MINERALOGY OF HIGH-GRADE MO-SKARN ORE IN THE QIU SHU WAN DEPOSIT, EASTERN QINLING MINERAL BELT, HENAN PROVINCE, CHINA

Skarn deposits form from hydrothermal fluids causing calc-silicate alteration of calcic protolith rocks – primarily carbonates – and locally precipitate economic amounts of Cu, Au, Mo, Zn, Sn, and W. Typically, skarn deposits are spatiotemporally associated with shallow igneous intrusions.

Here we present the results of an ongoing study that aims to understand the origin of high-grade Mo skarn ore in the Qiu Shu Wan deposit related to a Mesozoic, high-F porphyry molybdenum deposit in the Eastern Qinling porphyry Mo belt of Henan Province, China. Collectively this belt represents some the largest known molybdenum reserves in the world. Consequently, understanding the origin of the Qiu Shu Wan deposit (QSW) provides a rare opportunity to study the processes that control Mo enrichment in porphyry-associated skarn systems.

A suite of several dozen rocks collected from QSW are representative of both unaltered and altered porphyry intrusive rock, prograde and retrograde skarns, and host rocks. Petrographic observations and SEM/EDS studies document the weakly-altered porphyry containing 30% phenocrysts and 70% aplitic groundmass which represents porphyry zone dominated by plagioclase, K-feldspar, and quartz with subordinate amounts of biotite, titanite, and augite. Intense feldspar-stable alteration of porphyry is dominated by albite-rich plagioclase, K-feldspar, and quartz. This white albite-dominant alteration is associated with stockwork veins, and disseminated molybdenite. Several different skarn mineralogies are present including sulfide-poor garnet-dominated and pyroxene-dominated prograde skarn. Skarn-hosted sulfide mineralization occurs as quartz-pyrite-chalcopyrite-molybdenite stockwork veins, and as disseminated blebs. X-ray diffraction analysis of a porous, friable, very high-grade ore sample identified granular tan prehnite with lesser illite and residual unreplaced garnet impregnated with up to 20 modal % molybdenite. Presence of illite and hydroxyl-bearing prehnite suggests very high grade ore may have formed by retrograde alteration by a low pH, Mo- and H₂S-bearing fluid that altered garnet into prehnite and deposited molybdenite.

Our ongoing studies are aimed at further constraining the physical and chemical nature of both prograde skarn and skarn-destructive fluids.