22 μg/g, and the majority of total REE varies between 100 μg/g and 500 μg/g.

2) d_Eu varies between 0.09 and 1.64, and the average value is 0.84, which suggests that during the formation of alkali-rich rocks, but the fractional crystallization process does not or rarely happen.

3) d_Ce varies between 0.02 and 0.62, and the average value is 0.98. Its frequency distribution is in accordance with the Gaussian distribution.

4) (La/Sm)_N varies between 0.4 and 116.67, and the average value is 16.52, which is a typical Light REE enrichment.

5) In the w(La)-w(La)/ω(Sm) diagram, magma of alkali-rich rocks forms during the balance partial melting stage, and differential crystallization only forms in part of southern zone [15].

3.5 Isotope

3.5.1 Stable isotope

1) S isotope (δ³⁴S)

δ³⁴S in this research is not only from the deposits related to the alkali-rich rocks but also from some other typical deposits, within this belt, such as Jingding giant Pb-Zn deposit [1, 10-11, 21, 56-60]. As can be seen from the frequency statistics of sulphur frequency column diagram (Fig.2), in general, δ³⁴ has a wide range and varies between -0.304 3% and 0.165%, but it mainly concentrates on four intervals: -0.24%-0.12%,    -0.112 59%--0.038 33%, -0.017%-0.066%, 0.081 2%- 0.165%, of which, δ³⁴ varies from -0.24% to -0.12% in the first interval, which represents the sulphur from PbS and ZnS in Jingding giant Pb-Zn deposit. δ³⁴ varying between -0.24% and -0.12% illustrates that the  sulphur of both galena and sphalerite are all derived        from sedimentary stratigraphy in Jingding deposit, and simultaneously, reflects some formation characteristics of organic biogenetic sulphur [59]. HOU et al [8-9] pointed out that sulphur is principally derived from stratigraphy, and simultaneously, sulphates in stratigraphy have been reduced by organic matter and stored in the ore. δ³⁴S varying from -0.112 59% to -0.038 33% in the second interval indicates that sulphur is possibly derived from the sulphate of sea water, and sulphur might form in the process of bacteria reducing sulphate. δ³⁴S varying from -0.017% to 0.066% in the third interval is closely associated with alkali-rich rocks hosted deposit such as Yan’an, Beiya and Jingchanqing. The sulphur is equivalent to aerolite sulphur, which explains that not only diverse magmatic body has a good affinity but also sulphur in the magma is chiefly derived from upper mantle [14, 38]. δ³⁴S varying from 0.081 2% to 0.165% in the fourth interval is closely associated with the extensively distributed evaporate and sulphate. δ³⁴S ranging from 0.081 2% to 0.165% is close to later Trassic palaeo-ocean sulphate  δ³⁴S value of ~0.15%, which might be part of sulphur source of Jingding Pb-Zn deposit and sulphur in Jingding Pb-Zn deposit is derived from Mesozoic basin system [59-61].

Table 1 Comparison of petrochemical petrology among alkali-rich rock in northwest of Yunnan Province, alkali granite in south china and Australia A-type granite (modified from [16, 19])

Table 2 REE distribution pattern of Alkali-rich rock, northwestern of Yunnan Province

Fig.2 Frequency distribution of δ³⁴S from deposits associated with alkali-rich rocks

2) H, O isotope

Later ore-forming stage dolomite in Machangqing δ¹⁸O_mineral varies between 12.1×10^-3 and 12.4×10^-3, correspondingly, δ¹⁸O_water varies between 2.36×10^-3 and 3.84×10^-3, and δ¹⁸O_mineral varies between 2.71×10^-3 and 16.961×10^-3, which suggests that water in ore-forming fluid is princinpally derived from mantle or deep magmatic fluids related with mantle activity [38]. QIAN et al [56] proposed that H and O in calcite and quartz in Yan’an gold deposit are dominantly derived from sedimentary, volcanism and later heat event, and extensive interaction happens between H, O and water (hydrothermal, meteoric water) in the speculariron barite.

3.5.2 Radioactive isotope

1) Pb isotope

The statistic result of 176 Pb isotopes of sulphides from those deposits within alkali-rich rock belt shows that variation range of w(²⁰⁶Pb)/w(²⁰⁴Pb), w(²⁰⁷Pb)/ w(²⁰⁴Pb) and w(²⁰⁸Pb)/w(²⁰⁴Pb) are 17.191-19.184 7, 15.27-17.17 and 37.36-39.894 2, respectively. Their average values are 18.418 9, 15.565 6 and 15.549 3, respectively. Variation ranges of w(²⁰⁶Pb)/w(²⁰⁴Pb), w(²⁰⁷Pb)/w(²⁰⁴Pb) and w(²⁰⁸Pb)/w(²⁰⁴Pb) in the ore are 17.969-18.960, 15.226-15.992 and 37.591-39.607, respectively.

