Geochemical characteristics of chlorite in the Luohe iron deposit in the middle-lower Yangtze metallogenic Belt, Eastern China

Abstract

Chlorite is an alteration mineral commonly found in magmatic-hydrothermal deposits. In recent years, trace elements in chlorite have become an effective way to estimate the location of the mineralization centre (intrusive rocks) in many porphyry deposits, however, the usefulness of this approach has not yet been evaluated for iron oxide apatite (IOA) deposits. The Luohe iron deposit, located in the northwestern part of the Luzong (Lujiang-Zongyang) Basin of the Middle-Lower Yangtze Metallogenic Belt (MLYB), is the largest iron deposit in the MLYB. It is composed of a shallow (380–850 m) and a deep (1350–1800 m) ore body. However, no core intrusive rock has been found in the exploration area, which restricts field exploration and theoretical research of its metallogenic model. Based on the geological characteristics of the Luohe iron deposit, samples were collected from different locations along a 545 m long exploration tunnel and from four drillholes at depths ranging between 870 and 1850 m. The samples were analysed using in-situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to identify the trace element characteristics of chlorite. The results showed significant negative correlations of Fe vs Na, Fe vs Si, and Fe vs K, and a positive correlation of Fe vs Mg in chlorite. The ratios Ti/Pb, Mg/Sr, Ni/V, and Ti/Sr gradually increased from east to west, with the values highest in chlorite of drillhole zk2-1, and decreased northward and southward. The results confirm that the spatial distribution pattern of trace elements in chlorite in IOA deposits is similar to that in porphyry deposits. Therefore, a methodology similar to that for porphyry deposits was herein adopted for estimating the position of the mineralization centre in the Luohe iron deposit. According to the Ti/Sr ratios in Luohe chlorite, we estimate that the concealed mineralisation centre is located southwest of the lower ore body at a depth of approximately 2300–2500 m.