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.

Late Cenozoic tephrochronology of the Mount Diablo area within the evolving plate-tectonic boundary zone of northern California

ABSTRACT We present a tephrochronologic/chronostratigraphic database for the Mount Diablo area and greater San Francisco Bay region that provides a spatial and temporal framework for geologic studies in the region, including stratigraphy, paleogeography, tectonics, quantification of earth surface processes, recurrence of natural hazards, and climate change. We identified and correlated 34 tephra layers within this region using the chemical composition of their volcanic glasses, stratigraphic sequence, and isotopic and other dating techniques. Tephra layers range in age from ca. 65 ka to ca. 29 Ma, as determined by direct radiometric techniques or by correlation to sites where they have been dated. The tephra layers are of Quaternary or Neogene age except for two that are of Oligocene age. We correlated the tephra layers among numerous sites throughout northern California. Source areas of the tephra layers are the Snake River–Yellowstone hotspot trend of northern Nevada, southern Idaho, and western Wyoming; the Nevadaplano caldera complex of central Nevada; the Jemez Mountains–Valles Caldera in northwestern New Mexico; the Southern Nevada volcanic field and related source areas in eastern California and west-central Nevada; the Quien Sabe–Sonoma volcanic centers of the California Coast Ranges; and the young Cascade Range volcanic centers of northeastern California and Oregon.

Neogene volcanism on the eastside of Mount Diablo, Contra Costa County, California

ABSTRACT Two spatially separated areas of Neogene volcanic rocks are located on the northeast limb of the Mount Diablo anticline. The southernmost outcrops of volcanics are 6 km east of the summit of Mount Diablo in the Marsh Creek area and consist of ~12 hypabyssal dacite intrusions dated at ca. 7.8–7.5 Ma, which were intruded into the Great Valley Group of Late Cretaceous age. The intrusions occur in the vicinity of the Clayton and Diablo faults. The rocks are predominantly calc-alkaline plagioclase biotite dacites, but one is a tholeiitic plagioclase andesite. Mercury mineralization was likely concomitant with emplacement of these late Miocene intrusions. The northern most outcrops of Neogene volcanic rocks occur ~15 km to the north of Mount Diablo in the Concord Naval Weapons Station and the Los Medanos Hills and are probably parts of a single andesite flow. A magnetometer survey indicates that the flow originated from a feeder dike along the Clayton fault. The lava flow is flat-lying and occu pies ancient stream channels across an erosional surface of tilted Markley Sandstone of middle Eocene age. New radiometric dates of the flow yield an age of 5.8–5.5 Ma, but due to alteration the age should be used with caution. The flow is a calc-alkaline andesite rich in clinopyroxene and plagioclase. What appear to be uplifted erosional remnants of the flow can be traced northeastward in the Los Medanos Hills across a surface of tilted Cenozoic rocks that eventually rest on formations as young as the Lawlor Tuff dated at 4.865 ± 0.011 Ma. This stratigraphic relationship suggests that the andesite flow is probably late Pliocene in age and was impacted by the more recent uplift of the Los Medanos Hills but postdates the regional folding and faulting of the rocks of Mount Diablo. In terms of timing, location, and composition, the evidence suggests these two areas of dacitic and andesitic volcanics fit into a series of migrating volcanic centers in the California Coast Ranges that erupted following the northward passage of the Mendocino Triple Junction.

An Exhumation Pulse From the Nascent Franciscan Subduction Zone (California, USA)

