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1. Oxygen isotope evidence for input of magmatic fluids and precipitation of Au-Ag-tellurides in an otherwise ordinary adularia-sericite epithermal system in NE China

   https://doi.org/10.2138/am-2021-7825  

   Gao, Shen; Hofstra, Albert H.; Zou, Xinyu; Valley, John W.; Kitajima, Kouki; Marsh, Erin E.; Lowers, Heather A.; Adams, David T.; Qin, Kezhang; Xu, Hong (December 2021, American Mineralogist)

   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. 

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2. San Albino, Nicaragua: A low-angle, thrust-controlled orogenic gold deposit

   https://doi.org/10.5382/econgeo.5042  

   Lipson, Rael_D; Goldfarb, Richard_J; Frieman, Ben_M; Payne, John (March 2024, Economic Geology)

   Abstract The San Albino deposit is an orogenic gold occurrence hosted by a low-angle thrust that is the site of a new open-pit mine in northern Nicaragua. The deposit is hosted in greenschist facies rocks of the Jurassic metasedimentary Neuvo Segovia Formation. The schist was uplifted and exposed during arc accretion and Cretaceous thin-skin deformation, forming the NE-striking Colon fold-and-thrust belt. Deformation included emplacement of the 119 to 113 Ma NE-trending Dipilto batholith into the regionally metamorphosed clastic rocks about 5 km northwest of the San Albino deposit. Mineralization is dominated by three laminated quartz vein systems (i.e., San Albino, Naranjo, Arras) that broadly follow shallowly dipping (approx. 30°) carbonaceous shears roughly concordant to schistosity along the limbs of a doubly plunging antiform. The three main parallel shears are each separated by about 90 m and individually reach a maximum thickness of about 8 m. Maximum thickness of ore zones is where post-ore local folding and reverse motion along the shallow shears has duplicated the laminated low-angle gold-bearing veins (D2 and early D3). Additional gold was added to the veins, with abundant sulfides, during a subsequent brecciation event of the early formed quartz veins that accompanied progressive thrusting (late D3). This predated boudinage of the veins during continued compression and thrust loading (D4); high gold grades are particularly notable along pyrite- and arsenopyrite-bearing stylolites formed during D4 pressure solution. The D2 to D3 gold event is likely coeval with Albian uplift of the Dipilto batholith and with back thrusting in the schist aided by the stress inhomogeneities provided by the igneous complex. Low-angle thrust-controlled orogenic gold deposits may represent world-class exploration targets because of their large linear footprints, although they are traditionally looked at as less favorable exploration targets relative to gold systems developed more commonly along high-angle reverse faults. Our case study of the San Albino deposit shows that although low-angle deposits are not inherently misoriented for failure like the more common subvertical reverse fault-related deposits, they may be sites of significant pressure buildup due to hydrothermal mineral precipitation during initial water-rock interaction or slight temperature decreases along the low-angle flow path. Resulting fluid cycling may lead to thick laminated vein development, such as seen at San Albino, where especially high-grade zones may be associated with local steepening and/or dilational zones within the broader, low-angle vein-hosting shear system. 

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3. The Role of Quartz Cementation in the Seismic Cycle: A Critical Review

   https://doi.org/10.1029/2021RG000768  

   Williams, Randolph T.; Fagereng, Åke (March 2022, Reviews of Geophysics)

   Abstract Because quartz veins are common in fault zones exhumed from earthquake nucleation temperatures (150°C–350°C), quartz cementation may be an important mechanism of strength recovery between earthquakes. This interpretation requires that cementation occurs within a single interseismic period. We review slip‐related processes that have been argued to allow rapid quartz precipitation in faults, including: advection of silica‐saturated fluids, coseismic pore‐fluid pressure drops, frictional heating, dissolution‐precipitation creep, precipitation of amorphous phases, and variations in fluid and mineral‐surface chemistry. We assess the rate and magnitude of quartz growth that may result from each of the examined mechanisms. We find limitations to the kinetics and mass balance of silica precipitation that emphasize two end‐member regimes. First, the mechanisms we explore, given current kinetic constraints, cannot explain mesoscale fault‐fracture vein networks developing, even incrementally, on interseismic timescales. On the other hand, some mechanisms appear capable, isolated or in combination, of cementing micrometer‐to‐millimeter thick principal slip surfaces in days to years. This does not explain extensive vein networks in fault damage zones, but allows the involvement of quartz cements in fault healing. These end‐members lead us to hypothesize that high flux scenarios, although more important for voluminous hydrothermal mineralization, may be of subsidiary importance to local, diffusive mass transport in low fluid‐flux faults when discussing the mechanical implications of quartz cements. A renewed emphasis on the controls on quartz cementation rates in fault zones will, however, be integral to developing a more complete understanding of strength recovery following earthquake rupture. 

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4. Garnet secondary ion mass spectrometry oxygen isotopes reveal crucial roles of pulsed magmatic fluid and its mixing with meteoric water in lode gold genesis

   https://doi.org/10.1073/pnas.2116380119  

   Fan, Gao-Hua; Li, Jian-Wei; Valley, John W.; Scicchitano, Maria Rosa; Brown, Philip E.; Yang, Jin-Hui; Robinson, Paul T.; Deng, Xiao-Dong; Wu, Ya-Fei; Li, Zhan-Ke; et al (May 2022, Proceedings of the National Academy of Sciences)

   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. 

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5. 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)

   https://doi.org/10.1007/s00126-024-01333-9  

   Koděra, Peter; Mathur, Ryan; Zhai, Degao; Milovský, Rastislav; Bačo, Pavel; Majzlan, Juraj (December 2024, Mineralium Deposita)

   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 m_S/m_Sbof 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. 

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