Homestake Mining Company -- McLaughlin Mine
The San Quentin Sinter
San Quentin Sinter continued

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At right is a close-up view of another "geyser pearl" specimen that reveals the internal structure. Each sphere is about 1 mm in diameter, although examples as large as a centimeter have been found at the McLaughlin deposit. In a given specimen, all grains tend to be approximately the same diameter. D. Enderlin photo.
   
At left is a collapse breccia. Where hot springs pools were less turbulent, lily pad-like protrusions of sinter would grow outward across the water. Occasionally, these structures would collapse under their own weight or by external forces, causing them to fragment and settle to the bottom of the pool. In time, they would be cemented together by new sinter. The McLaughlin deposit contained a variety of breccias. Collapse breccia in sinter was distinguished by its clast content consisting only of sinter. In other types of breccia at McLaughlin, a variety of clast lithologies are found.
   
Sinter ranged through a variety of colors, but the most common color was white (as shown at right). In general the sinter was devoid of metal impurities, and so had no pigmenting agents. D. Enderlin photo.
   
Dark sinter such as that shown at left was pigmented by hydrocarbon residue. Vein matter at depth also often displayed resiny shades of brown and tan in response to the presence of hydrocarbons. Dark sinter typically deposited as the first layer after a hydrothermal explosion event.

Prior to the fracturing event, fluids would vertically partition in the water column, with oils rising to the top. The "first flush" of fluids to emerge following hydrofracturing would naturally be elevated in oils. These fluids also tended to be enriched in metals. The bubble-like cavities seen in the center of this specimen are "froth veins." These structures formed when silica precipitated around globules of oil and other fluid. D. Enderlin photo.

   
At left is a close-up image of another froth vein specimen from the San Quentin sinter that reveals tarry and pyrobituminous globules still in place in several of the cavities. Froth veins are not limited to sinter. Similar textures are found at many of the epithermal mercury deposits of the California Coast Ranges. D. Enderlin photo.
   
So how does one distinguish sinter deposits from veins in a system like McLaughlin? Symmetry is the answer. Veins have bilateral growth symmetry, while sinters are unilateral. The image at right shows a vein cross-cutting sinter. Note that the banding in the sinter is convex in only one direction, which is the direction of growth. D. Enderlin photo.
   
As the sinter pile grew progressively thicker, the older layers at the bottom of the pile would be cut by new generations of veins. Each generation of vein was a fracture that fed successively higher layers on the terrace. The image at left shows an opalized cinnabar vein (myrickite) crosscutting sinter. D. Enderlin photo.
   
One of the more peculiar forms observed in the San Quentin sinter was actually a fossil. The thread-like strands on the surface of the sinter sample at right are fossilized thermophyllic filamentous bacteria. The bacterial strands are completely replaced by chalcedony, but they preserve the strand-like forms from the time when they draped across the surface of a terrace, aligning with the direction of flow of the water. No other evidence of fossil life was found in the San Quentin sinter. Photo courtesy J. Farmer, NASA-Ames Research Center.
   
Another example of fossilized filamentous bacteria is shown at left. Although hot springs algae can also produce such forms, the temperature of these springs was too hot to support anything but bacteria near the vents. As a well-preserved record of primitive life, the McLaughlin fossils were studied by exobiologists from the Mars Program in order to visualize what fossilized extraterrestrial primitive life might look like. Photo courtesy J. Farmer, NASA-Ames Research Center.
   
One has to give credit to the bacteria for their tenacity! Not only were fossilized remnants found at the surface, but they were also identified in vein matter over 500 feet beneath the San Quentin. The photo at right shows fossil filamentous bacteria coating a quartz vein. The sample was collected from the sheeted vein zone near the ultimate bottom of the South Pit. D. Enderlin photo.

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