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10841. [Article] Mineralogy and geochemistry of copper deposits of the Lights Creek Stock, California : an assessment of porphyry versus iron-oxide copper origin
The Lights Creek Stock is an 18 square kilometer copper-bearing granitoid intrusion within the Plumas Copper Belt in the northern California Sierra Nevada. Engels Mine, Superior Mine, and Moonlight Valley ...Citation Citation
- Title:
- Mineralogy and geochemistry of copper deposits of the Lights Creek Stock, California : an assessment of porphyry versus iron-oxide copper origin
- Author:
- Stephens, Abigail E.
The Lights Creek Stock is an 18 square kilometer copper-bearing granitoid intrusion within the Plumas Copper Belt in the northern California Sierra Nevada. Engels Mine, Superior Mine, and Moonlight Valley represent the main copper-mineralization in the Lights Creek district and small prospects include the Ruby Mine, and Moonlight Creek. The Moonlight Valley deposit within the Lights Creek Stock has been previously considered a porphyry copper deposit on the basis of stockwork veinlets in a granitic host. The granitic host rock of the Lights Creek Stock contains hydrothermal tourmaline and biotite as well as disseminated and vein copper-sulfides (bornite-chalcocite-chalcopyrite), and these features are supportive of porphyry-style mineralization. Nonetheless, the mineral alteration assemblages and zonation observed at Moonlight Valley and the Superior and Engels Mines differ significantly from literature descriptions of porphyry systems. Ores in the Lights Creek district are notably sulfur-poor and copper-rich. Mineralization includes copper-sulfide veins with pink potassium-feldspar selvages, tourmaline veins with albitic-chlorite selvages, and actinolite-apatite veins, as well as albite-magnetite alteration in the Kettle Rock volcanic sequence intruded by the Lights Creek Stock. These features are similar to the class of ores known as iron oxidecopper- gold (IOCG) deposits. Lights Creek represents one of several copper- and iron -bearing ore districts associated with early Mesozoic intrusions in northern Nevada and California, including both porphyry copper-skarn ores and IOCG deposits. This region is underlain by an early Mesozoic marine province of volcanic and sedimentary rocks, including evaporites, intruded by shallowly emplaced Jurassic, and in some cases, Triassic plutons (Stewart et al., 1997; Proffett and Dilles, 2008). This scenario is ideal for formation of IOCG deposits produced by advection of sedimentary brines by the heat of the intrusions (Barton and Johnson, 1996). A uranium-lead zircon age of 178.1 ± 3.9 (2s) Ma was observed from two samples of hypabyssal quartz monzonite on the east side of the Lights Creek Stock where it intrudes the Early Jurassic volcanics of the Kettle Rock sequence. The Lights Creek Stock originally contained igneous hornblende, which exhibits hydrothermal alteration to tourmaline, chlorite and local biotite associated with narrow sulfide (pyrite, chalcopyrite) and tourmaline veins. Apatite-actinolite veins and albite-alteration zones also cut the Lights Creek Stock. In the Superior Mine, east-dipping ore-bearing veins (<10 cm to 2 m wide) are zoned from central copper-rich zones (bornite, chalcopyrite, magnetite) associated with hydrothermal tourmaline and biotite alteration to outer sulfide-rich zones (chalcopyrite, pyrite, sphalerite) with sericite-chlorite alteration. Gently dipping intermineral lamprophyre dikes cut the bornite-bearing veins, but are in turn cut by the pyrite-chalcopyrite-rich veins. Similar ore zonation was observed by Storey (1981) in the northwest part of the Lights Creek Stock at the large Moonlight Valley deposit. In the Moonlight Valley, Engels Mine, and Superior Mine deposits porphyry dikes are absent. The observations are consistent with an IOCG association of copper mineralization broadly synchronous with the emplacement of the 178 Ma Lights Creek Stock. The light-colored, relatively fresh and unmineralized China Gulch Granite intrudes the east side of the Lights Creek Stock, and yielded a uranium-lead zircon age of 148.1±1.3 (2s) Ma. Quartz porphyry dikes intrude Middle Jurassic volcaniclastic and volcanic rocks of the Mount Jura sequence (Christe, 2011) west of the Lights Creek Stock along Moonlight Creek, and