Stable Isotope (S, Mg, B) Constraints on the Origin of the Early Precambrian Zhaoanzhuang Serpentine-Magnetite Deposit, Southern North China Craton

Abstract

The origin of the Zhaoanzhuang serpentine-magnetite deposit in the southern North China Craton (NCC) is highly disputed, with some investigators having proposed an ultramafic origin, whereas others favor a chemical sedimentary origin. These discrepancies are largely due to the difficulty in determining the protolithic characteristics of the highly metamorphosed rocks. Sulfur, magnesium, and boron isotope geochemistry combined with detailed petrography was carried out in this study to constrain the original composition of the Zhaoanzhuang iron orebodies. Anhydrite is present as coarse crystals intergrown with magnetite, indicating that the anhydrite formed simultaneously with the magnetite during metamorphism rather than as a product of later hydrothermal alteration. The anhydrite has a narrow range of positive δ34S values from +19.8 to +22.5‰ with a mean value of +21.1‰. These values are significantly higher than that of typical magmatic sulfur (δ34S = 0 ± 5‰) and deviate away from primary igneous anhydrite towards mantle-sulfur isotopic values, but they are similar to those of marine evaporitic anhydrite and gypsum (~+21‰). The sulfur isotopic compositions of several samples show obvious signs of mass-independent sulfur fractionation (Δ33S = −0.47‰ to +0.90‰), suggesting that they were influenced by an external sulfur source through a photochemical reaction at low oxygen concentrations, which is consistent with the Neoarchean-Paleoproterozoic atmosphere. Coarse-grained tourmaline from the tourmaline-rich interlayers of the orebodies occurs closely with Mg-rich minerals such as phlogopite, talc, and diopside, indicating that it has a metamorphic origin. The δ11B values of the tourmaline range from −0.2‰ to +3.6‰ with a mean value of +2.0‰, which is much positive relative to that of magmatic tourmaline but is consistent with that of carbonate-derived tourmaline. The magnesium isotopic analyses of the serpentine–magnetite ores and the magnesium-rich wall rocks revealed a wide range of very negative δ26Mg values from −1.20‰ to −0.34‰ with an average value of −0.80‰. The value is higher than that of ultramafic rocks (δ26Mg = −0.25‰) and exhibits minor Mg isotopic fractionation. However, these values are consistent with those of marine carbonate rocks, which have lower δ26Mg values and larger Mg isotopic variations (δ26Mg = −0.45‰ to −4.5‰). Collectively, the S–Mg–B isotopic characteristics of the Zhaoanzhuang iron orebodies clearly indicate a chemical sedimentary origin. The protoliths of these orebodies most likely reflect a series of Fe–Si–Mg-rich marine carbonate rocks with a considerable evaporite component, indicating a carbonate-rich superior-type banded iron formation precipitated in an evaporitic shallow marine sedimentary environment.

Physicochemical characteristics and heavy metals of groundwater during the wet and dry seasons at the Lafarge cement factory environment, Sagamu, Ogun State, Nigeria

“…Magnesium varied from 0.34 at Abattoir to 1.32 mg/L in Trailer Wash during the wet season and from 0.24 mg/L at Modu to 1.46 at Cement Depot, and it is within the 30 mg/L permissible limit of the WHO standard and the 40 mg/L allowable limit of SON. The magnesium sources are the dolomite in sedimentary rocks and serpentines and tremolites in metamorphic rocks (Meng et al, 2019). Though the magnesium level is low, there is the possibility that some of these wells are underlined by rocks.…”

Assessment of Tanzania Magnesite’s Suitability for the Struvite Recovery and Other Industrial Applications

“…Since olivine is the most dominant mineral in peridotite, it is olivine which is carbonated (Grozeva et al, 2017;Hövelmann et al, 2012;Kelemen et al, 2011;Sissmann et al, 2014). It can also form as a result of the alteration of limestone, dolomite, marble, or any other carbonate-rich rock by magnesium-rich solutions during regional contact, or hydrothermal metamorphism (Meng et al, 2019;Rajendran et al, 2013). It might be formed in the regolith above weathering ultrama c rocks and other magnesium-rich rocks (Kelemen et al, 2011).…”