William J. Pickthorn

Nd, Sr, and O isotopic variations in metaluminous ash-flow tuffs and related volcanic rocks at the Timber Mountain/Oasis Valley Caldera, Complex, SW Nevada: implications for the origin and evolution of large-volume silicic magma bodies

Nd, Sr and O isotopic data were obtained from silicic ash-flow tuffs and lavas at the Tertiary age (16–9 Ma) Timber (Mountain/Oasis Valley volcanic center (TMOV) in southern Nevada, to assess models for the origin and evolution of the large-volume silicic magma bodies generated in this region. The large-volume (>900 km3), chemically-zoned, Topopah Spring (TS) and Tiva Canyon (TC) members of the Paintbrush Tuff, and the Rainier Mesa (RM) and Ammonia Tanks (AT) members of the younger Timber Mountain Tuff all have internal Nd and Sr isotopic zonations. In each tuff, high-silica rhyolites have lower initialɛNd values (∼1ɛNd unit), higher87Sr/86Sr, and lower Nd and Sr contents, than cocrupted trachytes. The TS, TC, and RM members have similarɛNd values for high-silica rhyolites (-11.7 to -11.2) and trachytes (-10.5 to -10.7), but the younger AT member has a higherɛNd for both compositional types (-10.3 and -9.4). Oxygen isotope data confirm that the TC and AT members were derived from lowɛNd magmas. The internal Sr and Nd isotopic variations in each tuff are interpreted to be the result of the incorporation of 20–40% (by mass) wall-rock into magmas that were injected into the upper crust. The lowɛNd magmas most likely formed via the incorporation of lowδ18O, hydrothermally-altered, wall-rock. Small-volume rhyolite lavas and ash-flow tuffs have similar isotopic characteristics to the large-volume ash-flow tuffs, but lavas erupted from extracaldera vents may have interacted with higherδ18O crustal rocks peripheral to the main magma chamber(s). Andesitic lavas from the 13–14 Ma Wahmonie/Salyer volcanic center southeast of the TMOV have lowɛNd (-13.2 to -13.8) and are considered on the basis of textural evidence to be mixtures of basaltic composition magmas and large proportions (70–80%) of anatectic crustal melts. A similar process may have occurred early in the magmatic history of the TMOV. The large-volume rhyolites may represent a mature stage of magmatism after repeated injection of basaltic magmas, crustal melting, and volcanism cleared sufficient space in the upper crust for large magma bodies to accumulate and differentiate. The TMOV rhyolites and 0–10 Ma old basalts that erupted in southern Nevada all have similar Nd and Sr isotopic compositions, which suggests that silicic and mafic magmatism at the TMOV were genetically related. The distinctive isotopic compositions of the AT member may reflect temporal changes in the isotopic compositions of basaltic magmas entering the upper crust, possibly as a result of increasing “basification” of a lower crustal magma source by repeated injection of mantle-derived mafic magmas.

Origin of lode-gold deposits of the Juneau gold belt, southeastern Alaska

Mesothermal gold-bearing quartz veins along the Juneau gold belt formed in the early Tertiary from H 2 O-CO 2 (±N 2 ,H 2 S,CH 4 )-rich, low-salinity fluids at temperatures of 200 to 325 °C and pressures in excess of 1 - 1.5 kbar. Isotopically heavy ore-forming fluids, with δ 18 O values between +8‰ and +12‰ and δD values of -20‰ to -30‰ , indicate a deep-seated fluid source. The data are compatible with a model of ore-fluid generation via metamorphic devolatilization within material subducted below the southern Alaska continental margin. Prograde metamorphic fluids infiltrated up zones of mechanically enhanced permeability associated with the Coast Range megalineament and formed the gold-bearing veins within brittle units at higher, retrograding crustal levels. Deposition of gold was accomplished by several mechanisms, including boiling, fluid-wall rock reactions, and decreases in pressure and temperature of the hydrothermal fluids.

Contamination of basaltic magma by mafic crust at Amboy and Pisgah Craters, Mojave Desert, California

Quaternary alkali basalts from Pisgah Crater and Amboy Crater in southern California exhibit unusual chemical and isotopic variations which probably result from assimilation of mafic crust. Although lavas from both volcanoes are alkali basalts and hawaiites with isotopic and chemical characteristics that are similar to ocean island basalts (OIB) (e.g., eNd=2.6 – 5.9, 87Sr/86Sr = 0.7038 – 0.7049, Hf/Ba=0.014 – 0.017), they display highly correlated and unusual variations in their chemical and isotopic compositions. At each volcano, MgO decreased during the eruptive sequence from ≈8.5 wt % to ≈4.5 wt %. Incompatible elements are positively correlated with MgO and therefore also decreased during the eruptive sequence. Nd, Sr, and Pb isotope ratios correlate strongly with MgO. Compositional and isotopic data cannot be explained by any combination of closed-system fractionation, partial melting of the mantle, or silicic contamination. These data indicate that the basalts represent mixing between a high-MgO, high-eNd component, common to both Pisgah and Amboy craters, with lower-MgO and -eNd components that are unique to each center. The high-MgO component most likely is a primitive mantle-derived magma, based in part on its similarity to nearby xenolith-bearing Quaternary basalts. The low-MgO components are interpreted to be partial melts of mafic crust. If our model is correct, then both volcanoes evolved from eruption of nearly pure mantle melts early in their history to eruption of nearly pure remelted mafic crust late in their history. The crustal source could not have been underplated Mesozoic or younger oceanic crust, but its age is otherwise unconstrained; nearby Mesozoic gabbros and Proterozoic diabases have appropriate isotopic compositions. The basalt data provide no evidence that ancient enriched lithospheric mantle currently underlies the Mojave Desert. If such mantle was present at any time beneath this region, it must have been removed during one or more of the many tectonic events that affected the Mojave Desert during the Phanerozoic. Regional variability of isotope ratios in basalts is commonly interpreted to reflect variability of the underlying mantle. Data from this study raise the possibility that some of this variability may result from cryptic contamination of OIB-like basalts by mafic crust.

