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[HRE13-P08] Mineralogy, fluid inclusion and sulfur isotope characteristics of the Namicupo gold prospect, Mozambique Belt, northeastern Mozambique
Keywords:Namicupo gold prospect, Fluid inclusions, Sulfur isotope, Mozambique Belt, Mozambique
The Namicupo gold prospect is hosted by the Xixano Metamorphic Complex in the Mozambique Belt, northeastern Mozambique. The geological characteristics and ore genesis of gold mineralization of the Namicupo prospect have to late been scientifically unreported. In this study, we report the ore mineralogy, fluid inclusion characteristics, and sulfur isotope constraints on gold mineralization.
The gold mineralization in the Namicupo prospect is present as quartz veins hosted in metasedimentary rocks. The mineralized veins are characterized by a mosaic texture of primary quartz. Primary quartz grains also exhibit dissolution and recrystallization textures at the edges, characterized by the presence of late-formed quartz bands surrounding the grains. The dissolution and recrystallization textures were likely due to an increase in pressure along the contact zones, resulting in the partial dissolution of the grains and subsequent precipitation of late-formed quartz bands. Gold occurs either associated with sulfides or as single grains hosted in the recrystallized quartz bands. Hematite and barite commonly replace pyrite, with chalcocite and covellite replacing chalcopyrite, and lead oxide replacing galena. Electron probe micro-analysis of gold grains showed that the gold grains coexisting with pyrite and chalcopyrite have Ag contents between 9 and 11 atomic % and are classified as native gold, with relatively low quantities of Hg (1,300 ppm on average) and Cu (up to 900 ppm), compared with that hosted in recrystallized quartz. On the other hand, one gold grain hosted in the recrystallized (late-formed) quartz band showed relatively high Ag contents (29-30 atomic %), thus is classified as electrum; curiously containing considerable Hg (2,500 ppm on average) and S (800 ppm on average), with no Cu. The mineralized quartz veins of the Namicupo prospect host three types of fluid inclusions at room temperature. Type A, three-phase irregular- to subregular-shaped inclusions (i.e., aqueous liquid, CO2 liquid and vapor), are the most abundant (> 75 % of total inclusions). The gas phase of Type A inclusions is composed mainly of CO2 with minor N2 and H2S as revealed by Raman spectroscopy. Type B inclusions are one-phase rectangular- to hexagonal-shaped inclusions, dark in color, mainly composed of most likely CO2. Type B inclusions are the second most abundant fluid inclusions (approximately 20 % of total inclusions) and occur commonly as primary inclusions. Type C two-phase (liquid and vapor) inclusions are very rare irregular-shaped inclusions (< 5 % of total inclusions). The δ34S values of pyrite and chalcopyrite vary in a narrow range, from -3.5 to +0.9 ‰, where a chalcopyrite sample yielded the highest value of +0.9 ‰.
We suggest that gold mineralization in the Namicupo prospect formed in two stages. The first stage was characterized by the precipitation of native gold, together with base-metal sulfides, from an aqueous-carbonic fluid. The second stage is associated with remobilization of the first-stage gold during deformation, resulting in the formation of electrum, which was co-precipitated with recrystallized quartz bands surrounding primary quartz grains. The presence of trace sulfur in remobilized gold and H2S in the gas phase of Type A fluid inclusions suggests that gold in both stages was transported as a bisulfide complex. The narrow range of δ34S of sulfides (-3.5 to +0.9 ‰) suggests a restricted sulfur source for the first stage mineralization, which can be magmatic or metamorphic in origin.
The gold mineralization in the Namicupo prospect is present as quartz veins hosted in metasedimentary rocks. The mineralized veins are characterized by a mosaic texture of primary quartz. Primary quartz grains also exhibit dissolution and recrystallization textures at the edges, characterized by the presence of late-formed quartz bands surrounding the grains. The dissolution and recrystallization textures were likely due to an increase in pressure along the contact zones, resulting in the partial dissolution of the grains and subsequent precipitation of late-formed quartz bands. Gold occurs either associated with sulfides or as single grains hosted in the recrystallized quartz bands. Hematite and barite commonly replace pyrite, with chalcocite and covellite replacing chalcopyrite, and lead oxide replacing galena. Electron probe micro-analysis of gold grains showed that the gold grains coexisting with pyrite and chalcopyrite have Ag contents between 9 and 11 atomic % and are classified as native gold, with relatively low quantities of Hg (1,300 ppm on average) and Cu (up to 900 ppm), compared with that hosted in recrystallized quartz. On the other hand, one gold grain hosted in the recrystallized (late-formed) quartz band showed relatively high Ag contents (29-30 atomic %), thus is classified as electrum; curiously containing considerable Hg (2,500 ppm on average) and S (800 ppm on average), with no Cu. The mineralized quartz veins of the Namicupo prospect host three types of fluid inclusions at room temperature. Type A, three-phase irregular- to subregular-shaped inclusions (i.e., aqueous liquid, CO2 liquid and vapor), are the most abundant (> 75 % of total inclusions). The gas phase of Type A inclusions is composed mainly of CO2 with minor N2 and H2S as revealed by Raman spectroscopy. Type B inclusions are one-phase rectangular- to hexagonal-shaped inclusions, dark in color, mainly composed of most likely CO2. Type B inclusions are the second most abundant fluid inclusions (approximately 20 % of total inclusions) and occur commonly as primary inclusions. Type C two-phase (liquid and vapor) inclusions are very rare irregular-shaped inclusions (< 5 % of total inclusions). The δ34S values of pyrite and chalcopyrite vary in a narrow range, from -3.5 to +0.9 ‰, where a chalcopyrite sample yielded the highest value of +0.9 ‰.
We suggest that gold mineralization in the Namicupo prospect formed in two stages. The first stage was characterized by the precipitation of native gold, together with base-metal sulfides, from an aqueous-carbonic fluid. The second stage is associated with remobilization of the first-stage gold during deformation, resulting in the formation of electrum, which was co-precipitated with recrystallized quartz bands surrounding primary quartz grains. The presence of trace sulfur in remobilized gold and H2S in the gas phase of Type A fluid inclusions suggests that gold in both stages was transported as a bisulfide complex. The narrow range of δ34S of sulfides (-3.5 to +0.9 ‰) suggests a restricted sulfur source for the first stage mineralization, which can be magmatic or metamorphic in origin.