Dissertations, Theses, and Capstone Projects

Date of Degree

9-2019

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor

Wayne Powell

Committee Members

Steve Kidder

Haydee Salmun

Jeffrey Chiarenzelli

Subject Categories

Geology

Keywords

ore deposit geology, massive sulfides, remobilization, anatexis, isotopic fractionation

Abstract

Shear zone-hosted orebodies at the Balmat, NY, zinc deposit were remobilized at the kilometer scale during amphibolite facies regional metamorphism ca. 1180 Ma. Despite there being little evidence for interaction of hydrous fluids and ores during deformation, such translocation distances are considered unlikely without the assistance of fluids. Two independent research methods, SEM-EDS micro-petrography and zinc isotope geochemical analysis, demonstrate that anatectic sulfide and silicate melts were generated during metamorphism. Micro-petrography of the Fowler ore body reveals low melting-temperature micro-inclusions of sulfosalt and sulfide assemblages which occur predominantly in Qtz-Py and Kfs-Py domains that cross-cut peak metamorphic assemblages. These results indicate that localized anatexis occurred on the prograde path, producing low volumes of melts of varying composition. Polymetallic sulfide melts were initiated by the prograde breakdown of minor phases containing low melting temperature chalcophile elements including As, Sb, Pb, and Cu. Alkaline silicate±carbonate±sulfide melts were fluxed by halogens, sulfur and other volatile components released during metamorphism of evaporitic and organic-rich horizons in the marble-dominated host sequence.

Measurements of Zn isotopic composition of sphalerite from six Balmat orebodies that originated from the same stratigraphic level (Upper Marble Unit 6) reveal variation between ore bodies, as well as intra-orebody trends of down-plunge decrease in ẟ66Zn. In general, increasing distance of remobilization correlates with decreasing ẟ66Zn. The syn-tectonic isotopic fractionation recorded in Balmat sphalerite is interpreted to have resulted from the interaction between the ore and low-volume sulfide magmas that were fluxed by H2S localized in Upper Marble Unit 7 (fetid dolomite). Lighter isotopes of Zn were enriched in the melt, leaving the residual ore enriched in heavier Zn isotopes. Down-plunge and along-fault migration of fluids resulted in lower ẟ66Zn at the down-plunge end of orebodies that occur in cross-stratigraphic fault zones. These observations support petrographic evidence for the presence of anatectic melts at Balmat and help to explain the unusual scale of translocation at Balmat by means of fluid-assisted remobilization during which rock competency was decreased due to the presence of intergrain fluid films.

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