GEOCHRONOLOGICAL BOUNDARIES OF FORMATION OF PORPHYRY CU-MO MINERALIZATION IN EASTERN ASIA

SOTNIKOV V.I., PONOMARCHUK V.A., BERZINA A.P.,
BERZINA A.N., GIMON V.O., SHAPORINA M.N.

Institute of Geology SD RAS, 630090, Novosibirsk, Prosp. Koptyuga, 3, Russia

Abstract: Despite of a great variety of these deposits, they form the genetically related group and are closely associated in space and time with granitoid intrusives (stocks, dikes) that are formed at relatively shallow depth and are composed of rocks of porphyry texture. Porphyry Cu-Mo deposits are formed during different metallogenic epochs. The established geochronological boundaries of intense porphyry Cu-Mo mineralization for territories of Siberia and Mongolia are definitely correlated with periods of high ore activity at the territories of East China, Middle Asia, Australia, North and South America: 150, 180, 210, 240-250, 280, 330, 390 and 440 Ma. In copper-molybdenum ore clusters of Russia (Siberia) and Mongolia the ore-bearing porphyry complexes finish the protracted multi-pulse granitoid magmatism. At the same time they inherit some petrogeochemical and metallogenic features of the early preceded magmatism with increasing potential ore-productivity of magmas from not very promising mineralization at initial stages to industrially important porphyry copper-molybdenum type at the late stage of development of magmatism. Multiple actions of magmatic and ore-metasomatic processes of different age within comparatively restricted geologic space (ore cluster) are one of the most important factors determining the formation of large porphyry Cu-Mo deposits.

INTRODUCTION

Porphyry Cu-Mo deposits are the leader source of copper and molybdenum worldwide. Despite of a great variety of these deposits, they form the genetically related group and are closely associated in space and time with granitoid intrusives (stocks, dikes) that are formed at relatively shallow depths and composed of rocks with porphyry texture. The deposits are characterized by streaky, streaky-disseminated, disseminated and brecciated ores developed among hydrothermally altered (K-feldspathized, biotitized, sericitized, silicified, propylitized, and argillized) host rocks. Skarns are formed in the case of presence of carbonate rocks at the intrusive exocontact. Similar deposits are recently referred to as Cu(Mo) skarn-porphyry ones. According to the ore composition and Cu:Mo:Au ratios the following types of deposits have been distinguished: copper porphyry and gold-copper porphyry, molybdenum-copper porphyry, copper-molybdenum porphyry, and molybdenum porphyry. The mineral composition of ores is relatively simple. The most abundant ore minerals are: pyrite, chalcopyrite, molybdenite; in addition, bornite, magnetite and hematite occur frequently. Galena, sphalerite and grey ores occur in the late mineral associations. Scheelite and cassiterite occur sporadically. Zonal distribution of mineralization is observed: Fe - Mo(Cu) - Cu(Mo) – Cu(Au) – Pb, Zn – Au, Ag.

Porphyry Cu-Mo deposits are formed during different metallogenic epochs. The highest ore productivity corresponds to the Meso-Cenozoic epochs. The formation of the largest porphyry deposits of the East Pacific metallogenic belt that extends along the western margins of the North and South America continents falls into this time span (the age of the deposits varies from the Jurassic to Neogene in the range from 170 to 5 Ma). In addition, many deposits of the West Pacific Belt (eastern regions of Russia and China, Philippines, Indonesia, Papua New Guinea, New Zealand and others) with the age from 230 to 1 Ma may also be attributed to this group. There are numerous porphyry Cu-Mo deposits of different ages in other regions of the world: Mediterranean metallogenic belt (South Eastern Europe, Armenia, Azerbaijan, Iran, and Pakistan) with the ages varying from 70 to 10 Ma; Ural-Mongolian metallogenic belt with the deposits of Middle-Late Paleozoic and Early Mesozoic age (Russia, Kazakhstan, Uzbekistan, Mongolia, China). Paleozoic deposits occur also on the territory of Canada, USA, Argentina, Australia and other countries.

