This abstract is written by Michael P
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This abstract is written by Michael P. Terry Dakota Territory’s principal researcher and considered by Richard Bachman, President of the Company as one of four or five top structural/metamorphic geologists in the world. His work on the Homestake Deposit is unique and will lead to new gold discoveries in the district. The Homestake deposit, host for > 40 million ounces of gold, is located on the west limb of a SE- plunging regional anticlinorium (F 2 ) formed during regional deformation (D 2 ) at greenschist to amphibolite facies during the Black Hills orogeny (~1750 Ma). Ore mineralization was localized in the Lead anticline and syncline pair that were formed from complex progressive deformation associated with sinistral transpression at ~1740 Ma. Structural analysis and application of models of transpression provide new insights into the distribution of gold mineralization. Lower strains observed in competent amphibolite unit in the core of the major anticlinorium allowed two deformation events (D 2a and D 2b ) to be preserved. Structural observations and measurements indicate that the cleavage (S 2a ) is highly deformed and concordant with compositional layering. D 2a folds (F 2a ) plunge moderately to the SE or ESE. D 2b is associated the dominant steeply east dipping cleavage (S 2b ) that is axial planar to folds (F 2b ) that trend SSE and plunge gently to steeply depending upon the type, degree and orientation of later shearing. Analysis within iron formation and weaker phyllite indicate that earlier structural elements related to D2a and D2b were localized into four different upright planar domains. Two relate to the earliest formed West Ledge shear system that parallel the regional deformation fabric (N15W) and the later formed, but contemporaneous East Ledge shear system that strikes N35W that formed as a result of localization of strain along the margin of the amphibolite unit. The third (N50W) is interpreted to contain the oblique flow apophysis for a monoclinic deformation. This indicates shear systems are pure shear-dominated (west) and wrench dominated transpression (east). This agrees with steeply plunging lineations in the West Ledge shear system and gently plunging lineations in the East Ledge shear systems. The fourth domain strikes N-S and may be related to shear zones localized along the N-S striking limb of the major anticlinorium or possibly a slightly younger event. The intersection of the East and West ledge shear systems is coincident with the deposit and explains for the geometry of the Lead anticline and syncline pair, the orientation of ore ledges and a partial explanation for the orientation of ore shoots. Michael P. Terry, Ph.D. [email protected] Mobile: 6059200574
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Transcript of This abstract is written by Michael P
This abstract is written by Michael P. Terry Dakota Territory’s principal researcher
and considered by Richard Bachman, President of the Company as one of four or five top
structural/metamorphic geologists in the world. His work on the Homestake Deposit is
unique and will lead to new gold discoveries in the district.
The Homestake deposit, host for > 40 million ounces of gold, is located on the west limb of a SE-plunging regional anticlinorium (F2) formed during regional deformation (D2) at greenschist to amphibolite facies during the Black Hills orogeny (~1750 Ma). Ore mineralization was localized in the Lead anticline and syncline pair that were formed from complex progressive deformation associated with sinistral transpression at ~1740 Ma. Structural analysis and application of models of
transpression provide new insights into the distribution of gold mineralization. Lower strains observed in competent amphibolite unit in the core of the major anticlinorium allowed two deformation events (D2a and D2b) to be preserved. Structural observations and measurements indicate that the cleavage (S2a) is highly deformed and concordant with compositional layering.
D2a folds (F2a) plunge moderately to the SE or ESE. D2b is associated the dominant steeply east
dipping cleavage (S2b) that is axial planar to folds (F2b) that trend SSE and plunge gently to steeply depending upon the type, degree and orientation of later shearing. Analysis within iron formation and weaker phyllite indicate that earlier structural elements related to D2a and D2b were localized into four different upright planar domains. Two relate to the earliest formed West Ledge shear system that parallel the regional deformation fabric (N15W) and the later formed, but contemporaneous East Ledge shear system that strikes N35W that formed as a result of
localization of strain along the margin of the amphibolite unit. The third (N50W) is interpreted to contain the oblique flow apophysis for a monoclinic deformation. This indicates shear systems are pure shear-dominated (west) and wrench dominated transpression (east). This agrees with steeply plunging lineations in the West Ledge shear system and gently plunging lineations in the East Ledge shear systems. The fourth domain strikes N-S and may be related to shear zones localized along the N-S striking limb of the major anticlinorium or possibly a slightly younger event.
The intersection of the East and West ledge shear systems is coincident with the deposit and explains for the geometry of the Lead anticline and syncline pair, the orientation of ore ledges and a partial explanation for the orientation of ore shoots.
Michael P. Terry, Ph.D.
Mobile: 6059200574
