LOW-T ALTERATION OF VOLCANOPLUTONIC ARC ROCKS; …



LOW-T ALTERATION OF VOLCANOPLUTONIC ARC ROCKS; PROCESSING FLUIDS IN THE UPPERMOST SUBDUCTION FACTORY

ROUGVIE, James R.1, SORENSEN, Sorena S.2, BARTON, Mark D.3, and SKOZA, Rachel A.1, (1) Dept. of Geology, Beloit College, 700 College St, Beloit, WI 53511, rougviej@beloit.edu, (2) Mineral Sciences, Smithsonian Institution, PO Box 37012, National Museum of Natural History MRC-119, Washington, DC 20013-7012, (3) Dept. of Geosciences, University of Arizona, Tucson, AZ 85721

The tuff of Skelton Lake from the Mammoth Crest area of the Sierra Nevada batholith displays Ca gains, Na losses, and scattered abundances of K relative to expected values for silicic volcanic rocks. In contrast, typically immobile element groups resemble those of rhyolite from calc-alkaline arc settings. The intensity of alkali and alkaline earth element alteration does not correspond to proximity to contacts with the Round Valley Peak granodiorite.

CL, PLM, and SEM petrography show: well-preserved igneous matrix and phenocryst textures even within meters of igneous contacts; variable replacement of groundmass grains and plagioclase phenocrysts by Ca-rich plagioclase (locally >An90) ± epidote ± muscovite; and near end-member K feldspar and albite grains, with microstructures and compositions that may record early alkali metasomatism, locally replaced by plagioclase. Locally abundant calc-silicate-mineral veins display selvages with ~An90 plagioclase. Early thulite-bearing veins are cut by epidote + quartz ± grossular ± calcite veins. Still younger sets of quartz veins and joints display Na-rich bleached margins. Some veins parallel folia similar in geometry to features related to batholith emplacement whereas other veins cut them.

Quartz and whole rock δ18O values for the tuff (9.7-13.2 ‰) and vein quartz (12-14 ‰) are greater than values for the granodiorite (quartz typically 90% by volume.

Hydrothermal alteration formed intermittently over 180 m.y., is most extensive in the shallow crust, and reflects superposition of many short-lived (50% Fe, sparse sulfides, and variable Cu-Au-V-REE-P enrichments. They have acid altered tops, metal-poor Na±Ca altered bottoms, and skarns may form where carbonate rocks are present. This region allows comparison of igneous-hosted Kiruna-type magnetite-apatite-actinolite occurrences with carbonate-hosted magnetite-dominated iron-skarns, and speculation on their connections with economically important iron-oxide(-copper-gold) (= IOCG) mineralization. Detailed mapping and geochronology in the southern Palen Montains (SPM), CA, reveals a relatively intact Middle Jurassic hydrothermal system with deep Na±Ca alteration and magnetite-apatite mineralization and shallow (near surface) level pyrophyllite(-hematite) advanced argillic alteration. Alteration and mineralization features of the SPM are similar to region-wide patterns involving crustal level, host rocks, and structures as observed in dismembered Jurassic iron-oxide-rich hydrothermal systems.

Synthesis of data from 50 iron-oxide districts and occurrences, many with minor Cu(-Au), reveals co-location and alteration/mineralization similarities, suggesting similar origins for iron-skarn and Kiruna-type occurrences. In the Cordilleran context, arid paleoclimate including and evaporitic surficial fluid sources, mildly extensional tectonics, and diverse magmatic compositions are common to the setting of Mesozoic iron-oxide systems in North and South America. Differences in Cu(-Au) contents may reflect the scarcity of S necessary for sulfide formation in the largely terrestrial setting in southwestern North America whereas the relatively Cu-rich South American IOCG systems formed where marginal marine organic-bearing rocks were present (Barton, 2009, SGA 10, p. 5-7).

