Not far away at Little Borax Lake, the group examined a terrestrial maar now part of a local golf course! The lake sediments are rich in borax, a hydrated sodium borate mineral. From 1868 to 1873 about 140 tons of borax were removed from this site. The site was the nation’s major supplier of borax until more accessible deposits were discovered in Death Valley. On the flanks of 300,000 year-old Mt. Konocti southwest of Little Borax Lake, loomed the remains of rockfalls shed off Mt. Buckingham, one of several peaks forming the larger volcanic structure. These large dacite blocks are rubble from columnar-jointed lava that has toppled down the slope, an omen of future rockslides.
Little Borax Lake.
Mt. Buckingham (Little Borax Lake is just to the right).
Lunch on the shore at Soda Bay was followed by a jaunt to the southern tip of Clear Lake to the aggregate quarry at Round Top Mountain. Round Top is a small, very recent basaltic andesite cinder cone dissected by the quarry operation. A small flow that escaped through the base of the cone ran eastward across the countryside and is now hidden by vineyards.
Looking for xenoliths at the quarry.
Three basalt flows in the volcanic field have the lowest Sr87/Sr86 ratios of all the Clear Lake lavas and are considered uncontaminated primitive magmas. The Round Top lava is an olivine-bearing basaltic andesite with a Sr87/Sr86 ratio only slightly higher than these primitive basalts. Its relatively undifferentiated character and lower crust xenolith assemblage indicate it erupted rapidly, with little time to react with the surrounding wall rocks. The xenoliths are gneisses and cordierite-bearing metapelites (metasediments) which are not part of the typical Franciscan basement assemblage. This indicates complex basement tectonics that is not reflected in surface exposures. Xenolith mineralogical relationships indicate a source at a depth of 12 to 18 km. and rapid eruption as a gas-charged magma. Round Top is the southernmost of a series of fault-controlled north-south trending cinder cones representing a mature stage of volcanic activity. Other linear cinder cone arrays attest to similar apparent fissure control of regional eruptive patterns.
At the southern tip of Clear Lake, just south of Clearlake town, a mall excavation provides a fascinating look at the 400,000 year-old Cache Creek dacite. Microstructural and mineralogical relationships in this unit clearly illustrate that magma mixing played a major role in the genesis of the Clear Lake volcanics.
Chemical and mineralogical evidence indicates the parent magma was a nearly crystallized rhyolite that was intruded by basalt or basaltic andesite in about a 50:50 ratio. The hybrid dacitic flow exhibits several telltale textural features that support this hypothesis. The dacite contains three types of plagioclase (sodium-calcium) feldspar. One is the fine-grained groundmass plagioclase. The second is a “fritted” feldspar making up almost all of the phenocryst population. It is riddled with glass-filled tubules that were formed when the magmas mixed near the surface and the more sodic component melted out. They were original components of the rhyolite. The third plagioclase is a minor phenocryst component up to 2mm in size that is not fritted. These crystals show repeated resorption and mantling with oscillatory rims zoned from calcic to sodic compositions. The latter formed in the mixed dacitic magma and reflect repeated surges of basaltic magma injected into the chamber that caused the feldspars to re-equilibrate after each pulse. The quartz in the rock also shows two generations. One is apparently an original rhyolite phenocryst or xenocryst phase now rimmed with an overgrowth of clinopyroxene. The greenish clinopyroxene also forms visible masses in handspecimen that probably represent complete reaction of quartz crystal swarms. The other generation is a clear, strongly resorbed phenocryst lacking pyroxene rims that Rolfe interprets as a late phase in the dacite magma. Sparse pyroxene-mantled olivine phenocrysts and partially resorbed clinopyroxene were likely associated with the mafic intruder, and clusters of resorbed clinopyroxene and plagioclase feldspar are interpreted as xenoliths entrained within the mafic magma. Although this dacite has not been isotopically analyzed, Rolfe feels confident that it would show rare sanidine (potassic feldspar) phenocrysts show a higher Sr87/Sr86 ratio than the whole rock value; clear geochemical evidence of magma mixing. These rocks have also been studied for the Rapikivi texture of their sanidine phenocrysts. These potash feldspars have been resorbed into ovoid shapes and then mantled with plagioclase by the injection of a hotter, more mafic magma. This phenomenon was originally observed in Scandinavian plutons, and its origin can be more fully understood by examining the same features under extrusive conditions.