Hydrothermal Metamorphism in Oceanic Crust from the Coast Range Ophiolite of California: Fluid-Rock Interaction in a Rifted Island Arc

Metamorphic-mineral parageneses and stable-isotope systematics in remnants of the southern portion of the California Coast Range ophiolite differ from those described in modern oceanic crust and many other ophiolites. Detailed studies of the Del Puerto and Point Sal ophiolite remnants reveal a style of submarine hydrothermal metamorphism characterized by (1) development within the extrusive section of a continuous low-P/T metamorphic facies series that includes zeolite-, prehnite-pumpellyite-, and greenschist-facies mineral assemblages; (2) pervasive alteration of the extrusive rocks; (3) absence of massive sulfide deposits; (4) large 18O enrichments in metavolcanic rocks (up to 20% [SMOW] in the Del Puerto remnant); and (5) absence of corresponding 18O depletions in associated plutonic rocks.

Genesis of Gold Deposits in Chugach Terrane of South-Central Alaska--Evidence from Fluid Inclusions: ABSTRACT

Gold-bearing quartz veins occur in shear zones, faults, and joints within the Upper Cretaceous Valdez Group flysch in the Kenai and Chugach Mountains of south-central Alaska. The veins are regionally restricted to areas of medium greenschist-grade metamorphic rocks and are notably absent in lower and higher grade metamorphic rocks. Fluid inclusion studies were conducted on samples of gold-bearing quartz from the Moose Pass, Hope-Sunrise, Port Wells, and Port Valdez districts. Ice and clathrate melting temperatures indicate that the ore-forming fluids had low salinities, ranging from 0 to 5-equivalent wt. % NaCl. These fluids contain appreciable amounts of dissolved gases, as shown by the nearly ubiquitous formation of clathrates during inclusion freezing and by the common presence of three-phase inclusions consisting of aqueous fluid, liquid CO2, and vapor. Total gas content varies from essentially nondetectable to as much as 10 vol. %. Freezing measurements on the inclusion fluids show the gas composition to vary from nearly pure CO2 to mixtures dominated by CH4 and N2We believe that the gold-bearing veins represent pathways for the escape of metamorphic fluids during rapid uplift of the Chugach and Kenai Mountains. The veins are believed to have formed along hydraulic fractures or along dilated preexisting fractures, created when fluid pressure exceeded load pressure. End_of_Article - Last_Page 665------------

Reconnaissance Oxygen Isotope Study of Gold-Bearing Quartz Veins Within Metasedimentary Rocks of Valdez Group, Alaska: ABSTRACT

A reconnaissance oxygen-isotope study of gold-bearing quartz veins within the metasedimentary rocks of the Valdez Group from around Prince William Sound was conducted to examine the variations and similarities within and between the different gold districts, and to investigate the origin of the ore-forming solutions. Analyses were made of mineralized samples from mines and prospects in the Port Valdez, Port Wells, Girdwood, Hope-Sunrise, and Moose Pass gold districts. In the Hope-Sunrise district the ^dgr18O values of the gold-bearing veins are relatively constant, ranging from +16.0 to +16.7 ^pmil, whereas in the Port Valdez district, the vein values are more variable (+13.9 to +17.0 ^pmil), implying variations in temperature or fluid compositions. Within individual mines, the ^dgr18O values are constant to within 0.1 ^pmil along strike in continuous veins, and vary by a maximum of 0.2 ^pmil between bands of quartz in ribbon vein samples. Petrographic and fluid inclusion data suggest a temperature of approximately 325°C for formation of the mineralized quartz veins. Calculated ^dgr18O values for fluids in equilibrium with the veins range from +8 to +12 ^pmil. A quartz separate from an altered quartz diorite intrusion at the Rough Tough mine in the Port Valdez district has a ^dgr18O value of +14.0 ^pmil; fluid in equilibrium with the intrusive at a reasonable temperature End_Page 675------------------------------ for its emplacement would have had a ^dgr18O value of approximately +12 ^pmil. These data suggest that the mineralized quartz veins formed from fluids derived from the Valdez Group during metamorphic dewatering. End_of_Article - Last_Page 676------------