Geochemistry and origin of piemontite-bearing and associated manganiferous schists from Arrow Junction, western Otago, New Zealand

Abstract

Closely spaced analyses for major, minor and trace elements have been made through 3 m of manganiferous quartzose schists and immediately overlying quartzofeldspathic schists at Arrow Junction, Otago, New Zealand. Chemically the quartzose schists correspond to pelagic siliceous muds or brown clays rather than to typical cherts. Whole-rock oxidation ratios Fe3+ · 100(Fe3+ + Fe2+) increase from 16 to 99 with increasing Mn. Apart from SiO2, Al2O3 is the most abundant oxide in these rocks. Al2O3 correlates negatively with SiO2 and positively with all other components analysed in the manganiferous quartzose schists, including Fe2O3, MnO, TiO2, MgO, CaO, Na2O, K2O, Ba, Cr, Cu, Ni, Pb, Rb, V, Y, Zn and Zr. Parameters such as Al2O3 · 100(Al2O3 + MnO) and Fe/Ti indicate that the quartzose schists have a higher hydrothermal plus hydrogenous component than average pelagic clays, but a larger terrigenous component than typical metalliferous sediments from the crest and proximal flanks of the East Pacific Rise. Close covariance of Al2O3 with TiO2 and Zr results in ratios TiAl≈ 50 · 10−3 and ZrAl ≈ 2 · 10−3, conforming closely with average shale and average New Zealand Alpine schist metasediments, but less closely with average andesite. The data exclude basalt or rhyolite as the dominant aluminous detrital component. Al, Ti and Zr are regarded as being of predominantly terrigenous origin. Normalized for constant Al2O3, the following elements are enriched in grey-green and in pink piemontite-bearing Arrow Junction quartzose schists relative to average New Zealand Alpine metase dimentary rocks, by average factors indicated: Mn (32, 44), Ni (15, 15), Cu (5, 12), Si (3.6, 2.6), Pb (2, 4), Zn (2.8, 2.5), Fe (1.8, 1.9), Y (1.7, 1.8), Ba (1.2, 1.6) and Sr (1.0, 2.0). The metal enrichment trend can be regarded as being imposed on the products of a silica dilution trend in which Al2O3 was reduced to ∼ 4% by dilution of end-member terrigenous sediment with non-terrigenous silica. About 97–98% of the Mn, 93% of the Ni, 79–91% of the Cu, 73-61% of the SiO2, and 43–48% of the Fe is considered to be of a non-terrigenous hydrothermal—hydrogenous—biogenic origin in the quartzose schists. Covariance of elements other than Si with Al2O3 within the single bed investigated is attributed firstly to accumulation of FeMn-oxides at a rate essentially constant relative to that of clay minerals and other aluminous phases, though diluted by varying amounts of largely biogenic silica, and secondarily to the sorptive, scavenging properties of the FeMn-oxides and of clay mineral components of the sediment. Although the base metals may ultimately have been introduced to the ocean from submarine hydrothermal systems these sediments originated at a site relatively remote from a fast-spreading ridge or other active submarine hydrothermal source.