264 Economic Geology

Wednesday, 12 October 2011
1:30 PM - 5:30 PM
Room 101H-J

SULFIDE MINERALOGY, ANALYTICAL DISCREPANCIES, AND TRACE ELEMENT VARIATIONS AT THE PUMPKIN HOLLOW FE-CU DEPOSIT, LYON COUNTY, NEVADA

Paper number: 264-4
Program: Technical Sessions
Time: 3:30 PM – 3:45 PM

Authors

Abstract

The Pumpkin Hollow deposit is an IOCG deposit located in the western Great Basin, outside the town of Yerington, Lyon County, Nevada. Mineralization is associated with rocks of the Jurassic Yerington Batholith, primarily granodiorite to diorite, which were emplaced into the Triassic Mason Valley Limestone and the argillites, shales, siltstones, and limestones of the Gardnerville Formation. As of March 2011, the measured and indicated reserve contains 5.9 billion pounds of copper, 1.6 million ounces of gold, 42 million ounces of silver, and 111 million tons of iron. The primary ore minerals are chalcopyrite and magnetite, with minor sphalerite; pyrite and pyrrhotite are common.

Bulk chemical analysis using ICP-OES was completed by a commercial laboratory, with follow up work by the USGS Denver office using LA-ICP-MS, SEM, neutron activation, XRD, and micro-XRF. A total of four different pyrite phases have been identified to date, each with a unique textural and/or geochemical signature. The trace elements associated with the different pyrite phases include: barren late-stage veins, high Co and Ni, high Ni with trace Co, and low Co and Ni. Arsenic values do not appear to follow any trend within the pyrite phases. Silver occurs primarily within chalcopyrite, with only local zoning of Ag values within grains. Sphalerite occurs as fine grained inclusions within chalcopyrite. Trace amounts of Bi, Te, and Ag have been identified as mineral inclusions within pyrite grains, with highly variable concentrations among the three elements.

Upon comparison of the bulk chemical data, we have concluded that there is a Fe interference with Te when using ICP-OES. Anomalously high Te values were reported by the commercial lab and suspiciously strong, apparent correlation were noted in plots of Te versus Fe. The multiple USGS methods yielded consistently low Te values in both hand samples and splits of the 5 foot composite pulps used by the commercial lab. Other elements were within the margin of error between different analytical techniques.