Abstract

The base metals-gold hydrothermal deposits near Baia Mare occur in the Gutii Mountains, a unit of the Neogene volcanic chain on the inner part of the Carpathian arc. There is a progressive development of volcanism in the mineralized area with time, from Sarmatian in the west to Pontian in the east. Two aspects of hydrothermal activity are present: hydrothermal alteration of the rocks and ore deposition.The ore deposits occur in a narrow belt, extending some 60 km along the southern part of the Gutii Mountains. In most cases, these are vein deposits; impregnations and stockworks are subordinate. Ten fields are presently in operation. The main veins have been worked to the lowermost optimum mineralized level and ore now mined contains 2 to 3 percent Pb, 2 to 4 percent Zn, up to 1 percent Cu, and 2 to 5 ppm Au.The metalliferous veins form parallel systems generally with constant northeast-southwest and east-west trends. These are grouped in four metallogenic alignments. The mineral assemblages include native elements (Au, Ag, Cu, As, S), sulfides, sulfosalts, oxides, arsenates, antimonates, tellurides, sulfates, carbonates, wolframates, phosphates, and silicates. The richest assemblages occur in the upper mineralized levels and in the center of the ore deposits belt. A generalized scheme for paragenesis begins with iron-bearing minerals and quartz chlorite gangue. With decreasing temperature the base metal sulfides plus other metallic minerals, quartz, adularia, and clay minerals occur. The lowest temperature minerals consist mainly of sulfosalts, native elements, and carbonates. The metallogenic process was polyascendant. Spatial disposition of the ore is sometimes zonal, although telescopic phenomena are frequent. The nature of the most important ore is mesohypothermal.The hydrothermal ore deposits from Baia Mare were emplaced in three phases which can be correlated with the three major phases of volcanic activity in the Gutii Mountains: Sarmatian pyroxene andesites, Pannonian dacites and quartz andesites, and Pontian pyroxene andesites. These deposits constitute a zone of the metallogenic province of Neogene Carpathic volcanism.

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