Eric P. Nelson
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Eric P. Nelson, Associate Professor
Department of Geology and Geological Engineering
Colorado School of Mines
Golden, CO 80401
Phone: (303) 273-3811 Fax: (303) 273-3859
President of Colorado Scientific Society in 2002
BS Caifornia State University/Northridge
MA Rice University
MPhil, PhD Columbia University
Current Research :
3-D and 4-D definition of structural fluid flow networks in Devonian carbonates using sulfide tracers; Emanuel Range of the Lennard shelf, Canning basin, Western Australia
Support: NSF Award EAR-0073763
Understanding the geometry and nature of fluid flow in the earth’s upper crust is critically important to a number of geoscience disciplines related to societal and industrial activities, including petroleum geology, ore deposit geology, hydrogeology, geothermal energy, and seismic hazard analysis. Crustal fluid flow is commonly controlled by structural permeability of fault and fracture networks. Although fault-fracture flow networks have been studied extensively through modeling and 2-D field studies, few 3-D field studies have been undertaken on ancient flow networks.
Zn-Pb deposits in the Canning basin of Western Australia provide a unique natural laboratory to investigate crustal fluid flow in 3-D. Metalliferous basinal brines flowed along fault-fracture networks as the basin compacted and dewatered, and deposited metal sulfide minerals when the fluids reached permeable, reactive carbonate formations. The area was unaffected by deformation following mineralization.
In this project, we have proposed to build a 3-D model of the paleo-flow network by mapping the distribution of sulfide minerals, deposited along faults and fractures, using superb 2-D surface exposure, 3-D underground exposures, 500+ kilometers of diamond drill core, and extensive seismic reflection, aeromagnetic, and gravity data. Cross-cutting vein relationships and petrographically determined sulfide parageneses will be used to determine the relative timing of mineralizing events, thus extending the model to 4-D (time).
Dr. John McLeod Miller is a post-doctoral research associate who is working on the project. He is based at the Australian Crustal Research Centre at Monash University (http://www.virtualexplorer.com.au/ACRC/), with who we are presently forging a formal inter-institutional agreement of cooperation.
field work in Western Australia
core showing sphalerite-matrix breccia
Three-dimensional strain during basin formation – orthorhombic fault patterns and associated MVT mineralization, Lennard shelf, Western Australia
J.McL. Miller1, E.P. Nelson2
1Australian Crustal Research Centre, School of Geosciences, Monash University, Australia
2Dept. Geology/Geological Engineering, Colorado School of Mines, Golden, CO
The northern margin of the Canning basin (Lennard shelf) is defined by northwest-striking normal faults parallel to the basin margin and northeast-striking faults that define accommodation zones at right angles to the basin margin. Northwest-striking faults commonly reactivate 50° to 60° south-dipping Proterozoic shear zones, whereas northeast-striking faults are localized in massive Proterozoic granitic basement. Structural permeability in fault-fracture meshes within Devonian carbonate rocks is the key control on the location of Mississippi Valley-type Zn-Pb mineralization. These deposits developed by infiltration of metalliferous basinal brines related to basin compaction and dewatering during the final stages of Late Devonian to Early Carboniferous extension. Surface and underground mapping of both structural trends, combined with visualisation of 3-D models using gOcadTM software, demonstrates that these faults form an orthorhombic geometry (Reches and Dieterich, 1983) and related extension fractures form a rhombic pattern in map view. The relative timing of different vein and fault generations is constrained by paragenesis of mineral fill. Pre-mineralization marine calcite fill in the fault-fracture mesh indicates that the mesh formed early in the deformation history. The orthorhombic fracture and fault geometry is interpreted to have developed initially in response to three-dimensional non-plane strain in which the intermediate finite stretch magnitude was non-zero and thus influenced fault orientation, and ultimately the distribution and geometry of oreshoots in Zn-Pb deposits. Late-stage calcite-galena veins and slickenline data indicate a change to plane strain. Opposite strike separation across northeast-striking graben-bounding faults, and a horizontal intersection of mineralized extension veins with these faults, indicate only a minor component of strike-slip movement. The northeast-striking accommodation zones are inferred to have developed from a component of extension sub-parallel to the basin margin rather than as transform-style transfer zones.
