Background Research
The Molybdenite from this study originates from the Cave Peak breccia pipe complex in Culberson County, Texas. As described in Sharp (1979) and shown in Figure 1 to the left, these breccia pipes, located on the Sierra Diablo mountain range, consist of three individual pipes: a main quartz latite-quartz monzonite porphyry associated plug and two rhyodacite associated plugs. The intrusion began around 39 Mya with the emplacement of a rhyodacite plug and breccia pipe. North of which was another set of plug and pipe. Finally, the Cave Peak breccia pipe formed adjacent to the aforementioned pipe. After brecciation of the main pipe, mineralized zones of molybdenum developed. Later, around 32 Mya, these pipes as well as the molybdenum were cut by porphyritic quartz monzonite. This paper provides a good overview into the geological setting at and around Cave Peak which will provide us with a base of knowledge as to which events may have led to the creation of the different molybdenite habits.
Prior to actually going into the lab and getting our scans done, we were intending to follow the method in Ciobanu et al. (2012) almost to a tee due to this paper being based on analyzing molybdenite deposits. In the paper, they looked at two previously analyzed deposits of molybdenite to demonstrate how LA-ICP-MS can be used to document trace element patterns to corroborate successive events in the deposit’s history. Ciobanu et al. (2012) used an Agilent 7500 mass spectrometer coupled with a UP-213 laser ablation system to analyze the molybdenite samples and acquire trace element data. They used an in-house standard STDGL2b2 for calibration. The isotopes that were measured are as follows: 57Fe, 59Co, 60Ni, 65Cu, 66Zn, 75As, 77Se, 107Ag, 118Sn, 121Sb, 125Te, 182W, 185Re, 197Au, 204Pb, 206Pb, 209Bi, and 238U. Analysis of standards were done at 10 Hz laser frequency and 80 μm spot size while molybdenite samples were done at 5 Hz and 35 μm spot size. The total measuring time per sample was 90 seconds; 30 seconds measuring of the background with the laser turned off and 60 s measuring with the laser on.
Uncertainty in the molybdenum standard STDGL2B2 is around 6%. This uncertainty is spread to all analyzed elements in the study since molybdenum is used as the internal standard. Rhenium uncertainty is believed to be of similar magnitude. Thus a total error between 20-40% is estimated for the Rhenium concentrations.
Molybdenite samples from both the Hilltop and Boddington deposits show lamellar or aggregates grains with common kinks or folding. Bismuth and galena inclusions were present throughout the samples. LA-ICP-MS was targeted on homogenous areas of grains that would be analyzed to help understand any variation in regard to different grain textures. Molybdenite grains from a Hilltop sample (HT-X) showed richer concentration of Re compared to the others. A grain from this sample stands out by having a coarse lamellar core surrounded by fine-grained cemented chalcopyrite and silicates. The grain’s inner core shows low Re concentration compared to its outer core. Contrary to Hilltop, Boddington samples show Re to be homogeneously distributed throughout the grains. This difference in trace element distribution shows that trace element distributions of Molybdenite can be used to interpret mineralization histories of the samples. It is our goal in our own study to figure out if the same can be done by us to interpret both morphological types of molybdenite. While Ciobanu at al. (2012) was able to use spot scans, when starting to analyze our own samples we quickly found that the laser would destroy our sample grains, especially the ones on thin sections, due to the time the beam is focused on the spot. Instead, we decided to use line scans.
Early in the project process we struggled to find so much as a mention of another standard to use in addition to STDGL2LB2. Sulfide standards appeared to be far and few in between, with most studies using resorting to using silicate standards. Zhi Ren et al. (2018) selected the GSG-1G basaltic standard alongside STDGL2LB2 for use in calibration. The study looked at the amounts of trace elements in the Shapinggou deposit located in the Dabie Orogenic Belt in eastern China. The isotopes that were measured were 27Al, 57Fe, 59Co, 60Ni, 65Cu, 66Zn, 75As, 77Se, 88Sr, 90Zr, 107Ag, 111Cd, 118Sn, 121Sb, 125Te, 172Yb, 182W, 185Re, 205Tl, 208Pb, 209Bi, and 238U. These isotopes were selected in order to avoid isobaric and polyatomic interferences that may have arisen otherwise. Laser frequency, spot size, and run time for the standards and the molybdenite samples were identical to those used in Ciobanu et al. (2012). The data from the study shows that highly variable metal concentrations, such as Re, Pb, and Cu, are present in the molybdenite from the area surveyed. In addition to this, Re concentrations as well as Mo/Cu ratios decreased with decreasing temperatures. In contrast to this, the concentrations of Cu, W + Sn, Re/Cu and Co/Ni ratios increased.
Audétat (2010) findings suggests that the brecciated Cave Peak and Marble Canyon, locations which are linked in their history and formation, show an evolution from mafic magmas to more felsic as you progress to the Cave Peak locality. By plotting incompatible trace element abundances from melt inclusions Audétat found linear trends showing 100 times the average bulk upper continental crust composition suggesting these felsic magmas originated from a mafic source and transitioned to felsic by fractional crystallization. Molybdenum concentrations also increased from 4 ppm to 12 ppm as the magma shifted from mafic to felsic composition. This Molybdenum content later decreased to 5 ppm which suggests that it was removed from the melt.
Audétat (2012) explores the early stages of porphyry copper deposit formation and emphasizes the role that magmatic sulfides have in controlling ore-forming metal distribution. High metal concentrations in mafic arc magmas typically are favored by low degrees of partial melting in the mantle as well as limited removal of magmatic sulfides in the lower crust. As ascending magmas accumulate in upper crustal magma chambers, interactions between mafic and felsic magmas lead to the formation of magmatic sulfides rich in copper and gold. Successful porphyry copper mineralization comes when these sulfides are subsequently destroyed and release their metal content into mineralizing fluids. Important factors that influence the efficiency of copper removal from magma and metal transpiration into mineralizing fluids are fluid composition, oxygen fugacity, and sulfur ratios influence.
Tan Et al. (2023) talks about molybdenum’s sulfide hosting of many trace elements such as Re, Cu, Ag, Se, Pb, Bi, Te, Ni, Co, Sn, Sb and W. These trace elements in molybdenite can be used to constrain the source and conditions of ore-forming fluids in individual deposits. However, the relationship between the trace element composition of molybdenite and deposit types has not been looked into in a large dataset. Tan uses simple statistics and partial least squares–discriminant analysis to find out whether or not different types of deposits can be distinguished by the trace elements in molybdenite and also find out what factors control the variations in trace element composition. The study also makes sure to discuss the limitations to using molybdenite as an indicator mineral due to the complex occurrences of elements in it, the large compositional variations within a deposit, and an imbalanced dataset.
Possibly the most in depth study on Cave Peak molybdenite comes from Ugurhan (2018). In his thesis, Ugurhan suggests a possible connection between two plutons in the area: Cave Peak and Marble Canyon. A look at Al2O3, K2O, Fe2O3, MgO, CaO, TiO2 and P2O5 amounts in each location shows a trend that goes from silica poor magma to silica rich magma. Ugurhan states that this trend is an indication of differentiation of the magma through time that becomes more silica rich when reaching the Cave Peak location. This falls in line with Audétat (2010), as that study’s trace element trends point towards the occurrence of fractional crystallization. Both Audétat and Ugurhan’s findings both point towards Cave Peak molybdenite coming from fractional crystallization.