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December Meeting: FLC Geology Students

December 7, 2023 @ 5:30 pm - 7:30 pm

Free – $20.00

Please join us for a special night of hearing about some cool geology research results from some seniors in Fort Lewis College’s Geology program! Details below:

  • Thursday, Dec 7, 5:30 PM – 8:00 PM
  • Happy Hour / Dinner will be available to attendees from 5:30 to 6:30 pm, then the talks will start, followed by a raffle drawing.
  • Vallecito Room, Fort Lewis College Student Union

Speakers and talk titles:

Nicole Pottala – An Investigation of the McIntyre Gulch debris flow, Red Mountain, Colorado

Debris Flows are fast-moving destructive masses that cause millions of dollars’ worth of damage every year. The debris flow in McIntyre Gulch near Ouray has created multiple hazards due to the large mass spreading onto Highway 550 two times during the Summer of 2022 covering the road in three feet of debris, (CDOT, 2023). My goals were to understand the controls of this debris flow and the triggers for the activation of the flow. To do this, I conducted field work with another professor and collected data to better understand the debris flow. We found that the debris flow is active during heavy Monsoon season years, (depending on rain). And the high precipitation intensity during the Monsoon season is the predicted trigger for the flow. Investigation shows that the magnitude of the flow is estimated to be 3002.45 CFS and it activates this often based on my findings. My results are attempting to show that the debris flow poses a major hazard and needs mitigation to be put in place.

Jess Zeigenfuss – Analyzing the eruptive history, trends, and magma composition of Mount St. Helens, Cascadia Subduction Zone 

Mount St. Helens (MSH) is the most active volcano in the continental U.S. Previous work has focused on sections of the mountain’s history, but a recent detailed summary of the patterns and trends in the eruptions is lacking. This data is important in understanding influences upon the magma chamber and predicting the trend of future eruptions. In this investigation, information from published literature for MSH over the last 40 ka was reviewed and compiled to explore and interpret patterns and trends in the eruptive cycle.

Data regarding eruptions of MSH history indicates that MSH follows cyclic eruptive patterns, beginning with a Plinian-style eruption that progresses into smaller pyroclastic deposits and lahars, and ends with lava dome building or lava flows. Recent (~4 ka to present) eruptions also progress from more silicic magma in the beginning of the cycle to less silicic magma. The data reveals that repose intervals of eruptions from MSH over the last 40 ka have been decreasing in length over time.

Basaltic magma that erupted from the volcano was hypothesized to show changes in the repose intervals of the magma system, which had previously only erupted dacitic/andesitic products. It was also hypothesized the basalt would have impacted the composition of future magmas, but the data did not lend support for this interpretation. Rather, data on isotopic ratios revealed that MSH magma shows mantle signatures throughout the last 40 thousand years, including dacites. Positive εNd values indicate a mantle derived magma, while higher 87Sr/86Sr values indicate the magma mixed with crustal melt. It is hypothesized that MSH magma is sourced from a rising pluton of vertically graded, mantle derived magma. This hypothesis is supported by the data, as it would explain the gradation of silicic magma during eruptive events, as well as offer an explanation to the mantle signatures in erupted dacites.

Rachel Mosch – Investigating contact metamorphism related to the emplacement of the Sultan Mountain stock near Silverton, western San Juan Mountains, Colorado

Contact metamorphism and alteration formed skarn along the west margin of the ~26 Ma Sultan Mountain stock near Silverton, Colorado. This metamorphic event is evident in fragments on mine dumps near the Thunder Tunnel mine. Preliminary examination of the skarn hinted at the presence of vesuvianite, which is uncommon in skarns in the region. The presence of vesuvianite indicates specific hydrothermal and geochemical conditions within the contact zone.

In this study, skarn samples were investigated to determine mineral associations, paragenesis, and the chemical compositions of key minerals from microprobe analyses. This data was used to assess the host rock for the skarn, metamorphic conditions, and the relationship of metamorphism to magmatism and mineralization.

The results of the field survey identified a relation between intrusive units and Paleozoic limestones near the contact zone and mine dump where samples were collected. The petrographic analyses identified mineral assemblages of vesuvianite + diopside + garnet + calcite along with a late-stage alteration. The skarn was produced by the intrusion of the Sultan Mountain stock into Lower Paleozoic limestone at 500ºC at 50 MPa to 800ºC and 1 GPA. This skarn represents the highest recorded contact metamorphic grade in the western San Juan Mountains.

Nicole Clary – An assessment of Carlin-type gold mineralization associated with plutons at Vista underground mine, Nevada

Nevada Gold Mines has documented elevated gold concentrations near several Mesozoic plutons at Vista underground mine. The cause of this relationship is uncertain. My investigation employed drill hole, petrographic, and geochemical data to explore the nature of the plutons and assess possible factors that could have contributed to the distribution of gold.

The two plutons within the study area crosscut older strata and appear to be dikes. Dike #1 is porphyritic and composed of feldspars with a lesser presence of quartz whereas dike #2 is porphyritic-equigranular and is dominated by quartz, feldspars, and mica minerals. Samples from both dikes are extensively altered to calcite + sericite + chlorite ± iron oxide ± epidote. Gold concentrations proximal to dike #2 range from 0.096 Au ppm to 9.25 Au ppm whereas gold values near dike #1 range from 0.024 Au ppm to 6.069 Au ppm. The highest grades in dike #2 are concentrated along the margins where the majority of Au in dike #1 is concentrated throughout the entirety of the drill core with highest grades along margins.

Later generations of non-magmatic pyrite in veins within the dikes are associated with higher gold concentrations in marginal zones. Samples that contain abundant pyrite have increased arsenic concentrations and are positively correlated with gold values which suggests the gold may be hosted on arsenopyrite. There are several factors that might explain the spatial relationships of gold mineralization with the plutons. The dikes were probably barriers to fluid flow due to their low porosity and permeability which influenced mineralization on the margins. Alteration within the dikes might also have caused chemical reactions with ore-bearing fluids causing instability in the complexes moving the gold, allowing the gold to precipitate.

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Details

Date:
December 7, 2023
Time:
5:30 pm - 7:30 pm
Cost:
Free – $20.00