Presented at Society of Economic Geologists (SEG) Global Exploration 2002: Integrated Methods for Discovery, April 2002 Denver, Colorado, USA
Structural controls on MVT-mineralization in a transtensional graben, Pillara deposit, Lennard shelf, Western Australia
J.M. Miller1, E.P. Nelson2, and M. Hitzman2
1Monash University, Australian Crustal Research Centre, Department of Earth Sciences Monash University, Australia
2Dept. Geology/Geological Engineering, Colorado School of Mines, Golden, CO
Download Abstract (PDF)
Mississippi Valley-type Zn-Pb mineralization at the Pillara deposit formed in Devonian carbonate rocks in a transtensional graben within a major transfer zone along the fault-controlled northern margin of the Canning basin (Lennard shelf). Sphalerite and galena precipitated in graben-bounding faults and associated fractures from metalliferous basinal brines related to basin compaction and dewatering during the final stages of extension in the Late Devonian to Early Carboniferous. The east-dipping Western fault, which contains most of the ore, changes from a NNE-striking, sinistral-normal fault in the south to a NW-striking normal fault in the north; stratigraphic offset increases to the north. Where fault strike changes significantly (>20°), dilatant jogs contain broad, low-grade regions of mineralized fault breccia. Where fault dip changes, or refracts through different stratigraphic units, dilatant jogs contain narrow, high-grade zones of mineralized rock. Colloform sulfide textures and large crystal-filled vugs indicate mineralization occurred mainly by open space filling within fault breccias and vertical extension fractures. Hydrothermal karsting occurred with some evidence for solution collapse, although dolomitization did not occur. Early laminated marine calcite veins are affected by extensive pressure solution (stylolitization) prior to reactivation by several syn-mineralization fracturing events. Vein fill events during the main mineralization phase are calcite-marcasite, marcasite followed by sphalerite-galena, and sparry calcite. In some areas, late stage breccias contain clasts preserving earlier sulfide minerals completely rimmed by sphalerite (or calcite then sphalerite). Later mineralization formed veins with galena and calcite that overprint the earlier mineralized faults, have slightly different kinematics (E-W extension), and are associated with slickenlines on faults throughout the deposit.
Geological analysis of mineralization controls at Arcata, Ares, and Caylloma mines, Peru
Support: Mauricio Hochschild & Cia. Ltda., S.A.C.
Download Abstract (PDF)
Although a number of volcanic-hosted epithermal Ag-Au ore deposits are present in the Tertiary volcanic arc of the southern Peruvian Andes, little is known about the regional and deposit scale structural controls on formation of such deposits. This is despite these deposits having been mined for a very long time, in some cases since the time of the Incas! These deposits have characteristics of both low- and high-sulfidation chemistry, and have structural characteristics typical of fault and vein hosted deposits. Field reconnaissance indicates that both strike-slip fault kinematics and dip-slip (normal) fault kinematics have affected different districts. Three districts are the focus of this project: Arcata, Ares, and Caylloma.
The research program involves three primary tasks: 1) geological mapping, 2) computer model construction, and 3) isotope geochronology and fluid inclusion analysis. Geological mapping will consist of surficial and underground mapping of volcanic geology, geological structures, and alteration. Structures to be mapped include faults, veins, slickenlines (fault and vein movement striations), and tectonic breccias. Volcanic geology will be mapped in order to construct a facies model for the volcanic stratigraphy. Mineral alteration will be mapped to determine the extent of alteration related to the mineralizing system. Cross sections and longitudinal sections and, eventually, 3D computer models of the deposits will be made showing lithology, vein thickness and orientation, ore grade distribution, alteration, and metal ratios. A program of isotope geochronology will be undertaken to date volcanic and mineralization events, and fluid inclusion studies will be undertaken to determine temperature and composition of mineralizing fluids.
Dr. Leandro Echavarria is a post-doctoral research associate who is working on the project. Dr. Echavarria received his Ph.D. (1977) in Ore Deposits at La Plata University, Argentina, and has had post-doctoral positions at La Plata and Cornell Universities.
Structural Controls On The Arcata Epithermal Vein System, Peru
Leandro Echavarria and Eric Nelson
Colorado School of Mines
Arcata is a low sulfidation epithermal deposit located in the southwestern portion of the Huanzo Cordillera, at about 180 km northwest of Arequipa and between 4,600 and 4,900m above sea level. It is situated within a broad belt of Neogene calc-alkaline intermediate to silicic volcanic rocks composed of a thick sequence of andesitic lava flows with thin intercalations of volcaniclastic rocks. These rocks overly a voluminous and widespread sequence of ash flow tuffs.
Principal veins of the Arcata district contain crustiform, symmetrical banding, comb and open space-filling textures. Precious metals are mostly contained in Ag-sulfosalt minerals (mainly pyrargyrite), tetrahedrite, and acanthite with important amounts of sphalerite, galena and chalcopyrite. Ore shoots are laterally continuous and span 250 to 350 meters vertically. The principal veins have average widths from 1 to 2.5 meters. Most of the principal veins are localized by subparallel normal faults that trend generally between N40°W and N70°W and dip between 40° and 65°. Veins in the northeastern part of the district dip to the southwest, whereas veins in the southwest dip to the northeast. The resultant fault pattern forms a graben with several hundred meters of total fault offset. Slickenlines on the walls of two major veins (Baja and Tres Reyes veins) indicate consistent down-dip movement with rakes between 75° and 90°. Structural analysis reveals low plunge tension axes trending between N16° and N29°E and sub-vertical compression axes.
