An official website of the United States government
Abstract Tellurium-rich (Te) adularia-sericite epithermal Au-Ag deposits are an important current and future source of precious and critical metals. However, the source and evolution of ore-forming fluids in these deposits are masked by traditional bulk analysis of quartz oxygen isotope ratios that homogenize fine-scale textures and growth zones. To advance understanding of the source of Te and precious metals, herein, we use petrographic and cathodoluminescence (CL) images of such textures and growth zones to guide high spatial resolution secondary ion mass spectroscopy (SIMS) oxygen isotope analyses (10 µm spot) and spatially correlated fluid inclusion microthermometric measurements on successive quartz bands in contemporary Te-rich and Te-poor adularia-sericite (-quartz) epithermal Au-Ag vein deposits in northeastern China. The results show that large positive oxygen isotope shifts from –7.1 to +7.7‰ in quartz rims are followed by precipitation of Au-Ag telluride minerals in the Te-rich deposit, whereas small oxygen isotope shifts of only 4‰ (–2.2 to +1.6‰) were detected in quartz associated with Au-Ag minerals in the Te-poor deposits. Moreover, fluid-inclusion homogenization temperatures are higher in comb quartz rims (avg. 266.4 to 277.5 °C) followed by Au-Ag telluride minerals than in previous stages (~250 °C) in the Te-rich deposit. The Te-poor deposit has a consistent temperature (~245 °C) in quartz that pre- and postdates Au-Ag minerals. Together, the coupled increase in oxygen isotope ratios and homogenization temperatures followed by precipitation of Au-Ag tellurides strongly supports that inputs of magmatic fluid containing Au, Ag, and Te into barren meteoric water-dominated flow systems are critical to the formation of Te-rich adularia-sericite epithermal Au-Ag deposits. In contrast, Te-poor adularia-sericite epithermal Au-Ag deposits show little or no oxygen isotope or fluid-inclusion evidence for inputs of magmatic fluid.
more »
« less
This content will become publicly available on December 26, 2025
Coupled antimony and sulfur isotopic composition of stibnite as a window to the origin of Sb mineralization in epithermal systems (examples from the Kremnica and Zlatá Baňa deposits, Slovakia)
Abstract Stibnite is a relatively common mineral in epithermal deposits, with little known about Sb transport and efficient stibnite precipitation. The famous Kremnica Au-Ag low-sulfidation deposit and Zlatá Baňa intermediate-sulfidation Pb-Zn-Cu-Au-Ag-Sb deposit are hosted in two different Neogene volcanic fields in Western Carpathians, Slovakia. In both deposits, stibnite-rich veins occur outside of major vein structures, accompanied by illite, illite/smectite, and kaolinite alteration, and affiliated to late-stage fluids (< 2 wt% NaCl eq., < 150 °C). Sulfur isotopic composition of stibnite and sulfides is different at both deposits, likely due to a different magmatic-hydrothermal evolution of the parental magmatic chambers in the Central and Eastern Slovak Volcanic Fields. The Sb isotopes (δ123Sb), however, show similar values and trends of gradual simultaneous increase with δ34S values, explained by a progressive precipitation of stibnite and its fractionation with the fluid. The data were modeled by two coupled Rayleigh fractionation models, (for Sb and for S), assuming a predominant Sb transport in HSb2S4–with a variable amount of S species. Higher molality ratio mS/mSbof fluids was found in Kremnica (~ 3–4) than in Zlatá Baňa (~ 2). At both deposits, the heaviest δ123Sb values are accompanied by a decrease in the δ34S values probably due to the commencement of pyrite/marcasite precipitation. According to thermodynamic models of solubility of Sb(III) complexes and observations from active geothermal fields, stibnite precipitation was triggered by temperature decrease accompanied by mixing with a mildly acidic fluid (pH 4–5) of a steam-heated CO2-rich condensate on margins and in the final stages of epithermal systems. The proposed model for the origin of stibnite-bearing veins in epithermal systems can be used for their better targeting and efficient mineral exploration.