The East Asian porphyry Cu-Mo deposits (situated in China, Mongolia and Russian East, Fig. 1) considered in this paper are also characterized by the different ages of their formation.

 AGE BOUNDARIES OF FORMATION OF PORPHYRY CU-MO MINERALIZATION ON THE CHINA TERRITORY

The majority of porphyry Cu-Mo deposits are located in Eastern China, particularly along the northeastern and southeastern margins of the Yangtze Craton [1, 2]. A few deposits occur in the northeast of the North China Craton (e.g., Xinancha and Laozhaishan deposits). Some large and gold-rich copper porphyry and related skarn-porphyry deposits occur in the Sanjiang fold belt (e.g., Yulong and Malasongduo deposits).

Most of the porphyry deposits in China were formed during the Yanshanian movement [3] and are mainly related to the subduction of the Pacific ocean basin plates. Many porphyry and skarn-porphyry deposits (e.g., Jilongshan, Jiguanzui, Xinqiao, Tonglushan, and Mashan deposits) are concentrated in the Yangtze River province. The province extends for more than 600 km between Wuhan and Shanghai in the Yangtze River trough located in the eastern part of the Yangtze craton [2]. The trough is composed of a few Mesozoic fault-bounded volcanic basins, with mainly ca. 135-127 Ma felsic to intermediate lower crustal melts emplaced along the basin margins and as zones of uplift within the basin. This tectonism may be the result of back-arc, post-collisional extension that occurred subsequent to amalgamation of the North and South China cratons, and during subduction of oceanic plates underneath Eastern China. Porphyry deposits are located within the southeastern margin of the craton. The majority are concentrated in the Dexing area of Jiangxi province (e.g., the Yinshan, Tongchang, and Fujiawu deposits). The copper- and gold-rich skarn and porphyry deposits apparently continue as far north as the western Shandong province. These deposits are associated with Jurassic and Early Cretaceous shoshonitic volcanic and intrusive rocks along, and immediately west of the Tanlu fault system [4].

The large Duobaoshan Cu-Mo-Au deposit in the Xinganling block of northeastern China occurs in 290-Ma granodiorite porphyry [5]. In northwestern China, most of the small gold-rich skarn and porphyry deposits formed in migrating island-arcs or in the continental margin during, or just subsequent to, collision between the Tarim, Yuli, and North-Asian cratonic blocks.

In southwestern China, the major porphyry and skarn deposits formed during the Himalayan orogeny (younger than 90 Ma) as a result of the collision between the Indian and South China cratons. In the Sanjiang fold belt, a series of large, gold-rich, copper ± molybdenum porphyry and skarn deposits (e.g., Yulong and Malasongduo) formed along the edges of a few Tertiary basins [5] during Himalayan tectonism.

AGE BOUNDARIES OF FORMATION OF PORPHYRY CU-MO MINERALIZATION ON THE TERRITORY OF RUSSIA (SIBERIA) AND MONGOLIA

Porphyry Cu-Mo mineralization represented by numerous deposits of various size and age play an important role in metallogeny of the Central-Asian fold belt (CAFB). According to ⁴⁰Ar/³⁹Ar dating of the deposits of this type that has been undertaken by the authors for the most important Russian ore clusters (Siberia and Mongolia), several metallogenic epochs of intense development of porphyry Cu-Mo mineralization have been established: Devonian – 410-360 Ma (the typical deposits are the large Sora deposit, Kuznetsk Alatau; Aksug, Northeastern Tuva; Tsagan-Suburga, South Mongolia), Triassic – 240-220 Ma (Erdenetuin-Obo, North Mongolia), Middle-Late Jurassic – 160-150 Ma (Zhireken, Bugdaya, Shakhtama, Kultuma in Eastern Transbaikalia), Early Cretaceous – 130-115 Ma (Badis, Chubachi, Borgulikan in Amur region). It is worth noting that while the Mesozoic metallogenic epochs were continuously reported based on the results of K-Ar analysis, the problem of the Devonian age of large-scale porphyry Cu-Mo mineralization arose solely owing to detailed ⁴⁰Ar/³⁹Ar dating of the Sora deposit. Recently in South Mongolia the large Cu-Mo(Au) Ouyut-Tolgoi deposit with the K-Ar age of 411±3 Ma [6] has been discovered that also supports existence of the Devonian metallogenic epoch and its high ore productivity. Smaller deposits formed, obviously, between these periods of most intense ore formation. The examples are the Kharmagtai deposit in South Mongolia with the ⁴⁰Ar/³⁹Ar age of 298±5 Ma, the Verkh-Chemskoe (258.7±7.5 Ma) and Novolushnikovskoe (256±5.5 Ma) deposits in Salair (Western Siberia) etc.

