FE OXIDE-CU MINERALIZATION AT THE MINNESOTA AND PUMPKIN HOLLOW DEPOSITS, YERINGTON DISTRICT, NEVADA

New petrographic and field studies at the Minnesota and Pumpkin Hollow magnetite-rich skarns (Yerington district, Nevada) show that Mt-rich, Si-poor mineralization formed early in a multi-step hydrothermal history, likely related to intense Na-Ca alteration and, presumably, from non-magmatic brines. This contrasts with evidence for later formation of porphyry Cu and Mt-poor, Cu skarns associated with release of magmatic fluids (cf. Dilles et al., 2000, SEG FG 32).

Both Minnesota and Pumpkin Hollow are magnetite(-hematite)-rich, sulfide- and silicate-poor deposits hosted in calcareous Mesozoic sedimentary rocks intruded by the Jurassic Yerington batholith. Metamorphism marbleized limestones and created calc-silicate or Bi-hornfels units in impure sediments. Hydrothermal alteration consists of pervasive endoskarn: deep Na-alteration (Ab-Rut-Chl-Act) at Minnesota, Ca alteration (Grt-Olig-Act±Di±Cc) at Pumpkin Hollow, and Na-Ca (Olig-Act-Sph±Di) at both Minnesota and Pumpkin Hollow. Mt-mineralization dominantly occurs in marble, but also replaces more aluminous rocks (intrusions, hornfels, volcaniclastic rocks). At Minnesota, Fe oxide- mineralization zones from central Mt-replacement to peripheral hornfels-clast Mt-cemented breccia, and distal Mt-clast Mg-silicate-cemented breccias along the marble contact. Magnetite mineralization at Pumpkin Hollow differs in size, in having higher contents of Cu-Fe sulfides and in hornfels hosted Mt-rich breccias and replacements.

Field relations suggest that the Mt-rich mineralization may have formed early, with quartz monzodiorite and added comparatively high Fe/Si; in contrast, the Mt-poor Cu-rich skarns in the district have abundant calc silicates, are linked to the later granitic phases of the batholith which also generated porphyry Cu deposits, and they added lower Fe/Si.. These results support the hypothesis that Mt-rich, variably Cu-bearing systems in the Yerington district are related to early circulation of non-magmatic brines, which transport Fe and Cu, make extensive Na-Ca alteration, widespread Fe-oxide mineralization on upwelling, and locally concentrate Cu. These features parallel those seen in IOCG-type systems worldwide.