A Comparison of Geochemistry, Carbonate Mineralogy, and Argillic Alteration between the Dome Prospect and the Main Au Resource of the Donlin Creek Project in Southwest Alaska
AuthorDrexler, Heidi L.
AdvisorThompson, Tommy B
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The Donlin Creek project, located in southwestern Alaska, contains a measured and indicated resource of ~35 Moz of Au, with an inferred resource of 4 Moz. This district is spatially associated with a base-metal prospect at Dome, 6 km to the north-northeast. Previous studies indicated differences in salinities, temperatures, and some isotopic signatures at Dome compared to the resource area. A paragenetic sequence showing the temporal relationship of through-going veins, sulfide and carbonate mineralogical associations, and sulfur, carbon, and oxygen isotopic analyses demonstrate the genetic relationship between these two dissimilar ore deposit types. The precious-metal mineral assemblage is composed of auriferous arsenopyrite-pyrite ± stibnite, realgar, and native arsenic intimately associated with Fe-dolomite. Crystallization of euhedral arsenopyrite on porous pyrite-marcasite ± pyrrhotite grain boundaries, mainly in the resource area, demonstrates the late timing of the precious-metal minerals compared to the base-metal mineral assemblage. The base-metal district is noted for pyrrhotite-sphalerite-chalcopyrite-pyrite-marcasite ± tetrahedrite stockwork veins. Relative vein ages exhibited in this study further subdivide and identify the vein assemblages currently logged in the resource area. Diagrams of trace elements obtained from LA-ICP-MS indicate four carbonate phases within veins that formed before, during, and after gold deposition. These include an early calcite phase, which has only been observed at Dome; a pre-mineralization manganiferous calcite event; Fe-dolomite, which is consistently found with auriferous arsenopyrite; and a late-stage post-mineralization ankerite phase. Diagrams of trace elements from LA-ICP-MS data also show two quartz phases that formed before and after gold deposition. Previous studies have indicated overlapping sulfur (δ34S -8 to -16 /) and carbon (δ13C -4.6 to -13.4 /) isotopic data from the ACMA/Lewis resource area and the Dome prospect. Additional carbon (δ13C 1.8 to -16.7 /) and oxygen (δ18O 2.2 to 24.9 /) isotopic analyses completed in this study also show overlapping data. These data suggest similar origins and evolution patterns for the carbonate and sulfide mineralogies associated with the through-going veins, and they further imply a genetic relationship between the precious- and base-metal districts at Donlin Creek. Carbonate staining methods show broad associations between manganiferous calcite ± ankerite and higher gold grades. Previous studies demonstrated that NH4-illite alteration is strongly associated with gold intervals. These mineral indicators provide methods for gold exploration at Donlin Creek. Fifty-meter elevation maps illustrate kriging of gold grades and clay alteration data and they show NNE- and NW-trending spatial patterns consistent with the orientation of dikes and sills in the district.The geochemical and mineralogical analyses completed prior to and in this study demonstrate the lateral relationship between the precious and base-metal districts. In addition, these methods provide information for further classification of this district. As such, this study concludes that the Donlin Creek precious-metal resource and closely located Dome base-metal assemblage fall into the high-level intrusion-related ore deposit-type that is characteristic of the Kuskokwim Mineral Belt.