Mount Diablo mercury deposits

ABSTRACT The California Coast Ranges mercury deposits are part of the western North America mercury belt, in which mercury occurs most commonly as red cinnabar (α-HgS), sometimes associated with its high-temperature polymorph, metacinnabar (β-HgS). In the Coast Ranges, ores were deposited from hydrothermal solutions and range in age from Miocene to Holocene. Ore deposition at Mount Diablo generally occurred along active faults and associated extension fractures in the Franciscan complex, most often in serpentinite that had been hydrothermally altered to silica-carbonate rock. The Mount Diablo mine lies ~48 km (~30 miles) northeast of San Francisco in Contra Costa County and is mineralogically unique in California, because metacinnabar, the higher-temperature polymorph of mercury sulfide, is a major primary ore mineral in the deposit, while at all other mercury mines in California, it is quite rare. In addition, hydrothermal activity is so recent that sulfurous gases and methane continued to be released into the mine at least into the 1940s. Historically, long before active large-scale mining began in the 1800s, the Mount Diablo mercury deposits were known to the Indigenous people of the Ohlone tribes, who used the cinnabar in rituals as well as for red pigment to decorate their bodies, and as a prized trade item. The deposit was later rediscovered in 1863 and mined intermittently until 1958. The Mount Diablo mine and adjacent Rhyne (also variously spelled Ryne or Rhine) mine were the sites of most of the mercury operations in the region, and at both mines, mercury ore occurs in structurally controlled lenticular bodies of silica-carbonate rock and serpentinite. The total district production probably exceeded 12,300 flasks (at 76 pounds or ~34.5 kg per flask) at an estimated grade of 2711 g per metric ton. Low-grade ore reserves are believed to still exist, with 17,000 short tons of indicated and inferred ore. Other minor deposits of copper, silver, and gold occur on Mount Diablo, principally in and around Eagle Peak, but mercury is not associated with these deposits.

Controls on Wintertime Nonbrightband Rain Rate and Frequency in California’s Northern Coast Ranges

Non-brightband (NBB) rain is a shallow, orographic precipitation that does not produce a radar brightband as a result of melting ice crystals. However, NBB rain is not the same as warm rain, which excludes ice from being involved in the microphysical growth of precipitation. Despite this difference, NBB rain is often treated as warm rain in the literature, and past studies have mostly ignored the role of ice. Here, we use two wet-seasons (2015-16 and 2016-17) at four precipitation observing sites in the Northern Coast Ranges of California to show the role of echo top height and ice in determining NBB rain intensity. It was found that NBB rain was only absent of brightbands 32-46% of the time depending on location of the site. Additionally, all NBB rain rates that exceeded 10 mm hr−1 exhibited observable brightbands during the hour period. We also define Growth Efficiency (GE) as the ability of shallow rain clouds to produce raindrops larger than drizzle size (D > 0.5 mm). High-GE rain drop size distributions were composed of fewer small drops and more large drops than low-GE rain, which was mostly drizzle. High-GE rain occurred with echo top heights above the freezing level where rapid growth of precipitation was observed by radar. Echo tops that only extended 1 km or less above the freezing level suggested hydrometeor growth from mixed-phase processes, indicating that ice may be present in coastal precipitation at warmer temperatures than previously considered.

Late Pleistocene rates of rock uplift and faulting at the boundary between the southern Coast Ranges and the western Transverse Ranges in California from reconstruction and luminescence dating of the Orcutt Formation

