Abstract

The Xietongmen district is located 260 km west-southwest of Lhasa in the Tibet Autonomous Region, China. The district occurs within the Gangdese belt, which forms the eastern part of the Trans-Himalayan magmatic belt and is the product of complex magmatic activity that began during the Late Triassic or Early Jurassic and ended in the Eocene. The Xietongmen Cu-Au and Newtongmen Cu-Au-Mo deposits contain a total measured and indicated resource of approximately 610 million metric tons, with additional mineralization in the Langtongmen and Olitongmen Cu-Au prospects. Porphyry mineralization in the Xietongmen district formed during Middle Jurassic volcanic arc activity in the Lhasa terrane, prior to its accretion to the southern margin of Eurasia, and establishes that an economically important, but only recently recognized, metallogenic event is present in the region.

Rock types in the Xietongmen district range from Early Jurassic to Eocene in age. Early Jurassic (~188-177 Ma) volcanic, volcaniclastic, and coeval intrusive rock types are crosscut by Middle Jurassic (176-171 Ma) hornblende diorite and quartz diorite porphyry dikes and stocks, including intrusions related to porphyry Cu-Au ± Mo mineralization. The Jurassic igneous assemblage was intruded by mafic dikes between the Late Jurassic and the Cretaceous, then by an Eocene (50-47 Ma) biotite granodiorite batholith and related dikes, and finally by, volumetrically minor lamprophyre dikes. The most important structures in the Xietongmen district are four E-striking, moderately N-dipping, sinistral-oblique thrust faults. Crosscutting and suturing relationships between the TSF-2 thrust fault, located in the south part of the district, and intrusions dated to between 174 and 180 Ma constrain the main stage of thrust fault activity to the Middle Jurassic. The Contact and Adit-1 thrust faults truncate the Xietongmen deposit and form the footwall and hanging wall to mineralization, respectively. Numerous zones of cataclasis deform the Xietongmen deposit between these bounding thrust surfaces. The strongly deformed Langtongmen Cu-Au prospect is located ~1.3 km west of the Xietongmen deposit and occurs in the immediate hanging wall of the Adit-1 thrust fault. The Newtongmen deposit and the Olitongmen Cu-Au prospect occur to the north in the hanging wall to the SBF thrust fault and are not strongly deformed.

Mineralization and hydrothermal features in the Xietongmen district are fully compatible with porphyry Cu-Au ± Mo deposits. Alteration, vein types, and mineralization are zoned around quartz diorite porphyry intrusions. Early K silicate alteration and related veins occur within and proximal to the intrusions and contain the highest grade mineralization. In the Xietongmen deposit, the grade of mineralization decreases outward from a core of early biotite-rich K silicate alteration, through a transitional zone in which early K silicate alteration is partially overprinted by quartz-sericite-pyrite alteration, to a peripheral zone of poorly mineralized quartz-sericite-pyrite ± pyrrhotite alteration. Incipient sodic alteration occurs as albite alteration envelopes to quartz-sulfide veinlets in the deepest part of the deposit. Late polymetallic veins and veinlets contain sphalerite, galena, and other base metal sulfides and sulfosalts, occur throughout the Xietongmen deposit, and reflect telescoping during late-stage collapse of the hydrothermal system. Partially developed supergene mineralization forms less than 10% of the Xietongmen deposit. Underlying hypogene mineralization comprises ubiquitous pyrite, chalcopyrite, lesser and more erratically distributed pyrrhotite, and rare molybdenite. The characteristics of the Langtongmen prospect are identical to those found in the deeper parts of the Xietongmen deposit. Characteristics of the Newtongmen deposit are generally similar to those in the Xietongmen deposit but Newtongmen contains only minor supergene mineralization, is cut by very few late polymetallic veinlets, and contains zones of strong, weakly mineralized sodic alteration related to a relatively later stage of quartz diorite porphyry intrusion. The Olitongmen prospect has characteristics similar to the Newtongmen deposit. The Xietongmen deposit and the Olitongmen prospect were thermally recrystallized in a hornfels aureole to the Eocene biotite granodiorite batholith, whereas thermal effects are minor to absent at Langtongmen and Newtongmen. Copper and gold (Au/Cu; ppm/%) are closely correlated within each of the two main deposits and ratios range from 1.5 to 1.2 in the Xietongmen deposit to between 0.8 and 0.6 in the Newtongmen deposit.

Mineralization in the Xietongmen district formed in several coeval mineralized centers and the vein types, alteration and metal assemblages among these centers span a continuum in hydrothermal characteristics. The differences between the mineralized zones are interpreted to reflect exposure at different relative paleodepths as a result of displacement and deformation by posthydrothermal, sinistral-oblique movement on thrust faults. The Xietongmen deposit was transposed to the south by deformation on and between the bounding Adit-1 and Contact thrust faults. The Langtongmen deposit was separated from the deep hanging wall of the Xietongmen deposit and displaced approximately 600 m vertically and 1,300 m to the west by the Adit-1 thrust fault. The Newtongmen deposit and the Olitongmen prospect were uplifted relative to Xietongmen and Langtongmen in the hanging wall to the SBF thrust fault and were not significantly deformed. The genesis and relationship between porphyry deposits in the Xietongmen district can be reconciled by the combined effects of vertical and lateral displacement by thrust faults, preservation of the deposits at different relative paleodepths, and varying degrees of posthydrothermal mechanical and thermal recrystallization.

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