A study of the adanac porphyry molybdenum deposit and surrounding placer gold mineralization in northwest British Columbia with a comparison to porphyry molybdenum deposits in the North American Cordillera and igneous geochemistry of the western United States.

摘要

The Adanac molybdenum deposit has been studied in detail in this thesis in order to classify the deposit as Climax-type or Endako-type. Placer gold from a nearby Creek that drains the Adanac deposit was sampled in order to compare initial Os signatures with that of magnetite from the porphyry deposit, so that it may be determined whether some of the placer gold is from eroded margins of the porphyry molybdenum deposit. Characteristics of porphyry molybdenum deposits throughout the North American Cordillera were summarized and tabulated. Finally, some of the geochemical characteristics of porphyry molybdenum deposits were used to query igneous rock databases for the Western United States to identify areas that may be host to more molybdenum deposits.The Adanac deposit is hosted in multiple intrusions of alkalic magma with high silica and K and moderately high Rb/Sr ratios. The Westra and Keith classification of 1981 using the K2O value at 57.5 wt% SiO 2 is 5, meaning the Adanac deposit is classified as the alkalic, high F, Climax-type molybdenum deposit. The trace element and alteration patterns conform to this classification as well. Adanac has a high-Mo zone as disseminated large to medium sized molybdenite rosettes in a smoky quartz vein stockwork that straddles and blankets at least 2 intrusions. There is a zone of high W (huebnerite) that is smaller than the molybdenite zone and coincides with it. A high F zone exists above and peripheral to the Mo and W. Small amounts of Pb and Zn (galena and sphalerite) occur primarily in faults. No other base metals or trace elements exist in appreciable amounts in the deposit. Alteration consists of a high silica core and potassic alteration as feldspar floods and potassic envelopes around veins that coincide with mineralization. QSP alteration and stilbite-calcite alteration is weak and occurs in veins or fractures that extend outward from Mo mineralization. A weak propylitic overprint (chlorite, kaolinite) occurs with mineralization but grows stronger outward from mineralization. Illite and kaolinite occur in the core of the deposit. Montmorillonite occurs in faults.Re-Os dating of molybdenite confirmed at least two episodes of mineralization at 70.87 +/- 0.36 Ma and 69.66 +/- 0.35 Ma, and also confirmed very low Re concentrations (5-39 ppm) in the molybdenite, which is typical of Climax-type molybdenites. X-ray diffraction of the molybdenite confirms it is the 2H polytype, which is also typical of Climax-type molybdenites and may be linked to the low Re concentrations. U-Pb dating of zircons confines the magmatism at Adanac from 81.6 +/- 1.1 Ma to 69 +/- 1.2 Ma, giving the Mount Leonard stock a lifespan of 13.9 Ma. No appropriate age match was found for an intrusion and mineralization episode using a weighted mean of 30 zircon analyses for each lithology, which is the standard for reporting U-Pb zircon ages. There are too many inherited zircons in Adanac lithologies for a mean age to be reliable, and statistical methods for determining lead loss discredit ages that are most likely valid. It is likely true that most, if not all, of the lithologies that were dated at Adanac were still undergoing some crystallization just before (1 Ma) or during mineralization.The isotopic comparison of the gold sample from Ruby Creek and magnetite from Adanac does not provide a link between the deposits. The gold has a primitive initial Os signature (0.1249) that clearly points to an origin associated with mantle rocks such as peridotites. The magnetite sample has an initial Os of 1.237 that has been enriched from terrestrial Os reservoirs. If any placer gold from the Atlin camp is intrusion related it was not identified in this study, but the possibility that some of the gold is intrusion-related still exists.For exploration purposes, the North American Volcanic and Intrusive Database (NAVDAT) was queried for rock types with high silica (>70 wt%) and high Rb/Sr (>1) and locations of suitable intrusive lithologies for porphyry molybdenum deposits were plotted on a map of the western United States, and compared with locations of known porphyry molybdenum deposits. The resulting areas highlighted numerous potential porphyry molybdenum camps. Some of these areas could be extensions of known camps, such as those in Colorado or in Idaho. Other areas, such as in southern Arizona, have no known porphyry molybdenum deposits but descriptive characteristics of rock types clearly enumerate potential for new discoveries.