Most Northerly Zinc Mine
Sunday, October 17, 2010 7:01:25 PM
The mineralisation was described in a paper published in 1996 by the Geological Survey of Denmark and Greenland (GEUS) freely available online as pdf file
- http://www.geus.dk/publications/review-greenland-96/gsb176p44-49.pdf .
The Citronen Fjord zinc-lead deposit is located in Lower Palaeozoic sediments. The deposit comprises at least five major, massive sulphide mounds that form a 10 km long NW–SE trending lineament. It is noteworthy that this trend is roughly parallel to a Late Palaeozoic – Mesozoic fault zone. Although no evidence of pre-Carboniferous activity along this fault zone has been documented to date, one can speculate that the fault zone is a reactivated Early Palaeozoic structure. The distances between the mounds vary from a few hundred metres to 4 km, and the mounds occur at several different stratigraphic levels within mudstone sequences between three thick debris flow conglomerate units.
The individual sulphide mounds are up to 1500 m long and 600 m wide, and have a maximum thickness of about 25 m. The central parts of the mounds consist of sulphides (pyrite, sphalerite and galena) and carbonates (calcite, dolomite and ankerite). Minor amounts of chalcopyrite and quartz have also been observed. The sulphide content varies from about 25 % to nearly 100 %. The sulphides in the mounds are confined to beds that are normally 30–60 cm thick, but are locally up to c. 2 m thick. Interlayered with the sulphide beds are irregular mudstone beds from a few millimetres to 10 cm in thickness.
The deposit is interpreted to be of the sedimentary-exhalative type. Sedimentary exhalative deposits (abbreviated as SEDEX from SEDimentary EXhalative) are ore deposits which are interpreted to have been formed by release of ore-bearing hydrothermal fluids into a water reservoir (usually the ocean), resulting in the precipitation of stratiform ore. SEDEX deposits are the most important source of zinc and lead.
Massive sulphide deposits of this sort are currently forming in the deep ocean around submarine volcanic arcs, where hydrothermal vents exhale sulphide-rich mineralising fluids into the ocean. Massive sulphide deposits are laterally extensive and are comprised of a central vent mound around the area where the hydrothermal circulation exits, with a wide apron of unconsolidated sulphide silt or ooze which precipitates upon the seafloor. Black smoker chimneys are common known discharge zones but other sorts of seepage mounds are of course possible.