Be careful if sampling, some of the blue green algae can be hazardous or toxic.  Personal protective gear may be needed and best to avoid contact until you know what it is.  Blue green algae can fix their own oxygen and CO2, so if the temperature and alkalinity/pH systems are out of balance, green algae cannot live and blue green develop.  I dont remember explanation for specific color, but you can look that up on the internet.  Also I dont know if they are using mercury or other chemicals to concentrate the gold, but if they are using hazardous materials, they may have altered the conditions for algae.  If looks like this is settling ponds, so the water is not being circulated or diluted, so the temperatures and nutrients are elevated, and open to sunlight also encourages algae growth.  Please dont let them dump this in the river, it will probably eventually go away.  If you test water quality for alkalinity and pH, if your alkalinity is low and/or pH high, blue green algae may invade.  It has been a long time since I studied this, so best to consult an expert and make sure you have the proper permissions if treating.  Hopefully this will help.  

http://www.advancedaquarist.com/2002/5/chemistry

http://www.plantphysiol.org/content/67/5/917.full.pdf

http://www.isws.illinois.edu/pubdoc/MP/ISWSMP-111.pdf

Simon:

Just examine the colored water under the microscope and you would possibly know which alga is causing bloom and coloration.

Best

Syed

Can you describe the appearance of the bloom: are individual colonies visible to the naked eye (even if very small), or is the water simply generally green?

Hi,

I also believe that the green colour is caused by by algae.Check if the colour remains in filtered water.

They might get nitrogen from cyanide (if that is used for extraction of the gold), which is quite easily hydrolysed and oxidized to nitrate. If its a small-scale mining Hg is more likely. Also blasting results in nitrogen compounds e.g. nitrate.

Good luck