Abstracts with Programs - Geological Society of America

NaCl+KCl+HCl+H 2 O. Sulfide mineral run products were analyzed by an Electron Microprobe to
determine the concentration of Au and their textures after quench. Au exsolution features were
observed in bornite and ISS throughout the mineral grains, whereas no exsolution textures were
observed in pyrrhotite. Au in pyrrhotite ranged from 300-500 μg/g and did not vary appreciably
over the entire range of the experiments. The solubility of Au in bornite increased from 1000 μg/g
at 500 °C to 1800 μg/g at 700 °C and with an increase in the log sulfur activity of -11.0±1 to
-6.0±0.1. The solubility of Au in ISS increased from 300 μg/g at 500 °C to 4000 μg/g at 700 °C
with the activity of sulfur exerting the principal control on Au solubility as, at 700 °C, Au increased
from 1100 to 4000 μg/g as it increased from log -6.0±0.1 to 0.4±0.8 (1σ). Our results demonstrate
that Au will partition preferentially into ISS relative to pyrrhotite in porphyry systems with an ISS +
pyrrhotite assemblage and into bornite for the bornite + ISS assemblage.
5-3 10:40 AM Mateas, Douglas J. [218078]
HYDROTHERMAL ALTERATION AND MINERALIZATION AMONG THE GOLD ZONES OF THE
BACK FORTY VOLCANOGENIC MASSIVE SULFIDE DEPOSIT
MATEAS, Douglas J., Eastern Illinois University, 600 Lincoln Ave, Charleston, IL 61920,
djmateas@eiu.edu
The Back Forty Volcanogenic Massive Sulfide (VMS) deposit, located along the Menominee
River in the Upper Peninsula of Michigan, is the second largest deposit in the early Proterozoic
Penokean Volcanic Belt. The VMS mineralization, which is hosted by felsic volcanic rocks,
is characterized as Kuroko-style and consists of massive, semi-massive and stringer sulfide
mineralization. The dominant ore in the VMS deposit is pyrite, a gangue mineral. Valuable ores in
the deposit consist of sphalerite, chalcopyrite and galena. Adjacent to the main massive sulfide
mineralization, there are three designated “gold zones” that have proven preferential to precious
metal mineralization of gold and silver. These precious metals have been remobilized from the
main area of mineralization. The three gold zones, which are named the Porphyry Margin Zone
(PM), 90 Zone and Near Surface Zone (NS), have markedly different presentations in hand
sample. PM Zone deposits are found in a quartz-feldspar porphyry, while 90 Zone and NS Zone
deposits are found in a rhyolite crystal tuff host rock. The 90 Zone host rock is so intensely
altered by chlorite, though, that it can be referred to as a chlorite crystal tuff. The objectives for
this research are three-fold. The first objective is to describe the mineralization and alteration
in representative samples from each zone in thin section. The second objective is to attempt to
identify similarities, if they exist, and point out differences in the mineralogy and alteration among
the zones. The overall objective is to determine if there are any characteristics to suggest that
the gold mineralization in the zones represent a single mineralizing event or multiple, separate
pulses of mineralization. At this point, it appears that vast differences in chlorite appearance and
distribution may signal a different mineralizing event for the PM Zone than in the 90 Zone and
NS Zone.
5-4 11:00 AM Hagni, Richard D. [217091]
ORIGIN OF PLATY GALENA IN THE VIBURNUM TREND, SOUTHEAST MISSOURI
HAGNI, Richard D., Geological Sciences and Engineering, Missouri University of Science
and Technology, 161 McNutt Hall, Missouri University of Science and Technology, Rolla, MO
65409-0410, rhagni@mst.edu
The Viburnum Trend of Missouri is the world’s largest producer of lead. The lead occurs as
galena predominantly in two crystallographic forms, octahedrons and cubes. Many studies have
shown that octahedral galena is paragentically early, the more abundant of the two crystal forms,
and is commonly modified by the cube. Those studies also have shown that the cubic form is
paragenetically later, less abundant than the octahedrons, and may exhibit minor octahedral
modifications. Viburnum Trend galena crystals that exhibit a platy form have received almost no
study. The reason for their lack of the study is the rarity of their occurrence. This communication
discusses their character, mine distribution, paragenetic position, trace element contents, nature
of twinning, and speculated conditions of formation. It also compares their character to similar
platy galena occurrences in Bulgaria, Russia, Mexico, and the Pine Point District in the Northwest
Territories of Canada.
Flat, platy galena crystals have been recognized to occur in very small amounts in the
Magmont, Buick, Fletcher, Brushy Creek, and Sweetwater mines in the Viburnum Trend. In
contrast, platy galena has never been observed to occur at the Casteel, West Fork, #27, #28, and
#29 mines in the Trend. The platy crystals have formed early in the paragenetic sequence of the
ores, prior to and coated by subsequently deposited cubic galena and drusy quartz.
Spinel twinning of the octahedron produces flat platy crystals. The platy galena crystals of the
Viburnum Trend are very similar in crystal morphology to platy galena crystals interpreted to be
spinel twins in the Dalnegorsk Pb-Zn (skarn deposit) mine in SE Russia, the Madan ore field of
skarn Pb-Zn-Ag deposits of southern Bulgaria, and the large Naica Pb mine of northern Mexico.
In some lead districts, less common forms of galena have been ascribed to the incorporation of
elevated contents of certain trace elements in those galena crystal forms. Analysis of Viburnum
platy crystals has shown that they contain very low levels of trace elements: 3.1 ppm Ag,