Ore-Forming Processes of the Daqiao Epizonal Orogenic Gold Deposit, West Qinling Orogen, China: Constraints from Textures, Trace Elements, and Sulfur Isotopes of Pyrite and Marcasite, and Raman Spectroscopy of Carbonaceous Material

Abstract

The Daqiao gold deposit is hosted in organic-rich Triassic pumpellyite-actinolite facies metamorphosed turbidites in the West Qinling Orogen, central China. Gold mineralization is characterized by high-grade hydraulic breccias (B and C ores) that overprint an earlier tectonic breccia (A ore). A complex paragenesis is defined by four sulfide stages: S1 diagenetic pre-ore pyrite (py), S2 hydrothermal early-ore disseminated pyrite and marcasite (mc), S3 main-ore pyrite and marcasite aggregates, and S4 late-ore coarse-grained marcasite with minor pyrite and stibnite. However, multiple generations of pyrite and marcasite may develop within one individual stage. Ore-related hydrothermal alteration is dominated by intensive silicification, sulfidation, sericitization, and generally distal minor carbonatization. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace element analyses show that the stage S1 py1 from the shale interlayers within turbidites contains low gold contents (mean of 0.05 ppm) and other trace elements (Mn, Co, Ni, Cu, Mo, Bi, and Pb) indicating an anoxic to euxinic sedimentary environment. Stage S2 contributed only minimally to the gold endowment with relatively low gold in various sulfides including py2 (mean of 0.09 ppm), py3 (0.84 ppm) to py4 (0.70 ppm), along with mc1 (0.02 ppm) and mc2 (0.14 ppm). Most of the gold was deposited in stage S3, which formed rapidly crystallized, irregular (e.g., framboids, colloform and cyclic zonation) cement-hosted py5a (mean of 27.35 ppm), py5b (9.71 ppm), and mc3 (5.94 ppm) during repeated hydraulic fracturing. Other trace elements (e.g., Ag, As, Sb, Hg, Tl, and W) are also significantly enriched in the main-ore stage pyrite and marcasite. Little or no gold is detected in the S4 py6 and mc4. Sulfur isotopes determined from in situ LA-MC-ICP-MS analyses of hydrothermal pyrite and marcasite from the Daqiao deposit vary significantly from-31.3 to +22.0‰ (δ 34 S values), but fall mostly between-10 to +10‰, and provide important information on the source and evolution of sulfur and of the ore-forming fluids. The results show that S2 ore fluids (mean δ 34 Ssulfide =-0.8 to +5.2‰) were most likely derived from deep-seated Paleozoic carbonaceous sediments during regional metamorphism associated with orogenesis of the West Qinling Orogen. Main-ore S3 ore fluids (mean δ 34 Ssulfide =-9.7 to-6.0‰) are relatively depleted in 34 S relative to those of S2, presumably due to fluid oxidation associated with hydraulic fracturing caused by the over pressurized fluids. The textural, chemical, and isotopic data indicate two distinct gold-introducing episodes at Daqiao, forming sulfide disseminations during early-ore S2 and cement-hosted sulfide aggregates during main-ore S3. The S2 mineralization took place in a tectonic breccia beneath low-permeability shale seals that capped the flow of deep-seated metamorphic fluids, facilitating reaction with pre-existing carbonaceous material and the host turbidites to form sulfide disseminations and pervasive silicific...

Metal remobilization and ore-fluid perturbation during episodic replacement of auriferous pyrite from an epizonal orogenic gold deposit

“…The results ( Fig. 4) are based on combination of the 72 spot analyses and the data reported in Wu et al (2018a). Data from two previously unreported LA-ICP-MS transects across complex sulfide aggregates are also presented in Figure 5.…”

“…1a). The West Qinling Orogen (WQO), separated from the East Qinling Orogen (EQO) broadly by the Baocheng Railway, can be further divided into the northern, central and southern domains by the Shangdan suture and Hezuo-Lintan-Liangdang Fault (HLLF), respectively (Wu et al, 2018a). The northern domain is an early Paleozoic oceanic arc, while the central and southern domains are characterized by greenschist-facies Devonian flysch metasedimentary rocks and unmetamorphosed to low-grade Triassic turbidites, respectively (Mao et al, 2002).…”

“…The Daqiao orogenic gold deposit of latest Jurassic and Early Jurassic age in the southern domain has more than 105 t of gold resources at an average grade of 3-4 g/t (Wu et al, 2018a). The comprehensive deposit geology has been described in Wu et al (2018a) and briefly summarized here.…”

“…The Daqiao orogenic gold deposit of latest Jurassic and Early Jurassic age in the southern domain has more than 105 t of gold resources at an average grade of 3-4 g/t (Wu et al, 2018a). The comprehensive deposit geology has been described in Wu et al (2018a) and briefly summarized here. Orebodies are structurally controlled by a number of second-order reverse faults steeply dipping 55 to 80° north or southwest and mostly localized in fracture zones at the contact between Triassic turbidites and Carboniferous limestone (Fig.…”

“…A detailed interpretation of hydrothermal stages and different generations of sulfides is provided by Wu et al (2018a). The early-ore stage sulfide euhedras and aggregates (Py2,Py3,Py4,Mc1,and Mc2) are dominantly hosted by tectonic breccia A ( Fig.…”