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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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Origin of Au-Rich Carbonate-Hosted Replacement Deposits of the Kassandra Mining District, Northern Greece: Evidence for Late Oligocene, Structurally Controlled, and Zoned Hydrothermal Systems
Abstract The Au-rich polymetallic massive sulfide orebodies of the Kassandra mining district belong to the intrusion-related carbonate-hosted replacement deposit class. Marble lenses contained within the Stratoni fault zone host the Madem Lakkos and Mavres Petres deposits at the eastern end of the fault system, where paragenetically early skarn and massive sulfide are spatially associated with late Oligocene aplitic and porphyritic dikes. Skarn transitions into predominant massive and banded replacement sulfide bodies, which are overprinted by a younger assemblage of boulangerite-bearing, quartz-rich sulfide and late quartz-rhodochrosite vein breccias. The latter style of mineralization is most abundant at the Piavitsa prospect at the western end of the exposed fault system. The sulfide orebodies at the Olympias deposit are hosted by marble in association with the Kassandra fault, where textural and mineralogical similarities to the sulfide bodies within the Stratoni fault zone suggest a genetic relationship. Estimated trapping temperatures and pressures based on fluid inclusion data indicate that carbonate replacement mineralization took place at depths less than about 5.9 km. Carbon and oxygen isotope patterns in carbonate from the Stratoni fault zone support isotopic exchange principally through fluid–wall-rock interaction, whereas decarbonation and fluid-rock exchange reactions were important at the Olympias deposit. Carbonate minerals associated with skarn and replacement sulfide throughout the district have isotopic compositions that are consistent with formation from a hydrothermal fluid of magmatic origin. Lower homogenization temperatures and salinities in the younger quartz-rich sulfide assemblage and quartz-rhodochrosite vein breccias, together with low δ18O values of gangue carbonate, suggest dilution of a primary magmatic fluid with meteoric water late in the evolution of the hydrothermal system in both the Olympias area and the Stratoni fault zone. The replacement sulfide orebodies in the district likely inherited their uniform Pb isotope composition from a late Oligocene igneous source and the isotopically heterogeneous metamorphic basement units. Metal distribution patterns at the scale of the Stratoni fault zone show diminishing Cu concentration with decreasing Pb/Zn and Ag/Au ratios from Madem Lakkos to Mavres Petres and the Piavitsa prospect in the west. The sulfide orebodies at the Olympias deposit exhibit elevated Cu values in the east with increasing Pb/Zn and Ag/Au ratios down-plunge to the south-southwest. Metal concentration and ratios support zoning related to temperature and solubility changes with increasing distance from a probable magmatic source. Structural and igneous relationships, together with fluid inclusion microthermometric and carbon-oxygen isotope data and metal distribution patterns, are supportive of a zoned hydrothermal system that exceeded 12 km along the Stratoni fault zone, sourced by an igneous intrusion to the southeast of the Madem Lakkos deposit. The Olympias replacement sulfide orebodies, associated with the Kassandra fault, resulted from a local hydrothermal system that was likely derived from a concealed igneous intrusion to the east of the deposit.
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- Award ID(s):
- 1719875
- PAR ID:
- 10148735
- Date Published:
- Journal Name:
- Economic Geology
- Volume:
- 114
- Issue:
- 7
- ISSN:
- 0361-0128
- Page Range / eLocation ID:
- 1389 to 1414
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Journal Article:
- https://doi.org/10.5382/econgeo.4664
