Hydrothermal fluid and ore paragenesis of the gold-bearing Rattlesnake Hills Alkaline Complex, Wyoming.

摘要

Rattlesnake Hills is a recently discovered alkaline rock gold system located in central Wyoming. Rattlesnake Hills is currently being explored by Agnico Eagle Ltd. Alkaline igneous complex hosted gold deposits are a rare, but potentially substantial type of gold deposit. Cripple Creek, Colorado is one of the best known world class alkaline rock gold deposits. While Cripple Creek is one of the largest gold alkaline systems in the world, gold enrichment in these systems is still not well understood. Both complexes lie along a trend of Tertiary alkaline complexes that extend from Mexico to Canada.;The project area is part of the larger Rattlesnake Hills Alkaline Igneous Complex (RHAC). The local geology of the project area consists of Archean mica-rich schist basement rocks and 15 Tertiary intrusive bodies with varying lithologies of phonolite to quartz monzodiorite. The age of Rattlesnake Hills alkaline complex is dated as 43-44 Ma. The three largest and most extensively drilled bodies of the alkaline complex: North Stock, South Stock, and Antelope Basin; however, South Stock is not currently being explored.;North Stock is dominated by explosive diatreme breccias and porphyritic bodies, both containing gold mineralization. Antelope Basin is a large quartz monzodiorite body that intruded schist basement and remains a target for gold mineralization. Gold mineralization aligns with the wall of the collapse cone of the diatreme breccia and the schist wall rock which are parallel to local faulting. These structures provide structural controls for the gold mineralization. Gold mineralization occurs in at least two styles: high grade gold+carbonate veins and low grade disseminated gold in altered rocks.;Mineralized and barren samples were analyzed to determine the form and setting of gold mineralization. Ore microscopy of thin sections shows the main ore minerals are gold, pyrite, marcasite, chalcopyrite, sphalerite, and other trace sulfides. Gold grains are found in marcasite grains and rarely as free gold in carbonate veins. The gold grains analyzed with scanning electron microscopy (SEM) have similar contents of gold and silver. Gold and silver are also found in marcasite and pyrite grains either in the lattice structure and/or nanonuggets. Laser ablation inductively couple plasma mass spectrometry (LA-ICP-MS) of pyrite grains show there is an increased concentration of arsenic and gold in the rims while the cores are depleted. In general, there are two trends of arsenic and gold in pyrite grains: 1) high gold and arsenic values and 2) low arsenic and low gold values. Additionally, lattice bound and/or nanonuggets of silver are found in marcasite grains.;Quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) analysis of 11 thin sections samples using back scatter imaging produces false-colored images taken across a 20-micron resolution. QEMSCAN images show several additional features that were not recognizable with ore microscopy: 1. Strong K-feldspar alteration in all samples, by percent volume ∼40-84%, 2. Pyrite or marcasite with arsenian rims is associated with muscovite-sericite zones, 3. There are generally two generations of carbonate in veins (in calcite-rich veins early calcite is rimmed by Mg and Fe-bearing calcite, and dolomite-rich veins show early dolomite with late Mg and Fe-bearing dolomite), 4. Pyrite associated with gold mineralization is at the vein contacts with wallrock or is associated with early carbonate generations, 5. Most gold, silver, and electrum grains range from 10-25 mum.;Fluid inclusion studies show the fluid at Rattlesnake Hills is low temperature of 180-210°C with a low to moderate salinity. The inclusions measured show no evidence of boiling; however, there are inclusions that are vapor-rich but do not yield any microthermometric data. There are rare solids, daughters and/or accidental solids, in complex three-phase inclusions. Some inclusions have low, but detectable CO2 present. LA-ICP-MS of inclusions show there is up to 2-9ppm of gold and silver in solution.;The stable oxygen and carbon isotopic compositions show two distinct isotopic trends: 1) large cluster with positive covariation of oxygen and carbon and 2) a scatter to strongly negative delta13C PDB values. The covariation signature is most likely caused by magmatic degassing at depth within the magma chamber causing the progressive release of CO2 rich fluids which then resulted in the precipitation of carbonate. The signature implies that Rattlesnake Hills is dominated by a mantle-derived magma.;Rattlesnake Hills is comparable to world class alkaline gold deposits throughout the world, but has most similarities with both high and low sulfidation epithermal style deposits. The fluid at Rattlesnake Hills was low temperature, low to moderate salinity, magmatic, and contains trace gold and silver in solution.