Black Sand and Gold Flakes: Where a River Hides Its Minerals

Cleanup Day on the Alder River

1Every October, the Alder River Riverkeepers spend a Saturday pulling tires, cans and fishing line out of the river below the mountains. This year, seventh grader Lucan Tremblay noticed thin streaks of black sand on a gravel bar. Dr. Wanjiru Castellane, a geologist who volunteers with the group, said the streaks were made of heavy mineral grains, mostly magnetite, a dark iron mineral found in small amounts in many kinds of rock. She showed the volunteers how to swirl gravel and water in a pan so that light grains wash over the edge and heavy grains stay at the bottom.

2Dr. Castellane explained where the minerals come from. Upstream, Quartz Creek drains a mountain area of granite, which formed when magma cooled deep underground. As the magma cooled, very hot water carrying dissolved minerals rose through cracks in the rock around it. The water cooled and left white quartz veins in the cracks, and some of the veins hold small amounts of gold. Over millions of years, the mountains were pushed up and worn down, and rain, frost and floods broke the veins and the granite into pieces that streams carry away.

3The volunteers panned 10 L of gravel at each of six sites (Table 1). Sites 1 to 5 are below the place where Quartz Creek flows into the Alder River; Site 6 is on the Alder River above that point. In a bend, the water flows fast along the outside and slowly along the inside. Behind a large boulder, water swirls slowly in a small pool.

4Farther downstream, the valley widens. A gravel company digs coarse gravel from an old river channel near the mouth of the valley, and a brick maker digs fine clay from a filled-in oxbow lake, a curved pond left behind when the river changed course. In the 1860s, miners dug gold from the gravel bars along this river for about 10 years. Dr. Castellane estimates that the bars had taken about 10,000 years to build up.