The established geochronological boundaries of intense porphyry Cu-Mo mineralization for territories of Siberia and Mongolia are definitely correlated with periods of high ore activity at the territories of East China, Middle Asia, Australia, North and South America: 150, 180, 210, 240-250, 280, 330, 390, and 440 Ma [7-9].

Formation of the Devonian porphyry Cu-Mo deposits in the considered regions is related to the development of the back riftogenic depressions and framing magmatic belts. It is significant that ore-bearing porphyries from all studied deposits have low (⁸⁷Sr/⁸⁶Sr)_o values corresponding approximately to the mantle ones: Sora (0.70450-0.70460), Aksug (0.70454-0.70462), Tsagan-Suburga (0.70393-0.70421).

Origination and functioning of the Triassic Erdenet porphyry Cu-Mo ore-magmatic system in North Mongolia is determined by the development of the Permian-Triassic Selenga belt in the northern framing of the western segment of the Mongol-Okhotsk orogenic belt. There are different views on the formation of the Selenga belt: autonomous tectono-magmatic activization [10], intracontinental riftogenesis [11], pericontinental subduction-related magmatic arcs [12], the relation with the transform fault along the continent-ocean boundary [13], hot spot (mantle plume) activity within the Earth’s mantle [14, 15]. It is supposed that there is the relation between the Triassic porphyry Cu-Mo deposits and development of the Siberian super plume [16]. It is worth noting that similar porphyry deposits were also found within areas where trappean magmatism (the evidence of super plume) was developed at the Siberian Platform (Bolgokhton deposit in the Norilsk region – 233-223 Ma age based on U-Pb and ⁴⁰Ar/³⁹Ar data) and Taimyr (Uboininskaya ore zone). Like the Devonian porphyry deposits, ore-bearing porphyries of the Erdenetuin-Obo deposit are characterized by low (⁸⁷Sr/⁸⁶Sr)_o values equal 0.70418-0.70426.

Late Mesozoic porphyry Cu-Mo mineralization in Eastern Transbaikalia and on the territory of Amur area was formed on the continental margin under pre-riftogenic conditions. At the same time, general rejuvenation of magmatism including ore-bearing porphyry one is observed in the west-east direction. Taking into account the newest ⁴⁰Ar/³⁹Ar datings [17] obtained for the southern framing of the eastern segment of the Mongol-Okhotsk fold belt (Umlekan-Ogodzha fold belt), it was established that igneous rocks with the age of 119-94 Ma

 (ore-bearing granodiorite-porphyry-diorite-porphyritic complex – 119-115 Ma) have geochemical features specific for intraplate magmatites. Unlike the Devonian and Triassic ore-bearing porphyries, the Late Mesozoic ones are characterized by higher (⁸⁷Sr/⁸⁶Sr)_o ratios: from 0.70642-0.70782 in Eastern Transbaikalia to 0.70733-0.70902 in the Stanovoi Ridge structures.

