Spatial and temporal evolution of fluids in hydrothermal ore deposits

Abstract

Magmatic-hydrothermal systems typically have vertical extents of several hundred<br />meters and their geochemical characteristics (e.g. mineral assemblages) vary considerably<br />over that vertical extent. As a consequence the expression in outcrop varies depending on<br />the level of erosion. Therefore understanding the geochemical zonation of magmatic-hydrothermal<br />ore deposits opens the possibility to detect deep magmatic-hydrothermal<br />systems, and to assess qualitatively the degree of erosion that has taken place in the area<br />and at which level the mineralization may occur. This dissertation presents the<br />characterization of two shallow hydrothermal systems and their potential relations with<br />deeper magmatic-hydrothermal systems. In addition, this dissertation develops the<br />equations to directly interpret thermometric data from the fluid inclusion type dominant in<br />one of those deposits (fluid inclusions that homogenize by halite disappearance).<br />Red Mountain, AZ is a porphyry copper system with a well-preserved lithocap<br />providing an ideal candidate to characterize the shallow expression of porphyry copper<br />systems in the southwestern US. The distribution of fluid inclusions, alteration mineralogy<br />and grade indicate that the intrusive responsible for the mineralization was only partially<br />intercepted during the exploration program and that one single magmatic event was likely<br />responsible for the mineralization detected. Fluid inclusion types and clay minerals are<br />systematically distributed within the deposit. The fluid responsible for the shallow<br />hypogene mineralization was a low pH-intermediate temperature-low density fluid while a<br />high salinity fluid was responsible for deep mineralization.<br />Wutong is a Pb-Zn-Ag deposit in the Nanling belt (southeast China). The combination<br />of fluid inclusion and mineral thermometry indicates that the Wutong deposit formed at<br />relatively low pressures. The age and isotopic composition of the mineralization indicates<br />that the deposit formed during the Cretaceous from crustal derived fluids. The occurrence<br />of a shallow magmatic-hydrothermal system of Cretaceous age in this region suggests that<br />Cretaceous intrusions, despite not outcropping very commonly in this particular region may<br />occur at deeper levels.