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Hydrothermal and structural evolution of the Zijinshan high sulfidation Au-Cu deposit, Fujian Province, China

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posted on 2023-05-28, 09:04 authored by Chen, J
The Zijinshan ore field is a world class mineral district located in southwest Fujian Province, southeast China. It contains a diverse array of ore deposits hosted by the Zijinshan granite complex and surrounding volcano-sedimentary rocks. Associated deposits include high sulfidation epithermal gold-copper, intermediate sulfidation epithermal polymetallic silver-base metal, and porphyry molybdenum-copper deposits. The Zijinshan high-sulfidation Au-Cu deposit is in the middle of the Zijinshan ore field. Its ore zones are hosted in the Zijinshan lithocap. The granitic host rocks are part of a Jurassic intrusive complex emplaced between 165 and 157 Ma. \\(^{40}\\)Ar/\\(^{39}\\)Ar age spectra of hydrothermal alunite indicate that hydrothermal activity at Zijinshan occurred around 103 ¬± 1.5 Ma and was associated with dacite porphyry dikes that intruded around 105 ¬± 2.2 Ma. Hydrothermal activity produced pervasive silicic and advanced argillic alteration assemblages that are zoned from a central, massive quartz alteration domain, outward to massive quartz ‚- dickite, disseminated quartz ‚- alunite ¬± dickite and disseminated quartz ‚- muscovite ‚- dickite zones. There are three principal styles of mineralization at Zijinshan: veins, breccias and disseminations. The ore zones are typically dominated by breccias and veins. Evolution of the Zijinshan hydrothermal system resulted in a sequence of mineral deposition that can be separated into four hypogene stages. Stage 1 is characterized by veins with fine-grained pyrite intergrown with quartz and muscovite. Stage 2 was the earliest stage of hypogene copper mineralization, producing discrete quartz ‚- enargite veins associated with patchy disseminated quartz ‚- pyrophyllite alteration in the host granite. Stage 3 is the most economically important paragenetic stage at Zijinshan ‚- it is characterized by abundant covellite, digenite and abundant or minor pyrite. Stage 4 veins are dominated by dickite or alunite and contain only minor covellite and digenite. The temporal evolution from pyrite ‚- muscovite to enargite ‚- pyrophyllite and covellite ‚- digenite ‚- alunite ‚- dickite assemblages most likely involved cooling coupled with increasing acidity of the mineralizing fluids. The increase in acidity may relate to condensation of acidic magmatic gases, possibly via mixing with shallow-level meteoric waters. Supergene oxidation caused gold enrichment beneath a leached cap in strongly weathered silicic altered rocks in the upper parts of Zijinshan, and depleted copper almost entirely from theweathered zone. Consequently, Zijinshan can be divided into a near-surface oxide gold ore zone that overlies three deeper-seated hypogene copper ore zones. A set of sub-vertical NE-striking faults were active prior to mineralization at Zijinshan. These faults have been reserved at the fold hinge zone of the Xuanhe anticlinorium, which help to localize the emplacement of the Jurassic Zijinshan granite complex. The pre-mineralization faults may be related with a NW-directed compression, which at least still active during Jurassic. A NW-striking fault system developed in the Cretaceous and controlled the emplacement of veins, breccias and dacite dikes. Kinematic indicators show that most of the NW-striking faults were active as normal oblique faults during mineralization. Syn-mineralized structures formed under conditions of subvertical contraction and subhorizontal NNE-trending extension. Post-mineralization ENE-striking dextral strike-slip faults are associated with minor conjugate NNW-striking faults. They are Riedel shears governed by a strike-slip regime defined by a NW-oriented axis of compression and NE-trending axis of extension. Post-mineralization faults are related to transient local variations in stress regimes at the deposit-scale. Changes in the kinematics of pre- and syn- mineralization faults from compression to extension were fundamental to mineralization. Because the changes of regional stress triggered the fault slip, which further enhancing the permeability of the host rocks and facilitating hydrothermal fluid flow. Widespread brecciation occurred during the evolution of the Zijinshan hydrothermal system. The resultant syn-mineralization breccia veins and dikes are important host rocks to mineralization. In order of abundance, breccia subfacies at Zijinshan include hydrothermally-cemented breccia (CM), cement and matrix-bearing breccia (CMX) and matrix-rich breccia (MX). In the northern part of Zijinshan, hydrothermally-cemented breccias occur as sub-parallel breccia veins that strike NW and dip less than 45¬∞ NE. Matrix-rich breccias crop out in the southern part of the deposit as 0.2 - 2 m wide dikes that strike NW and dip moderately NE. Cement and matrix-bearing breccias are rare, typically occurring in the center of the deposit. There is minimal superposition of the hydrothermally-cemented breccia and matrix- rich breccia types. The CM, CMX and MX breccias are classified as tectonic-hydrothermal breccias. Their paragenetic relationships indicate that brecciation evolved from attrition to dilation, potentially corresponding to the change from compressional to extensional environments during mineralization. Advanced argillic and silicic alteration were intimately associated with mineralization at Zijinshan and caused significant chemical changes to the host granite and dacite. Mass-balance calculations highlight significant silica addition (up to +80.4 g per 100 g) during silicic alteration, which manifested as both massive and vuggy quartz alteration. Alkali and alkaline earth elements depletion (up to -6.37 g per 100 g) was widespread during all stages of alteration at Zijinshan. Gold and silver were strongly enriched in the silicic altered granite and dacite (up to + 0.67 g per ton and +1.52 g per ton, respectively). Quartz ‚- dickite and quartz ‚- alunite altered rocks are typically the most enriched in copper relative to the least altered protoliths (up to 0.03 g per 100 g and 0.05 g per 100 g, respectively). Alunite has several modes of occurrences at Zijinshan and formed during three hypogene paragenetic stages. Type 1 alunite replaced primary K-feldspar in the granite and dacite. Type 2 alunite is intergrown with covellite and digenite in veins. Type 3 alunite infilled late-stage fractures in the wall rocks. The alunite 1480 nm feature position measured by short-wavelength infrared analyses are consistently higher in type 1 alunite samples collected on the eastern side of the formation (up to 1482.2 nm) compared to the west (up to 1478.8 nm), implying elevated temperatures of alunite deposit to the east, possibly indicating the location of the heat and fluid source for Zijinshan. Given that the mineralized structures at Zijinshan dip moderately (~ 45¬∞) to the NE, the most likely location of a porphyry-style fluid source for Zijinshan would be at depth towards the NE, underneath the Zijinshan lithocap.


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