IMPORTANCE OF RETROGRADE SKARN CONCENTRATION OF MO FROM PROGRADE PROTORE SKARN IN THE QIUSHUWAN PORPHYRY DEPOSIT, HENAN, CHINA

The Qiushuwan deposit in the Qinling Mountains, Henan, China hosts porphyry Mo deposits that intruded limestone, forming molybdenite-bearing skarn deposits. We petrographically-analyzed samples of porphyry, prograde and retrograde skarn to determine if retrograde remobilization of molybdenum is necessary to produce high grade Mo ore from lower-grade prograde skarn protore.

Weakly-altered porphyritic granite contains 1-2 mm, biotite 1-2 mm albite, and 3-5 mm locally-embayed euhedral quartz phenocrysts set in an aplitic quartz—K-feldspar groundmass.

The two types of porphyry ore lack quartz phenocrysts. Sericitically-altered porphyry is cut by multiple stages of 1-3 mm quartz-moly veins which are cut by quartz-pyrite-sericite. White porphyry has partial albitic-alteration and contains the highest porphyry-hosted Mo grade in this study. Molybdenite occurs as disseminated grains throughout the groundmass and less commonly as hairline veins. 1-3 mm endoskarn veins with garnet cores are rimmed with 1 mm molybdenite-rich prehnite selvages.

Barren prograde garnet-pyroxene skarn contains very sparse sulfides and consists of brecciated growth-zoned garnets (avg. Grs₂₃And₇₅Pyr₀₀Alm₀₁Sps₀₁), infilled with ferroan diopside (Di₇₂Hd₂₇Jo₀₁) and cut by late-stage quartz-pyrite-chalcopyrite veins.

High-grade Mo-bearing garnet skarn consists of oscillatory-zoned grandite garnet (Grs₄₅And₄₉Pyr₀₀Alm₀₃Sps₀₃) intergrown with subordinate interstitial ferroan diopside (Di₆₃Hd₃₃Jo₀₄) and disseminated molybdenite that occurs both interstitially between pyroxene and garnet grains and as pure moly veins. Other sulfides are extremely sparse. Trace amounts of K-feldspar and albite fill voids between garnet grains and are closely associated with molybdenite.

Retrograde skarn consists of 10 cm rounded clasts of brecciated garnet ore set in a matrix of friable, crustiform, vuggy prehnite intergrown with sericite and illite. Molybdenite-bearing zoisite masses in the breccia matrix formed during retrograde alteration in which molybdenite is the sole sulfide in the Mo deposition stage.

We conclude that although prehnite retrograde skarn processes analogous to sericitic alteration in porphyry can augment Mo grades, prograde skarn can host primary ore-grade rock without retrograde enhancement.