THE PETROGENESIS OF SILICA ROCKS IN BLACK ROCK SERIES IN PERMIAN LUCAOGOU FORMATION, SANTANGHU BASIN, XINJIANG, NW CHINA

The Permian Lucaogou Formation in Santanghu Basin of north Xinjiang province develops an important set of hydrocarbon source rocks and reservoirs. It has been considered as a normal lacustrine deposit for long time. However, our team found a complicated lithology combination recently: volcanic-hydrothermal exhalative sedimentary rocks which including primary dolostone related to, mantle-originated exhalative hydrothermal activities carbonate exhalative rocks, analcite rock related to exhalative hydrothermal activities, etc. Although lots of work has been done on these rocks, but the silica rock is still in less understanding. In this paper, petrology and mineralogy study, together with multiple test methods are used to confirm the characteristics of each type of silica rock and discuss their source and genesis.

The Lucaogou Formation overlies the Kalagang Formation and is covered by Tiaohu Formation. Both of Kalagang Formation and Tiaohu Formation contain volcanic and sedimentary deposits. Fine-grained sediments which deposit in deep-semi deep lake developed dominantly in Lucaogou Formation, as well as abundant volcanic-hydrothermal exhalative rocks and sedimentary rocks related to volcanic activities which indicate complicated lithology combination during volcanic interval.

Silica rocks in black rock series in Lucaogou Formation is characterized by veins, intervals, plaques, lensing, and clumps. and all of them are made up of microcrystal quartz. According to cathodoluminescence results, microcrystal quartz collections has no luminescence inside, but some of the edge of quartz collections show bright blue light. Meanwhile, the chalcedony has blue light. This significant phenomenon illustrates that the edge of quartz collections and chalcedony are formed by hydrothermal activities.

We selected various types of silica rocks for silicon isotope experiments, and combined with fluid inclusion experiments to draw the following conclusions. The Petrogenesis of Siliceous Rocks in black rock series in Permian Lucaogou Formation multi-stage magmatic hydrothermal activities.