Reading about turbidity, I came across this web page with the frightening image shown below.
Text immediately before reads:
Reasons for Natural Variation
Algal turbidity varies seasonally and with depth in a complex manner as discussed previously in response to physical, chemical and biological changes in the lake. Inorganic and detrital particles from the watershed vary largely in response to hydrological events such as storms and snowmelt.
The figure caption reads:
Even relatively small amounts of wave action can erode exposed lakeshore sediments, in this case a mine pit lake from northeastern Minnesota. Can you guess what mineral was mined here?
Any ideas? I know there are bright red oxides of heavy metals like lead and mercury, certainly hope that's not the right answer.