| The north Fujian area is located on the eastern part of the Cathaysia Block and adjacent to the Zhenghe-Dapu fault zone, which roughly bounded the east and west Cathaysia. Episodic magmatism (especially volcanism) and multi-stage tectonic reactivation made profound influences on metallogeny in this area. Based on the detailed field investigation on the Mesozoic Dongkeng volcanic-sedimentary basin and gold deposits interior, such as Dongji, Maluntou and Shangshangang, in this paper, we present isotopic ages, geochemical data for the volcanic rocks and intrusions, and present fluid inclusion characters, ore H-O-S-Pb isotopes to (1) document the magma sources and petrogenesis, and (2) provide potential information about the geodynamic evolution from the’early Yanshanian’ to ’late Yanshanian’, and (3) analyzed the genetic links between gold metallogenesis and volcanisms. Moreover, the volcanic-sedimentary sequences and its relation with gold metallogenesis were analyzed as well. Based on the relations between gold metallogenesis with structure, volcanic rocks and stream sediment geochemical data, the potential gold resources was evaluated in this area. The main results are summarized herein:1. Three magmatic zircon U-Pb age ranges of 159 Ma,143 Ma and 101-96 Ma obtain from volcanic rocks and intrusions from the Dongkeng Basin. The 159 Ma rhyolitic porphyry is highly peraluminous, and characterized by high abundances of SiO2, P2O5 and lower abundances of CaO, Na2O. All samples are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), and depleted in high field strength elements (HFSE), and display apparent negative Nb, Ti, P, Sr and Eu anomaly. Geochemical data suggest that the rocks are S-type rocks derived from partial melting of paragneisses. These rocks represent products of the northwest ward subduntion of the Paleo-Pacific Ocean. The 101-96 Ma intermediate-acid volcanic rocks is metaluminous, and characterized by wide range of SiO2 contents and relatively high abundances of CaO, Na2O and HFSE. All samples are enriched in LILE and LREE, and depleted in HFSE, and display apparent negative Nb, Ti, P, Sr and Eu anomaly. Geochemical data suggest that these rocks are fractionated I-type rocks. Fractionation of plagioclase was responsible for negative Sr and Eu anomaly. The young Nd model ages and wide ranges of Hf isotope data suggest a mixing source of lower crust and lithospheric mantle. The above elemental and isotope characters document a extensional geodynamic settings for the generation of the 101-96 Ma intermediate-acid volcanic rocks. The 143 Ma rhyolites have both characters of the S-type and I-type rocks, and are dominantly characterized by S-type features. The magmas evolving from 159 Ma S-type rhyolitic porphyry to 101-96 Ma I-type intermediate-acid volcanic rocks in the Dongkeng Basin indicate a transition from compressional to extensional geodynamic settings.2. The north Fujian area is underlain by the Meso-Neoproterozoic metamorphic basement, and the Jurassic-Cretaceous volcanic-sedimentary sequences. The regional structures are dominated by the multi-stage active NNE-trending Zhenghe-Dapu fault and its subordinate SN- (and NE-) trending fault. Almost all gold ore bodies distribute in the interior of, or around the Dongkeng Basin, and are hosted by the Jurassic-Cretaceous volcanic-sedimentary sequences, Jurassic intrusions and metamorphic basement. The gold deposits can be divided into three types, namely, auriferous veins, lenticular and banded ores. Pyrite, galena, sphalerite and chalcopyrite are common sulfides in different gold deposits, and the gold often occurs as native gold or electrum in pyrite. The alteration is mainly siliceous metasomatism, forming quartz, sericite, chlorite, pyrophyllite and associated sulfides. In view of the mineralogy and textural relationships, three stages of mineralization are recognized:(1) quartz-pyrite, (2) quartz-polymetallic sulfide, and (3) quartz-carbonate.3. The fluid inclusions from gold deposits in Dongkeng Basin have a range of homogenization temperatures (Th) of 143.2-391.5℃ with three peaks at 362-378.5℃, 187-244℃, and 149-175 ℃, respectively. This is consistent with homogenization temperatures ranges obtained from the Zijinshan gold deposit. Samples from gold veins have δDH2Oo values of -78.3%o to -55‰ and δ18OH2O values of -4.5‰ to +1.9%o. The similarity of homogenization temperatures, hydrogen and oxygen isotope data between gold deposits in the Dongkeng Basin and the Zijinshan gold deposit, indicate similar ore fluid characters, which derived from mixing of volcanic fluids and meteoric water. Fluid unmixing and temperature drop are the main controlling factors about the gold minerals deposition.δ34S values of sulfides from the Dongji, Maluntou and Shangshangang range from +0.4‰ to +2.5‰,-1.8‰ to +4.3%o, and +2.0‰ to +3.8‰, respectively. This overlaps the compositional fields of magmatic-derived sulfur, indicating a deeply seated source for sulfur. Lead isotopes of sulfides ((206Pb)i/(204Pb)i= 17.592 to 18.580, (207Pb)i/(204Pb)i = 15.570 to 15.974) is consistent with that of the Cretaeous magmas (206Pb)i/(204Pb)i= 17.960 to 19.606, (207Pb)i/(204Pb)i= 15.603 to 15.696). Taken together, we suggest a magmatic-derived derivation for the metallogenic minerals about gold deposits in the Dongkeng Basin.4. The Jurassic-Cretaeous volcanic-sedimentary sequences are bounded to the east of Zhenghe-Dapu fault or exposed in small fault-bounded basins, and are made up of rhyolites, dacites, rhyolitic-dacitic ignimbrites, pyroclastic and sedimentary rocks. Based on the field investigation on rock types and their contact relations, the Cretaeous strata can be divided into two sedimentary-volcanic sequences, namely, episode I (Huangkeng Formation) and episode II (Zhaixia Formation). The Huangkeng Formation of episode I, overlain by sequences of episode II, can be divided into two subsequences, including underlying sedimentary subsequences and conformably overlying volcanic subsequences. The sedimentary subsequences are thin and limited in the Dongkeng area, and rest unconformably on the late Jurassic-early Cretaeous rhyolites. The Zhaixia Formation (episode II) has complicated rock types and can be exactly divided into three subsequences. The lowermost subsequences mainly consist of pyroclastic rocks interlayered with sedimentary fine-grained lapilli tuffs. The intermediate and uppermost level of the Zhaixia Formation mainly consists of rhyolites, dacites and rhyolitic-dacitic ignimbrites, which is underlain unconformably by the lowermost subsequences.5. The relatively high contents of Au, Ag, Cu, Pb, Zn, As and Sb of the Zhaixia Formation reveals that these rocks may provide metallogenic minerals for gold mineralization in the Dongkeng Basin. In addition, almost all gold deposits located in the interior of the Zhaixia Formation, or other strata around it. Taken together, gold minerals may be mainly derived from the Zhaixia Formation.The Dongkeng Basin is dominantly by the NNE-trending Zhenghe-Dapu sinistral strike-slip fault system and the volcanic structural system. Nearly all gold ore bodies are hosted by the subsidiary faults of regional NNE-(and NE-) trending deep-cutting faults (like the Dayaokeng and Dongji gold deposits) and the radial fractures (like the Maluntou and Shangshangang gold deposits). Hence, faults provide important passage for metallogenic fluids and sites for gold deposition in this area.6. Based on the relations between gold metallogenesis with structure, volcanic rocks and stream sediment geochemical data, the potential gold resources was evaluated in this area. Thirteen favorable areas at different scales are selected in the Dongkeng Basin for further prospecting. |
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