are here cut by quartz-tourmaline sulfide (pyrite, chalcopyrite) veins associated with strong sericitic alteration. Muscovite from this alteration zone yielded an ⁴⁰Ar-³⁹Ar plateau age of 146.05±0.88 (2s) Ma, the same age as the China Gulch Granite to the east, and suggest latest Late Jurassic age of porphyry copper mineralization. Copper-sulfides from Moonlight Valley and Superior are characterized by lead isotope values of ²⁰⁶Pb/²⁰⁴Pb ~19.4, while lead isotope values of copper-sulfides from Moonlight Creek and Engels are close to the igneous values of the Lights Creek Stock at ²⁰⁶Pb/²⁰⁴Pb ~18.6. The lead isotope data support the hypothesis that the fluids that formed the Moonlight Creek deposits and a portion of the ore of the Engels deposit were magmatic-hydrothermal, while the Moonlight Valley and Superior Mine ores are in part derived from non-magmatic fluid source reflecting an older lead source. Copper mineralization in the Lights Creek district likely included components of both porphyry-type magmatic-hydrothermal and IOCG-like non-magmatic sedimentary brine fluids. The Lights Creek Stock was emplaced in the late Early Jurassic (~178 Ma) and was in turn intruded by the younger late Late Jurassic China Gulch Granite (148 Ma). Magmatic fluids derived from the Lights Creek Stock produced tourmaline ± biotite ± magnetite ± chalcopyrite ± bornite veins in potassic alteration containing zones of hydrothermal potassium feldspar + biotite. Contemporaneously, the intrusion of the Lights Creek Stock drove advection of sedimentary brines through the adjacent metavolcanic rocks and margins of the stock. These sedimentary brines produced zones of albitic alteration and ores characterized by apatite + actinolite + titanite + magnetite + ilmenite ± chalcopyrite ± bornite ± epidote observed at Sulfide Ridge and the Superior Mine. Mixed fluids then produced zones of of albite ± chalcocite ± chalcopyrite ± tourmaline occuring at Sulfide Ridge, Superior Mine, and Moonlight Valley. In the late Late Jurassic (148 Ma), renewed magmatic activity resulted in the intrusion of the China Gulch Granite and granite porphyry dikes. The latter produced small-scale porphyrystyle mineralization along Moonlight Creek and Ruby Mine west of the Lights Creek Stock.
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10842. [Article] 2006 Borax Lake Chub Investigations Progress Reports 2006
Abstract -- Borax Lake chub (Gila boraxobius) is represented by a single population that inhabits a 4.1 hectare geothermally-heated alkaline lake in Harney County, Oregon. The Borax Lake chub is a small ...Citation Citation
- Title:
- 2006 Borax Lake Chub Investigations Progress Reports 2006
Abstract -- Borax Lake chub (Gila boraxobius) is represented by a single population that inhabits a 4.1 hectare geothermally-heated alkaline lake in Harney County, Oregon. The Borax Lake chub is a small minnow endemic to Borax Lake and adjacent wetlands in Oregon’s Alvord Basin (Williams and Bond 1980). Borax Lake is a natural lake, perched 10 meters above the desert floor on sinter deposits, which is fed almost exclusively by thermal groundwater. The Borax Lake chub was listed as endangered under the federal Endangered Species Act in 1982 (U.S. Fish and Wildlife Service 1982). Population abundance estimates obtained in 1991-1996 indicated a fluctuating population ranging from a low of 8,144 fish to a high of 34,634 fish (Salzer 1997). The basis for the Borax Lake chub’s listed status was not population size, but the security of a very limited, unique, isolated, and vulnerable habitat. Because Borax Lake is situated above salt deposits on the desert floor, alteration of the salt crust shoreline could reduce lake levels and the habitat quantity and quality available to Borax Lake chub. At the time of the listing, Borax Lake was threatened by habitat alteration caused by geothermal energy development and alteration of the lake shore crust to provide irrigation to surrounding pasture lands. The Borax Lake chub federal recovery plan, completed in 1987, advocated protection of the lake ecosystem through the acquisition of key private lands, protection of groundwater and surface waters, controls on access, and the removal of livestock grazing (U.S. Fish and Wildlife Service 1987). Numerous recovery measures implemented since listing have improved the conservation