Petrographic and geochemical evidence for a hypogene origin of granite-hosted, vein-type Mn mineralization at the New Ross Mn deposits, Lunenburg County, Nova Scotia, Canada

Abstract

The New Ross Mn deposits consist of two centers of past mining activity located in the 370 Ma South Mountain batholith 10 km north of New Ross, Lunenburg County. The deposits occur as Mn oxide veins and lenses formed along steeply dipping northeast-trending faults. Previous investigators disagree as to whether the most economically important Mn oxides, pyrolusite and manganite, are of supergene or hypogene origin. Recent diamond drilling and geologic mapping have provided new data that support a hypogene origin.The South Mountain batholith in the area of the deposits consists mostly of coarse-grained megacrystic biotite monzogranite. A swarm of small, elongate leucomonzogranite plutons both intrude and are in fault contact with the monzogranite to the north and west of the deposits and at depth below one of the mine workings. These plutons are elongate parallel to a series of persistent northeast-trending topographic linears, which appear to represent faults and/or shear zones. This spatial association suggests that these structures played a role in the emplacement of the plutons. Brecciation and hydrothermal alteration of the leucomonzogranite plutons indicates that movement and hydrothermal activity continued on the faults after final crystallization of the plutons.Several types of hydrothermal alteration are well developed in the faults hosting the Mn mineralization and along several other northeast-trending structures in this region. The diamond drilling shows that, although the Mn mineralization is absent at depth, the fault zone and hydrothermal alteration persist to at least 452 m below the mine workings. Low-temperature hematization and argillic alteration is best developed within the main ore-bearing zone from surface to a depth of about 150 m. With increasing depth, the amount of argillic alteration decreases but hematization remains well developed. Desilicification (episyenitization) accompanied hematization at several localities in the megacrystic monzogranite and the leucomonzogranite intrusions. The desilicification is developed to the point that no primary quartz remains in the rock. Higher temperature alterations such as silicification, greisenization, K feldspathization, and albitization were caused by emplacement of the leucomonzogranite. The silicification occurs as total replacement of the rock and grades through greisenized selvages into unaltered leucomonzogranite or monzogranite. Textural evidence indicates that precipitation of carbonate minerals occurred prior to, during, and after crystallization of the Mn and Fe oxides.The activity of phosphate during the alteration sequences is of particular importance. Below the mine, P 2 O 5 enrichment occurs in the higher temperature silicified, greisenized, K feldspathized, and albitized rocks (up to 5.20 wt % P 2 O 5 ) and is manifest as abundant secondary fluorapatite. Phosphate remained present in the hydrothermal fluid at lower temperatures and resulted in crystallization of fluorapatite with Mn and Fe oxides in the mineralized zones and formation of a 1.2-m-wide quartz-fluorapatite-manjiroite vein (9.95 wt % P 2 O 5 ). Samples of manganite and pyrolusite often contain vug fillings and inclusions of rhodochrosite and minor amounts of other hypogene minerals such as knebelite and pinakiolite. Furthermore, these samples also contain anomalous amounts of Be (90 ppm), W (140 ppm), Mo (28 ppm), Pb (3,000 ppm), and Cu (1,500 ppm) which is consistent with their formation from fluids derived from a magmatic source. These textural relations and geochemical signatures indicate that the alteration sequence and Mn oxides, including those that were previously mined, could all be part of an evolving hydrothermal system developed in upwelling hydrothermal fluids. These processes operated along other northeast-trending faults throughout much of this region of the South Mountain batholith, which suggests a high potential for as yet undiscovered deposits of Mn and other granophile elements.