Two Hydrothermal Events at the Shuiyindong Carlin-Type Gold Deposit in Southwestern China: Insight from Sm–Nd Dating of Fluorite and Calcite

Abstract

The Shuiyindong Gold Mine hosts one of the largest and highest-grade, strata-bound Carlin-type gold deposits discovered to date in Southwestern China. The outcrop stratigraphy and drill core data of the deposit reveal Middle–Upper Permian and Lower Triassic formations. The ore is mainly hosted in Upper Permian bioclastic limestone near the axis of an anticline. The gold is mainly hosted in arsenian pyrite and arsenopyrite, mainly existing in the form of crystal lattice gold, submicroscopic particles and nanoparticles. Fluorite commonly occurs at the vicinity of an unconformity between the Middle–Upper Permian formations, which is proposed to be the structural conduit that fed the ore fluids. Calcite commonly fills fractures at the periphery of decarbonated rocks, which contain high grade orebodies. This study aimed to verify the occurrence of two distinct hydrothermal events at the Shuiyindong, based on Sm–Nd isotope dating of the fluorite and calcite. For this purpose, rare-earth element (REE) concentrations, Sm/Nd isotope ratios, and Sm–Nd isochron ages of the fluorite and calcite were determined. The fluorite and calcite contain relatively high total concentrations of REE (12.3–25.6 μg/g and 5.71–31.7 μg/g, respectively), exhibit variable Sm/Nd ratios (0.52–1.03 and 0.57–1.71, respectively), and yield Sm–Nd isochron ages of 200.1 ± 8.6 Ma and 150.2 ± 2.2 Ma, with slightly different initial εNd(t) values of −4.4 and −1.1, respectively. These two groups of Sm–Nd isochron ages suggest two episodes of hydrothermal events at the Shuiyindong gold deposit. The age of the calcite probably represents the late stage of the gold mineralization period. The initial εNd(t) values of the fluorite and calcite indicate that the Nd was probably derived from mixtures of basaltic volcanic tuff and bioclastic limestone from the Permian formations.

Invisible Gold Paragenesis and Geochemistry in Pyrite from Orogenic and Sediment-Hosted Gold Deposits

“…As seen in Figure 5A, Te is commonly negligible in both the core and rim. There are several other examples of Carli-type deposits, especially in southern China, that show Au-As enrichment on pyrite rims (e.g., Shuyindong deposit in SW China, [33]).…”

Sm–Nd Isochron Age Constraints of Au and Sb Mineralization in Southwestern Guizhou Province, China

“…Sm-Nd isotope systematics are liable to be kept closed and are partially capable of resisting weathering and/or alteration [23]. The Sm-Nd isochron method is an effective tool for dating hydrothermal Cabearing minerals and has been used in various hydrothermal deposits for geochronology studies (e.g., calcite [18,19,[24][25][26], scheelite [27][28][29], fluorite [30][31][32], and tourmaline [27,28]).…”

“…They usually have higher rare earth element (REE) concentrations and relatively variable Sm/Nd ratios, which are favorable for Sm-Nd isotope dating [18][19][20]33]. Ore-related calcite is generally intergrown with realgar and orpiment at Au deposits and is thought to be the product of decarbonatization (carbonate dissolution) of host rocks, providing Fe 2+ to form Au-bearing pyrite [18,19,33]. Calcite deposited from decarbonatization reflects the age of Au mineralization [18,19].…”

“…Ore-related calcite is generally intergrown with realgar and orpiment at Au deposits and is thought to be the product of decarbonatization (carbonate dissolution) of host rocks, providing Fe 2+ to form Au-bearing pyrite [18,19,33]. Calcite deposited from decarbonatization reflects the age of Au mineralization [18,19]. Ore-related stibnite is symbiotic and coeval with fluorite crystallization, such that fluorite Sm-Nd dating can represent the age of Sb mineralization [20].…”

Tennantite–Tetrahedrite-Series Minerals and Related Pyrite in the Nibao Carlin-Type Gold Deposit, Guizhou, SW China

“…Over 30 Au deposits (total resource: >800 tonnes) were discovered in the Youjiang basin, SW China, making the region the world's second largest Carlin-type/-like gold deposit cluster (Figure 1; [1][2][3][4][5]). These deposits in SW China have three major types of host rocks: (1) Upper Permian-Middle Triassic carbonates, which host the majority of deposits such as the Shuiyindong [6][7][8][9][10][11][12]; (2) Middle Triassic calcareous clastic rocks, which host deposits such as the Lannigou [13][14][15]; and (3) Upper Permian pyroclastic rocks and Indosinian (Triassic) diabase that are related to the end Permian Emeishan large igneous province (ELIP), which host deposits such as the Nibao and Badu [16][17][18]. Despite the significant lithological differences between the three types of ore host, all the deposits show Carlin-type gold mineralization features, including: (1) rare visible free gold, and the gold is mainly bound structurally within the (arseno)pyrite lattice as Au + [3,11,15,16]; (2) the Au ores are rich in As, Sb, Hg, and Tl, but are poor in Ag (Ag/Au < 5) and metals (Cu+Pb+Zn < ≈500-1000 ppm) [2,19,20]; (3) ore-forming fluids were of low temperature (≈190-240 ℃), low salinity (≈2-3 wt %, NaCl equiv.…”