Date of Award

Winter 2014

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Earth and Environmental Sciences

First Advisor

Samson, Iain M.

Second Advisor

Gagnon, Joel E.

Keywords

Earth sciences, Tin deposit, Tungsten molybdenum deposit, Fluid immiscibility, Fluidinclusions, Fluorite chemistry, Focused ion beam, Laser ablation

Rights

CC BY-NC-ND 4.0

Abstract

At Mount Pleasant, New Brunswick, Canada, Sn-rich and W-Mo-rich mineralized zones occur in hydrothermal breccia pipes and granites. Several fluorite types occur associated with Sn, W, and Mo minerals, base-metal sulphides, or are barren of sulphides or oxides. Fluorite associated with Sn and W has complex cathodoluminescence (CL) patterns, suggesting episodic fluid infiltration and interplay among fluids of different character. Other fluorite types have simpler CL patterns, suggesting a less complex fluid environment. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) results show that fluorite associated with W-Mo has higher W/Sn ratios than fluorite associated with Sn mineralization. The rare earth element (REE) content of fluorite associated with Sn, W, and Mo mineralization is higher than other fluorite types. These trace-element data allow discrimination of mineralized and barren assemblages and indicate that the metal endowment reflects fluid chemistry. All fluorite types have flat chondrite-normalized REE patterns with insignificant tetrad patterns, and a negative Eu anomaly, similar to the patterns of the associated Mount Pleasant granites, indicating that fluorite composition reflects that of magmatic fluids. The low REE content of barren fluorite suggests mixing of magmatic and meteoric fluids during the late stages of fluorite deposition. Fluorite associated with cassiterite contains primary, co-existing, dilute, liquid-vapour and saline liquid-vapour-solid fluid inclusions, with contrasting salinity and homogenization temperatures (< 250 °C and > 425 °C, respectively), showing that immiscible liquids coexisted prior to cassiterite precipitation; the temperature differences are interpreted to have resulted from post-entrapment stretching. Cassiterite precipitated from low salinity, low- to moderate-temperature fluids. Raman spectroscopy, scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), and focused ion beam (FIB) with SEM-EDS showed that solids in LVS inclusions include halite, rokühnite or hibbingite, douglasite, and hematite, as well as Pb-, Zn-, K-, and Sn-bearing chlorides. The FIB-SEM-EDS analyses demonstrate that halite could be misidentified, resulting in erroneous salinity calculations. LA-ICP-MS and FIB data show that fluid inclusions in fluorite contain abundant Na, K, Fe, Mn, and Zn with lesser Pb, Sn, and W. The application of NaCl-H2O phase equilibria to such a complex fluid would result in inaccurate salinity estimates for saline inclusions.

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