Petrological and geochemical investigation of the Michelin uranium deposit, Central Mineral Belt, Labrador

The Michelin deposit of Labrador’s Central Mineral Belt is a Na-metasomatic uranium deposit which has an approximate resource of 103 M lbs. U₃O₈. The deposit is hosted in a broad series of ca. 1857 Ma fine grained porphyritic (FP) and coarse grained porphyritic (CP) felsic volcanic rocks of the Aillik Group, which have been intruded by several generations of mafic and felsic intrusives. Individual felsic volcanic units are tabular and generally maintain their thickness, striking 070 and dipping southeast40-60°. These rocks were metamorphosed to lower amphibolite facies during the ca. 1800 Ma Makkovikian Orogeny where they were also deformed, imparting a regional penetrative foliation. Individual FP and CP units are nearly identical consisting of a 1:1:1 quartzpotassium feldspar-albite matrix which hosts phenocrysts of potassium feldspar, albite and quartz; the only distinguishable difference being the sizes and abundances of the phenocrysts. Geochemically these rocks are classified as metaluminous, high-K, subalkaline, calc-alkaline rhyolites with an average SiO₂, Na₂O and K₂O content of 74.0, 3.5 and 5.3 wt. % respectively. Mineralization occurs in an approximate 300-350 m wide zone that extends from surface continuously to a depth of ~700 m. This zone ranges in thickness from 20-80 m, which reaches a maximum thickness of 60-80 m at 300-500 m depth, narrowing away from this zone, eventually dying out. The mineralized zone is hosted in a tight sequence of alternating CP and FP units termed the Mine Series. These units are geochemically related and texturally identical to unaltered units but have a drastically different mineralogy and geochemistry due to intense alteration and mineralization. These units are comprised predominantly of albite, which can reach up to 85 %, which forms through the sodic metasomatism of potassium feldspar grains and as secondary albite in spaces create from quartz dissolution. Geochemically these rocks are now metaluminous to peralkaline, low-K, alkaline, calc-alkaline rhyolites with an average SiO₂ concentration of 67.5 wt. %, and Na₂O and K₂O averaging 9.5 and 0.10 wt. % respectively. Uranium mineralization is concentrated within the CP units, and along the contacts with adjacent FP units, adjacent to the contacts with syn-kinematic mafic dikes, and in strongly deformed areas including shear and mylonite zones. Uranium mineralization consists of micron-sized uraninite grains that contain variable amounts of cation substitutions, consisting of radiogenic Pb, Ca, Si, Fe, Zr, Ti, and LREE. Three distinct groupings are recognized 1) uraninite grains that only contain radiogenic Pb, these are often partially-completely surrounded by 2) uraninite grains with Si-Ca-Pb-Zr- Ce, these still contain some radiogenic Pb but interaction with the altering hydrothermal fluid most likely caused the substitution of Ca-Si-Zr-Ce into the uraninite structure, finally group 3) contains large Si concentrations with Ca-Fe-Zr-Ti, these partially surround group 2 and most likely represent continued interaction between uraninite grains and the altering fluid. Uranous ions in the hydrothermal fluid precipitated through the oxidation of hematite and are closely associated with titanite grains. Two styles of alteration affect the felsic volcanic rocks. The first being a widespread pervasive sodic metasomatism, marked by the ion exchange of K⁺ for Na⁺ in potassium feldspar, that envelopes the deposit. This completely strips the rocks of K₂O, Rb and Tl and increases the Na₂O content by as much as 3-5 wt. %. The second style of alteration is concentrated within the mineralized zone and consists of several different styles. The first being quartz dissolution, during which quartz is nearly completely removed from the rock. This is marked by a decrease in SiO₂ by ~8 wt. %. The space created from the dissolved quartz in the groundmass and phenocrysts is filled with secondary alteration minerals including albite, sodic amphibole and pyroxene, calcite, and uraninite. Increased sodic alteration, involving the alteration of biotite and hornblende to sodic amphiboles and pyroxenes, and the formation of secondary albite and sodic amphibole and pyroxene occurs concurrently with quartz dissolution, increasing the Na₂O content of the rock to 9-11 wt. %. The rocks also undergo variable hematitization, due to the oxidation of magnetite. Hematite alteration and uranium mineralization are closely related and higher mineralized zones within the mineralized zone show a more intense hematite alteration, and greater magnetite destruction. Zr is also mobilized during alteration, forming new secondary growth around pre-existing grains, the Zr content within the mineralized zone doubles, from an average of 450 ppm in unaltered rock to 950 ppm in mineralized rock. Mass balance calculations indicate that there are slight enrichments of Al₂O₃, CaO, FeO, Sr, V, Y, LREE, As, and Be associated with alteration, and minor depletions in Ba and Sb. Finally, calcite forms during the later stages of alteration filling remaining available pore space within the rock.

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