Influence of crude oil on the genesis of the Lanjiagou porphyry molybdenum deposit, western Liaoning Province, China

Abstract

The Lanjiagou porphyry molybdenum deposit in western Liaoning Province, China, is hosted in fine-grained Jurassic granites. LA-ICP-MS zircon U–Pb analyses indicate that the crystallization of the ore-hosting granites took place 185.0±1.8Ma (MSWD=1.4). Molybdenum mineralization in the deposit can be divided into three stages: the stockwork quartz vein stage, the planar quartz vein stage, and the fissure-filling quartz vein stage. Re–Os isotopic ages for the molybdenite from the stockwork quartz vein-type ores yielded an isochron age of 188.8±9.9Ma (MSWD=3.0), while six samples from the planar quartz vein-type ores yielded a similar isochron age of 185.6±1.2Ma (MSWD=0.5). Re–Os isotopic ages for the molybdenite identical, within error, to zircon U–Pb isotopic ages indicate that the molybdenum mineralization is related to the host intrusions. Apart from primary inorganic fluid inclusions (IFIs), a large number of primary organic fluid inclusions (OFIs) are found in the latter two stages of vein quartz, and minors found in the first stage. The components and characteristics of OFIs in the three stages of vein quartz differ from each other, which is also true for the IFIs. OFIs in stockwork vein quartz are characterized by halite-bearing inclusions, and organic liquids in the inclusions are brown and do not fluoresce under ultraviolet (UV) light. Homogenization temperatures (Th) for the primary IFIs coeval with OFIs of this stage ranges from 300°C to >450°C, while the salinity varies from 10 to 53wt.% NaCl equiv.. In planar vein quartz, OFIs are predominately two-phased (liquid and gas), and salt daughter minerals (halite) are absent. Organic liquids are light brown to colorless and show blue fluorescence under UV light; The Th range for the IFIs of this stage is 250–360°C, and the salinity range is 3–17wt.% NaCl equiv. Finally, OFIs in fissure-filling vein quartz are marked by liquid–gas inclusions. Organic liquids are generally colorless and show yellow fluorescence under UV light. The Th range for the primary IFIs is 180–240°C and the salinity range is 4–11wt.% NaCl equiv. Organic geochemical analyses indicate that organic matter in the Lanjiagou deposit was derived from mature crude oil. We suggest that large volumes of crude-oil-bearing non-magmatic fluids were flushed into the Lanjiagou porphyry hydrothermal system during all phases of ore formation and likely played important roles in mineralization.