HYDROTHERMAL FEATURES IN THE CATALÃO I CARBONATITE COMPLEX, GOIÁS
C
ARLOS C. RIBEIRO and JOSÉ C. GASPAR
Instituto de Geociências, Universidade de Brasília, Brasília.
Presented by ALCIDES N. SIAL
The Catalão I Carbonatite Complex is situated in southwestern Goiás, about 10Km from the Catalão town. Its emplacement occurred in the Cretaceous and it is essentially composed of phlogopitites and carbonatites. In the central part of the complex, in the open pit of the Ultrafértil Mine, there is one remarkable system of hydrothermal activity, related to the final magmatic stages. Compared to other carbonatite complexes the system is unique due to its intensity and singular features. The hydrothermal rocks are mainly composed of apatite, ilmenite, monazite, and barite, which vary from 20 up to 30m thick. Their most important structural features include horizontal layering and banding, dykes, veins, veinlets, and breccia pipes. The system was clearly multistage as demonstrated by crosscutting veins, dykes, and pipes. Whenever the fluid intruded carbonatites a reaction between fluid and rock resulted in important assimilation and erosion of the walls but, if in contact with phlogopitites, no visible reaction is detected. In the first case, the product is a horizontal layered rock composed of apatite and ilmenite that can reach about 0.3 up to 2.0m in thickness, which is differentiated upwards to layers mostly composed of ilmenite and monazite. In the second case, the hydrothermal rocks are mainly composed of apatite and barite. Dyke feeders and pipe roots are often observed. The breccia pipes present a cone shape with a cross section that is roughly circular and that varies from 1cm up to 1.0m in diameter. The fragments in the breccias are derived directly from the wall rocks and show small displacements. Fragments are angular and present a large range of grain size. Few pipes are composed of three main portions: a) a base that is filled with more equigranular fragments (millimeter to centimeter sizes) and that is almost massive; b) a medium portion that is finer (sizes up to few millimeters) and very well layered. In this portion the pipe widens up; c) the uppermost portion that is filled with the breccia that is typical of these pipes. Horizontal fractures, probably due to caldera collapse, were the main conduits to the fluids resulting in structures largely determined by conduit flow mechanisms. Increasing pressure was released through vertical fracture intersections originating the breccia pipes. (May 19, 2000)
Publication Dates
-
Publication in this collection
05 Jan 2001 -
Date of issue
Dec 2000
