Magnetite texture and trace element evolution in the Shaquanzi Fe-Cu deposit, Eastern Tianshan, NW China

Abstract

The Shaquanzi deposit is an important Fe-Cu deposit in the Yamansu Fe mineralization belt, Eastern Tianshan. Three types of magnetite were distinguished based on the textural and mineral assemblages, including the platy mushketovite (TA), granular magnetite coexisting with K-feldspar and epidote (TB), and granular magnetite intergrown with pyrite (TC). TA magnetite consists of three generations, including a bright core (TA-1), a dark-gray inner rim (TA-2), and a light-gray outer rim (TA-3), under BSE observations. TA-1 magnetite was transformed from the primary hematite due to a sharply reduced oxygen fugacity and retains its platy pseudomorph. With the changing temperature (increase or decrease), TA-1 magnetite underwent dissolution and re-precipitation (DRP) processes to form TA-2 and TA-3 magnetite. TB magnetite consists of two generations (TB-1 and TB-2). TB-1 magnetite contains abundant mineral inclusions and microporosity. With the decrease of fO2 and temperature, TB-1 magnetite underwent a fluid-assisted recrystallization process to form TB-2 magnetite with foam texture. TC magnetite consists of four different generations (TC-1, TC-2, TC-3, and TC-4). TC-1 magnetite with densely-distributed oscillatory zonings (generally smaller than 1 μm) is surrounded by the darker TC-2 magnetite. TC-2 magnetite with foam texture may be formed by recrystallization of TC-1 magnetite. TC-3 magnetite replaced or cut TC-2 magnetite, indicating that it was formed by the DRP process of TC-2 magnetite. Following periodic changes of fO2 and temperature, TC-4 magnetite with oscillatory zones formed around TC-3 magnetite.This study proposes a new Fe/(Mg + Al + Mn) vs Ti discrimination diagram to distinguish hydrothermal and magmatic magnetite. Magnetite compositions of the Shaquanzi deposit are quite different from those of magmatic magnetite, but very similar to those observed in the hydrothermal system such as IOCG and skarn deposit, indicating that magnetite of the Shaquanzi deposit was formed by hydrothermal process rather than magmatic differentiation.