GEOLOGY 101

Banded iron formation-hosted gold deposits, Part I

Banded iron formation-hosted gold deposits consist of gold intergrown with quartz and/or sulphide minerals in deformed and structurally complicated iron-rich sedimentary rocks. In general, most geologists would define these deposits as a variety of the mesothermal lode gold type.

These deposits mainly occur within Archean-aged (more than 2,600 Ma, or million years old) greenstone belts, though some are Early Proterozoic (ca.

2,100 Ma). Greenstone belts are linear volcanic and sedimentary centres that are engulfed and completely surrounded by granitic-gneissic basement rocks; these belts are typical of the shield areas of northern Ontario and Quebec and the Northwest Territories. The banded iron formation (BIF) host rocks are thinly layered (layers can be measured in centimetres) sedimentary rocks with alternating iron-rich and cherty (silicious) layers; the BIFs can have considerable lateral extents.

There are different types of BIFs, defined on the basis of the mineralogy of the iron-rich layers: if the iron-rich layer is dominantly

magnetite-hematite, then the BIF is termed oxide facies (a sedimentary term meaning a distinctive group of characteristics that distinguish one sedimentary unit from another); if the layer is composed of pyrite and/or pyrrhotite (iron sulphides), then the BIF is called sulphide facies. There are also carbonate- and silicate-facies BIFs.

All BIF’s are classified as chemical sediments, which means that they formed through chemical precipitation from seawater on the sea floor. Other sedimentary textures in the BIFs suggest deposition in shallow water on submarine continental shelves.

Gold occurs as native (free) gold intergrown with pyrite and/or pyrrhotite; arsenopyrite and/or magnetite are also present in some deposits. Other accessory and trace minerals are similar to those found in mesothermal lode gold deposits, such as sphalerite, chalcopyrite, tetrahedrite, scheelite, and molybdenite. Mineralogy of the host-rock alteration is predicated upon the fact the rocks are iron-rich. In the case of oxide-facies BIF, primary hematite-magnetite is replaced by pyrite-pyrrhotite with minor siderite (iron carbonate). Quartz, in the form of crosscutting veins, is also a common alteration mineral and, most typically, the gold is intergrown with sulphides in the quartz veins. Chlorite is a common alteration product of silicate minerals here.

Most generally, BIF-hosted gold deposits are thought to form by the reaction of auriferous and sulphur-bearing hydrothermal fluids with the iron oxide (or sulphide) in country rocks, causing precipitation of gold and sulphides.

The gold is present in quartz veins or the immediate wallrock, wherein the precipitation reactions occur. As such, the deposits are said to be stratabound (i.e., the gold is contained within a single stratigraphic unit, but the mineralization can cut across the layering in the unit) because the specific chemical horizon responsible for gold precipitation is represented by a single sedimentary horizon. Access to the favorable chemical environments of the BIF for the hydrothermal fluids was provided by large-scale fault and shear systems in a manner similar to that visualized in mesothermal lode gold models.

There is debate as to the origin of a few BIF-hosted gold deposits. Some geologists suggest that gold actually precipitated with the original chemical sedimentary host rocks as sort of a submarine hot-spring that exhaled onto the sea floor. In this model, subsequent deformation of the gold-enriched BIF led to the local remobilization and secondary concentration of gold in highly deformed zones. In other words, the gold was originally precipitated at above-normal concentrations in the BIF but was concentrated up to ore grade with deformation of the BIF. In this case, the gold in the BIF would be classified as stratiform (truly bedded and related to the deposition of the host unit), based on its original pre-deformation form.

— The author is a professor of geology at Memorial University in St.

John’s, Nfld.