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Alteration and infiltration : documenting controls on skarn formation at Mineral Hill, Sechelt, southwestern British Columbia McConaghy, Katharine R.


The Mineral Hill wollastonite deposit is hosted by a north-west trending calcareous roof pendant enclosed within Late Jurassic plutons of the southwestern Coast Plutonic Complex. The study area consists of calcite marble, other meta-sediments and skarn in contact with a dioritic component of the Crowston Lake Pluton. The area is cross-cut by two Cretaceous-aged dike generations (D2 and D3). Detailed mapping, petrography, petrology and O and C stable isotope analyses has led to the interpretation of a complex infiltration history of the study area. High temperature mineral production (i.e. wollastonite), skarn 18O/16O ratios, and extensive SiO2 metasomatism indicate magmatic volatiles infiltrated and exchanged with the roof pendant during Late Jurassic pluton emplacement creating spatially extensive wollastonite and garnet skarn. Homogeneously depleted marble 18O values near the wollastonite skarn boundary require interaction with a low δ18O fluid (meteoric) at high temperatures. Because very low δ18O values (< 5 permil) for marble are spatially associated with the pluton, and because both D2 and D3 dikes preserve textures that indicate a cold crust at the time of emplacement, a high temperature meteroic fluid must have 18 infiltrated pre- to syn- skarn formation during the Late Jurassic. Finally, low δ18O values preserved in D2 and D3, require at least one low δ18O fluid interaction event either during Cretaceous syn-dike emplacement (D2 and D3, or D3) as a response to thermal activity or during a post-Cretaceous high temperature event. This study also documents the nature and evolution of permeability at the wollastonite skarn/marble boundary within a 450 m by 150-200 m map area. Because syn-metamorphic permeability is destroyed by compaction, I used reaction transport theory to deduce paleo-fluid flow geometry. The distribution of multiple tracers (i.e. SiO2, degraphitization, and 18O/16O) are used in order to distinguish between infiltration sides in which flow is parallel to the alteration boundary and infiltration fronts in which flow is perpendicular to interface geometry. At Mineral Hill, a dominance of infiltration sides and field observations support an irregular and interfingering contact between wollastonite skarn and marble. This geometry may be controlled by reaction infiltration instabilities (RII) at the reaction front which are derived from positive feedback coupling between infiltration and reaction [Ortoleva et al, 1987]. RII requires dissolution at the reaction front which allows fluid to focus into areas of high permeability.

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