Wednesday, November 27, 2013

On The Source Of Orogenic Gold

Interesting!... In the Research Focus section of Geology Andrew G. Tomkins writes about  (open access) the current state of understanding on the sources of orogenic gold deposits. These are deposits formed in accretionary and collisional orogens where two tectonic plates are pushing against each other.

On the source of gold in such settings:

There are two plausible sources for the gold: (1) metamorphic rocks, from which fluids are generated as temperatures increase; and (2) felsic-intermediate magmas, which release fluids as they crystallize. Gold-bearing magmatic-hydrothermal deposits are enriched in many elements, including S, Cu, Mo, Sb, Bi, W, Pb, Zn, Te, Hg, As, and Ag (e.g., Goldfarb et al., 2005; Richards, 2009). Such deposits have been referred to as gold-plus deposits (e.g., Phillips, 2013), but most orogenic gold deposits fall into the alternative group of gold-only deposits, and are more enigmatic. These are characterized by elevated S and As, and have only minor enrichments in the other elements. The current dominant opinion is that metamorphic rocks are the source for these deposits (Goldfarb et al., 2005; Phillips and Powell, 2010). 

And on the major gold forming episodes in earth history, wherein several geological situations converged to create conditions suitable for orogenic gold deposition:

The vast majority of orogenic gold (excluding Witwatersrand, South Africa) is from three periods in geologic time: the Neoarchean (ca. 2700-2400 Ma), a second period in the Paleoproterozoic (ca. 2100-1800 Ma), and a third period from ca. 650 Ma continuing throughout the Phanerozoic (Goldfarb et al., 2001). Two explanations have been offered for this timing: (1) because orogenic gold deposit formation requires accretionary tectonics, the major periods of formation coincided with periods of continental growth (Goldfarb et al., 2001), and (2) during the Phanerozoic, increased ocean oxygenation facilitated uptake of gold in biogenic and diagenetic pyrite, which became the gold source during later accretion and metamorphism (Tomkins, 2013). The first explanation must be correct to some extent, but cannot explain the relative lack of gold during the formation of Rodinia; the second requires that gold can be sourced from carbonaceous metasedimentary rocks.

This open access commentary is written as an accompaniment to a paper by Gaboury 2013 in the same issue which identifies ethane C2H6 as a diagnostic geochemical tracer sourced from carbonaceous metasedimentary rocks, a common component in subduction accretionary settings.

In more local news from India, gold mining in the Kolar mines from Karnataka state is set to resume. The mines are located in the 2600 mya greenstone belts, which are composed of greenschist and lower amphibolite facies mafic and felsic volcanic rocks intruded by plutons. These Archean greenstone belts are thought to originate in either ocean spreading centers or island arc settings which later got accreted (plastered) on to continental nuclei during orogeny. The major metamorphic minerals are green colored chlorite and amphiboles, hence the name. Gold occurs in quartz veins and all the geological indicators point to them sourced from magmatic fluids derived predominantly from the crystallization of felsic magmas i.e. source (2) of the orogenic style deposits detailed in the article.

No comments:

Post a Comment