The trip headed northward along the southeastern tip of Clear Lake, passing the early (1.66 m.y. old) basaltic andesites of Schoolteacher Hill and Quackenbush Mountain. These early eruptives were mostly primitive lavas scattered to the east and southeast of the main volcanic complex. Rolfe noted that the late-stage Clear Lake cinder cones are oriented in a north-south direction and that the maars are aligned northwest to southeast. These alignments imply a structural control of regional volcanism.
The final stop was at Sulphur Bank mine, now an EPA Superfund site. Here one can witness firsthand subvolcanic hydrothermal activity. Sulphur Bank was discovered in 1856 and subsequently mined for the next 100 years, first for its sulphur deposits and then for its mercury ore. The contamination of bottom sediments in the flooded mine pit and adjacent portions of Clear Lake with soluble methyl mercury is the principle reason Sulphur Bank has become a Superfund site.
Methyl mercury enters the aquatic food chain, eventually ending up in fish. This is the main risk to the public, and has compelled authorities to advise individuals not to eat more than two fish caught in Clear Lake per month, and that pregnant women abstain. The pit water level is fifteen feet higher than the lake level, and is separated by a thin septum of rock and tailings from Clear Lake. The acid mine waters have leached silica and alumina out of the andesitic volcanics, which then percolate through the thin barrier into the lake. The flocculated amorphous aluminosilicate transports the mercury, and because of its buoyancy, can be ferried around the lake by currents. The EPA has taken measures to ensure that rainwater runoff does not enter the mine pit or the lake. The government also has plans to remediate an adjacent Indian reservation built on tailings, and to cap off existing tailing piles with an impermeable barrier. The next step is to lower the pit water level below that of the lake, but this will require an expensive permanent pump-and-treat facility.
Sulphur Bank was created when a pyroxene andesite flow covered the site less than 44,000 years ago (C14 date of a log underlying the flow). This is the youngest dated unit in the Clear Lake volcanic field. The flow smothered an active hot spring system, which later began percolating through it. Hydrogen sulfide gas in the spring waters created sulfuric acid. This “solfataric” vapor eventually reduced the andesite to a residue of opaline silica and alunite. The fumarole gases precipitated elemental sulfur in fractures, and cinnabar (mercury sulfide (HgS)), metacinnabar, stibnite (antimony sulfide), and iron sulfide below the water table. The ore was initially worked by underground techniques, and later by open pit methods. About 4500 tons of mercury were produced at Sulphur Bank, making it one of the largest mercury deposits ever worked. Rolfe took the group to a nearby hot spring-fed pond where sulfur and carbon dioxide gases were bubbling up to the surface. The area is obviously still hydrothermally active. Rolfe can confirm that sulfur and mercury sulfides are presently being deposited at the mine site. Various isotopic geochemical studies indicate the ore fluids are metamorphically-derived from underlying Franciscan bedrock, which is also considered the source of the sulfur and mercury.
The NCGS deeply appreciates the time and effort Dr. Rolfe Erickson put into the guidebook and this wonderful field trip. Thanks are also due to Field Trip Coordinator Jean Moran and her husband Bill Martin for their usual meticulous attention to registration, food, and logistic details. Treasurer Phil Reed brought the morning refreshments and drove the rental van. Special thanks also go to NCGS Counselor and former President/Field Trip Coordinator Tridib Guha, who has quietly advised Jean and her predecessors on various aspects of our field trip program.
Checking out the wares - a certified NCGS mug!
Past President Mark Detterman & Field Trip Coordinator Jean Moran congratulating Dr. Rolfe Erikson