Unidirectional Solidification Textures and mineralization: the Kule porphyry-Molybdenum system (Xinjiang Uygur, China)

The Kule valley is located close to the western end of Xinjiang Uygur Autonomous Region, some 100 km southeast of Yining. The area belongs to the western sector of the > 2500 km long, east-west trending Tian Shan mountain range. In this part of range (central Tien Shan) the Yili Terrane, a wedge-shaped Precambrian craton, is exposed, and bound by the North and South Tien Shan ranges. Variscan calc-alkaline granite and granodiorite bodies, spread in age between 400–250 Ma, crop out over much of the Yili block, representing a still poorly understood magmatic arc derived from orogenic events on both sides of the block. The Kule area is composed of a suite of magmatic rocks of inferred upper Carboniferous to Permian age. Based on field relations and petrographic features, the following units are recognized: - Granite Unit, composed of: a) medium-grained, equigranular to porphyritic granite, affected by a pervasive “propylitic” alteration; b) microporphyritic granite, made of plagioclase, K-feldspar and quartz phenocrysts in a micrographic matrix; c) microporphyritic leucocratic granite, showing peculiar (UST) textures which are described below. - Rhyolite Unit: it is a mixture of rhyolite lava and volcanoclastic rocks with the same composition. It includes massive rhyolite bodies related to emplacement of rhyolitic magma under hypoabissal conditions. - Tuff Unit: shows the same composition of the Rhyolite Unit, but displays features typical of pyroclastic (to epiclastic) deposition. Its primary bedding shows a NNE-SSW trending subvertical attidude, suggesting (in agreement with other evidences) that the Kule sequence suffered a tilting towards WNW. - Andesite Unit: intrudes the Tuff Unit and is composed of porphyritic andesite affected by propylitic alteration. Dykes related to the Andesite Unit intrude the Rhyolite Unit. The sequence is affected by veining, alteration and Mo mineralization. The following main veins types occur (older to younger): - Qtz - Kfs ± Mgt (magnetite) veins : they crosscut the microporphyritic granite and rhyolitic rocks, producing a strong “potassic-type” alteration given by brown Bt + Qtz + Mgt ± Kfs; - Qtz – Ms + Py ± Mlb (molybdenite) veins and breccias: crosscutting the microporphyritic granite, rhyolitic rocks and Tuff Unit, they produce a mild to very strong alteration given by the same assemblage. - Qtz ± Chl ± Ep ± Cc veinlets: they only occur in the medium-grained granite and Andesite Unit, where produce a pervasive Chl – Ep – Ab – Ttn ± green Bt (“propylitic”) alteration. When restored to its primary position, the Kule sequence shows features (e.g., calc-alkaline volcano-plutonic setting; multiple veining and alteration, with a “potassic” core surrounded and in part overprinted by a “phyllic” envelope; hydrothermal breccias; molybdenite) typical of a (low-F) porphyry-Mo system. The microgranite porphyry, that in the restored setting lies below the potassic core, shows a “layered texture”, given by cm-thick coarse-grained, crenulated quartz – magnetite layers separated by leucocratic granite. Both quartz and magnetite crystals within the layers grow in the same direction, thus representing Unidirectional Solidification Textures, like those firstly recognized in the Henderson porphyry-Mo deposit (Shannon et al., 1982). Peculiar of the Kule microgranite is the occurrence of coarse-grained magnetite within the layers. However, the absence of hydrous magmatic phases in the microgranite (the only mafic mineral being fine-grained magnetite) strongly suggests, as proposed for other UST occurrences, release of (ore) fluid to the time of crystallization and solidification of the apex of a stock. The occurrence of Unidirectional Solidification Textures at the Kule porphyry-Mo system further emphasizes their importance, as features indicative of high primary volatile content and ore fluid release in magmatic-hydrothermal systems.