status of Borax Lake chub and protection of its habitat (Williams and Macdonald 2003). When the species was listed, critical habitat was designated on 259 hectares of land surrounding the lake, including 129 hectares of public lands and two 65- hectare parcels of private land. In 1983, the U.S. Bureau of Land Management designated the public land as an Area of Critical Environmental Concern. The Nature Conservancy began leasing the private lands in 1983 and purchased them in 1993, bringing the entire critical habitat into public or conservation ownership. The Nature Conservancy ended water diversion from the lake for irrigation and livestock grazing within the critical habitat. Passage of the Steens Mountain Cooperative Management and Protection Act of 2000 removed the public BLM lands from mineral and geothermal development within a majority of the basin. These actions, combined with detailed studies of the chub and their habitat have added substantially to our knowledge of the Borax Lake ecosystem (Scoppettone et al. 1995, Salzer 1992, Perkins et al. 1996). However, three primary threats remain. These include the threat to the fragile lake shoreline, wetlands, and soils from a recent increase in recreational use around the lake (particularly off-road vehicle usage), the threat of introduction of nonnative species, and potential negative impacts to the aquifer from geothermal groundwater withdrawal if groundwater pumping were to occur on private lands outside the protected areas (Williams and Macdonald 2003). Although an increase in abundance is not a goal in the successful recovery of this species, monitoring trends in abundance over time is an important management tool to assess species status. From 1998-2004, data describing the abundance of the Borax Lake chub population are not available. Abundance estimates were obtained from 1986- 1997 by The Nature Conservancy (Salzer 1997) (Figure 1). Abundance estimates for 1986-1990 are not comparable with those obtained in 1991-1997. Prior to 1991, estimates were obtained only from traps set around the perimeter of the lake. In 1991, estimates were obtained from traps set on a regularly spaced grid throughout the lake. A study comparing the methods suggests that prior to 1991 abundance was under estimated, perhaps by as much as 50 percent (Salzer 1992). A recent review of the conservation status of the Borax Lake chub by Williams and Macdonald (2003) cited the lack of recent and ongoing population and ecosystem monitoring as one argument against downlisting or delisting the species at this time. The chub population has experienced substantial fluctuations in abundance over the time period (1986-1997) when abundance data are available (Figure 1). At the time of the review, the most recent abundance estimates that were obtained in 1996 and 1997 were some of the lowest estimates since 1991. Borax Lake chub population abundance estimates from 1986 to 1997 and 2005 to 2006. Horizontal bars represent 95% confidence limits. In 1986-1990 (solid symbols), only the perimeter of the lake was trapped. After 1990 (open symbols) the entire lake was trapped. Estimates are not directly comparable across these time periods. There are limited data on population age structure that offer valuable insight into the productivity of Borax Lake chub. Williams and Bond (1983) examined lengthfrequency data and concluded that the population consisted primarily of age 1 fish, with few age 2 and age 3 fish present. Limited opercle bone aging of chub collected in 1992- 1993 also indicated that most Borax Lake were less than one year of age (67-79%), yet a few individuals were aged at 10+ years (Scoppettone 1995). Because Borax Lake chub are only found in one location and the population is apparently dominated by a single year-class of adults, the species has a high inherent risk of extinction. 3 The objectives of this study were to: 1) obtain a mark-recapture population estimate of Borax Lake chub, and 2) to evaluate ways to reduce handling of Borax Lake chub when monitoring population abundance both by modifying previous mark-recapture protocols and by developing snorkeling survey protocols to use as an alternative to mark-recapture estimates. In addition, we collected data regarding lake temperatures, chub size (age) structure, and the condition of the fragile lake shoreline and outflows.
