Homestake Mining Company -- McLaughlin Mine
The San Quentin Sinter
A feature of many hot springs around the world is the terrace. As water cascades down slope from the emerging hot springs, mineral deposits build up through time. Calcareous spring deposits are called "tufa," while siliceous spring deposits are called "sinter." At the McLaughlin mine, the ancient terraces that capped portions of the ore deposit were siliceous, and are appropriately called sinter.

Because of their significance, a pictorial essay of textures found in them is presented below. All specimens are from the McLaughlin San Quentin sinter unless otherwise noted.

The San Quentin sinter at the McLaughlin mine was a prominent feature of the deposit. It rose above the surrounding slopes to form a low hill, and it was extensively "glory holed" in the old days to extract veins of mercury-rich cinnabar and metacinnabar ore that cut through it. The hill got its name sometime in the late Nineteenth Century, because the surface mining work was not unlike hard labor at California's first prison. In the historic photo below, San Quentin Hill appears in the background. In the foreground is the historic settlement of Johntown. Photo from Cal. State Mining Bureau Bulletin 27, 1908.

Below, is the San Quentin Hill as it appeared from the air in 1981. The original Manhattan mercury mine furnace appears near the center of the image. Homestake Mining photo.
Below, is a close-up of the old Manhattan furnace as it appeared around 1908. San Quentin Hill is on the skyline at the right of the photo. From Cal. State Mining Bureau Bulletin 27, 1908.
Hot springs terraces in the sinter occurred at many scales. The "terracettes" at left are in a boulder about 1 meter wide. This and many other examples of the sinter are on display in Homestake's "Stonehenge" rock display at the McLaughlin mine core library. Photo courtesy J. Farmer, NASA-Ames Research Center.
On a smaller scale, peculiar patterns in sinter occurred in a variety of forms. At right are ripple-like structures about 1 cm from crest to crest. D. Enderlin photo.
Unlike veins, which grow from opposing walls of a cavity, sinter forms in only one direction (upward and outward from the surface on which it is depositing). Textures such as the "eggshell" layer pattern shown at left are seen in modern hot springs as well. Sinter can form as leathery mats with gas-filled cavities sometimes intervening. When lithified through time, the cavities are sometimes preserved. Many cavities are coated with crusts of quartz and chalcedony that deposited during later stages of mineralization. D. Enderlin photo.
Hot springs terraces can dry out as they form. As they dehydrate, they can produce mud cracks just like clayey soils do. The photo at right is a view of the top of a mud crack pattern on a sinter boulder. The cracks formed before the sinter lithified, but the cracks were completely preserved as the siliceous mud turned to stone. If you look closely at the photo of eggshell sinter above, you can see vertical structures crosscutting the eggshell layers. These are profile views of the same mudcracks. D. Enderlin photo.
As mud cracks developed on the sinter terraces, they occasionally curled. In extreme cases, they rolled up to form scroll-like shapes. The image at left shows such a "mud scroll" in San Quentin sinter. The structure was first thought to be a plant fragment, but closer inspection revealed its true origin. D. Enderlin photo.
A close-up end view of "mud scrolls" from the San Quentin sinter appears at right. D. Enderlin photo.
At left is another peculiar texture of the hot springs sinter at the McLaughlin mine.  The spherical forms are called "geyser pearls" or "geyser eggs." Like marine oolites, these structures formed as concentric bands of precipitate built up on constantly agitated sand grains. Agitation prevented the grains from cementing to the floor of the hotspring pool as they grew. Each "pearl" contains a nucleus of rock. D. Enderlin photo.
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