Geochemistry, Mineralography and Rare Earth Elements Distribution of Gossans related to Volcanogenic Massive Sulfide Deposit, Case Study: Ghaleh-Rigi, Southwest of Jiroft, South of Iran

Erosion and oxidation of massive sulfides when uplifted and exposed to the surface, commonly lead to the formation of gossans. In this process, surface water will dissolve soluble elements, and oxides and hydroxides of iron (goethite and hematite) will form on top of the volcanogenic massive sulfide (VMS) deposits. The main tectonic settings for Iranian VMS deposits are magmatic arcs, which can be subdivided into volcanic primitive arc, arc/intra-arc rift, and back-arc settings and Sanandaj-Sirjan zone is one of the structural zones that host many VMS deposits in Iran (Mousivand et al., 2018).The study area is located southwest of Jiroft, Kerman province. The main rock units include vitric tuff, pelagic sediments, volcano-sedimentary rocks, gabbro and intermediate to mafic dykes. Mineralization has occurred in volcano-sedimentary beds. The pelagic sediments which are composed of limestone, shale, sandstone, siltstone and interlayers of pillow lava, are the main hosts for mineralization. Surface oxidation of mineralized zones has led to conversion of primary sulfides to iron oxides and hydroxides to form gossan. This study contributes to mineralogical and geochemical composition and mineralization of gossans to demonstrate how surface oxidation of primary sulfides can play a role in locating VMS mineralization at depth.

A geological map with a scale of 1:5000 was prepared during field and laboratory studies. Twenty polished section were studied to identify mineral distributions and textures, and some of them were chosen for scanning electron microscopic (SEM) examinations. Fifteen rock samples from the gossan horizons were chosen for geochemical studies. The samples were taken from across the mineralized horizon. Six rock samples were taken from old mining site outcrops to compare the geochemistry of gossans with other surface mineralization. All samples were sent to the laboratory for analysis by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). The rare earth element (REEs) values were measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). X-ray diffraction (XRD) spectroscopy was used to identify mineralogy of 30 rock samples. All analyses were performed in the central laboratory of the Geological Survey and Mineral Exploration of Iran, in Tehran and Karaj.

The ore and gangue minerals have massive, layered, disseminated, veinlet, breccia and replacement textures. Based on mineralography, XRD and SEM studies, the main minerals are hematite, goethite, quartz, and jarosite-group minerals. The upper horizon of gossan, with 13 meters thickness has large volume of hematite and gothite minerals. The enrichment of gold, arsenic, antimony, silver, lead and bismuth were observed in this zone. The lower horizon, with a thickness of about 1.5 meters show anomalies of copper and zinc elements. The highest amount of gold and silver were measured about 18.5 and 120 g/ton, respectively. The highest amount of lead element is 1.3 wt.%, which shows a positive correlation with silver variations. The other values are copper 0.16 wt.%, arsenic 0.61 wt.%, bismuth 580 g/ton, and antimony 280 g/ton.

Trace and REEs geochemistry are useful in identifying gossans and probable sources (Scott et al., 2001). Geochemical studies also can be used to separate mature from immature gossans. Although the composition of gossans is influenced by early composition of the ore, gossans with high content of Pb (more than 4 wt.%) are usually considered immature. The average Pb measured in the studied gossans is about 2210 g/ton. The Ag content is also low (less than 150 g/ton) and there is a relatively linear relationship between increasing Ag and Pb content. High values of copper often refer to a lower degree of maturity. In the studied gossans, the average amount of Cu is about 2900 g/ton, which is much lower than the immature gossans with average 1.6 wt.%. Therefore, the results of chemical analysis indicate that these gossans are in the category of mature ore bearing gossan.The REE from La to Lu, is relatively consistent with the shape of REE profiles for volcanogenic massive sulfide mineralization and concurrent massive sulfide gossans (Peter et al., 2003; Volesky et al., 2017; Gieré, 1993). The pattern of distribution of REEs shows small positive Eu enrichment and zoning of precious mineral elements confirms the possibility of orebody under the gossans. Further exploration of volcanoge