|Abstract:North Daba Mountains, which is an important part of South Qinling belt, was confined by An’kang Fault in the north and Chengkou-Fangxian Fault in the south. Besides minor lamprophyre and carbonatite, alkali volcanic rocks, characterized by trachyte, alkali basalt and related volcanoclastic rocks, are widespread in North Daba Mountains. These alkali volcanic rocks are the main hosting rocks of copper, Zinc and Fluorite mineralization. Debates on the magmatic evolution, formation age and tectonic setting about the alkali volcanic rocks still exist due to lack of comparison and systematical research of distribution of alkali volcanic rocks, which blocks the perspective of regional evolution. The lack of study on mineralization age and ore-forming material of zinc-fluorite deposit constrains the breakthrough of mineral deposit exploration.
Results of rock assemblage, geochemistry, isotopic composition and formation age of volcanic rocks from Pingli-Zhenping area are presented in this study. The magmatic evolution, formation age, magma sources and formation setting were concluded from the comparison between our new results and previous data. In addition, the ore genesis of Damogou-Naoyangping zinc-flourite deposit was deduced based on our new results of mineralization age and ore-forming fluid.
A suit of alkali volcano-sedimentary rocks composed of trachyte, alkali volcanic rocks and volcanoclastic rocks develops at Niutoudian area, Pingli-Zhenping County. From bottom to top, the sequence is composed of alkali lava (trachyte and minor basalt), volcanoclastic (tuffaceous breccia, breccia tuff and tuff) and bioclastic limestone, which is typical of seamount assemblage. The volcanic breccias in volcanoclastic rocks are mainly composed of trachyte, and subordinate alkali feldspar. Pillow basalt and limestone enriched with abundant volcanic breccias and bioclasts imply under-water setting. However, the occurrence of columnar basalt indicates parts of the volcanic edifice exposed above the water.
Alkali basalt with phenocrysts composed of plagioclase with minor apatite and titanite, is characterized by various SiO2 content (44.16～55.92wt%), relatively high Na2O+K2O (6.91～8.43wt%) and TiO2(2.63～3.10wt%) contents, and low Mg# values (40～48), Cr (0.97～10.70ppm), Co (3.07～4.40ppm) and (1.95～6.11ppm) contents, representing highly evolved magma. Enriched incompatible elements of Th, U, Nb, Ta, Zr, Hf and REEs (ΣREE=226～399ppm), with strongly differentiated REE (LaN/YbN=11～25), displaying OIB geochemical attributes. Ba in alkali basalt has an extremely high content of 33290-58380ppm and displays positive anomaly. Electron microprobe analysis results for minerals in alkali basalt indicate that Ba is hosted in hyalophane (Cn=4.77～33.81) which occurs along the rim and cracks in feldspar as the product of post-diagenesis fluid metasomatism. The upper Cambrian strata with abundant barite are the potential sources for Ba.
The phenocrysts in trachyte include K-feldspar, biotite and amphibole. Trachyte has various SiO2 content of 60.46～71.44wt%, relatively low TiO2 content of 0.79～1.09wt% and Mg# value of 19-32, high Na2O+K2O content of 8.08～12.75wt%. The rare earth element content of trachyte is extremely high (ΣREE=444-1111ppm) with intense differentiated REE (LaN/YbN=14.4-25.0), negative Eu anomaly (δEu=0.48-0.90) and more depleted MREE (GdN/YbN=1.4-2.4) compared with alkali basalt. The Primitive Mantle normalized spider diagram of trachyte is characterized by enriched incompatible elements of Th, U, Nb, Ta, Zr and Hf and delpeted Ba, Sr, P and Ti.