The time of manifestation of ore-bearing porphyry magmatism within the porphyry Cu-Mo ore clusters under consideration is clearly correlated to the global processes occurring during the Earth’s history. This relationship is shown on the Fig. 2, which represents the evolution of strontium isotope composition (⁸⁷Sr/⁸⁶Sr) in the Phanerozoic carbonate series. It is well known that increase in the ⁸⁷Sr/⁸⁶Sr ratio in sea-water is related to the activation of continental erosion while its decrease – with an inflow of mantle strontium into the ocean. Therefore, marine carbonates are the natural recorders of the most significant events in the Earth’s history. The Paleozoic and Mesozoic porphyry Cu-Mo deposits of Siberia and Mongolia, depicted on the Fig. 3, coincide by time with the minima of the ⁸⁷Sr/⁸⁶Sr evolutional curve. This fact supports the universality of endogenous processes responsible for the formation of porphyry deposits.

PETROGEOCHEMICAL FEATURES OF MAGMATISM OF THE PORPHYRY CU-MO ORE CLUSTERS

The porphyry Cu-Mo mineralization is closely associated in space and time with small porphyry bodies (stocks and dikes) formed at the final stage of magmatism in the areas of repeated intrusive and/or effusive activities. The specific feature of the porphyry Cu-Mo ore clusters in Siberia and Mongolia is localization of porphyry bodies and ore-metasomatic formations within large intrusive massifs (see Fig. 3).

Host and porphyry complexes of some ore clusters, despite the considerable difference in their age, are similar not only in rock associations but also in geochemical parameters. In spite of the general similarity of petrogeochemical characteristics of host and porphyry complexes, some ore clusters exhibit their specific evolution from the early to the late complex and possess a set of parameters characterizing the geochemical features of a particular ore cluster (Table 1).

The Sora magmatites belong to a subalkalic rocks with predominance of sodium. Among the studied magmatic centers they are distinguished by maximum alkalinity and high K contents (in this feature they are second only in comparison with magmatites of Mesozoic centers). The rocks have elevated content of aluminium and phosphorus. The agpaitic coefficients and f (Fe/(Fe+Mg)) values are great as well. The porphyry complex, compared to the host one, is rich in alkalies, especially potassium. Rocks of the host and porphyry complexes are characterized by elevated content of Sr, Ba, REE, and HFSE. According to these parameters, they are obviously the derivates of latitic magmas [19]. The most typical trace elements are Nb, Ta, and Zr. The rocks have high K/Rb and U/Th and low Rb/Sr ratios. Monzonites of the pluton and dikes of monzodiorite-porphyries have the highest lithophile trace element contents. The content of lithophile trace elements in rocks decrease while the content of silicic acid increases. In the porphyry complex the concentration of trace elements is higher than that in the host one.

The Aksug magmatites have reduced content of alkalies and correspond in their total content to normal rocks. They are depleted in trace elements and thus are assigned to derivates of andesitic or sodium calc-alkaline magma [19]. They have low Rb/Sr, HFSE/REE, and LREE/HREE and high K/Rb and U/Th values. Alkali contents in the porphyry complex are slightly higher than those in the host complex.

 Table 1. Content and ratio of elements in rocks of ore clusters

Note: Al₂O₃, K₂O, Na₂O, and P₂O₅ are given in wt. %, the rest – in ppm. REE – the sum of rare-earth elements (La, Ce, Nd, Sm, Eu, Gd, Tb, Yb, Lu, and Y). HFSE – the sum of high-field strength elements (Zr, Nb, Ta, Hf, U, and Th), LREE and HREE – light and heavy rare-earth elements, respectively.

The Early-Middle Paleozoic magmatic centers are generally characterized by elevated content of aluminium, high K/Rb and U/Th, and low Rb/Sr.

Total alkalinity of the Erdenetuin-Obo magmatites correspond to granitoids that have the intermediate composition between normal and subalkalic ones. The rocks have high Sr and Ba and moderate REE and HFSE concentrations and elevated HFSE/REE ratio. According to geochemical features, they are intermediate between the Aksug and Zhireken magmatites.

The Zhireken magmatites belong to a subalkalic series with predominance of potassium. Their total content of alkalies, as compared to the Sora magmatites, is lower and corresponds to the minimum values of subalkalic rocks. They have low aluminium and phosphorus content and relatively high REE and HFSE concentration. The content of Rb, Cs, Th, and U are specific. In contrast to Paleozoic magmatites, the HFSE/REE and K/Rb ratios are low, while Rb/Sr is high. The potassium and trace-element content in porphyries is higher as compared to the host complex.

