Research of Climax-type porphyry molybdenum systems during the past ten to fifteen years has led to identification of a cyclic magmatic-hydrothermal style of deposit evolution in which a discrete molybdenum ore zone is produced by a single intrusive. Results from numerous studies suggest there is a close temporal relationship between magmatism and hydrothermal mineralization, that metals precipitated from high temperature (500 - 600 C) fluids, and that both fluids and molybdenum are magmatic in origin. While these studies have defined the igneous processes responsible for generating these deposits, they have not addressed the physical and chemical nature of the aqueous fluids which exsolve from the magma and transport and precipitate the metals. Existing fluid data is either preliminary in nature, or has addressed only post-metal alteration assemblages. Knowledge of the physical and chemical characteristics of the aqueous fluids is necessary if we are to understand metal transport and precipitation mechanisms and to provide constraints for models now being formulated to quantitatively describe metal behavior in these systems. The Questa, New Mexico, molybdenum deposit has been classified as a Climax-type porphyry deposit based on petrology, ore and alteration mineralogy, and ore grade. However, preliminary fluid inclusion studies at Questa propose metals precipitated from lower temperature fluids (350 - 450 C) as a result of mixing of magmatic and meteoric fluids. A recent, detailed study of alteration assemblages in a magmatic- hydrothermal breccia (MHBX) ore zone at Questa determined mineralization is similar to other Climax-type deposits in that it is related to high-temperature alteration assemblages. Within this zone, altered andesite fragments are cemented by a mineralized pegmatitic matrix consisting of quartz, K-feldspar, biotite, molybdenite, and rutile. We propose to investigate the physical and chemical characteristics of hydrothermal fluids in and adjacent to the MHBX ore zone at Questa in order to outline the temporal and spatial evolution of the fluid transporting and precipitating molybdenum mineralization. This will be accomplished by: 1. a detailed microthermometric analysis to determine the gross chemistry of the fluids, 2. Raman microprobe analyses of fluid inclusions to confirm microthermometric analyses and to identify species not uniquely defined by microthermometry, 3. detailed reflected-and transmitted-light petrography supplemented by electron microprobe studies to correlate alteration/mineralization assemblages and fluid inclusion populations, 4. microthermometric and Raman microprobe analyses on samples previously characterized with respect to components gained and/or lost during alteration, and 5. analyses of melt inclusions in order to document the magmatic/hydrothermal transition. Results will be combined with available fluid chemistry for other similar molybdenum systems in order to generate a model describing fluid evolution during transport and precipitation of molybdenum mineralization.