more »
« less
- Award ID(s):
- 2233426
- PAR ID:
- 10591162
- Publisher / Repository:
- Springer Nature
- Date Published:
- Journal Name:
- Mineralium Deposita
- ISSN:
- 0026-4598
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract High-grade ores in low-sulfidation epithermal precious metal deposits include banded quartz veins that contain gold dendrites. The processes by which dendrite growth takes place have been subject to debate for decades, especially given that these deposits are known to form from dilute thermal liquids that contain only trace amounts of gold. It is shown here that growth of gold dendrites in epithermal veins at the McLaughlin deposit in California (western USA) originally took place within bands of gel-like noncrystalline silica. The gel provided a framework for the delicate dendrites to form. The high permeability of the gel allowed the diffusion and advection of gold from the thermal liquids flowing across the top of the silica layers to the sites of crystal growth within the gel. Over time, the gel hardened to form opal-AG. This silica phase is thermodynamically unstable and recrystallized to quartz that has a distinct mosaic texture.more » « less
-
This chapter discusses how radiogenic and stable isotopes can be used in the study of metallic mineral deposits. Although the chapter is mostly focused on the radiogenic (Pb, Os) and heavy stable (Fe, Cu, Zn) isotopes of metallic elements, we complement the discussion highlighting also the power of stable isotopes of light elements, which are major to significant components of hydrothermal fluids and rocks (e.g., H, B, C, N, O, S), as well as of radiogenic isotopes of elements (Sr, Nd, Hf ) that are useful in tracing fluid/magma sources and their interaction with the host rocks. In the first part of this chapter we discuss general aspects of isotopes clarifying the differences between stable non-radiogenic and stable radiogenic isotopes and, consequently, their different applicability to metallogenic studies. Due to their properties, stable non-radiogenic isotopes record mass-dependent fractionation that occur in many reactions associated with the formation of mineral deposits. Mass-dependent fractionation of stable non-radiogenic isotopes occurs both under equilibrium and non-equilibrium (kinetic) conditions of the reactions leading to ore mineral deposition and is controlled by various physico-chemical parameters, like, among the principal ones, temperature, oxygen fugacity, and biological activity. Therefore, stable non-radiogenic isotopes can inform us about the physico-chemical and, eventually, biological processes that control ore mineral deposition and also on the sources of some metals (e.g., transition metal isotopes of elements like Fe, Cu, Zn) or of the fluids (e.g., H, C, O, N, S isotopes) and even of metal ligands (e.g., S, Cl). We conclude the first part of the chapter providing some hints on the strategy of sampling and on the instrumentation related to isotopic studies. In the second part we discuss radioactive-radiogenic isotope systems and their applications in metallogenic studies of metallic mineral deposits. Stable radiogenic isotopes are characterized by relative variations that are controlled, in each geological system, by the addition of a radiogenic component of an isotope, derived from the decay of a radioactive parent, to the same radiogenic isotope already present in the Earth since its formation 4.55 Gyr ago. This relative variation is usually expressed as the ratio of a radiogenic isotope of an element to a non-radiogenic isotope of the same element. The ratio of these two isotopes has increased since the Earth formation and the magnitude of its variations depends on the radioactive/ radiogenic isotope ratios in different geological systems and on the time elapsed since the system has formed. The Earth is 4.55 Gyr old and has evolved from an initially homogeneous isotopic composition to reservoirs (e.g., mantle, crust) and crustal rocks with very variable radioactive/radiogenic isotope ratios due to magmatic, metamorphic, weathering, atmospheric and biologic processes, among others. This has resulted in extremely large variations of radiogenic isotopes in rocks and reservoirs of the Earth which can track various geological processes. In ore geology, stable radiogenic isotopes are best suited for tracing metal (e.g., Pb, Os) sources from different rocks and reservoirs (e.g., mantle, upper crust, lower crust), fluid-rock interactions (i.e., the hydrothermal plumbing system), or magma-host rock interactions (e.g., host rock assimilation by magmas associated with magmatic-hydrothermal deposits). Radioactive-radiogenic isotope systems allow us to determine also absolute ages of suitable minerals that are found in mineral deposits. This is an essential information in metallogeny that allows us to link the formation of a mineral deposit to a specific geological process and/or to specific periods of the Earth’s history. We discuss various dating methods that are extensively applied to date mineral deposits. These methods can be subdivided into those that allow a direct dating of ore minerals (e.g., RedOs dating of molybdenite, UdPb