THE HATU GOLD ANOMALY, XINJIANG-UYGUR AUTONOMOUS REGION, CHINA - TESTING THE HYPOTHESIS OF AEOLIAN TRANSPORT OF GOLD

In 1987, a cooperative project between the U.S. Geological Survey and the Institute of Geophysical and Geochemical Exploration was initiated to evaluate the origin of the Hatu gold anomaly. The anomaly is locat ed in the Hatu mining district in the northwest corner of Xinjiang-Uyg ur Autonomous Region in northwest China. The climate is semiarid to ar id and wind erosion predominates. A regional soil survey of the Hatu d istrict, based on samples collected on a 200 by 500 m grid and composi ted prior to chemical analysis to a density of one sample per square k m, delineated a series of south-southeast-trending Au anomalies. Anoma lous Au values range from 5 ppb to more than 700 ppb. The Hatu anomaly , the most prominent of these anomalies, is more than 30 km long and a bout 5 km wide. The mining town of Hatu and the economic gold deposits of Qiqu 1 and Qiqu 2 are at the northern end of this anomaly. The axi s of the Hatu anomaly cuts across mapped structure and stratigraphy in the district, but is parallel to the prevailing wind direction. This observation led to the hypothesis that the Hatu anomaly is the result of aeolian dispersion of gold from the vicinity of Qiqu 1 and Qiqu 2. The alternative interpretation, that the anomalies reflected additiona l primary gold occurrences, was not consistent with existing informati on on the known occurrences and the geology. The investigation led to the identification of three types of gold in heavy-mineral concentrate s derived from stream sediments that were collected along the axis of the Hatu anomaly: (1) free gold, (2) gold in pyrite, and (3) gold incl uded in quartz. Gold in quartz was only observed within 2 km of Qiqu 1 . The size of the gold particles and the number of gold particles in t hese samples did not decrease with distance from Qiqu 1 as would be ex pected from aeolian or fluvial dispersion from a point source. Instead , both the size and amount of gold increased significantly at a distan ce of 3.5 km from Qiqu 1 and this increase continued to approximately 5.5 km from Qiqu 1. The mean intermediate diameter of gold particles i ncreased from 0. 1 mm to approximately 0.25 mm and the gold particle c ontent increased from approximately 0.3 particles per kg of sample to almost 8 particles per kg of sample. The morphology of the gold change d from a delicate filigree texture near Qiqu 1 to coarse, blocky parti cles in the southern part of the anomaly. The Hatu anomaly is caused p rimarily by alluvial dispersion of free gold from local point sources along the anomaly. Aeolian dispersion is restricted to very fine-grain ed (+/- 2 mum) gold included in sulfide minerals or quartz grains and is significant only within 1-2 km of the known deposits.