A geological and geochemical study of the Skidder basalt and Skidder trondhjemites : and the geology, ore petrology and geochemistry of the Skidder Prospect and its accompanying alteration zone: Buchans area, central Newfoundland

Pickett, Jacob Wayne (1988) A geological and geochemical study of the Skidder basalt and Skidder trondhjemites : and the geology, ore petrology and geochemistry of the Skidder Prospect and its accompanying alteration zone: Buchans area, central Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Skidder Basalt, which outcrops in an area of central Newfoundland immediately to the northwest of Red Indian Lake and approximately 12 km southwest of the town of Buchans, is a spilitized, tholeiitic, sub-alkaline assemblage composed mainly of mafic yariolitic and non-variolitic pillow lava, mafic pillow breccia and massive mafic flows; these extrusive rocks are interlayered with lesser amounts of mafic pyroclastic rocks and chert, and are intruded by diabase dykes. Units typically strike northeast to east-northeasterly and dip steeply; facing directions are typically northwesterly, although local reversals, particularly in the vicinity of the Skidder Prospect, are noted. Foliations trend northeasterly and dip steeply. Several northeasterly, northerly and northwesterly trending lineaments are evident on aerial photographs, and many correspond to linear magnetic features. -- Rocks of the Skidder Basalt are dominated by secondary mineral assemblages. Typically, albitized plagioclase phenocrysts occur in an intergranular to intersortal groundmass of albite, chlorite, quartz, dense granular sphene, and variable amounts of subhedral to acicular opaque minerals. Primary clinopyroxene and chromite, and secondary calcite, amphibole and epidote are constituents of some samples. Clinopyroxene and albitized plagioclase in some low-Zr Skidder basalts exhibit quench-texture morphologies. Formation of varioles, present in some of the low-Zr Skidder basalts, is suggested to be a result of quenching. Spilitization of the Skidder Basalt has involved albitization of plagioclase, chloritization of basaltic glass, alteration of ferromagnesian minerals to chlorite and lesser amphibole, and alteration of opaque Fe-Ti oxides to dense intergranular sphene. Geochemically, spilitization has resulted in redistribution of SiO₂ and total iron, removal of K₂O and MgO, and extensive addition of Na₂O. The spilitization is suggested to have resulted from interaction of the rocks with circulating seawater under conditions of low seawater/rock ratios. -- The chemistry of Skidder Basalt clinopyroxenes and chromites suggests it has greater similarity to ocean-floor basalts than to basalts formed in an island arc environment. The Skidder Basalt rocks define tholeiitic trends and plot either within the ocean floor basalt field or overlap the ocean floor basalt and island arc tholeiite fields on trace element variation diagrams. It is geologically and geochemically more similar to the pillow lava sections of ophiolite complexes than to the Buchans Group basalts. The Skidder Basalt probably formed in an extensional tectonic environment at a slow-spreading oceanic or back-arc basin ridge. -- Low-Al₂O₃, oceanic-type trondhjemite dykes and pods intrude, or are interlayered with, the Skidder Basalt in places. They are chemically similar to trondhjemites considered to represent late magmatic differentiates of basaltic magma. -- The Skidder Prospect is an ophiolite-type volcanogenic massive sulphide deposit hosted by basaltic pillow lavas, mafic pillow breccias and aquagene tuffs of the Skidder Basalt. Brecciated, quartz-veined, unlayered and lesser bedded jasper and jasper-rich siltstone are spatially associated with the massive sulphides. Trondhjemite dykes intrude rocks in the Skidder Prospect area in several places. -- The deposit contains possible and probable reserves of 200 000 tonnes grading 2% copper and 2% zinc accompanied by very minor amounts of lead. The sulphides occur mainly in two lenses composed of semimassive to massive unlayered and layered pyrite containing lesser amounts of chalcopyrite and low-iron sphalerite. Rare galena, hematite and magnetite are also noted. Quartz, chlorite and lesser calcite are the predominant gangue minerals. Abundant disseminated sulphides, mostly pyrite, occur in a quartz and/or chlorite ± talc-rich stringer zone underlying the massive sulphides. -- Distinct alteration zones characterized by secondary mineral assemblages envelop the massive sulphide lenses and flank the underlying stringer zone, typically up to 150 m away from the sulphide-bearing zones. The alteration primarily involves large increases in the amount of intersertal chlorite and quartz; replacement of mafic minerals by chlorite; and replacement of albitized plagioclase by quartz and lesser phengitic sericite. Quartz has been removed and replaced by chlorite, in places. Chlorites from the alteration zone are significantly enriched in magnesium relative to those of typical spilitized Skidder Basalt. Calcite and epidote, which occur in abundance in the Skidder Basalt, are absent in the most intensely altered rocks of the Skidder Prospect alteration zone. -- Geochemically, the alteration is characterized by sporadic enrichment of K, Rb, Ba and Pb; depletion of Ca, Sr and Na; and redistribution of Si in most of the altered rocks. Magnesium and zinc are enriched in intensely chloritized zones. The Zn probably occurs in tiny sphalerite grains intimately associated with high-Mg chlorite. The sporadic enrichment of K and related elements is evident up to 400 m away from the sulphide-bearing zones; the other geochemical effects are recognizable only about 150 m away. Incompatible elements Zr, Y, P and, to a lesser extent, Ti; and compatible elements Cr and Ni have remained stable, even in intensely altered rocks. Chondrite-normalized rare earth element patterns associated with sulphide-poor samples, including some that are relatively unaltered to others that are silicified and intensely chloritized, are similar to those of spilitized Skidder Basalt. Sulphide-rich samples are depleted in REE concentrations relative to the others and some show relative depletion of Ce and the middle REE; characteristics shown by chondrite-normalized REE patterns for seawater. -- Lead isotope ratios of the Skidder prospect sulphides are some of the least radiogenic of Newfoundland mineral deposits. On the ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagram, they plot along the mantle lead evolution curve of Zartman and Doe (1981). -- The genetic model suggested for formation of the Skidder Prospect is similar to that proposed for other ophiolite-related massive sulphide deposits. Metals are suggested to have been leached from underlying rocks by heated, deep circulating, modified seawater possibly similar in composition to that being emitted at present on the East Pacific Rise. The metals were probably carried in solution as chloride complexes. Local faulting provided upward access to the seafloor for the metal-bearing hydrothermal fluids where the massive sulphides are suggested to have been deposited possibly in a manner similar to those now forming on the East Pacific Rise. Mixing between the hot, upwelling, metal-bearing hydrothermal fluid and cool, shallow-convecting seawater is suggested to have produced the associated alteration effects and the disseminated-sulphide stockwork zone. Much of the spatially associated jasper has probably been produced by oxidative leaching of sulphides exposed on the seafloor.

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