<p>We investigated a suite of metabasite blocks from serpentinite matrix and shale matrix mélanges of the California Coast Ranges. Our new data set consists of 40Ar/39Ar dates of amphibole and phengite and U‐Pb dates of metamorphic zircon. Combined with published geochronology, including prograde Lu‐Hf garnet ages from the same blocks, we can reconstruct the timing and time scales of prograde and retrograde metamorphism of individual blocks. In particular we find that exhumation from amphibole‐eclogite facies conditions occurred as a single episode at 165–157 Ma, with an apparent southward younging trend. The rate and timing of exhumation were initially uniform (when comparing individual blocks) and fast (with cooling rates up to ~140°C/Ma). In the cooler and shallower blueschist facies, exhumation slowed and became less uniform among blocks. Considering the subduction zone system, the high‐grade exhumation temporally correlates with a magmatic arc pulse (Sierra Nevada) and the termination of forearc spreading (Coast Range Ophiolite). Our findings suggest that a geodynamic one‐time event led to exhumation of amphibole‐eclogite facies rocks. We propose that interaction of the Franciscan subduction zone with a spreading ridge led to extraction of the forearc mantle wedge from its position between forearc crust and subducting crust. The extraction led to fast and uniform exhumation of subducted rocks into the blueschist facies. We also show that the Franciscan subduction zone did not undergo significant cooling over time and that its initiation was not coeval with blueschist‐facies metamorphism of the Red Ant schist of the Sierra Nevada foothills.</p>

Controls on streamwater age in a saturation overland flow-dominated catchment

<p>Water age and flow pathways should be related; however, it is still generally unclear how integrated catchment runoff generation mechanisms result in streamflow age distributions at the outlet. Here, we combine field observations of runoff generation at the Dry Creek catchment with StorAge Selection (SAS) age models to explore the relationship between streamwater age and runoff pathways. Dry Creek is an intensively monitored catchment in the northern California Coast Ranges with a Mediterranean climate and thin subsurface critical zone. Due to limited storage capacity, runoff response is rapid (~1-2 hours), and total annual streamflow consists predominantly of saturation overland flow, based on field mapping of saturated extents and runoff thresholds. Even though SAS modeling reveals that streamflow is younger at higher wetness states, flow is still typically older than one day and thus older than event water. Because streamflow is mostly overland flow, this means that a significant portion of overland flow must derive from groundwater returning to the surface, consistent with field observations of exfiltrating head gradients, return flow through macropores, and extensive saturation days after storm events. We conclude that even in a landscape with widespread overland flow, runoff pathways may be longer than anticipated, with implications for contaminant delivery and biogeochemical reactions. Our findings have implications for the assumptions built into classic hydrograph separation inferences, namely, that overland flow is not all new water.</p>

Metamorphic Temperatures and Pressures across the Eastern Franciscan: Implications for Underplating and Exhumation

The Eastern Belt of the Franciscan Complex in the northern California Coast Ranges consists of coherent thrust sheets predominately made up of ocean floor sediments subducted in the Early Cretaceous and then accreted to the overriding plate at depths of 25-40 km. Progressive packet accretion resulted in the juxtaposition of a series of thrust sheets of differing metamorphic grades. This study utilizes laser Raman analysis of carbonaceous material to determine peak metamorphic temperatures across the Eastern Belt and phengite barometry to determine peak metamorphic pressures. Locating faults that separate packets in the field is difficult, but they can be accurately located based on differences in peak metamorphic temperature revealed by Raman analysis. The Taliaferro Metamorphic Complex in the west reached 323-336°C at a minimum pressure of ~11 kbar; the surrounding Yolla Bolly Unit 215–290°C; the Valentine Springs Unit 282-288°C at 7.8±0.7 kbar; the South Fork Mountain Schist 314–349°C at 8.6–9.5 kbar, a thin slice in the eastern portion of the SFMS, identified here for the first time, was metamorphosed at ~365°C and 9.7±0.7 kbar; and a slice attributed to the Galice Formation of the Western Klamath Mountains at 281±13°C. Temperatures in the Yolla Bolly Unit and Galice slice were too low for the application of phengite barometry. Microfossil fragments in the South Fork Mountain Schist are smaller and less abundant than in the underlying Valentine Springs Unit, providing an additional method of identifying the boundary between the two units. Faults that record a temperature difference across them were active after peak metamorphism while faults that do not were active prior to peak metamorphism, allowing for the location of packet bounding faults at the time of accretion. The South Fork Mountain Schist consists of two accreted packets with thicknesses of 300 m and 3.5 km. The existence of imbricate thrust faults both with and without differences in peak metamorphic temperature across them provides evidence for synconvergent exhumation.

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>