Delineating the Tuwu porphyry copper deposit at Xinjiang, China, with seismic-reflection profiling

Abstract

The Tuwu deposit is one of a series of recently discovered porphyry copper deposits in the eastern Tian Shan range of Xinjiang, China. Since its discovery in 1997, more than ten boreholes have been drilled and a suite of geophysical surveys has been acquired to delineate the deposit. As part of the geophysical program, a set of eight seismic reflection profiles was acquired in 2000, followed by a physical rock-property study in 2001. The ores are characterized by slightly higher density (Δρ ∼ 0.1 g/cm[Formula: see text]) and significantly higher P-wave velocity ([Formula: see text] ∼ 1.0–1.5 km/s) than the dioritic host rocks. The seismic surveys used 0.6- to 0.9-kg shallow dynamite sources, with a 24-channel end-on spread and offsets up to 350 m. The orebody and associated igneous layers dip steeply (>45°) toward the south, so careful processing of the seismic data was required. Weak reflections from stratigraphic contacts are visible on most of the profiles, including the top of the intrusion and the base of the orebody. Since the observed reflections include a significant out-of-plane component, we developed a simple 2.5D migration procedure. This method was applied to line drawings of the seismic profiles, providing the basis for delineation of the orebody in three dimensions. Synthetic seismic sections computed using the inferred bounding surfaces of the ore deposit are in reasonable agreement with observed reflections, even for along-strike lines not used to build the model. The ability to verify interpreted reflections using line intersections was critical to the development of our model. The results of this work indicate that seismic methods may be useful as an aid for mapping the flanks of shallow, moderately dipping porphyry copper orebodies and associated strata, particularly for defining the structure of deeper sections of the mineralized zones in advance of drilling.