Heavy mineral composition of the Songhua River system identified by manual and TIMA automatic methods and implications for provenance tracing

The Songhua River is the most important fluvial system in northeast China and its heavy mineral composition is of great significance to the revealing of depositional environment, source-sink relationship and drainage evolution. However, the current study concerning the heavy mineral composition of the Songhua River system is very limited, and the factors affecting the composition of heavy minerals are not clear up to date. In order to solve the problem, sand samples are collected along the banks of the main stream and 10 major tributaries of the river system, in addition to the two from the T3 terraces of the Tonghe River. Using the new automated mineral identification technology TESCAN Integrated Mineral Analyzer (TIMA) and the manual method respectively, heavy mineral analysis of the sample components in depth of 63~250 m is carried out for the samples. The influences of sediment sources, fluvial processes and chemical weathering on the heavy mineral composition of the Songhua River system are discussed. The results show that the tributaries sediments are sourced from different mountains, such as the Great Hinggan Mountains, the Lesser Hinggan Mountains and the Changbai Mountains. These mountains are significantly different in heavy mineral composition, and the heavy minerals of hornblende, titanite, epidote, leucoxene, augite, ilmenite and hematite+limonite are effective indicators to distinguish the water systems of different sources. The Nenjiang tributaries are originated from the Great Hinggan Mountains, of which sediments are rich in augite (the Nuomin River), ilmenite (the Ganhe River, Duobukuer River and Alun River) and hematite+limonite (the Ganhe River, Duobukuer River and Yalu River), showing the control of the basal source over heavy mineral composition, which can be observed in the modern riverbed gravels. However, in the main stream of the Songhua River, those heavy minerals are significantly reduced, suggesting that the influence of the Great Hinggan Mountains, as a source area, is extremely weak and the source control of heavy mineral composition is heavily influenced by river process. It is worth to point out that the heavy mineral composition of the Songhua River main stream is the mixture of the Lalin River and the Second Songhua River, indicating that the main stream of the Songhua River has inherited more heavy mineral information from the southeastern mountains of the Songnen Plain (the remnants of Changbai Mountain). The composition of heavy minerals in T3 terrace samples of Tonghe, which was severely affected by chemical weathering, is extremely simple, the unstable heavy minerals such as augite and hornblende have completely disappeared, and stable minerals are highly enriched, reflecting the control of chemical weathering over heavy mineral composition. Compared with the traditional manual identification of monotonous heavy mineral composition (usually only more than 20 mineral species can be identified), the TIMA method can reveal more mineral information (usually more than 40 mineral species can be identified), but there are shortcomings in identifying homogenous and polymorphic minerals.