Abstract
The mineralogy and mineral chemistry of carbonates from various hydrothermal deposits, including volcanic-hosted Au-Cu epithermal, “Chilean Manto-type” Cu(-Ag), stratabound Mn, and Ag-Ba vein deposits from Spain and Chile, were investigated. Dolomite-ankerite (±siderite) was found in variable amounts within the epithermal deposits and associated hydrothermal alteration, whereas calcite was found either within barren veins or disseminated within the regional alteration. Calcite is the major gangue phase within the stratabound deposits, which tend to lack dolomite/ankerite and siderite. Carbonates precipitated from hydrothermal ore fluids are typically Mn-rich, up to 3.55 at. % in siderite, 2.27 at. % in dolomite/ankerite, and 1.92 at. % in calcite. In contrast, calcite related to very low-grade metamorphism or regional low-temperature alteration is Mn-poor but sometimes Mg-rich, possibly related to a higher temperature of formation. Chemical zonation was observed in the hydrothermal carbonates, although no unique pattern and chemical evolution was observed. This study suggests that the chemical composition of carbonates, especially the Mn content, could be a useful vector within ore-forming hydrothermal systems, and therefore constitutes a possible tool in geochemical exploration. Furthermore, Mn-poor calcites detected in some deposits are suggested to be linked with a later episode, maybe suggesting a predominance of meteoric waters, being not related to the main ore stage formation, thus avoiding misunderstanding of further isotopic studies.
Highlights
Carbonates are commonly present as part of the gangue in hydrothermal deposits associated with quartz, phyllosilicates, and barite
Only limited works on the composition of carbonates in ore-forming hydrothermal environments are available (e.g., [6,14,15,16,17]). These authors suggest a possible association of an elevated metal contents (Mn) content in carbonates with ore-forming hydrothermal fluids, and Mn has been used as an index of such activity in whole-rock compositions (e.g., [18,19])
Islica deposit show that dolomite/ankerite originated through hydrothermal activity, both characterized by very low Mn contents (0.00–0.08 at. %, Figure 8e,f) both in La Serena and in the orebody and the associated zones of hydrothermal alteration
Summary
We systematically characterize the mineralogy and composition of carbonate associations from contrasting ore-related hydrothermal environments, the corresponding volcanic host rocks, and zones of weak alteration in Cabo de Gata Volcanic Belt in southeastern Spain and the Costal Range of central Chile. These new geochemical data suggest that hydrothermal carbonates related to ore formation can be clearly distinguished from sedimentary marine, metamorphic, and low-temperature carbonates. Routine EPMA (Electron Probe Micro Analyser) chemical analysis of carbonates in ores could help to interpret their geochemical and isotopic composition by confirming the carbonates under investigation are directly related to ore-forming hydrothermal fluids
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