The western Transverse Ranges and southern Coast Ranges of California are lithologically similar but have very different styles and rates of Quaternary deformation. The western Transverse Ranges are deformed by west-trending folds and reverse faults with fast rates of Quaternary fault slip (1–11 mm/yr) and uplift (1–7 mm/yr). The southern Coast Ranges, however, are primarily deformed by northwest-trending folds and right-lateral strike-slip faults with much slower slip rates (3 mm/yr or less) and uplift rates (<1 mm/yr). Faults and folds at the boundary between these two structural domains exhibit geometric and kinematic characteristics of both domains, but little is known about the rate of Quaternary deformation along the boundary. We used a late Pleistocene sedimentary deposit, the Orcutt Formation, as a marker to characterize deformation within the boundary zone over the past 120 k.y. The Orcutt Formation is a fluvial deposit in the Santa Maria Basin that formed during regional planation by a broad fluvial system that graded into a shoreline platform at the coast. We used post-infrared–infrared-stimulated luminescence (pIR-IRSL) dating to determine that the Orcutt Formation was deposited between 119 ± 8 and 85 ± 6 ka, coincident with oxygen isotope stages 5e-a paleo–sea-level highstands and regional depositional events. The deformed Orcutt basal surface closely follows the present-day topography of the Santa Maria Basin and is folded by northwest-trending anticlines that are a combination of fault-propagation and fault-bend-folding controlled by deeper thrust faults. Reconstructions of the Orcutt basal surface and forward modeling of balanced cross sections across the study area allowed us to mea­sure rock uplift rates and fault slip rates. Rock uplift rates at the crests of two major anticlinoria are 0.9–4.9 mm/yr, and the dip-slip rate along the blind fault system that underlies these folds is 5.6–6.7 mm/yr. These rates are similar to those reported from the Ventura area to the southeast and indicate that the relatively high rates of deformation in the western Transverse Ranges are also present along the northern boundary zone. The deformation style and rates are consistent with models that attribute shortening across the Santa Maria Basin to accommodation of clockwise rotation of the western Transverse Ranges and suggest that rotation has continued into late Quaternary time.

A New Combination in Holodiscus (Rosaceae, Amygdaloideae, Spiraeeae)

Holodiscus dumosus var. cedrorum, The Cedars oceanspray, was described in 2011 in recognition of the distinct morphology of populations occurring on serpentine soils in The Cedars area in the Outer Coast Ranges of Sonoma County, California. Morphological and genetic data suggest that this taxon should instead be treated as a variety of Holodiscus discolor, here interpreted broadly as a widespread and highly variable species with several taxonomic varieties that intergrade. The new combination Holodiscus discolor var. cedrorum is made and a key to the varieties that occur in California is provided.

Depth, area, volume, and kinematics of slow-moving landslides from airborne synthetic aperture radar and mass conservation

<p>Slow-moving, deep-seated landslides travel downslope at rates of only a few meters per year and can remain active for decades and possibly centuries. As a result, they transmit large quantities of sediment to the channel network and are a major natural hazard that impact transport corridors and infrastructure. However, because slow-moving landslides rarely fail catastrophically, it is challenging, and often infeasible to directly measure their thickness and volume, two key parameters required to quantify sediment flux and to model landslide motion. Here we use remote sensing data from the NASA/JPL Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) to measure the 3-D surface velocity and geometry of over 90 slow-moving landslides in the California Coast Ranges. We then use mass conservation techniques to infer the thickness and volume of each landslide. These landslides have volumes that span between 10<sup>4</sup> and 10<sup>7</sup> m<sup>3</sup>, thicknesses between 3 and 90 m, and move at average annual rates < 5 m/yr. We also examined landslide depth-area and volume-area geometric scaling relations and compared our findings to a worldwide inventory of soil and bedrock landslides compiled by Larsen et al. (2010). We find that the landslide thickness, area, and volume are larger than soil landslides and smaller than bedrock landslides globally. Lastly, we estimate the subsurface geometry of the catastrophic Mud Creek landslide, central California Coast Ranges, during a period of slow motion that lasted at least 8 years before its ultimate failure. We find a volume of ~2.0 x 10<sup>6</sup> m<sup>3</sup>, which is close to the post-catastrophic failure volume measured using Structure From Motion (~2.1 x 10<sup>6</sup> m<sup>3</sup>) by Warrick et al. (2019). Therefore, in certain cases, it is possible to constrain landslide thickness and volume prior to catastrophic collapse. Our work shows how state-of-the-art remote sensing techniques can be used to better understand landslide processes and quantify their contribution to landscape evolution.</p>