Abstract The ages of Carlin-type gold deposits in the Golden Triangle of South China have long been questioned due to the general lack of minerals unequivocally linked to gold deposition that can be precisely dated using conventional radiogenic isotope techniques. Recent advances in U-Pb methods show that calcite can be used to constrain the ages of hydrothermal processes, but few studies have been applied to ore deposits. Herein, we show that this approach can be used to constrain the timing of hydrothermal activity that generated and overprinted the giant Shuiyindong Carlin-type gold deposit in the Golden Triangle. Three stages of calcite (Cal-1, Cal-2, and Cal-3) have been recognized in this deposit based on crosscutting relationships, cathodoluminescence colors, and chemical (U, Pb, and rare earth element [REE]) and isotope (C, O, Sr) compositions. Cal-1 is texturally associated with ore-stage jasperoid and disseminated Au-bearing arsenian pyrite in hydrothermally altered carbonate rocks, which suggests it is synmineralization. Cal-2 fills open spaces and has a distinct orange cathodoluminescence, suggesting that it precipitated during a second fluid pulse. Cal-1 and Cal-2 have similar carbonate rock-buffered chemical and isotopic compositions. Cal-3 occurs in veins that often contain realgar and/or orpiment and are chemically (low U, Pb, and REE) and isotopically (higher δ13C, lower δ18O and Sri values) distinct from Cal-1 and Cal-2, suggesting that it formed from a third fluid. U-Pb isotope analyses, by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) for U-rich Cal-1 and Cal-2 and by LA-multicollector (MC)-ICP-MS for U-poor Cal-3, yield well-defined age constraints of 204.3 to 202.6, 191.9, and 139.3 to 137.1 Ma for Cal-1, Cal-2, and Cal-3, respectively. These new ages suggest that the Shuiyindong gold deposit formed in the late Triassic and was overprinted by hydrothermal events in the early Jurassic and early Cretaceous. Given the association of Cal-3 with orpiment and realgar, and previous geochronologic studies of several other major gold deposits in the Golden Triangle, we infer that the latest stage of calcite may be associated with an early Cretaceous regional gold metallogenic event. Combined with existing isotopic ages in the region, these new ages lead us to propose that Carlin-type gold deposits in the Golden Triangle formed during two metallogenic episodes in extensional settings, associated with the late Triassic Indochina orogeny and early Cretaceous paleo-Pacific plate subduction. This study shows that the calcite U-Pb method can be used to constrain the timing of Carlin-type gold deposits and successive hydrothermal events.
Abstract Tourmaline is common in magmatic-hydrothermal deposits, and its composition and boron isotope geochemistry have been widely used to fingerprint the source and evolution of hydrothermal fluids and associated metals. However, whether these chemical or boron isotopic compositions or their combinations can be used as vectors for mineral exploration remains to be explored. In this study, we documented the major and trace element compositions and boron isotopic values of tourmaline along a vertical extension (i.e., 510, 830, 1230 m above sea level, a.s.l.) of the newly discovered porphyry Au mineralization in the Hewan feldspar quartz porphyry, Yixingzhai deposit, to shed light on the evolution of the ore-forming fluid, the mechanisms of Au deposition, and potential indicators for Au exploration. Field observations showed that tourmaline in the Hewan porphyry occurred mainly as orbicules or veins and intergrew with Au-bearing pyrite, hydrothermal quartz, and some clay minerals, indicating a magmatic-hydrothermal origin. Tourmaline sampled from 510 m a.s.l. showed δ11B values (–11.5 to –9.3‰) consistent with those of the average continental crust and tourmaline in magmatic systems, which suggests that the ore-forming fluid was most likely exsolved from the host Hewan porphyry. The δ11B values became heavier upward, reaching –9.9 to –1.5‰ at 830 m and –8.0 to +6.8‰ at 1230 m a.s.l. This boron isotopic variation, integrated with increasing Fe, Mg, Na, Ca, Li, Co, and Sr but decreasing Al, U, Th, REE, Zn, and Pb contents of the tourmaline samples from deep to shallow levels, implies that the initial magmatic fluids were gradually mixed with circulating meteoric water that contained materials leached from peripheral Archean metamorphic rocks and Mesoproterozoic marine sedimentary rocks. Considering the spatial distribution of the Au grade of the porphyry, we propose that a suitable mixing proportion of magmatic and meteoric fluids caused Au deposition and accumulation. We note that tourmaline samples collected from the economic Au zones had much lower and more concentrated δ11B (–11.5 to –3.0‰), Co/(Pb+Zn) (<0.01), and Sr/(Pb+Zn) (0.27 to 1.07) values than those in low-grade or barren zones. Coeval plutons and breccia pipes, where tourmaline also occurs, are well developed inside and outside the Yixingzhai Au mine. We suggest that the obtained parameters can potentially be used as proxies for further Au exploration in this region. This study highlights the feasibility of using the chemical and isotopic compositions of tourmaline for mineral exploration.
Abstract Precise constraints on the source and evolution of ore-forming fluids of Carlin-type gold deposits in the Golden Triangle (south China) are of critical importance for a better understanding of the ore genesis and a refined genetic model for gold mineralization. However, constraints on the source of ore fluid components have long been a challenge due to the very fine grained nature of the ore and gangue minerals in the deposits. Here we present He, Ne, and Ar isotope data of fluid inclusion extracts from a variety of ore and gangue minerals (arsenian pyrite, realgar, quartz, calcite, and fluorite) representing the main and late ore stages of three well-characterized major gold deposits (Shuiyindong, Nibao, and Yata) to provide significant new insights into the source and evolution of ore-forming fluids of this important gold province. Measured He isotopes have R/RA ratios ranging from 0.01 to 0.4 that suggest a maximum of 5% mantle helium with an R/RA of 8. The Ne and Ar isotope compositions are broadly comparable to air-saturated water, with a few analyses indicating the presence of an external fluid containing nucleogenic 38Ar and radiogenic 40Ar. Plotted on the 20Ne/4He vs. helium R/RA and 3He/20Ne vs. 4He/20Ne diagrams, the results define two distinct arrays that emanate from a common sedimentary pore fluid or deeply sourced metamorphic fluid end-member containing crustal He. The main ore-stage fluids are interpreted as a mixture of magmatic fluid containing mantle He and sedimentary pore fluid or deeply sourced metamorphic fluid with predominantly crustal He, whereas the late ore-stage fluids are a mixture of sedimentary pore fluid or deeply sourced metamorphic fluid bearing crustal He and shallow meteoric groundwater containing atmospheric He. Results presented here, when combined with independent evidence, support a magmatic origin for the ore-forming fluids. The ascending magmatic fluid mixed with sedimentary pore fluid or deeply sourced metamorphic fluid in the ore stage and subsequently mixed with the meteoric groundwater in the late ore stage, eventually producing the Carlin-type gold deposits in the Golden Triangle.
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