The Chemistry of Serpentine Soils Developed Over A Podiform Chromite Deposit,Western Sierra Nevada,USA

The Chinese Camp mining district in the western Sierra Nevada of California,USA,contains a serpentinized,ultramafic dunite intrusion with podiform chromite deposits.Serpentine soils have developed over this intrusion,creating a unique ecosystem of endemic vegetation and soils characterized by low Ca/Mg ratios and high Ni and Cr contents.The vegetation and red coloration make it easy to visually distinguish between soils developed over intruded,serpentinized bedrock and unmineralized,adjacent andesite bedrock(Fig.1).The purpose of our study was to compare soil chemistry and vegetative parameters among 3 study-design levels:1)undisturbed serpentine soil,2)undisturbed background soil(non-serpentine,developed over andesite),and 3)serpentine soil disturbed by mining activities.Within each of these l e v*e ls,3 random locations were chosen where weestablished 3,30-m transects(spaced 120-degrees apart).One soil sample was collected at a random location along each transect(0-15 cm depth after removing litter/O horizon).This scheme resulted in the collection of 9replicate soil samples per study-design level.Samples were analyzed for total metal content by ICP-AES/MS(inductively coupled plasma atomic emission spectroscopy/mass spectroscopy),p H,electrical conductivity,and total C/N/S.The vegetative parameter of%canopy cover was measured with a line-point intercept survey along each transect,using 0.6m intervals.Above-ground net primary productivity(ANPP)was estimated by harvesting all aboveground living plant material within a 0.5 m quadrant at 3 random locations along each transect,drying,and weighting the material.Significant differences among design levels were observed for ANPP,canopy cover,total P,total N,and Ca/Mg,where the median values for these parameters decreased in the order undisturbed background>undisturbed serpentine>mining-disturbed serpentine.The highest concentrations of Cr and Ni were found in undisturbed serpentine(medians of 1960 ppm and 2529ppm,respectively)followed by mining-disturbed serpentine(medians of 420 and 2120,respectively)then undisturbed non-serpentine(medians 47.0 and 32.2 ppm,respectively).Soil p H varied significantly among the design levels with a median 5.74 in undisturbed background,median 6.25 in undisturbed serpentine,and median of 7.17 in mining-disturbed serpentine.These data document the distinct differences in soil chemistry and vegetation parameters between undisturbed serpentine soil and adjacent,undisturbed background soil.Efforts toward mining reclamation must recognize these differences and include the correct baseline conditions in the reclamation plan.

Origin and Emplacement of Archean Ophiolites of the Central Orogenic Belt, North China Craton

Understanding Archean crustal and mantle evolution hinges upon identification and characterization of oceanic lithosphere. We report and update here more than 10 years work on a complete, yet dismembered and metamorphosed Archcan ophiolite sequence in the North China craton, in the Dongwanzi （东湾子）-Zunhua （遵化） structural belt and correlatives in the Wutaishan （五台山） area. Banded iron formation structurally overlies several tens of meters of variably deformed pillow lavas, mafic flows, and picritic amphibolites. These are in structural contact with a 2 km thick mixed gabbro and dike complex with gabbro screens, exposed discontinuously along strike for more than 20 km. The dikes consist of metamorphosed diabase, basalt, Hb-Cpx-gabbro, and pyroxenite. The dike/gabbro complex is underlain by several kilometers of mixed isotropic and foliated gabbro, which preserve compositional layering approximately 2 km below the dike complex, and then over several hundred meters merge into strongly compositionally layered gabbro and olivine-gabbro. The layered gabbro becomes mixed with layered pyroxenite/gabbro marking a transition zone into emulate ultramaflc rocks including serpentinzed dunite, pyroxenite and wehrlite, and finally into strongly deformed and serpentinized olivine and orthopyroxene-bearing ultraInafic rocks interpreted as depleted mantle harzburgite tectoultes. A U/Pb zircon age of 2.505 Ga from gabbro of the Dongwanzi ophioUte makes it one of the world＇s oldest recognized, laterally. extensive complete ophiolite sequences, though older dismembered ophiolites are recognized in South Africa and Greenland, extending back to 3.8 Ga. This age is confirmed by a ca. 2.6 Ga Re-Os isochron from chromites from the belt, and a number of dated 2.5-2.4 Ga cross-cuttlng younger igneous units. The Dongwanzi ophiolite is one of the largest well-preserved greenstone belts in the central orogenic belt that divides the North China craton into eastern and western blocks. More than 1 000 other fragments of gabbro, pillow lava, sheeted dikes, harzburgite,and podiform-chromite bearing dunite occur as tectonic blocks （tens to hundreds of meters long） in a biotite-gneiss and BIF matrix, intruded by tonalite and granodiorite, in the Zunhua structural belt. Blocks in this metamorphosed Archcan ophiolitic melange preserve deeper levels of oceanic mantle than the Dongwanzi ophiolite. The ophiolite-related melange marks a suture zone across the North China craton, traced for more than 1 600 km along the central orogenic belt. Many of the chromitite bodies are localized in dunite envelopes within harzburgite tectonite, and have characteristic nodular and orbicular chromite textures, known elsewhere only from ophiolites. The chromites have variable but high chrome numbers （Cr/（Cr＋Al）=0.74-0.93） and elevated P, also characteristic of suprasubduction zone ophiolites. The high chrome numbers, coupled with TiO2〈0.2 wt.% and V2O5〈0.1 wt.% indicate high degrees of partial melting from a very depleted mantle source and primitive melt for the chromite. A Re-Os isochron from the chromites indicates an age of 2.6 Ga, showing that they are the same age as the Dongwanzi ophiolite. The range in initial Os isotopic compositions in the chromites in these ophiolitic blocks is small and well within the range seen in modern ophiolites. The ultramafic and ophiolitic blocks in the Zunhua melange are therefore interpreted as dismembered and strongly deformed parts of the Dongwanzi ophiolite. We suggest the name ＂Dongwanzi-Zunhua ophiolite belt＂ for these rocks. Geochemical and structural features of the Dongwanzi ophiolite suggest that it formed in a forearc environment and was incorporated in an accretionary prism soon after it formed. Neoarchean and Paleoproterozoic （2.50 and 1.90 Ga） high-pressure granulites form a belt more than 700 km long along the western side of the central orogenic belt. Several Neoarchean sedimentary basins consisting of conglomerate, greywacke, and shale are located along the eastern side of the central orogenic belt, and are interpreted as remnants of a foreland basin. The three belts record the Neoarchcan subduction and collision between an arc terrane and eastern blocks of the North China craton in the Neoarchean, and further deformation and metamorphism in the Paleoproterozoic related to collisions on the northern margin of the already amalgamated North China craton.