Session No. 207

T97. Evolution of REE-Enriched Carbonatite-Alkalic Rock Systems: In Honor of Daniel R. Shawe

Tuesday, 29 October 2013: 8:00 AM-12:00 PM

Colorado Convention Center Room 304

Geological Society of America Abstracts with Programs. Vol. 45, No. 7, p.500

MESOZOIC MAGMATIC FLUXES, NORTHERN CHILE

GIRARDI, James D. and BARTON, Mark D., Dept. of Geosciences, University of Arizona, Tucson, AZ 85721, jgirardi@email.arizona.edu

Mesozoic magma fluxes in the Coastal Batholith of the central Andes reveal a history of episodic magmatism characterized by broad changes in the bulk composition of the arc and changes in plutonic fluxes by as much as a factor of 5. Estimates of magmatic flux were determined through the analysis of more than 400 published radiometric ages and 56 new U-Pb ages, detailed mapping along a transect at ~27oS and spatial analysis of igneous rocks between 26oS - 28oS, and comparison with published data between 18oS - 30oS. Plutonic fluxes were calculated at 10, 5, and 3 My average intervals to establish different temporal patterns and possible magnitudes of plutonic flux. Four episodes of magmatism, 240 - 230 Ma, 200 - 190 Ma, 160 - 100 Ma, and 70 - 60 Ma reached peak plutonic fluxes of 10 - 60 km3per km of strike length per My, with intervening lulls of relative quiescence. Triassic through Early Jurassic plutons were mainly granites and granodiorites, while Early Cretaceous plutons were dominantly quartz diorites and diorites, and latest Cretaceous plutons were mainly granodioritic. Changes in pluton fluxes and compositions correlate with tectonism including back-arc extension, basin inversion, and trans-tensional episodes.

Comparison of the Coastal Batholith with Mesozoic batholiths of the southwestern United States and Canadian Cordillera reveals a similar pattern of episodic magmatism, although the tectonic setting, timing, fluxes, and compositions vary. During orogenic periods, analogous to the modern Andes, the North American batholiths display roughly double the observed plutonic fluxes and compositions become more felsic (tonalitic to granitic) in composition. In contrast, the Chilean Coastal Batholith evolved toward more mafic and juvenile magmas as flux increased into the Early Cretaceous. This distinctive evolutionary pattern also characterizes early phases of Cordilleran magmatism elsewhere, for example in the Jurassic arc of the southwestern United States.

Session No. 115

T169. Volcanology, Mineralogy, Geochemistry & Petrogenesis of Circum-Pacific Magmatism: A Tribute Session to Gerhard Wörner

Monday, 28 October 2013: 8:00 AM-12:00 PM

Colorado Convention Center Room 605

Geological Society of America Abstracts with Programs. Vol. 45, No. 7, p.292

FE OXIDE-CU MINERALIZATION AT THE MINNESOTA AND PUMPKIN HOLLOW DEPOSITS, YERINGTON DISTRICT, NEVADA

RUNYON, Simone E., Geosciences, University of Arizona, 1040 E 4th St, Tucson, AZ 85721, srunyon@email.arizona.edu and BARTON, Mark D., Dept. of Geosciences, University of Arizona, Tucson, AZ 85721

New petrographic and field studies at the Minnesota and Pumpkin Hollow magnetite-rich skarns (Yerington district, Nevada) show that Mt-rich, Si-poor mineralization formed early in a multi-step hydrothermal history, likely related to intense Na-Ca alteration and, presumably, from non-magmatic brines. This contrasts with evidence for later formation of porphyry Cu and Mt-poor, Cu skarns associated with release of magmatic fluids (cf. Dilles et al., 2000, SEG FG 32).