Principal veins contain hydrothermal and tectonic breccia, symmetric banding, open space-fill textures, slickensides on the vein walls, and gouge material, suggesting that extensional tectonic activity and mineralization were simultaneous, with repeated reopening and also post-mineralization movements.
The position of ore is related to the structurally most favorable fault position and orientation. Vein width is mostly controlled by changes in strike and dip of the faults, and the wider portions of veins contain the higher grade ores. The main veins, like Marion and Tres Reyes veins, horsetail to the southeast, forming several splits that also represent normal faults. The movement along those faults produced a progressive down-dropping of fault blocks toward the central part of the graben. In general, ore shoots are much more continuous in the central part of the veins.
Nelson, E., Forsythe, R., and Arit, I., 1994, Ridge collision tectonics in terrane development, J. S. American Earth Sci., 7, 271-278.
Nelson, E.P., 1996, Suprasubduction mineralization: Metallo-tectonic terranes of the southernmost Andes, in Subduction from Top to Bottom, Bebout, G.E., Scholl, D.W., Kriby, S.H., and Platt, J.P., eds., AGU Geophysical Monograph 96, 315-330.
Coe, J.A. and Nelson, E.P., 1997, Characterization of fracture networks using close-range photogrammetric mapping and GIS analysis, in Fractured Reservoirs: Characterization and Modeling, Hoak, T.E., Klawitter, A.L. and Blomquist, P.K., eds., Rocky Mountain Association of Geologists, 1997 Guidebook, p.43-55
Nelson, E.P., Kullman, A.J., Gardner, M.H., and Batzle, M., 1999, Fault-fracture networks and related fluid flow and sealing, Brushy Canyon Formation, west Texas, in Faults and Subsurface Fluid Flow in the Shallow Crust, Haneberg, W.C., Mozley, P.S., Moore, J.C., and Goodwin, L.B., eds., Geophysical Monograph 113, 69-81.
Pichler, T., Ridley, W.I., and Nelson, E., 1999, Low-temperature alteration of dredged volcanics from the southern Chile Ridge: additional information about early stages of seafloor weathering, Marine Geology, 159, 155-177.
Yigit, O., Nelson, E.P., Hitzman, M., 2000, Early Tertiary epithermal gold mineralization, Bahcecik prospect, northeastern Turkey, Mineralium Deposita, v. 35, p.689-696.
Yigit, O., Nelson, E.P., Hitzman, M., and Hofstra, A.H., in press, Structural controls on Carlin-type gold mineralization in the Gold Bar district, Eureka County, Nevada, Economic Geology.
Kullman, A.J., Nelson, E.P., 1997, Fracture Network Predictability in Relation to Bed Thickness, Lithology, and Fault Proximity, Brusy Canyon fm., West Texas: in Geological Society of America Abstracts with Programs, v. 29, No. 6, p. A-220.
Young, O.D. and Nelson, E.P., 1999, Fault Controls on Gold Mineralization, Cortez District, Nevada, Geological Society of Nevada, Geology And Ore Deposits 2000: The Great Basin and Beyond, Symposium, May 2000, p.A14-A15.
Yigit, O. and Nelson E.P., 1999, Paleozoic Structural Controls on Tertiary Gold Mineralization in Gold Canyon Deposit, Eureka County Nevada, Geological Society of Nevada, Geology And Ore Deposits 2000: The Great Basin and Beyond, Symposium, May 2000.
Nelson, E.P., 1999, Faults, Fluid Flow, and Gold Vein Formation: Studies in Oblique Convergent Tectonics, New Zealand, CSM Geology/Geological Engineering Newsletter.
Freitag, K., Hitzman, M., and Nelson, E., 1999, Structural geology of the Lower Southwest orebody, Greens Creek Mine, Southeast Alaska. Pathways Meeting, January 2000, Vancouver, Canada.
Echavarria, L. and Nelson, E., 2002, Structural Controls On The Arcata Epithermal Vein System, Peru, GSA Abstracts.
J.McL. Miller, E.P. Nelson, 2002, Three-dimensional strain during basin formation - orthorhombic fault patterns and associated MVT mineralization, Lennard shelf, Western Australia, GSA Abstracts.
Crustal Age Poster
Color Topo Of Colorado
Color Topo Of Western U.S.
Nasa photos of Chile