Structural controls on gold mineralization at the Homestake Mine and their implications for the geology of the Black Hills

Bell, Christopher C. (2013) Structural controls on gold mineralization at the Homestake Mine and their implications for the geology of the Black Hills. PhD thesis, James Cook University.

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Abstract

The thesis consists of five sections, each intended as bodies of work for publication in international journals. Section E, published in Mineralium Deposita April 14th, 2010, resulted from discussions with Jeremy Richards in 2005 during a field trip to Boroo and Erdenet Mines in Mongolia that led to a collaboration with Ryan Morelli, Robert Creaser (University of Alberta) and Antonio Simonetti (University of Notre Dame) utilizing a new approach to dating gold mineralization employing rhenium-osmium sulfide geochronology. The author contributed 45 % of this section.

Section A uses detailed work around the Homestake Mine (sections B, C, D and E) combined with regional traverses to interpret folds across the rest of the Black Hills uplift in South Dakota, USA. The trends of regional folds around the competent Harney Peak Granite in the southern portion of the Black Hills are directly analogous to those resulting from partitioning deformation around rigid objects such as porphyroblasts. The trends provide a direct connection to the orientations of structures preserved within garnet and staurolite porphyroblasts at the Homestake mine that allow integration of the history of fold development across the whole region. This has considerable significance for the timing of emplacement of the Harney Peak plutonic complex. This section provides a structural framework for the detailed work on the Homestake Mine region where the younger deformations that affected the Black Hills region are much more intensely developed. The early history indicated by porphyroblasts, but lost in the matrix of schists at Homestake, can be seen 50 kms to the south in the regional folds. Combining regional fold trends with quantitative orientation data from foliations preserved in porphyroblasts provided a very useful approach to resolving the complex structural geologic relationships across the whole region. This approach could be used in any multiply deformed and metamorphosed terrain.

Section B documents and describes the structural relationships on the surface around the Homestake Mine. Relatively little modern structural work has been done at the Homestake Mine, considering the protracted 1876 to 2002 history of mining at the deposit, the very large amount of gold extracted and its worldwide significance as the 'type' deposit for carbonate-facies iron formation hosted gold mineralization. For many years, gold mineralization was thought to be syngenetic strata-bound style and little structural work was undertaken as the distribution of rock units in the mine area and region was well known. Surface fieldwork revealed a deformation event with a gently dipping axial plane that controlled the location of where subsequent folds with sub-vertical axial planes formed directly associated with gold mineralization. Without this flat lying event, similarly oriented pre-existing folds with sub-vertical axial planes would simply have intensified during later deformation. Mineralization only developed where this flat lying event formed structural heterogeneities that led to local gaping, implosion and thus breccia vein development during the mineralizing deformation event. The asymmetry of folds associated with this flat lying deformation changes across the mine from N to S suggesting that somewhere within the mine, parasitic coaxial folds that formed in its hinge controlled the location of orebodies. This section documents the discovery of that deformation, using it to resolve several structural issues in interpreting the structural history of the mine.

Section C documents the structural work from underground within the mine and interprets the development of orebodies from this data. This section provides resolution for known ore ledge (plunging fold structure) and ore shoot geometries, in terms of the structural relationships that controlled their development and localization. It demonstrates the key role of the deformation event with sub-horizontal axial planes in Section B and shows that three successive deformations control the location, gross trend and geometry of ore bodies within the Homestake Mine. Mineralization during the last of these phases of deformation would not have formed without the two prior fold and foliation producing events. Section C concludes with an innovative approach to exploration resulting from work presented herein that can be applied elsewhere in the region and in similar terrains worldwide.

Section D documents unexpected aspects of structural and metamorphic development of rocks within the Homestake Mine. These features were discovered by the microstructural work conducted on foliations in both the matrix and porphyroblasts. The Homestake Mine is unusual in providing unparalleled exposure of a north-south striking, vertically orientated garnet isograd through the crust in workings to a depth of 2.5 kms below surface. Isograds are generally thought of as gently to moderately dipping features. The initial interpretation was that the garnet isograd had been rotated by deformations forming sub-vertical foliations to this steep orientation. However, microstructural work using shear sense indicators revealed that rotation to this steep orientation was not a viable explanation. Every deformation that could be documented, counting those in the matrix and those foliations that predate it (those preserved as inclusion trails in the garnet porphyroblasts that help define the isograd), show a west side up shear sense or coaxial strain. The biotite zone lies on the west side of the garnet zone, yet east side up shear to rotate the garnet isograd to its sub-vertical orientation is not supported by these observations. The data indicate that the garnet isograd formed sub-vertically and maintained this position through many deformations. This section documents and interprets the significance of this research.

Section E refines the timing of gold mineralization at Homestake Mine through the application of rhenium-osmium geochronology of ore-associated sulfide minerals relative to known tectonothermal events in the northern Black Hills. Rhenium-osmium geochronology of common sulfide minerals is a technique that offers a theoretical means of directly dating many hydrothermal ore deposits in the Earth’s crust. In practice, however, the successful application of this technique has historically been impeded by major analytical challenges. This section utilized recent analytical developments that permitted precise and accurate isotopic measurements of the low concentrations of rhenium and osmium typical in many crustal sulfide minerals. Arsenopyrite yielded a Re-Os age of 1738 ± 8 Ma, consistent with existing age constraints for gold mineralization. This indicates that the Re-Os arsenopyrite chronometer is robust to at least 400ºC. The Re-Os arsenopyrite age and initial Os ratio were interpreted to indicate that gold was introduced at ca. 1730 Ma, coincident with the onset of exhumation of crustal blocks and early intrusive phases of Harney Peak Granite magmatism. New in situ U-Pb monazite analyses from an aplite dike in the east-central Black Hills indicate that granite magmatism was a protracted event, persisting until at least ca. 1690 Ma.

The study forms part of collaborative research between the Structural and Metamorphic Research Institute (SAMRI) and the Economic Geology Research Unit (EGRU) at James Cook University on structural controls on mineralization from world-class ore deposits.

Item ID: 29892
Item Type: Thesis (PhD)
Keywords: Homestake Mine; Black Hills, South Dakota; gold mineralization; structural formations; geochronology; metamorphic development
Date Deposited: 24 Oct 2013 05:06
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 34%
04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 33%
04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 33%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970105 Expanding Knowledge in the Environmental Sciences @ 50%
84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840107 Titanium Minerals, Zircon, and Rare Earth Metal Ore (e.g. Monazite) Exploration @ 50%
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