Mineral Assemblage Of Magnetite Research Articles

The role of early sulfide saturation in the formation of the Yulong porphyry Cu-Mo deposit: Evidence from mineralogy of sulfide melt inclusions and platinum-group element geochemistry

Sulfide melt inclusions enclosed in silicate minerals have been observed in many oxidized Cu-bearing porphyries. The origin of such sulfide melt inclusions could bear critical information of metal enrichment or depletion during magma evolution of porphyry systems. The giant Yulong porphyry Cu-Mo deposit in eastern Tibetan Plateau consists of a series of felsic intrusions, among which the mineralized Yulong intrusion emplaced at a late stage of the magmatic activity. This study reports detailed texture and composition of sulfide melt inclusions from barren and mineralized intrusions, as well as platinum-group elements of these intrusions, to evaluate potential role of early sulfide saturation in the porphyry mineralization system. These sulfide melt inclusions, occurring as globules or droplets, are preserved in zircon, amphibole and quartz grains. Most of them are composed of a major phase of pyrrhotite and minor pyrite, chalcopyrite and pentlandite. Smooth boundaries between these phases suggest the origin of fractionation from sulfide melt upon cooling. Small amounts of Cu and Ni are heterogeneously distributed in the pyrrhotite. The sulfide melt inclusions could be observed in both the pre- and syn-mineralization intrusions at Yulong, indicating that sulfide saturation had commonly occurred during magma evolution. Magnetite mineral inclusions could be observed in zircon and amphibole grains as well. The mineral assemblage of magnetite + pyrrhotite + pyrite of the mineral inclusions constrains the Yulong magmatic system to redox conditions near the magnetite-pyrite/pyrrhotite buffer and above the fayalite/ magnetite-quartz buffer (FMQ). High Eu/Eu* values (0.56–0.78) of the hosting zircons and high calculated ΔNNO values (>0.5) of the hosting amphibole indicate highly oxidized nature of the parental magma.Both pre- and syn-mineralization intrusions have low PGE contents (Pt = 0.04 to 0.17 ppb, Pd = 0.19–0.60 ppb) with compositions plotting in the Au-poor porphyry Cu fields in discriminating diagrams. Early sulfide saturation may have been reached and small amounts of sulfides have been segregated from magma. Most PGEs and Au but little Cu have been removed from the evolved magma because the partition coefficient of Cu between sulfide and silicate melt is about two orders of magnitude lower than PGE or Au. As a consequence, the evolved magma would have high Cu but very low PGE and Au, resulting in the Au-poor porphyry Cu mineralization at Yulong.

In-situ LA–ICP–MS trace elements analysis of magnetite: The Fenghuangshan Cu–Fe–Au deposit, Tongling, Eastern China

The Fenghuangshan deposit is a typical Cu–Fe–Au skarn deposit in the Tongling area, Anhui province, Eastern China. The deposit has a paragenetic sequence of a prograde skarn stage, followed by a retrograde skarn stage, and a final quartz–sulfide and carbonate stages. Magnetite in the Fenghuangshan deposit mainly formed in the retrograde and carbonate stages. According to the morphology of magnetite and mineral assemblage of ores, we divided magnetite-bearing ores into three groups. Group 1 (early retrograde skarn stage) is represented by a mineral assemblage of magnetite and chalcopyrite. Group 2 (late retrograde skarn stage) has a mineral assemblage of magnetite, chalcopyrite, and wollastonite with characteristic ring-like magnetite. Group 3 (carbonate stage) is characterized by large amounts of calcite veins crosscut or associated with magnetite and intensive hematization of magnetite crosscut by the veins.Laser ablation (LA)–ICP–MS was used to determine trace element concentrations of magnetite from the three mineralization stages. Positive correlations among Mg, Al, Ca, and Si in magnetite indicate that these lithophile elements have similar behavior during the skarn formation process. Calcium is an important discriminant element for magnetite in skarn deposits. Positive correlations are also evident for Pb, Sn and W in magnetite, which also indicates their similar behavior. In general, magnetite grains of different stages have similar normalized trace element patterns, indicating that they share a common source. However, some elements such as Co and Mn in magnetite decrease from early retrograde skarn stage, late retrograde skarn stage to carbonate stage, which may be attributed to the precipitation of coexisting minerals (sulfides and carbonates) or the decreasing temperature. Magnetite grains of the retrograde stage have higher Mg+Mn and Si+Al contents than those of the carbonate stage, indicating a decreasing degree of fluid–rock interaction during the skarn formation process. Trace element data of skarn magnetite indicate a more widely compositional variation than previously suggested. Magnetite from the Fenghuangshan Cu–Fe–Au deposit has similar composition to those from other Cu, Cu–Fe or Cu polymetallic skarn deposits, but different from those from Fe skarn deposits, such that magnetite composition is very powerful in establishing the origin of skarn deposits.