Characterization of the acidic cold seep emplaced jarositic Golden Deposit, NWT, Canada, as an analogue for jarosite deposition on Mars

摘要

Surficial deposits of the OH-bearing iron sulfate mineral jarosite have been observed in several places on\udMars, such as Meridiani Planum and Mawrth Vallis. The specific depositional conditions and mechanisms\udare not known, but by comparing martian sites to analogous locations on Earth, the conditions of formation\udand, thus, the martian depositional paleoenvironments may be postulated. Located in a cold semi-arid desert\ud100 km east of Norman Wells, Northwest Territories, Canada, the Golden Deposit (GD) is visible from\udthe air as a brilliant golden-yellow patch of unvegetated soil, approximately 140 m 50 m. The GD is\udunderlain by permafrost and consists of yellow sediment, which is precipitating from seeps of acidic,\udiron-bearing groundwater. On the surface, the GD appears as a patchwork of raised polygons, with acidic\udwaters flowing from seeps in troughs between polygonal islands. Although UV–Vis–NIR spectral analysis\uddetects only jarosite, mineralogy, as determined by X-ray diffraction and inductively coupled plasma emission\udspectrometry, is predominantly natrojarosite and jarosite, with hydronium jarosite, goethite, quartz,\udclays, and small amounts of hematite. Water pH varies significantly over short distances depending on\udproximity to acid seeps, from 2.3 directly above seeps, to 5.7 several m downstream from seeps within\udthe deposit, and up to 6.5 in ponds proximal to the deposit. Visual observations of microbial filament communities\udand phospholipid fatty acid analyses confirm that the GD is capable of supporting life for at least\udpart of the year. Jarosite-bearing sediments extend beneath vegetation up to 70 m out from the deposit and\udare mixed with plant debris and minerals presumably weathered from bedrock and glacial till. This site is of\udparticular interest because mineralogy (natrojarosite, jarosite, hematite, and goethite) and environmental\udconditions (permafrost and arid conditions) at the time of deposition are conceivably analogous to jarosite\uddeposits on Mars. Most terrestrial analogues for Mars jarosites have been identified in temperate environments,\udwhere evaporation rates are very high and jarosites form along with other sulfates due to rapid\udevaporation (e.g. Rio Tinto, Spain; Western Australian acidic saline lake deposits). The GD is a rare example\udof an analogue site where jarosite precipitates under dominant freezing processes similar to those which\udcould have prevailed on early Mars. Thus, the GD offers a new perspective on jarosite deposition by the\udupwelling of acidic waters through permafrost at Meridiani Planum and Mawrth Vallis, Mars. The GD also\uddemonstrates that martian deposits may show considerably more chemical and mineral variability than\udindicated by the current remote sensing data sets.