Genetic relationship between porphyry and epithermal deposits at Jamestown, Colorado.

摘要

Jamestown mining district, Colorado, contains a unique association of porphyry-style (molybdenum in the Porphyry Mountain) and a set of epithermal-style deposits. It also shows a typical mineral zonation around the Porphyry Mountain source of mineralization. In order of decreasing age and increasing distance from the Porphyry Mountain, the mineral zones are Mo below the surface in the Porphyry Mountain stock, lead-silver, fluorite, pyrite-gold and gold-telluride deposits surrounding the Porphyry Mountain. The porphyry and epithermal deposits occur on and close to the surface at Jamestown. This setting is excellent to demonstrate the genetic relationship between porphyry and epithermal deposits.; Several porphyry-style deposits have peripheral, zoned epithermal (low-temperature) deposits, e.g. Bingham, Utah; Butte, Montana and Northern Luzon, Philippines. Although this spatial association has been frequently noted, no strong evidence has been provided to evaluate possible genetic relationships.; Such evidence of genetic relationship between porphyry and epithermal mineralization in the mining district has been assembled through fieldwork, core logging, geochemical studies of major and trace elements, including rare earth elements, petrographic studies of igneous rocks and ore minerals, radiogenic dating (40Ar/39Ar), and remote sensing using imaging spectrometry or "hyperspectral remote sensing AVIRIS" and laboratory spectrometry.; The results of this study show that: (1) The Porphyry Mountain stock consists of multiple intrusions enriched in silica, alkalies and fluorine. (2) Estimated depth of formation of the Porphyry Mountain stock ranges from 227 to 1,510 m. Epithermal veins were formed at shallow depths depending on type of mineralization (50--1,133 m). (3) The intrusions of Porphyry Mountain show similar mineral and chemical composition, indicating possible one parent source. (4) The mineral deposits and the igneous rocks display similar rare earth element patterns. (5) Mineral deposits form concentric zones around Porphyry Mountain. (6) Porphyry Mountain and surrounding fluorite deposits have a close temporal relationship (about 54.44 +/- 0.1 Ma). (7) Temperature and salinity of the ore-forming fluids of the epithermal veins are inversely related to the distance from the Porphyry Mountain. (8) The main alteration minerals are clays (kaolinite and montmorillonite) and sericite.; The above results indicate that the Porphyry Mountain stock is the main source of epithermal mineralization at Jamestown, Colorado. This genetic connection provides implications for exploration. Where porphyry-style mineralization is found, there is a potential for an epithermal-style nearby.