Where's Our Gold? Black sand beaches and gold
Written February 23, 2012, published in March 2014
Those who visit the beaches from Leadbetter Point to Cape Disappointment probably know that southern beaches are darkest colored in winter. Benson Beach is the darkest of all, often with no light-colored sand, particularly at the north end. Black sand beaches around the world often have gold deposits, and if so, where’s the gold on this beach?
|Beard's Hollow, north of North Head, is a good place to see black sands any time of year,. In this photo, the dark sands are interspersed with lighter quartz and feldspar sands in the foreground. Photo by Kathleen Sayce|
Black sand beaches are typically made from basalt, either from fresh lava, ground by the ocean into fine bits, which are common on Hawaiian beaches, or eroded out of hard rock by water and carried downstream in rivers. Black sands are of particular interest to miners because they often contain important minerals and elements, including iron, gold, platinum and titanium, and as such are called placers.
Gold has been noted in black sands along the Columbia River from northeast Washington all the way downriver to the coast, and on the ocean beaches. Several river beaches became placer mines. The first mention of gold in black sands at Cape Disappointment was in a Coast Survey report to Congress in 1858. The amounts seen were not sufficient to support gold mining, the report noted. Profitable mining is based on finding high concentrations of gold and separating it in a cost-effective manner from the surrounding non-gold materials.
|Sands sort with wind and water. In this close up, approximately 2 feet across, you can see black bars of heavy black sands, brownish feldspars in the upper left, and lighter quartz sands throughout the image. Photo by Kathleen Sayce|
Water sorts minerals out by weight to make placer deposits; in geo-speak this is called gravity separation. You can often see gravity separation on the beach as the tide recedes in the summer. Mineral grains of different weight sort out with every wave, into black, brown, greenish, reflective light brown and whitish layers. Gold is about six times as heavy as quartz, the lightest element; it settles out first. The magnetic black layers are the heaviest, twice as heavy as quartz, and drop out next; they have heavy elements, including iron, manganese and titanium. The whitish and brown layers are lighter and drop out last as the water recedes; they contain lightweight silica minerals like quartz and feldspar, which are the most common minerals in our beach sands. There’s also some mica, very light, which makes the beach glitter.
Valuable placer minerals erode out of hard rocks, including basalt, granite and metamorphic rocks. Sands on the ocean beaches in Washington and Oregon were analyzed for their component minerals, in part to help determine where the beach sands come from, and also to help determine if there might be economically valuable deposits of minerals. From a book by Paul Komar, The Pacific Northwest Coast, 1998, comes a description of beach sand grains around the Columbia entrance: clear quartz, green and brown feldspar, light brown biotite, dark hypersthene (which includes black magnetite and ilmenite), dark green augite, light brown enstatite, white zircon and clear to light pink garnet. Magnetite and ilmenite minerals can contain gold or titanium along with iron, manganese and magnesium.
Placers accumulate in locations where the heaviest sands drop out easily. These include river edges at or below low water, river mouths and deltas, coast beaches, and offshore. Where placers form on beaches, surf picks up sand grains on the benthic surface and deposits them high in the surf zone. The black sands are generally too heavy to blow around in the wind. Water does move them, though it has to be moving fast to keep sand in suspension. Storms, floods and tsunamis move around massive amounts of sand.
Tsunamis come immediately after local subduction zone earthquakes, and flood uplands with ocean sands. The erosions that follows pulls light sands off local beaches and leaves behind heavier minerals in a large-scale gravity-separation process. In geologic time, local earthquakes generated in the Cascadia subduction zone have been followed by hundreds of feet of beach erosion before the shoreline stabilizes and a new outer dune rebuilds. This has been well documented by students of Curt Peterson, Portland State University, and others.
In the months following earthquakes the surf carries sands back onshore to form a new dune. Placers are buried at the bottom of this new dune. More geo-speak: placers are called lags or lag deposits when they are placed at the base of dunes. As with the sorting at wave edges, lighter sands move more easily in wind and water, and are re-sorted and placed higher. Black sands end up being concentrated at the dune base. The tsunami-derived placers under our old dunes are several feet thick. These iron-rich placers often give a distinctive orange tinge and iron taste to water from shallow wells pumping water from this layer. Some gold is at the bottom of dunes in lag deposits.
Sea level also determines where gold goes. During the past 1.9 million years of the Pleistocene Epoch, sea level was as much as 350 ft lower than today. The Columbia River and other local rivers carried sediments past the local area and out to the edge of the continental shelf in river channels. This Pleistocene gold is largely in the deep ocean today, or well buried in those river channels at historic low sea levels levels, and covered by younger sediments.
When the great floods from the Glacial Lakes in the Rockies occurred, sea level was still so low, 300 to 200 feet below today’s level, that those floods roared past in the Columbia River Valley, and out the Astoria Canyon. This ‘glacial floods’ gold is also in the deep ocean and at the lower end of the continental shelf in the Astoria canyon and alluvial fan.
Today, ocean currents spread sands from the Columbia northwest across the continental shelf. Surf carries some sands east to the beaches, constantly reworking and sorting the fine to heavy grains. Only the heaviest surf can move the heaviest sands, so most of the heavy grains stay behind in deeper waters. This gold is on the continental shelf, spread northwest of the Columbia River. If there are deposits worthy of mining, this is likely where they will be, in the ocean northwest of the entrance. Mining is possible, once concentrated deposits are located, but extraction damages fish and crab habitat. When damaged, it takes years to recover natural productivity on the benthic surface.
River water slows as it reaches the ocean, dropping most of its sand at the Columbia River Entrance. Historically, Benson Beach was on the main channel, and received considerable black sand from the river as it wrapped around Cape Disappointment. This beach and buried sands at depth around Baker’s Bay probably have more gold than any other beach in this area, but still not enough to justify mining. There are too many non-valuable minerals mixed in with it.
One of Paul Komar’s graduate students sampled sands along the beaches from Seaside to Leadbetter, and noted another black sand concentration at Leadbetter Point. He proposed that currents from Willapa Bay helped stop longshore movement of sand, and re-concentrated black sands in the Willapa Entrance. Lighter sands made it across and went on north; heavy sands stayed at the Point.
With the main channel pushed south early in the twentieth century, most black sands from the Columbia River today are deposited around that channel and Clatsop spit. New black sand deposits are forming today on the south side of the river. Channel dredging shifts a little modern river gold offshore with every load, creating small placers in the dredge disposal areas.
This is where our gold is: scattered all over, under dunes, at the Columbia and Willapa Entrances, in the ocean, in deeply buried sands along rivers and in channels. Unfortunately, it’s not up on the beach where it’s easy to find and remove.