Metallogenic model and ore-prospecting indicators of Luoda-type siderite deposits in South Qinling

The iron ore is one of the critical resources in our country. The siderite ore deposit is weakly magnetic, making it difficult to prospect for the ore. Considering that discoveries of large-size deposits mainly stemed from understanding the metallogenic regularities of small-size deposits, this paper aims to study the metallogenic process, metallogenic model and metallogenic factors of Luoda-type siderite ore deposits, and put forward ore-prospecting ideas. Based on the regionally geological background of mineralization, the geology of ore deposits, and the Re-Os isochron dating of paragenic pyrite with siderite, the geochemistry of trace elements and stable isotopes, as well as the characteristics of fluid inclusions, are utilized to trace the metallogenic material sources, discuss the metallogenic process and genetic types of the ore deposits, construct the metallogenic model, extract the metallogenic factors, and propose ore-prospecting ideas. The results are as follows: The genetic types of Luoda-type siderite deposits should be classified as meso-epithermal deposits, which formed during the tectonic uplift process resulting from NNE-SSW lateral extrusion from the Late Jurassic to the Early Cretaceous. The Fe was likely mainly derived from mantle fluids and concealed Indosinian granitoid intrusives. The metallogenic fluid was a mixture of mantle fluid and metamorphic fluid, and the siderite's metallogenic process was related to carbonate wall-rocks. We conclude that the metallogenic model of Luoda-type siderite ore deposits can be summarized as a process from the gathering of metallogenic fluids to cooling constrained by uplifting related to lateral extrusion, and wall-rocks. During the Yanshanian lateral extrusion and uplifting, the copper-bearing quartz veins formed in the Late Indosinian were crushed, and the metallogenic fluid was injected into the metallotectonics. The subsequent cooling process led to the precipitation of pyrite and siderite, and initiated the dissolution of carbonate; and this later process promoted the deposition of siderite again. The metallogenic model indicates that mantle fluids, concealed Indosinian granitoid intrusives, tectonic extrusion and uplift, and carbonate wall-rocks are the basic factors for the formation of siderite ore deposits. The metallogenic model of Luoda-type siderite deposits is only formally equivalent to the pyritization alteration zone in the metallogenic model of porphyrite-type iron deposits in the Middle-Lower Reaches of the Yangtze River, and is one of the tectonic-fluid metallogenic systems related to mantle-derived fluids. When prospecting for the Luoda-type siderite deposits, the metallogenic regularities of tectonic-fluid coupling should be followed, and the indirect ore-prospecting indicators are negative anomalies of residual gravity, NWW faults, and wall-rock alterations.