The Shakhtama rocks are characterized by potassium alkalinity, elevated content of femic elements, reduced content of sodium, aluminium and phosphorus, and low f and Fe₂O₃/FeO values. The content of trace elements are high, especially of LREE, Rb, Cs, and Th. The HFSE/REE and K/Rb ratios are low, while Rb/Sr is high. In porphyries, the content of potassium, magnesium, titanium, and trace elements is much higher than those in rocks of the host complex.

The Mesozoic magmatic centers, in spite of the significant difference in some geochemical parameters of the Zhireken and Shakhtama rocks, show high Rb, Cs, and Th content, elevated content of potassium, and predominance of REE over HFSE and of LREE over HREE.

MULTI-PULSE DEVELOPMENT OF MAGMATISM AND MINERALIZATION IN THE ORE CLUSTERS

In copper-molybdenum ore clusters of Russia (Siberia) and Mongolia the ore-bearing porphyry complexes finish protracted multi-pulse granitoid magmatism. At the same time they inherit some petrogeochemical and metallogenic features of the early preceding magmatism with increasing potential ore-productivity of magmas from not very promising mineralization at initial stages to industrially important porphyry copper-molybdenum type at the late stage of development of magmatism. Multiple actions of magmatic and ore-metasomatic processes of different age within comparatively restricted geologic space (ore cluster) are one of the most important factors determining the formation of large porphyry Cu-Mo deposits.

The three age pulses were established for porphyry magmatism at the Sora deposit using ⁴⁰Ar/³⁹Ar dating: pre-ore dikes – 405-402 Ma, subalkaline porphyries I – 389-388 Ma, subalkaline porphyries II – 357-356 Ma. The first porphyry rhythm is the most ore-productive one. In the course of development of the granitoid predecessors composing the Uibat pluton several magmatic pulses were established: 482-479; 474-473; 465-462; 454, and 422-418 Ma. Cu-Mo-skarn mineralization is related to syenitoid rock assemblage (465-462 Ma) while quartz-biotite-K-feldspathic metasomatites (420 Ma) – to disseminated molybdenite and chalcopyrite mineralization.

Porphyry magmatism at the Aksug deposit was also preceded by multi-pulse granitoid magmatism (the host Aksug massif): 533-522 (skarnized copper-pyrite ores are related to this pulse in the region); 497; 490-488 and 462 Ma (manifestations of poor streaky-disseminated Cu and Mo mineralization). Dating of various hydrothermally altered rocks (sericite, sericitized plagioclase) revealed three pulses of endogenous events: 404-401, 364-354, and 331-324 Ma. These pulses correspond to the periods of formation of stock-work Cu-Mo mineralization and ore mineralization with native copper and chalcocite. The latest rock assemblage in the Aksug ore cluster is aplite-granitic one (336-324 Ma) associated with poor streaky- and disseminated pyrite-chalcopyrite mineralization. It is worth noting that the Lower Cambrian Khamsara volcanogenic-sedimentary series hosting the Aksug massif contains hydrothermal-sedimentary copper-pyrite ore beds.

In spite of considerable total time span of action of different magmatic processes, the (⁸⁷Sr/⁸⁶Sr)_o values for all igneous rocks of different age within given ore cluster vary in very limited range: Sora – 0.7040 ¸ 0.7046, Aksug - 0.7043 ¸ 0.7046. Similarity of primary strontium isotope composition as well as inheritance of some petro-geochemical features of preceding granitoids by porphyries indicates, probably, the common deep area of magma formation (at the lower crust - upper mantle level). The latter assumption may be supported by a common geochemical profile of mineralization related to magmatic pulses of different age.

This work is supported by the Ministry for Science and Technology of Việt Nam in the framework of the Project "Intraplate magmatism of North Việt Nam and its metallogeny", by grant 04-05-64238 from the Russian Foundation for Basic Research and "Leading Scientific Schools" (grant NSH-1573.2003.5).

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