Some evidences for Carlin-type alteration and gold mineralization in carbonate breccia at Yemi area, Taebaegsan basin, South Korea

Abstract

Carbonate-hosted ore deposits, including various metal deposits, are useful resources globally. Among these, Carlin-type gold deposits are an important type responsible for 4% of global gold production. In the United States, China, and Canada, development and exploration have been conducted on the tectonic environment, stratigraphy and structure, and geochemistry of Carlin-type deposits. In South Korea, fundamental geochemical and geophysical exploration of the Paleozoic limestone area of Taebaegsan basin has been intermittently conducted to confirm the potential for Carlin-type deposits, but no work has been done for the development of any deposits. Carlin-type gold deposits are generally associated with carbonate breccia as the host rock for mineralization or alteration. In this study, carbonate breccia distributed in the Yemi area of the Taebaegsan basin was assumed to have a high possibility of Carlin-type gold mineralization, and research was conducted on the Yemi carbonate breccia to investigate the origins of this breccia through geological and geochemical studies, and to evaluate the possibility of Carlin-type gold mineralization using the breccia as a host rock, and also to provide useful basic data for exploration or development. Approximately 200 specimens collected from the research area were sampled and classified by field and microscopic observation. Analyses of the major elements, trace elements, and rare earth elements were carried out to determine the geochemical characteristics of the classified samples. Potential areas were evaluated by statistical regression analysis and factor analysis based on the geochemical data. Electron probe microscopic analysis (EPMA) was conducted to determine the geochemical characteristics of gold-bearing pyrite and hematite in the host rock. Analyses for platinum group elements were conducted to evaluate the dikes sampled in the research area as a possible source of gold. The carbonate breccia in the Yemi area is classified into crackle, mosaic, and chaotic breccia based on its morphology. The breccia can be also classified based on its matrix mineralogy as silica–calcite–hematite matrix, silica–calcite–pyrite matrix, or calcite-matrix breccia. The hydrothermal alteration in the research area shows a successive spatial relationship and silicification, argillization, and decarbonatization are observed. The stages of mineralization were classified into the pre-ore stage, reduced-fluid ore stage, oxidized-fluid ore stage, and post-ore stage on the bases of mineral paragenesis and alteration characteristics. Pyrite and hematite were classified by morphology and stage of formation through microscopic observation. In the reduced-fluid stage, pyrite was mainly formed, while in the oxidized-fluid stage, mainly hematite was formed. Electron microscopic analysis of classified pyrite and hematite shows that gold mineralization in pyrite and hematite formed in the reduced-fluid and oxidized-fluid stages. The gold occurrence is divided into three types, gold contained in pyrite (Type A), gold independently observed in oxidized pyrite and host rock (Type B), and gold contained in hematite (Type C). The gold content was 0.37–0.67 wt% for type A, 1.5–12 wt% for type B, and 0.07–0.12 wt% for type C. The pyrite (As: 0.38–8.87 wt%) formed in the reduced-fluid stage is higher in gold content and arsenic content than hematite (As < 0.11 wt%) formed during the oxidized-fluid stage. Mapping images of gold-bearing pyrite shows zonation of the arsenic content, and gold is confirmed in arsenic-rich pyrite. Analysis results of major elements and trace elements were classified into altered limestone, non-gold-bearing breccia, and gold-bearing breccia. The values of major and trace elements increased as the content of SiO2, Al2O3, Fe2O3, As, Sb, and Pb changed altered limestone to gold-bearing breccia. On the other hand, the contents of CaO and MgO decreased. The rare earth elements show a similar pattern in non-gold detected and gold detected breccia, but are more enriched in gold-bearing breccia. The gold concentration shows positive correlation with elements typically enriched (Ag, As, Tl, Sb) and depleted (W, Cu, Pb, Zn) in Carlin-type deposits except for Tl and W. The enrichment factor shows a pattern similar to that of a general Carlin-type deposit, but has a low Tl content and enriched base metal content. Analysis of the platinum group elements shows that Rh is depleted and Ru enriched. Six factors were calculated (Factor 1: Al2O3, -LOI (loss of ignition), -CaO, SiO2, TiO2, P2O5, Tl, Ba, Fe2O3; Factor 2: Pb, Zn, Sb; Factor 3: Na2O, Zn, Cu, Ba; Factor 4: W, MnO; Factor 5: Bi, Fe2O3; Factor 6: -MgO, K2O) using factor analysis of the major elements and trace elements (As, Ba, Bi, Cu, Pb, Sb, Tl, W, Zn). A distribution map of the calculated factor scores, arsenic content, carbon isotopes, and oxygen isotopes was prepared. Arsenic distribution and Factor 2 distribution tend to be similar. An isocon diagram of limestone – non-gold-bearing breccia and limestone – gold-bearing breccia shows that Bi, As, Sb, Mo, Rb, Pb, Zr, K2O, Fe2O3, and SiO2 of gold-bearing breccia are relatively enriched while the CaO content tends to decrease. The carbonate breccia in the Yemi area is a hydrothermal-solution breccia formed by decrease in volume resulting from decarbonatization by hydrothermal fluids supplied along surfaces such as faults, folds, thrusts, and bedding; Carlin-type gold mineralization appeared to accompany this process. The appearance of and mineralogical differences in the matrix are thought to be caused by the degree of alteration; these differences seem to be due to differences in the rock types and structure. Gold mineralization is accompanied mainly by silicification, argillization, and decarbonatization. Looking at the magma fertility using platinum-group elements, the research area dikes plot in either the barren or porphyry copper areas. Magma fertility does not directly indicate gold mineralization, but the magma cannot be excluded as a source rock for the gold mineralization. Considering the occurrence and alteration characteristics of carbonate breccias in the research area, it is estimated that most of the gold-bearing breccias were influenced by oxidized fluids. Because the gold-bearing pyrite was formed during the reduced-fluid stage, the bonanza of Carlin-type gold mineralization is presumed to be located in the lower part of the oxidation zone. Considering to occurrence of gold and geochemistry, gold-bearing fluids dissolved iron from the host rock by decarbonatization, and sulfur and arsenic in the hydrothermal fluid combined to form pyrite or arsenian pyrite. At this same time, it appears that gold, transported in the form of bisulfide, separated and precipitated in pyrite. Gold precipitated in the pyrite (Type A) formed in the reduced-fluid stage is oxidized during the oxidized-fluid stage, during which the gold in pyrite is eluted and redeposited along the edge of pyrite grains or in the host rock (Type B). When pyrite has been completely replaced by oxidizing fluid, a relatively low content of gold remains in hematite (type C). It is judged that gold was eluted by oxidation in type B. The characteristics such as the distribution of alteration zones, hydrothermal minerals, geochemistry, and gold mineralization identified in the Yemi breccia are very typical in Carlin-type gold mineralization, which is also accompanied by formation of hydrothermal-solution breccia and other alteration zones in the limestone.