The high Nb/Yb ratios (22-28) of alkali basalt are consistent with OIB-type geochemistry. Alignment along MORB-OIB array on the Th/Yb-Nb/Yb diagram excludes apparent crustal contamination. Alkali basalt and trachyte share common isotopic composition, which is characterized by identical initial 143Nd/144Nd ratios (0.512301~0.512340 in alkali basalt and 0.512252~0.512328 in trachyte) and εNd(t) values (+2.55~+3.32 in alkali basalt and +1.60~+3.07 in trachyte). The HIMU mantle is considered as the potential source for alkali basalt and trachyte. Affected by later stage fluid metasomatism, variable initial 87Sr/86Sr ratios of 0.701418-0.714437 were obtained.
Regionally, the volcano-sedimentary sequence in Pingli-Zhenping area consists of primary trachyte and minor alkali basalt, while the volcano-sedimentary sequences of Taohekou Group in Ziyang-Langao and Zhuxi Group in Zhuashan area are composed of alkali basalt (with mantle xenolith encircled) and minor trachyte. The phenocrysts of theses alkali basalt include clinoproxene and subordinate phlogopite. Geochemistry and Sr-Nd isotopic compositions of these alkali volcanic rocks are similar with those in Pingli-Zhenping area, but displaying a more primitive magma component.
The change discipline of geochemical component between alkali basalt and trachyte indicates fractional crystallization plays an important role in magma evolution process. Removal of Clinopyroxene and ilmenite (titanite) was dominant during the early stage. Fractional crystallization of plagioclase and apatite took over in the subsequent stage. As the consequence of fractional crystallization, REEs became more enriched in trachyte. Removal ofapatite in trachytecaused depletion of MREE and fractional crystallization of plagioclase induced Eu and Sr negative anomalies.
Paleozoic crinoid had been discovered in limestone lenticle in Niutoudian volcano-sedimentary rocks indicating the alkali volcanism erupted at Paleozoic. The age of 363±3Ma obtained from Ar-Ar dating of K-feldspar phenocryst constrains the alkali volcanism in Pinglin-Zhenping area occurred at Late Devonian. The alkali volcanism in North Daba Mountains was probably the product of multi-stages volcanism commenced at the early stage of Middle Silurian (432Ma) based on previous research.
The Paleozoic North Daba Mountains was part of the passive margin of Yangtze plate. Rifting occurred at late stage of Paleozoic triggered intrusion of extensive gabbro-diabase. Paleozoic alkali volcanic rocks from North Daba Mountains formed under extensional setting on passive continental margin of Yangtze plate. Magma supply rate and evolution degree were considered to be the main factor controlling the distribution of alkali volcanic rocks. During Middle Silurian to Early Devonian (?), high magma supply rate gave rise to extensive alkali basalts in Ziyang-Langao and Zhushan area. With a low magma supply rate in Late Devonian (363Ma), magma experienced sufficient fractional crystallization, generating massive trachyte in Pingli-Zhenping area.
Damogou-Naoyangping zinc-fluorite deposit is spatially associated with Paleozoic trachyte. The orebody is strictly bounded by faults, developing along the contact zone of trachyte and Zhuxi group with brecciaed trachyte as the hosting rocks. Two mining districts: Damogou and Naoyangping, are divided based on mineralization type. The Damogou district is predominated by zinc and fluorite mineralization with ore minerals of sphalerite and fluorite, and minor pyrite, chalcopyrite and galena. The zinc orebody exhibit crumby or veinlet structure, which is posterior to fluorite lens and vein. However, fluorite is the primary mineralization type in Naoyangping district, with crumby or veinlet structure. The occurrence of comb and miarolitic texture of fluorite suggested hydrothermal mineralization. The Damogou-Naoyangping deposit is considered as the fault-controlled hydrothermal deposit.
The homogenization temperature of primary inclusion in fluorite is 243～383℃, whereas those inclusions in quartz from sphalerite veinlet have lower homogenization temperature of 140～273℃. The Sr-Nd isotopic composition of fluorite and sulfur isotopes indicate a crustal source for ore forming material, which is probably from the Zhuxi group limestone. Fluorite Sm-Nd isochron age of 215.3±3.6Ma indicate the mineralization took place in the Late-Triassic.