A fluid inclusion and trace element geochemical study of the granite-hosted, St. Lawrence fluorspar deposits and related host rocks

Collins, Celestine James (1992) A fluid inclusion and trace element geochemical study of the granite-hosted, St. Lawrence fluorspar deposits and related host rocks. Masters thesis, Memorial University of Newfoundland.

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Abstract

The St. Lawrence Fluorspar Deposits, on Newfoundland's south coast, are vein-type deposits hosted by and genetically related to the Devonian St. Lawrence Granite. The alkaline to peralkaline granite was emplaced to high crustal levels in a post-tectonic, extensional regime that followed a major Silurian orogenic event. In the late stages of a protracted cooling history, this relatively dry, hypersolvus granite magma evolved the F-rich fluids from which the veins were formed. -- The fluorspar veins are structurally controlled, occurring as open-space filling in fracture systems that developed in response to both regional stress regimes and localized regimes generated by the cooling and crystallizing of the granite pluton. The vein mineralogy is dominated by fluorite, with lesser amounts of quartz, calcite, barite and minor sulphides. The fluorite is highly variable in colour and primarily coarse grained and delicately banded, although fine grained varieties and breccia zones are common in the larger veins. The vein contacts are mostly sharp with little or no evidence of intense wall-rock alteration along the vein margins. -- The chemical and physical conditions that prevailed during the deposition of the veins were investigated using geochemical and fluid inclusion studies of growth-zoned, brecciated and other more homogenous samples from various veins. The zones and growth directions were defined by macroscopic and microscopic examination of hand specimens, combined with field observations where possible. Growth zones, in the vein samples, range in thickness from 100 to 1 mm, with the majority ranging from 10 to 5 mm. -- Microthermometric investigations of fluid inclusions in fluorite and other vein minerals indicate homogenization temperatures ranging from 500ﾟC to 50ﾟC and fluid salinity ranging from 30 to 0 equivalent weight percent NaCl. As a consequence of the high-level, relatively low-pressure environment in which the veins were formed, it is inferred that homogenization temperatures closely approximate formation temperatures. Variations between growth zones indicate that fluorite precipitation took place through a wide range of fluid temperatures and salinity. The cyclic nature of these variations can be related to individual fluid pulses, which in turn reflect changing fluid conditions within larger evolutionary cycles. The temperature and salinity variations can he accounted for by the unmixing of a supercritical magmatic fluid into a low salinity vapour and a high salinity liquid at near magmatic temperatures. These fluids were further modified by condensation, boiling and mixing as they migrated through the higher levels (low pressure/temperature) of the conduit system. Fluorite precipitation appears to have been primarily in response to the increasing pH of the fluid, caused by boiling or incursion of lower pH ground waters. -- High precision trace element analyses of fluorite, calcite and host rocks were conducted utilizing inductively coupled plasma-mass spectrometric analysis (ICP-MS), supplemented by whole rock analysis by X-ray fluorescence spectometry (XRF). The fluorite is relatively enriched in Y and REE with the former averaging 660 ppm and the latter 240 ppm. Systematic changes in REE concentrations in fluorite are interpreted to reflect changing fluid conditions from early to late stages of mineralization. The early-stage fluorite is characterized by LREE enrichment and HREE depletion while the late-stage fluorite is characterized by LREE depletion and MREE to HREE enrichment. The systematic variations of REE concentrations in fluorite reflect dramatic changes in fluid composition with progressive evolution. Such REE behavior may be a result of reversals in the dominance of F or Cl complexing of the REE, either in the magmatic or hydrothermal regimes. -- The apparent partitioning of the REE in fluorite was estimated by normalizing the fluorite to a fluid composition calculated, using appropriate vapour/melt partition coefficients, from a late-stage St. Lawrence Granite phase (granitic porphyry dyke). The REE in fluorite suggest a strong influence of the size (ionic radius) of the substituting ion (i.e. crystal structure control) on trace element partitioning, with a preferred substitution site size at or near the ionic radius of Dy (0.103 nm [1.03Å]). Since REE substitution in the fluorite structure is believed to primarily involve 'free-ion' REE species, the relative stability of REE fluoro-complexes, in the aqueous fluid, dramatically affect the availability of REE for substitution. The stability of such aqueous REE-complexes, and consequently the concentration of REE in fluorite, is strongly dependant on the temperature, pH and F activity of the fluid.

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