Experimental study of high to intermediate temperature alteration in porphyry copper systems and geological implications

Abstract

Porphyry copper systems, which provide most of the world’s copper resource, are commonly associated with characteristic concentric zonation of alteration and mineralization. In-depth knowledge of the distribution and transport mechanism of elements in the alteration zones is essential for understanding the ore-forming processes. We employed flow-reaction apparatus to simulate the fluid-rock interactions during porphyry ore formation so as to investigate the mechanisms that govern the transport of elements and the development of zonation. The results indicate more heterogeneous distribution of elements in the experimental products at 450°C compared to those at lower temperatures, which implies a crucial role of temperature in controlling elements redistribution in hydrothermal systems. Heating advances potassic alteration and Ca leaching of wall rocks. To achieve the same degree of sodic alteration, it requires a higher concentration of Na+ in the fluid toward higher temperature. Temperature also facilitates the incorporation of Ti, Sr and Pb into silicate minerals through cation substitution. We infer from experimental results that from the center of intermediate to mafic volcanic wall rocks toward periphery, the contents of K and Ti should decrease and the contents of Ca, Zn and Mn should increase, whereas the trend for Si and Na could be non-monotonic. This study provides experimental and theoretical insights into a variety of vital geological observations, including anhydrite formation and the widespread development of potassic rather than sodic alteration in porphyry copper deposits.