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Geologic mapping of the Longview-Kelso area and the measurement and description of a composite 650-meter thick stratigraphic section of the Cowlitz Formation (Tc) in Coal Creek using bio-, magneto-, litho-, ...
Citation Citation
- Title:
- Stratigraphy and sedimentology of the middle eocene Cowlitz Formation and adjacent sedimentary and volcanic units in the Longview-Kelso area, southwest Washington
- Author:
- McCutcheon, Mark S.
Geologic mapping of the Longview-Kelso area and the measurement and description of a composite 650-meter thick stratigraphic section of the Cowlitz Formation (Tc) in Coal Creek using bio-, magneto-, litho-, and sequence stratigraphy reveals a complex interplay of Cowlitz micaceous, lithic arkosic shelf to tidal/estuarine to delta plain facies associations, and Grays River basalt lava flows and interbedded basalt volcaniclastics from nearby Grays River eruptive centers (e.g., Mt. Solo and Rocky Point). The lower 100 meters of the Coal Creek section (informal unit 1, Chron 18r) consists of micaceous, lithic arkosic sandstone and siltstone and minor coals, was deposited as part of a highstand system tract (HST) at the base of 3rd order cycle number 3. This unit consists of four dominantly tidal shoaling-upward arkosic sandstone parasequences reflecting upper shoreface to delta plain depositional environments. The overlying unit 2 (Chron 18n) is defined by abundant Grays River basalt volcaniclastic interbeds that intertongue with Cowlitz lithic arkoses. This unit represents the latter part of 3rd order cycle 3, and consists of mostly fining- and thinning-upward parasequences of middle shoreface to delta plain successions of an aggradational to transgressive parasequence set. Near the top of unit 2 is a maximum marine flooding surface depositing lower shoreface lithic arkosic sandstone to shelf siltstones over upper shoreface micaceous lithic arkose. Unit 3 comprises 3rd order cycle 4 (Chron 17r), a lowstand system tract, and consists of 6 mostly fining- and thinning-upward parasequences of lower shoreface to delta plain facies associations. A parasequence or erosional boundary at the base of unit 5 (Chron 17r) consists of submarine channel-fill scoured into underlying micaceous siltstones, produced during a lowstand system tract (LST) of 3rd order cycle 5. This deep marine channel-fill sequence is overlain by thinlybedded to laminated overbank distal turbidites and hemipelagic siltstones that define the top of the Coal Creek section. These 5 informal units in Coal Creek lithologically and chronologically correlate to 5 similar informal units defined by Payne (1998) in the type section of Cowlitz Formation in Olequa Creek near Vader -30 km to the north. Middle Eocene Grays River Volcanics of the study area are mapped as two separate units: a lower unit over 150 meters thick in places, consisting of subaerial basaltic flows and invasive flows (Tgvl), intrusions (Tgvis and Tgvid), and volcaniclastics (Tgvsl); and an upper unit consisting of commonly mollusk-bearing, shallow marine basaltic sedimentary interbeds that intertongue with the Cowlitz Formation (Tgvs2), particularly Cowlitz unit 2 of the Coal Creek section. These volcaniclastic deposits are intrabasinal, derived from volcanic highlands to the west and northwest, and local phreatomagmatic tuff cones. The lower Grays River volcaniclastic unit typically overlies Grays River flows in the study area and is divided into 5 informal facies. Geochemically, Grays River flows in the study area fall within normal parameters (3 to 4% TiO2 and high iron tholeiitic basalts). However, basalt flows and bedded scoriaceous breccias near Rocky Point are anomalously low in TiO2 and are considered in this study to be a separate volcanic subunit (Rocky Point Basalts), time equivalent to and interfingering with Grays River lavas, but may represent mixing with shallower western Cascade calc-alkaline magma. Over 60 younger Grays River dikes intrude the Cowlitz Formation in Coal Creek. A dike low in the Coal Creek section is dated at 40 ± 0.36 Ma, and an invasive flow at Mt. Solo