Hofstra, Albert H.¹, Landis, Gary P.¹, Premo, Wayne R.¹, Ressel, Michael W.², and Henry, Christopher D.³, (1) U.S. Geological Survey, MS-973, Box 25046, Denver, CO 80225, (2) Kinross Gold USA, Inc., 5370 Kietzke Lane, Suite 102, Reno, NV 89511, (3) Nevada Bureau of Mines and Geology, MS 178, University of Nevada, Reno, NV 89557
The source of ore fluid components in Eocene Carlin-type gold deposits remains a contentious issue that is critical to development of realistic genetic and exploration models. To refine understanding of volatile and metal source(s) in Carlin trend and Jerritt Canyon gold deposits, we analyzed the concentration and isotopic composition of He in fluid and melt inclusions as well as Pb, Sr, and Nd in hydrothermal minerals (ore stage pyrite, orpiment, stibnite) and neighboring 36 to 41 Ma basalt to rhyolite igneous rocks (separated minerals and whole rock).
He R/Ra values range from 0.005 to 3.18 for all igneous rocks, 0.005 to 0.76 for felsic rocks, and 0.33 to 1.41 for hydrothermal minerals. On a plot of He R/Ra vs. He concentration, igneous rocks yield a V-shaped pattern of binary mixtures between a crustal-air saturated meteoric water (ASMW) mixture at the base point of the V and crustal He (R/Ra ≈ 0.02) and mantle He (R/Ra ≈ 8) at each extremity of the V. Hydrothermal minerals plot near the point of the V and extend towards ASMW He (R/Ra =1), though most hydrothermal minerals have He/Ne ratios (0.6 to 5790; 282 avg.) that preclude significant atmospheric helium (He/Ne = 0.288, R/Ra =1). The felsic rocks and hydrothermal minerals near the point of the V have low He concentrations and an intermediate crust-mantle helium mixture (2-10% mantle He). This similarity permits that felsic magmas and hydrothermal fluids were charged with a similar proportion of crustal and mantle He at a similar position in the crust; e.g., near crustal structures at depths of about 6 km, which may correspond to the bottom of ASMW convection cells and the top of felsic magma chambers.
Eocene igneous rocks yield an ill-defined Pb-Pb isochron age of ~1640 Ma and are generally more 207Pb-rich than their Paleozoic host rocks indicating the probable involvement of an older, U-poor, Paleoproterozoic source. They have crustal initial values (Sri > 0.705 and Ndi < -2) with no evidence for a depleted mantle component. While Carlin trend mafic to felsic igneous rocks are relatively homogeneous (Sri ≈ 0.708 and Ndi ≈ -7), those from Jerritt Canyon and the Ruby Mountains are more enriched (Sri = 0.724 and Ndi = –17) suggesting they are underlain by Proterozoic granitic sources and/or minor Archean sources. Hydrothermal minerals from both districts either plot near the ill-defined Pb-Pb isochron for Eocene igneous rocks or between it and the array of values for Paleozoic host rocks. Late pyrite in the Carlin trend is more radiogenic than the Paleozoic host rocks. The results imply that the Pb in hydrothermal minerals was derived in various proportions from Eocene igneous and both deep and shallow sedimentary sources.
Together, the He, Pb, Sr, and Nd results permit that Eocene felsic magmas contributed a portion of the volatiles and metal fixed in gold deposits of the Carlin trend and Jerritt Canyon district but appear to rule out mafic magmas as a significant source.