dating of cassiterite) and those that allow dating of minerals that are demonstrably related with the mineralization (e.g., UdPb dating of zircon from magmatic rocks associated with magmatic-hydrothermal deposits; Ar/Ar dating of K-bearing minerals resulting from alteration associated with various types of mineral deposits). We discuss pros and cons of using these various methods and also mention methods that are less used (because potentially less accurate and precise), but sometimes represent the only possibility to provide an age to deposit types that are notoriously difficult to date (e.g., MVT and Carlin-type deposits). We highlight the power of both stable radiogenic and non-radiogenic isotopes in unravelling the genesis of metallic mineral deposits through a series of conceptual and real examples applied to a broad range of mineral deposit types such as porphyry systems (i.e., porphyry deposits, high- and intermediate-sulfidation epithermal deposits, skarn, carbonate replacement deposits, sediment-hosted Au deposits), low-sulfidation epithermal deposits, IOCG deposits, ortho-magmatic deposits, volcanic-hosted massive sulfide deposits (VHMS), sediment-hosted deposits (stratiform copper, MVT), and supergene deposits. In the third part of the chapter, we discuss the use of transition metal stable non-radiogenic isotopes to mineral deposits. Although in its infancy, the application of transition metal isotopes to mineral deposit investigation is quickly growing because these isotopes allow us to address different aspects of the formation of mineral deposits compared to radiogenic isotopes. In particular, isotopes of transition metals (like stable isotopes of light elements) undergo mass-dependent fractionation processes that may be associated with different types of equilibrium and non-equilibrium chemical, physical and biological reactions occurring during the formation of mineral deposits. We focus on the applications of the isotopes of Cu, Fe and Zn to various deposit types, because isotopes of these transition metals are those that have been most extensively used in mineral deposit studies. Mass-independent fractionation may also occur for isotopes of some elements and could be a developing field that has not yet been extensively explored in the study of mineral deposits.more » « less
-
Lode gold deposits, which are currently the world’s major gold supply, have been shown to be generated mostly by phase separation of metamorphic fluids and/or interaction between these fluids and wall rocks. Here we use garnet oxygen isotopes by secondary ion mass spectrometry to document the crucial role of magmatic hydrothermal fluids and their mixing with meteoric water in the formation of the world-class Dongping gold deposit in the North China Craton. Garnet grains from quartz veins of various paragenetic stages and the mineralized alteration envelope at Dongping have dynamic δ 18 O variations of 3.8 to −11.0‰, with large intragrain fluctuations up to 5.3‰. These values correspond to calculated δ 18 O values of 6.1 to −9.1‰ for the hydrothermal fluids from which the garnet formed. The isotope data, notably the cyclic alternation in δ 18 O within individual garnet grains, are best interpreted to reflect multiple pulses of magmatically derived fluids and subsequent mixing of each pulse with variable amounts of meteoric water. The results presented here allow us to quantify the significant interplay between magmatic hydrothermal fluids and meteoric water that spanned the entire mineralization history and triggered gold deposition of a lode gold deposit. This study highlights the potential use of in situ oxygen isotope analysis of garnet in tracing the origin and evolution of hydrothermal fluids in the Earth’s crust that may have formed other giant ore deposits.more » « less
-
Abstract Sulfur belongs among H2O, CO2, and Cl as one of the key volatiles in Earth’s chemical cycles. High oxygen fugacity, sulfur concentration, and δ34S values in volcanic arc rocks have been attributed to significant sulfate addition by slab fluids. However, sulfur speciation, flux, and isotope composition in slab-dehydrated fluids remain unclear. Here, we use high-pressure rocks and enclosed veins to provide direct constraints on subduction zone sulfur recycling for a typical oceanic lithosphere. Textural and thermodynamic evidence indicates the predominance of reduced sulfur species in slab fluids; those derived from metasediments, altered oceanic crust, and serpentinite have δ34S values of approximately −8‰, −1‰, and +8‰, respectively. Mass-balance calculations demonstrate that 6.4% (up to 20% maximum) of total subducted sulfur is released between 30–230 km depth, and the predominant sulfur loss takes place at 70–100 km with a net δ34S composition of −2.5 ± 3‰. We conclude that modest slab-to-wedge sulfur transport occurs, but that slab-derived fluids provide negligible sulfate to oxidize the sub-arc mantle and cannot deliver34S-enriched sulfur to produce the positive δ34S signature in arc settings. Most sulfur has negative δ34S and is subducted into the deep mantle, which could cause a long-term increase in the δ34S of Earth surface reservoirs.more » « less