is dated at 36.98 ±.78 Ma. Volcanics capping the hills east of the Cowlitz River are chemically distinct as slightly younger western Cascade basaltic andesite flows, and two dikes east of the river are chemically distinct as western Cascade andesite. Overlying Grays River Volcanics and Cowlitz Formation in much of the study area, are clayey and commonly tuffaceous siltstones and silty sandstones, possibly of the late Eocene-early Oligocene Toutle Formation, a new unit to this area. The Toutle Formation is a mixture of wave and stream reworked micaceous and arkosic Cowlitz Formation and fresh silicic pyroclastic ash and pumice from the active western Cascade arc. An angular unconformity separates the Paleogene Grays River Volcanics, Cowlitz Formation, and Toutle Formation from the early to middle Miocene Columbia River Basalt Group. Based on lithology, geochemistry, stratigraphic relationships, and magnetic polarity, 6 individual Columbia River Basalt flows have been mapped in this study. The three lower Grande Ronde flows are of normal polarity and Ortley low MgO chemical composition. The lowermost flow (N2 Ortley #1) is absent in the Columbia Heights area, low MgO, about 10 meters thick and consists of pillow-palagonite sequences in the upper quarry on Mt. Solo. Aphyric N2 Ortley flow #2 is over 35 meters thick with well-developed upper and lower colonnade, and of intermediate MgO. N2 Ortley flow #3 is pillow-palagonite in the Storedahl Quarry and low MgO. A -4-meter thick tuffaceous overbank siltstone and basalt conglomeratic channel interbed separates the three low MgO Ortley flows from the overlying high MgO N2 Grande Ronde Sentinel Bluffs flow. A single exposure of well-developed large colonnade with sparse 1 cm labradorite laths, and reddish oxidized soil, defines the N Sand Hollow flow of the Frenchman Springs Member of the Wanapum Formation. The overlying Pomona Member is mapped based on previous work by other authors. Pliocene gravels and arkosic sand of the Troutdale Formation form upland terrace deposits up to 100 meters thick in southern parts of the study area, and represent the uplifted paleo-thalweg and overbank flood deposits of the downcutting, antecedent ancestral Columbia River. Well-rounded clasts are a mixture of extrabasinal granitic and metamorphic quartzite, and intrabasinal porphyritic basaltic andesite, dacite, and basalt from the western Cascades and Columbia River Basalts. Troutdale terrace gravels grade northward into contemporaneous volcanic pebble and cobble gravel terrace deposits produced along the ancestral Cowlitz River that are dominantly composed of porphyritic andesite gravel and volcanic sand from the western Cascades. Lower terraces along the Cowlitz River were deposited by the late Pleistocene Missoula Floods. All of these unconsolidated to semiconsolidated gravels and sands are prone to landslides, and the Aldercrest-Banyon landslide, the second worst landslide disaster in American history, occurred in the Troutdale Formation gravels. After eruption of the Grays River Volcanics and deposition of the Cowlitz Formation, the forearc underwent a period of transtension in the late-middle Eocene related to magmatic upwelling and reorganization of the subducting Farallon Plate. This event produced a northwest-trending set of oblique slip normal faults, along which Grays River dikes intruded. Starting in the early Miocene the region underwent a transpressional event, reactivating many of the northwest-trending faults, and producing the Columbia Heights Anticline, Hazel Dell Syncline, the Coal Creek Fault, and the Kelso Fault Zone. The paleotopography resulting from this event was stream eroded to a nearly flat plain before emplacement of the Columbia River Basalts, which are nearly horizontal today. Continued offset along the northwest-trending fault set has also offset the Columbia River Basalts. Continued oblique slip post-Miocene broad arching of the Coast Range and downcutting by the Columbia and Cowlitz Rivers has resulted in Pliocene and Pleistocene terraces, and produced an east-west fault set that offsets all earlier structural features.