The Laodou gold deposit, located in the West Qinling Orogen of central China, is a newly recognized intrusion-related gold deposit. It consists of auriferous quartz-sulfide-tourmaline and minor quartz-stibnite veins that are structurally controlled by fault zones transecting the host quartz diorite porphyry. Two types of tourmaline were identified in this study: Type 1 tourmaline occurs as quartz-tourmaline nodules within the quartz diorite porphyry, whereas type 2 tourmaline occurs as quartz-sulfide-tourmaline veins in auriferous lodes. Here, we present a major and trace element analysis by electron microprobe and laser ablation inductively coupled plasma mass spectrometry on these two types of tourmaline. Both tourmaline types fall into the alkali group, and are classified under the schorl-dravite solid solution series. The substitutions of FeMg–1, FeAl–1, AlO((Fe, Mg)(OH)) –1, and X-site vacancyCa–1 are inferred by the variations of their major element compositions. Field and mineralogy observations suggest that type 1 tourmaline is a product of the late crystallization process of the quartz diorite porphyry, whereas type 2 tourmaline coexists with Au-bearing arsenopyrite and is crystallized from the ore-forming fluids. Their rare earth element compositions record the related magmatic hydrothermal evolution. The Co and Ni concentrations of the coexisting type 2 tourmaline and arsenopyrite define a regression line (correlation coefficient = 0.93) with an angular coefficient of 0.66, which represents the Co/Ni ratio of the tourmaline and arsenopyrite-precipitating fluids. This value is close to the Co/Ni ratios of the host quartz diorite porphyry, indicating a magma origin of the ore-forming fluids. The substitution of Al3+ by Fe3+ in both tourmaline types shows that type 1 tourmaline approaches the end member of povondraite whereas type 2 tourmaline occurs in opposite plots near the end member of Oxy-dravite, reflecting a more oxidizing environment for type 2 tourmaline formation. Moreover, the redox-sensitive V and Cr values of type 2 tourmaline are commonly 1–2 orders of magnitude higher than those of type 1 tourmaline, which also suggests that type 2 tourmaline forms from more oxidizing fluids. Combined with gold occurrence and fluid properties, we propose that the increasing of oxygen fugacity in the ore-forming fluids is a trigger of gold precipitation.
The Shagou vein-type Ag-Pb-Zn deposit in the Xiong’ershan district, southern margin of the North China craton, is hosted within amphibolite facies metamorphic rocks of the Late Archean to early Paleoproterozoic Taihua Group. The Ag-Pb-Zn veins are localized in NE- to NNE-trending brittle faults and typically display symmetrical zoning consisting of siderite, quartz + sphalerite, galena, and quartz + calcite from the margin toward the center of each vein. Ore-related hydrothermal alteration is well developed on both sides of the veins, dominated by silicification, sericitization, chloritization, and carbonatization. Sericite separates extracted from a major Ag-Pb-Zn vein yield a 40 Ar/ 39 Ar plateau age of 140.0 ± 1.0 Ma (1 σ ) and isochron age of 141.1 ± 1.6 Ma (1 σ ), indicating that mineralization occurred at the beginning of Early Cretaceous. Field and textural relationships indicate four hydrothermal stages marked by assemblages of quartz + siderite (stage I), quartz + sphalerite + ankerite (stage II), quartz + galena + silver minerals + ankerite (stage III), and quartz + calcite (stage IV), respectively. Silver minerals are abundant in all veins and are composed of, in paragenetic order, argentiferous tetrahedrite, polybasite, jalpaite, argentite, and native silver. These silver minerals commonly occur as replacements of galena, chalcopyrite, and other sulfides, or as fillings of microfractures in sulfides and quartz. Microthermometric measurements of primary fluid inclusions in quartz, carbonates, and sphalerite from various hydrothermal stages indicate that ore minerals were deposited at intermediate temperatures (267°–157°C) from aqueous-carbonic to aqueous fluids with moderate salinities (7.2–15.9 wt % NaCl equiv). Coexisting galena-sphalerite pair yields sulfur isotope equilibrium temperatures of 205° to 267°C, consistent with the overall homogenization temperatures of fluid inclusions. The microthermometric data also indicate that both fluid mixing and fluid-rock interaction were important mechanisms for ore precipitation. Carbonate minerals (siderite, ankerite, calcite) spanning the entire mineralization history have δ 13 C V-PDB values of −5.2 to −1.4‰ and δ 18 O V-SMOW of 10.9 to 15.0‰, corresponding to calculated values for the ore fluids of −6.5 to −1.8‰ and 1.4 to 5.4‰, respectively. δ 34 S V-CDT values of sulfide minerals (pyrite, sphalerite, galena) range from 1.1 to 5.5‰, consistent with a deep-seated sulfur source. Galena separates have 206 Pb/ 204 Pb ratios of 17.472 to 17.813, 207 Pb/ 204 Pb ratios of 15.411 to 15.498, and 208 Pb/ 204 Pb ratios of 38.178 to 38.506. The isotope data, together with geological and geochronological evidence, favor a primary metamorphic source for sulfur and other components in the ore fluids. A synthesis of available data suggests that the Shagou deposit is a typical vein-type Ag-Pb-Zn deposit that formed under an extensional geodynamic setting associated with tectonic reactivation of the North China craton during the late Mesozoic, a time period that is manifested by pervasive magmatism, widespread formation of metamorphic core complexes, and development of faulted basins throughout the eastern part of the craton. Metamorphic devolatilization of the Meso-Neoproterozoic marine sedimentary rocks previously subducted beneath the Xiong’ershan district, facilitated by extensive magmatism and elevated heat flow due to lithospheric extension, could have provided large amounts of ore fluids responsible for the Ag-Pb-Zn mineralization. The NE- to NNE-trending faults affiliated with the transcrustal Machaoying fault may have acted as pathways for the upward migration of deep-seated metamorphic fluids. Mixing of the metamorphically derived fluids with meteoric waters ultimately resulted in deposition of the Ag-Pb-Zn veins in brittle extensional structures.
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