Analysis of a Black Smoker, Sea Floor, Hydrothermal Vent

Ancient volcanogenic massive sulphide (VMS) deposits are an important source of minerals in the world today. Understanding how these deposits have formed is an important step to finding new deposits. Unfortunately, the original structure of these deposits has been lost due to millions of years of metamorphism and tectonic upheaval. However, it is known that these deposits originally formed through hydrothermal activity at the sea floor, and therefore, by studying modern, sea floor, hydrothermal vents, we can learn about the original ore forming processes of the ancient VMS deposits. 

This figure shows EBSD results, at low and high magnification, from the region indicated by the red rectangle in the last figure. The grey images (a and b) are pattern quality maps, the blue images (c and d) are phase maps, and the coloured images (e and f) are orientation inverse pole figure (IPF) maps (IPF Z). The pattern quality maps (a and b) show the grain structure of the sulphide phase from the chimney wall. The structure is a duplex structure, consisting of large grained regions and small grained regions. This structure is even more apparent in the orientation maps of (e) and (f). The phase maps of (c) and (d) indicate the presence of 4 phases, sphalerite, wurtzite, chalcopyrite and pyrite. Sphalerite and wurtzite have identical chemistries (both are nominally ZnS), but sphalerite has a cubic crystal structure and wurtzite is hexagonal. Therefore, these phases can be differentiated by EBSD, but not by EPMA or EDS. This is a powerful reason why these two techniques are frequently used in tandem to characterise a sample. The wurtzite, seen as red laths in (c) and (d) seems to be zoned. It seems to be concentrated at the edge of the globules and at other regions within the globules. There is also evidence of a depleted zone within the globules as shown in (d). For a detailed explanation of how the wurtzite and sphalerite form in the chimney wall see Reference 1.

In addition to the zoning of wurtzite, this region of the chimney wall is chemically zoned. This can be seen in the EPMA results for this region shown in the figure. (a) and (b) are backscattered electron images at the same low and high magnifications used in the previous figure. Chemical zoning is evident in these images, and these zones are indicated in (b). (c), (d) and (e) are all EPMA elemental maps for Pb, As and Fe, respectively. The chemical zoning is most evident in these maps. Zones enriched and depleted in Pb, As and Fe are all shown in these maps, and these zones correspond to the zones marked with dotted lines in (b). They suggest subtle changes in the composition of the hydrothermal fluid during the growth history of the chimney wall.

(a) and (b) show chemical line profiles across the yellow line in the previous figure. The globule boundaries show enrichment of Pb and As, as shown in (b). The globule boundaries also show a depletion of Fe in these regions. These chemical variations are attributed to solute redistribution during crystallisation of the sphalerite chimney wall.