Abstract
Carbonaceous material (CM) is common in meta-sediments and is generally interpreted to be intimately associated with gold mineralization. For the Bumo deposit in Hainan Province, South China, CM is mainly hosted by greenschist facies—to amphibolite-facies metamophic rocks of the Paleo—to the Mesoproterozoic Baoban Group, and by auriferous veins which could be used as an important gold prospecting indicator. However, the genesis of CM and its relationship with gold mineralization are still unclear. From the field work and thin section observations two types of CM occur, i.e., layered and veinlet. The layered CM occurred in CM-bearing black shales, up to meters thick, and prevails in the deposit. More importantly, Au-bearing sulfides are commonly distributed along the boundary between the quartz veins and layered CM. In contrast, the veinlet CM, co-precipitated with native gold and sulfides, has the thickness of micro- to centi-meters, and these thin veins occur in quartz veins and hydrothermally altered rocks. In addition, layered CM has a stringy shape and laminate structure, while veinlet CM occurs as isometric particles based on the Scanning Electron Microscope (SEM) analysis. The Raman carbonaceous material geothermometer indicates that layered CM with a high maturity is formed at elevated temperatures of 400–550 °C, consistent with X-ray diffraction (XRD) analysis. In contrast, veinlet CM with a low maturity is formed at 200–350 °C and generally consistent with gold mineralization. In addition, layered CM has δ13C values ranging from −30 to −20%, demonstrating a biogenic origin. Consequently, it is interpreted that layered CM is formed by a pre-ore metamorphic event during Caledonian, and its reducing nature promotes gold precipitation via destabilization of aqueous Au complexes or facilitating sulfidation. Veinlet CM is of hydrothermal origin, and its precipitation modified the chemical conditions of ore fluids, leading to the destabilization of Au complexes, which therefore are favorable for mineralization.
Highlights
Carbonaceous material (CM) was reported to be found in many metasediment-hosted orogenic gold deposits, such as the Macraes deposit in New Zealand [1], Cosmo Howley in Australia [2], and Syama, Obuashi, and Inata deposits in west Africa [3]
The relationships between CM and gold mineralization have been proposed to be: (1) Pre-mineralization CM as a reducing agent to cause the precipitation of native gold and sulfides [5], (2) pre-mineralization CM absorbing Au complexes and Minerals 2020, 10, 708; doi:10.3390/min10080708
Minerals 2020, 10, 708 leached by syn-ore fluids [6,7,8], (3) pre-mineralization CM as a physically weak part to facilitate shear zones that localize ore veins [9], and (4) syn-mineralization CM co-precipitated with native gold and sulfides due to hydrothermal activities [10]
Summary
Carbonaceous material (CM) was reported to be found in many metasediment-hosted orogenic gold deposits, such as the Macraes deposit in New Zealand [1], Cosmo Howley in Australia [2], and Syama, Obuashi, and Inata deposits in west Africa [3]. Minerals 2020, 10, 708 leached by syn-ore fluids [6,7,8], (3) pre-mineralization CM as a physically weak part to facilitate shear zones that localize ore veins [9], and (4) syn-mineralization CM co-precipitated with native gold and sulfides due to hydrothermal activities [10]. The Bumo deposit, with a gold reserve of about 13 t and an average grade of 9.97 g/t, is hosted by metasediments in the Paleo- to Mesoproterozoic Baoban Group. This paper uses an integrated analytical approach, combined field and petrographic work, SEM, laser Raman, XRD, and carbon isotope, to identify the two types of CM and their genesis and relationships with the gold mineralization in the Bumo deposit
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