Editorial for special issue “Geochemistry and mineralogy of hydrothermal metallic mineral deposits”

The Special Issue of Minerals on Geochemistry and Mineralogy of Hydrothermal Metallic Mineral Deposits presents the results of diverse geochemical and mineralogical research from across the globe. It is aimed to demonstrate that geochemical and mineralogical variation, both within and among hydrothermal ore deposits can be applied to genetic models, to exploration and drilling programs, and more. The eight contributions reflect a wide range of deposits, as well as different types of geochemical and mineralogical research applied to them. While most of these studies are focused on gaining a better understanding of deposit genesis, the results have a far greater application, as highlighted below

Sources of Hydrothermal Fluids Inferred from Oxygen and Carbon Isotope Composition of Calcite, Keweenaw Peninsula Native Copper District, Michigan, USA

The Mesoproterozoic North American Midcontinent Rift hosts the world’s largest accu-mulation of native copper in Michigan’s Keweenaw Peninsula. During a regional metamorpho-genic‐hydrothermal event, native copper was deposited along with spatially zoned main‐stage minerals in a thermal high. This was followed by deposition of late‐stage minerals including minor copper sulfide. Inferences from the oxygen and carbon isotopic composition of main‐stage hydrothermal fluids, as calculated from 296 new and compiled isotopic measurements on calcite, are consistent with existing models that low‐sulfur saline native copper ore‐forming fluids were domi-nantly derived by burial metamorphic processes from the very low sulfur basalt‐dominated rift fill at depth below the native copper deposits. Co‐variation of oxygen and carbon isotopic compositions are consistent with mixing of metamorphic‐derived fluids with two additional isotopically different fluids. One of these is proposed to be evolved seawater that provided an outside source of salinity. This fluid mixed at depth and participated in the formation of a well‐mixed hybrid metamorphic-dominated ore‐forming fluid. Secondary Ion Mass Spectrometry in‐situ isotopic analyses of calcite demonstrate a high degree of variability within samples that is attributed to variable degrees of shallow mixing of the hybrid ore‐forming fluid with sulfur‐poor, reduced evolved meteoric water in the zone of precipitation. The oxygen and carbon isotopic compositions of 100 new and compiled measurements on late‐stage calcite are mostly isotopically different than the main‐stage hydrothermal fluids. The late‐stage hydrothermal fluids are interpreted as various proportions of mixing of evolved meteoric water, main‐stage hybrid ore‐forming fluid, and shallow, evolved seawater in the relatively shallow zone of precipitation

Changes in geothermal vegetation at Myvatn, Iceland, and comparisons to other geothermal areas

We sampled 207 plots from 15 transects at the Myvatn geothermal area in Iceland in 1985, one year after the September 1984 eruption of Krafla, and again in 1987, and compared the species and community structure to that of other sites we had visited in Japan and New Zealand. We found 5 lichen, 47 bryophyte, and 28 tracheophyte species at Myvatn. Overall Shannon diversity (H’) was high at 4.34, with Brillouin (information-theory-based) species diversity (H’) being very similar at 4.32. The greatest diversity occurred at > 25–35°C surface temperature and the least at > 60°C. Zones are defined mostly by temperature and humidity, with Cyanobacteria closest to the vents in the hottest zones, followed by bryophytes, then lichens, then tracheophytes. Bryum argenteum, Ceratodon purpureus, and Fossombronia sp. 1 (probably immature F. foveolata) occupied the highest temperatures near the vents in 1985. Lichens tended to avoid hot soils with high humidity. Soil chemistry most likely plays a role, but with so many variables and many values at unquantifiable levels, we considered it premature to determine their individual effects. By 1987, Bryum argenteum, a near-vent species, had almost totally disappeared, along with the steam emissions where they had thrived. The geothermal areas in cold climates could serve as refugia for more temperate species that may have existed prior to the Ice Age, or they may simply be suitable habitats for northern extensions of the species. This study gives us a glimpse of potential vegetational changes resulting from climate changes as demonstrated in three regions of the world

Tectonic context of native copper deposits of the North American Midcontinent Rift System

Economic deposits of native copper are an important feature of the Precambrian North American Midcontinent Rift System. The rift was progressively filled by a thick succession of basalt and then red clastic sedimentary rocks from about 1,109 to 1,060 Ma. Regional contraction of the rift at about 1,060 Ma produced faulting, fracturing, folding, and uplift of rocks on the edges of the rift, including transformation of original graben-bounding faults into high-angle reverse faults. Native copper ore deposits are contemporaneous with these faults/fractures that integrated the paleohydrologic system and provided pathways for upward movement and focussing of ore fluids into permeable and porous tops of basalt lava flows and interflow sedimentary rocks. Potential ore fluids were likely generated throughout the tectonic history of the rift by burial of strata and elevated basal heat flow. It is postulated that the coincidence of available burial metamorphic fluids, generated via the thermal pulse related to the rifting event, and faults/fractures, generated via regional Grenvillian contraction, provided the critical component in the genetic model of the native copper deposits