Both Minnesota and Pumpkin Hollow are magnetite(-hematite)-rich, sulfide- and silicate-poor deposits hosted in calcareous Mesozoic sedimentary rocks intruded by the Jurassic Yerington batholith. Metamorphism marbleized limestones and created calc-silicate or Bi-hornfels units in impure sediments. Hydrothermal alteration consists of pervasive endoskarn: deep Na-alteration (Ab-Rut-Chl-Act) at Minnesota, Ca alteration (Grt-Olig-Act±Di±Cc) at Pumpkin Hollow, and Na-Ca (Olig-Act-Sph±Di) at both Minnesota and Pumpkin Hollow. Mt-mineralization dominantly occurs in marble, but also replaces more aluminous rocks (intrusions, hornfels, volcaniclastic rocks). At Minnesota, Fe oxide- mineralization zones from central Mt-replacement to peripheral hornfels-clast Mt-cemented breccia, and distal Mt-clast Mg-silicate-cemented breccias along the marble contact. Magnetite mineralization at Pumpkin Hollow differs in size, in having higher contents of Cu-Fe sulfides and in hornfels hosted Mt-rich breccias and replacements.

Field relations suggest that the Mt-rich mineralization may have formed early, with quartz monzodiorite and added comparatively high Fe/Si; in contrast, the Mt-poor Cu-rich skarns in the district have abundant calc silicates, are linked to the later granitic phases of the batholith which also generated porphyry Cu deposits, and they added lower Fe/Si.. These results support the hypothesis that Mt-rich, variably Cu-bearing systems in the Yerington district are related to early circulation of non-magmatic brines, which transport Fe and Cu, make extensive Na-Ca alteration, widespread Fe-oxide mineralization on upwelling, and locally concentrate Cu. These features parallel those seen in IOCG-type systems worldwide.

Session No. 223--Booth# 14

Economic Geology (Posters)

Tuesday, 29 October 2013: 9:00 AM-6:30 PM

Colorado Convention Center Hall D

Geological Society of America Abstracts with Programs. Vol. 45, No. 7, p.537

U-PB GEOCHRONOLOGY OF LARAMIDE MAGMATISM RELATED TO CU-, ZN-, AND FE- MINERALIZED SYSTEMS, CENTRAL MINING DISTRICT, NEW MEXICO

MIZER, Jason D., Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721, jdmizer@email.arizona.edu and BARTON, Mark D., Dept. of Geosciences, University of Arizona, Tucson, AZ 85721

Eleven new U-Pb dates from key igneous units in the Central Mining District (CMD) of New Mexico document a relatively short duration of magmatism related to multiple, distinct types of mineralized systems. The new dates are compatible (within error) with observed crosscutting relations. Most zircons exhibit simple zoning and yielded only Laramide ages; three samples have inherited cores ranging from 1.4 to 1.6 Ga.

Regionally distributed quartz monzodioritic sills formed first at 60.5 ± 1.3 Ma (3 samples). The granodioritic to granitic Santa Rita (59.5 ± 1.5 Ma) and Hanover-Fierro (58.5 ± 0.9 Ma) stocks are associated with the principal porphyry Cu and Fe skarn and replacement deposits. Felsic dikes, including quartz diorite porphyry (58.5 ± 0.7 Ma), granite porphyry (58.3 ± 0.8 Ma), and late granodiorite porphyry (59.0 ± 0.7 Ma) overlap in age but cut earlier intrusions; they are associated with Zn ± Cu skarn and replacement mineralization. The Copper Flat stock formed at 57.5 ± 0.7 Ma and is associated with breccia and skarn Cu ± Fe. An age of 46.6 ± 0.9 Ma on the monzonite plug in the North Star basin suggests an independent, later phase of magmatism. These ages, when combined with relative ages based on crosscutting relationships, constrains the timing of mineralization of porphyry copper in the CMD associated with the Santa Rita stock to between 61.0 and 59.5 Ma. Available K-Ar and Ar-Ar dates are mostly concordant with these results, indicating rapid cooling from magmatic to Ar closure temperatures.

The ages determined here are similar to many other Laramide porphyry copper-related plutons in the Southwest (~55-65 Ma). Unlike some of the other districts, there is no evidence for older mafic to intermediate magmatism that predates mineralized plutons by 5-10 m.y. Indeed, the ................
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