Geochemical exploration for polymetallic ores in volcano-sedimentary rocks : studies in China and Finland

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>A comparison between the two very important sulfide belts Raahe-Ladoga Ore Zone (RLZ) in Finland and Southern Edge of Altay (SEA) in China, including geological setting, metallogenic characters and geochemical exploration has been made. The two sulfide belts share similarities but differ from each other in the tectonic setting and metallogenic epoch. Polymetallic ores in RLZ and SEA are the products of the submarine volcanism, but mainly Zn-Cu type is present in RLZ and Pb-Zn, Cu-Pb-Zn and Cu-Zn types occur in SEA. A main Ni-Cu ore belt related to the mafic-ultramafic intrusions is also present in the RLZ. RLZ is metamorphosed to a higher grade than SEA.</p><p>The Viholanniemi Zn-Au deposit is a veinlet-disseminated type, possibly beneath the stratabound sulphide ores, and the Keketale Pb-Zn deposit is a stratabound sulphide ore hosted by sedimentary rocks in the volcano-sedimentary formation. They show many differences. It is suggested that stratabound sulphide ores overlie stratigraphically the Viholanniemi stringer ores and Au-bearing stringers underlie the Keketale stratabound ores. Geochemical explorations of the two deposits exhibit different methods, subjects and procedures. Boulder tracing and till geochemical exploration proved to be very effective in finding the Viholanniemi deposit while stream sediment and soil geochemical surveys were the major and effective tools in finding the Keketale deposit. An extensional environment and the intensity of volcanism are the essential conditions for the formation of polymetallic ores related to the volcanism. It is feasible to classify the ores into the ores hosted by volcanics and sedimentary rocks in a volcano-sedimentary formation. The stratigraphical thickness of volcanic rocks and the amount of agglomerates are the two most crucial factors needed to be considered in prospecting. The chemical variations of the host rocks can indicate the sulphide ores hosted by sedimentary rocks in some circumstances.</p></div>