Variation ranges of w(²⁰⁶Pb)/w(²⁰⁴Pb), w(²⁰⁷Pb)/ w(²⁰⁴Pb) and w(²⁰⁸Pb)/w(²⁰⁴Pb) in the magmatic body are 18.094-18.644, 15.537-15.709 and 37.566-39.094, respectively. It can be seen from the diagram that Pb composition within these deposits related to alkali-rich rocks is obviously different from crust Pb and has a good linear relation (see Fig.3), which reflects that the Pb source within alkali-rich rocks is very complex, and might be derived from a mixture of mantle, wall rock and crust [32]. The age of Pb within alkali-rich rocks is about 250-220 Ma [24, 50].

Fig.3 Plots of w(²⁰⁶Pb)/w(²⁰⁴Pb) vs w(²⁰⁷Pb)/ w(²⁰⁴Pb) (a) and w(²⁰⁶Pb)/w(²⁰⁴Pb) vs w(²⁰⁸Pb)/w(²⁰⁴Pb) (b) of both Jinding Zn- Pb sulphide ores and some alkali-rich rocks in northwest of Yunan Province

2) Sr, Nd, and Pb isotope

Sr, Nd, and Pb isotope compositions suggest that the source area of alkali-rich rocks has EM II type mantle enrichment geochemical feature [55]. w(⁸⁷Sr)/w(⁸⁶Sr) value within magmatic body nearly keeps a constant and ranges from 0.706 4 to 0.711 624, with an average value of 0.707 75, which is below 0.708. All the features mentioned above show that it is derived from mantle, but it might be derived from crust-mantle [14-15, 21], indicating that the crustal contamination is not obvious in the process of magma rising [1, 62]. The Sr, Nd, and Pb compositons of alkali-rich rocks fall in between typical depleted mantle and crust [24, 50], of which I_Sr of Yan’an magmatic body ranges from 0.708 8 to 0.709 3, ε_Nd varies from -9.2 to -10.6, I_Sr ranges from 0.706 1 to 0.708 2, and ε_Nd ranges from -3.1 to -10.2 in Machangqing magmatic body. These two magmatic bodies have a high I_Sr and low ε_Nd, which has similar variation tendency with Sr and Nd isotope compositon (I_Sr=0.705 4-0.707 2, ε_Nd=-1.8--6.3) within Ailaoshan- Jinshajiang belt measured by other people. And it is close to EMII mantle end member [63-64], which is a kind of metasomatic rich mantle related to recycle between crust and mantle, and I_Sr is high and ε_Nd is low [1].

3) ³⁹Ar-⁴⁰Ar and K-Ar

Timing result on alkali-rich rocks by using K-Ar shows that the age of alkali-rich rocks varies between 26.5 and 37.6 Ma [6, 24, 50, 65-67]. ZHANG et al [12, 43] reported that the K-Ar age of syenite and orthophyre in this belt is 28-37 Ma. According to GE et al’s research [14-15], the ages of alkali-rich rocks are concentrated between 30 and 50 Ma, however, ZHANG and ZHAO [42] divided  the intrusive activity of alkali-rich rocks in the northwest of Yunnan Province into both early and later two stages. In the earlier stage (65.0-45.7 Ma), the rocks consist primarily of granite porphyry and ivernite; in the later stage (42.0-23.2 Ma), the rocks consist of orthophyre, lamrophyre and alkali complex rocks.

XU et al [44] did a systematic dating on quartz albitophyre, quartz orthophyre, biotite-orthophyre and lamprophyre in Beiya deposits using ³⁹Ar-⁴⁰Ar, and it turns out that the porphyry rock in Beiya deposits can be divided into three stages: quartz albitophyre and lamprophyre in the earlier stage (65.56 Ma), quartz orthophyre in the middle stage (25-33 Ma) and biotite orthophyre in the later stage (3.66 Ma, 3.78 Ma). BI dated the Amphibole in Yaoan deposit using ³⁹Ar-⁴⁰Ar. The result shows that the age of the magmatic body is (28.9±0.1) Ma. PENG et al [68] dated quartz in Machangqing magmatic body using ³⁹Ar-⁴⁰Ar, and then the dating result indicates that the ore-forming age of Machangqing is (33.7±0.1) Ma.

4 Metallogenetic relation of alkali-rich rocks

The principal mineralization within the alkali-rich belt in the northwest of Yunnan Province, China is Cu-Mo-Au and minor Pb-Zn. Gold mineralization is in pace with magnetism. Generally speaking, gold mineralization within this belt is associated with the different kinds of rocks, however, in terms of multiple stage of magmatic evolution, the rocks close to Au mineralization are the later-middle stage products of same magmatic fluids. Ore-bearing mantle fluids rising from deep part and magma form in the transition area between the crust and mantle or mixed layers of crust and mantle, then during the alkali metasomatism and magmatic enrichment, a large amounts of ore-forming substances such as Cu, Mo, Pb, Zn and Au are participated into the magma, forming ore-forming fluids. The hydrothermal fluid coexists with magma raised up and is accompanied by magma during the multiple stage of the intrusive process of magma. Minor mineral composition from stratigraphy is participated into this mixed fluid. When they ascend to the particular height, the magma begins crystallizing, and fluids become into ore-forming fluids. Copper and Mo mineralization and their alteration exist in or around the magmatic body, which is determined by high-temperature and low- pressure in the earlier stage. However, gold deposites in a certain distance to the magmatic body in the later stage [38]. BI et al [1] proposed that within the Ailaoshan– Jingsha River belt, gold mineralization is intimated related to the volcanic activity, which are the products of relatively high-temperature, low-pressure and high oxygen fugacity fluids, and copper mineralization, however, is associated with the magmatic activity of alkali to sub-alkali rich rocks, which are derived from low-temperature, high-pressure and low oxygen fugacity fluids.

