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.

Thursday, November 21, 2013

Quote: John McPhee On The Left Handed Geologist

I was flipping through John McPhee 's Assembling California and came across this passage:

Gradually, though - outcrop to outcrop, roadcut to roadcut - Moores revived enough related scenes in the distinct origins of the random rock to frame a cohesive chronological story. That is what geologists do. " You spend a lot of time working over rocks and you have a lot of time to do nothing but think," he said. "These mountains , for example are Tertiary normal faulted, confusing topography with regard to structure. They show different levels of structure in different places. To see through the topography and see how the rocks lie in three dimensions beneath the topography is the hardest thing to get across to a student". After a mile of silence he added cryptically, "Left -handed people do it better". 
I said nothing for a while, and then asked him, " Are you left-handed?"
He said. "I am ambidextrous".
As it happens, I am left-handed, but I kept it to myself. 

I am also left handed... True or not, that lefties are disproportionately represented in the geology profession has been the subject of discussion before.. I am so glad that one of the great writers about geology is left handed too..

Thursday, November 14, 2013

What A Porphyry Copper Ore Body Tells Us About How India Was Assembled

Nature Geoscience has some interesting articles on giant magmatic ore deposits  ( 1 , 2 ) with a focus on porphyry copper- molybdenum deposits which occur within magmatic arcs above subduction zones.

Ever since I found that copy of Tyrrell I've been reminiscing a bit about my early days in geology. These papers on copper ores started another chain of thought. We were preparing for our first year M.Sc. field trip which is really supposed to be a tour to learn field mapping. So the area selected is usually one where rock bodies are exposed clearly, have lateral continuity, where relations and contacts between geological units can be observed, basically an area where principles of field mapping are relatively easy to learn. As it happens our department at Pune University had gotten a big grant from ONGC to do a reconnaissance of Gondwana rift basin sediments of Carboniferous-Permian age just north of Itarsi in Madhya Pradesh. Our department chair organized our field trip to this area, reasoning that we could use this for training as well as contribute to the project.

Unfortunately, it was a disaster. The area was thickly forested, rock exposures limited to few stream cuttings and occasional road cuts, just not what you want for a rigorous training in mapping. The one bright spot was the copper mine we visited at Malanjkhand. This is an open pit mine.

Google Interactive Map of Malanjkhand Copper Mines:

View Larger Map

We were allowed to walk right up to the exposed walls of the pit and observed the stringers of copper and molybdenum sulphide ore embedded in networks of quartz veins. The host rock was a granodiorite. It was altered to various clay assemblages but you could make out blobs of relatively unaltered textures. Overall, after two weeks of tramping through forests it was great to be looking at massive walls of rock and glistening ore!

Ok, so what does this copper ore body have to do with ideas of how India was assembled and what does that even mean?

Monday, November 11, 2013

Not Just a Puppet! The Animated Life Of Alfred Russel Wallace

Have you seen this documentary?

The Animated Life Of A.R. Wallace.

Produced by Flora Litchman and Sharon Shattuck and narrated by George Beccaloni of the  Natural History Museum London and Andrew Berry of Harvard University, it celebrates the life and work of Alfred Russell Wallace who along with Charles Darwin discovered the principles of evolution through natural selection in the mid 1800's. Wallace also made pioneering contributions to the field of Biogeography.

A fine example of creative science outreach using paper puppet animation. Beautifully produced and narrated.

Thursday, November 7, 2013

Reliance Cites Geological Surprises In Krishna Godavari Basin

My friend S.C.N Jatar, former CMD Oil India and ONGC Videsh writes about the suspicion that Reliance overstated reservoir potential in the KG -D6 field of the Krishna Godavari basin.

“Geological surprise” is cited as the cause of Reliance’s production shortfall. KG-D6 block commenced production in September 2008 with 0.58 million standard cubic meters per day (mmscmd) reaching a peak of 69.43 in March 2010 and then declined to 13 mmscmd currently. Reliance attributed the decline to substantial variance from prediction in reservoir behaviour, higher than envisaged pressure decline and unpredicted early water production in some wells. Was there a geological surprise? In an article published 10 years ago in Business Standard on January 13, 2003, I wrote, “Producing even 40 mmscmd for 10 years will need an unusually large number of wells....” The latest thinking on such reservoirs is that one can expect unpleasant surprises even after 3D surveys confirm the ‘structure’ because it cannot confirm the ‘reservoir’. I had then stressed: “There is a big question mark over the projected recoverable reserves of the Dhirubhai fields.”

Rest of the article here