Mineralogical characterization and paragenesis of the Cripple Creek deposit, Colorado

Abstract

The world-class Cripple Creek gold telluride deposit in central Colorado is centered on an Oligocene alkaline-magmatic system composed of diatremal Breccia that is intruded by dikes and sills that range in composition from phonolite to lamprophyre. The deposit has historic production of >24 Moz of gold, making it the third most productive gold deposit in the United States. Most of this production came from underground mining operations that started with the discovery of the deposit in 1891, and continued until the early 1960's. The discovery of broad low-grade, bulk tonnage deposits in the 1980's led to renewed mining in the form of open pit operations, which remain active on a large scale. Two styles of mineralization are therefore recognized within the deposit: high-grade gold telluride veins that were historically mined underground, and low-grade bulk tonnage "disseminated" gold that is targeted by current pit mining operations. Geological study of the deposit began shortly after its discovery, and has continued through the present day. Much of this research has focused on describing and analyzing the alkaline rock suite that hosts mineralized ore bodies, and describing the ore bodies themselves. More recently there has been extensive work in describing the magmatic and hydrothermal evolution of the deposit. This work also revealed that gold-bearing As-rich pyrite exists within some ore zones. Through multiple studies, several different models have been proposed for the mineralizing history of the deposit, and many of the previously published paragenetic sequences that describe the relationship of different ore, gangue, and alteration minerals are contradictory. Conflicting parageneses and deposit models indicate that revision is needed in order to better understand how the Cripple Creek deposit became mineralized. This thesis aims to address some of the existing discrepancies, with focus on refining the paragenetic sequence of mineralization and developing a new model that describes the mineralogical evolution of the deposit. In addition, this thesis explores the nature of gold-bearing As-rich pyrite as sampled from different regions within the deposit. This thesis presents information derived from detailed mineralogical characterization of ore, gangue, and associated alteration minerals from five sampling locations. Samples that were analyzed in this study preserve evidence of seven distinct mineralizing stages. These stages are characterized by: 1) Early trace element-poor pyrite formation that formed through sulfidation of preexisting iron-rich minerals such as hematite, specular hematite, and biotite across broad zones of rock, 2) The formation of trace element-rich gold-bearing As-rich pyrite as overgrowths on trace element-poor pyrite followed by the formation of microcrystalline hypogene gold- these minerals are restricted to fractures and wall rock adjacent to fractures within broad zones of early trace element-poor pyrite, 3) The development of micron-scale Ag-Pb-(Au) telluride minerals hessite, altaite, and petzite with galena within micro-fractures that cut fracture-hosted pyrite grains formed during the previous stage of mineralization, 4) The formation of tetrahedrite-bearing quartz veins, 5) The growth of additional stages of trace element-poor and trace element-rich pyrite, confined purely fractures and veins that would later host gold telluride minerals, 6) The large-scale development of gold telluride minerals within quartz ± fluorite veins that may or may not contain trace element-rich pyrite from the second and fifth stages of mineralization, and 7) Molybdenum, copper, and silver-rich base metal sulfide and sulfosalt deposition within veins of quartz ± fluorite ± barite that cut all previously formed minerals. Recognition of these distinct stages allows for several refinements to be made to the existing deposit model. Key refinements include: (1) the recognition that the deposition of gold-bearing As-rich pyrite associated with microcrystalline hypogene native gold is not paragenetically related to gold telluride mineralization, (2) recognition that at least three different types of quartz exist within the deposit, (3) recognition that quartz within gold telluride veins was deposited before the gold telluride minerals themselves, and (4) recognition that textural relationships between compositionally distinct stages of pyrite growth can be used to track the mineralizing history of the deposit through time and thus add constraint to the paragenetic sequence of mineralization. Characterization of gold-bearing As-rich pyrite from the Cripple Creek gold telluride deposit has revealed that trace element-rich pyrite is likely common in the deposit. Scanning electron microscope and electron microprobe analyses conducted during this study demonstrate that at least five distinct generations of pyrite exist within the deposit. In addition, electron microprobe analyses reveal that most of the gold contained within pyrite is in solid solution as opposed to existing as nano- to sub-micron-scale inclusions of native gold. The relatively high concentrations of gold within As-rich pyrite indicate that gold-bearing As-rich pyrite may be a significant and previously unrecognized resource within the deposit. A model was created to estimate the mass of gold that is contained within gold-bearing As-rich pyrite, and indicates that between 1.0 and 4.7 g/ton of gold may be contained within gold-bearing As-rich pyrite from specific sampling locations, thus providing strong evidence that pyrite within mineralized areas is in fact a significant gold resource.