LAI et al [20] stated briefly that the mineralization related to alkali-rich rocks is Au, Ag, Cu, Mo, Pb, Zn, S (FeS₂). LREE (light rare earth element), trace element and non-metal element are intensely controlled by rock type and rock distribution, which is resulted from the chemical composition of rocks and contents of the ore-forming elements in the rocks. For example, in Yongping district, Au, trace element, LREE and P are discovered, and in Jiangchuang, gold and Cu mineralizations have been found but do not become an industrial ore body because Au and Cu mainly exist in the rocks derived from mantle, and simultaneously, Pb and Zn in the rocks are derived from crust.

The diversity of alkali-rich rock in the northwest of Yunnan Province in southwest of China determines the diversity of deposits and their distribution. The rocks in Yongping district are mainly composed of alkali pyroxenolite, alkali gabbro, and alkali syenit, which are in favour of gold mineralization. The rocks in Jingping district are predominantly composed of quart porphyre, alkali syenit, which are in favour of Cu Mo and magnetite deposition. The rocks in Yaoan district are mainly composed of orthophyre and trachyte, which are in favour of Pb, Ag and Au deposits. The rocks in Dali district are composed of ivernite, granite porphyry, which are in favour of Cu-Mo and Pb-Zn-Ag deposits. And the rocks [13] in Jiangchuan district are composed of simaite, trachyte porphyry and granite, which are in favour of Cu-Au mineralization [69].

In addition, even in the same ore field, with the diversity of chemical petrology, the accommodates still vary. For example, in the Machangqing ore field, along with the zoning of porphyry rocks, central part is microphyric alkali granite→granite→granodiorite→ ivernite and the mineralization is changed from Mo, Cu-Mo→Cu→Au→Pb, Ag. In view of rock type, Au deposit is close to orthophyre and ivernite, next to granite. Considering the chemical petrology, Au mineralization is associated with middle (alkali) rocks (ivernite and orthophyre) and acid rocks (granite porphyry), and Cu poly-metal mineralization is mainly associated with middle to acid rocks [70].

5 Conclusions

1) Alkali-rich rocks are closely associated with non-ferrous mineralization. On one hand, alkali-rich rocks are the source rocks for mineralization; on the other hand, during the evolution of alkali-rich rocks, more and more other materials participate into the ore-forming fluids, and gold mineralization is related to alkali series magmatic activities with relatively high temperature, low pressure and high oxygen fugacity, and Cu mineralization, however, is associated with alkali to sub alkali magmatic activities with relatively low temperature, high pressure and low oxygen fugacity.

2) Alkali-rich rocks refer to a series of rocks composed of orthophyre, ivernite and volcanic rocks sharing similar chemical petrological composition. Moreover, this suite of rocks has an obvious distribution in spatial which might be related to the magma activity and structure of straightigraphy, and is affected by the regional tectonics evolution as well.

3) Low silica, rich aluminium and high alkali and potassium are the typical features of this rock.

4) Trace elements in extensively widespread alkali-rich rocks are similar to those in the low crust. In alkali-rich rocks within this district, LILE, LREE and Sr are rich, but HFSE and HREE are poor, especially Nb, Ta, P and Ti. Light REE enriched and other rare earth element feature suggest that alkali-rich rocks form during the balance partial melting stage, and differential crystallization only forms in part of southern zone.

5) Sulphurs derived from low-crust to upper mantle are proved by sulphurs isotope analysis. H and O isotope data illustrate that H and O are from a very complex sources including mantle fluids, deep magmatic fluids associated with mantle fluids activities, sedimentary, volcanism and later stage hot event.

6) ³⁹Ar-⁴⁰Ar and K-Ar datings constrain the age of alkali-rich rocks between 30 Ma and 50 Ma.

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(Edited by YANG Bing)

Foundation item: Project(1343-74334000019) supported by the PhD Innovation Subject of Central south University, China; and Project(1960-71131100088 (CX2010B085)) supported by the Hunan Provincial Innovation Foundation For Postgraduate Students, China

Received date: 2010-04-15; Accepted date: 2010-03-01

Corresponding author: ZHANG De-xian, PhD; Tel: +86-13467514585; E-mail: dxzhang303@gmail.com