Non-Traditional Stable Isotopes Completed
Understanding the genesis of ore deposits and their behavior in the environment is a subject of great importance to the Nation. A relatively new tool to aid in these efforts to investigate the origin and environmental effects of ore deposits is the use of "heavy" metal stable isotopes. Our research objectives are to utilize various isotopic systems to advance our understanding of ore genesis and the weathering of mineral deposits. Our studies focus on two themes: 1) use of stable isotopes as source indicators, and 2) use of isotopes as process indicators.
Science Issue and Relevance
Understanding the genesis of ore deposits and their behavior in the environment is a subject of great importance to the Nation. Precious and base metals and critical elements may become enriched in the Earth’s crust by a wide range of processes. However, the formation of an ore deposit requires a sequence of extraordinary conditions and/or events, which remain poorly understood in many cases. A relatively new tool to aid in these efforts to investigate the origin and environmental effects of ore deposits is the use of "heavy" metal stable isotopes. These non-traditional stable isotopes are tracers of specific geologic and biologic processes and can be used to further advance our understanding of metal cycling within magmatic, hydrothermal, and low-temperature systems.
Methodology to Address Issue
Our research objectives are to utilize various isotopic systems to advance our understanding of ore genesis and the weathering of mineral deposits. Our studies focus on two themes: 1) use of stable isotopes as source indicators, and 2) use of isotopes as process indicators.
Isotopic and Geochemical Systematics of Rare Earth Element Ore Deposits: With the increased demand for rare earth deposits outside of China, understanding the processes that control ore-grade enrichment of rare earth elements is critical for not only identifying exploration targets, but also determining which zone of a particular target may be enriched in specific elements of interest. The processes responsible for 1) formation of rare earth element-enriched magmas and 2) ore-grade concentration of rare earth elements are complex and poorly understood. Our objectives are to utilize stable and radiogenic isotopes coupled with trace element geochemistry to constrain the processes responsible for ore-grade enrichment of rare earth elements. We are focusing on carbonatite-related deposits because these deposits:
- are the world's primary source of light rare earth elements (La, Ce, Pr, Nd, Sm, and Eu) and Nb,
- have been mined for PO4, F, Cu, V, Ti, and Ta,
- are potential sources of other critical elements (Th, Y, and other rare earth elements).
Radiogenic, stable, and non-traditional stable isotopic systems can help constrain important questions including:
- what is the source of rare earth elements for these deposits;
- what processes control ore-grade enrichment; and
- why are there over 500 known carbonates but only 9 currently producing ore.
Stable Metal Isotopic and Geochemical Signatures of Element Dispersion from Ore Deposits: Non-traditional isotope ratios may be used to identify sources of metals as they migrate from a mineral deposit or altered area in the weathering environment. These isotopic ratios also may be affected by processes that occur during weathering and transport, including biological uptake. By examining the ratios of selected metal isotopes we hope to resolve the effects of source and process to increase our understanding of the weathering of mineral deposits and surrounding altered areas. Our objectives are to conduct isotopic analyses of several metals, semi-metals, and non-metals, including copper, iron, zinc, and molybdenum to determine the range of values observed in natural systems, and to evaluate the conditions and processes that lead to isotopic fractionation.
We have completed research on the following:
- Method development for copper, iron, and zinc isotopes in complex aqueous samples,
- Fate of metals in mining-affected areas,
- Adsorption of metals onto Fe-oxyhydroxides,
- Using metal isotopes to examine background/baseline conditions and to estimate pre-mining baseline in mined areas.
- Fate of metals in the biosphere - uptake of Zn by aquatic insects.
We are currently working the following:
- Multi-isotope characterization of mineralized areas - what do Fe, Cu, and Zn isotopes tell us?
- Refining methods - lowering blanks for Zn and Cu
Simultaneous Measurement of Trace Elements and Isotopes as Constraints on Ore Forming Processes: In many non-magmatic ore systems the sources of metals and ligands (ion or molecule attached to a metal atom) remains incompletely understood yet is at the heart of determining transport pathways, processes of deposition, and genetic relations between deposits in a mining District. In many deposits the ore minerals are frequently chemically zoned, suggesting complex precipitation mechanisms. Hence, bulk rock compositions, or even bulk chemical properties of individual minerals, cannot reveal the details of the ore forming process or the sources of metals and ligands.
The goal of this study is to evaluate the genetic relations (sources and transport) of metals and ligands that ultimately precipate to form an ore deposits. This task will be looking at the application of the split-flow method to economic geology for simultaneous isotope and trace elements analysis.
Multicollector-Inductively Coupled Plasma-Mass Spectrometry Methods Development: The high resolution multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) laboratory supports the research of this project and other priority research endeavors in the Geology, Geophysics, and Geochemistry Science Center. The laboratory currently houses two MC-ICP-MS units, a quadruple ICP-MS, and a laser ablation system. Isotopic data can assist geologists and geochemists to gain a better understanding in ore genesis processes, metal sources, and metal migration. We focus on continually improving isotope capabilities by improving or developing new methods in support of novel applications for isotope systematics, with current focus on ore genesis and metal mobility and migration.
Completed Activities
Application of Lithium Isotopic Studies to Ore Genesis: Lithium is a rare metal with importance in the nuclear, electronic optical, medical, ceramic, and glass industries. Lithium is identified as an energy-critical element due to the growing demand for lightweight rechargeable lithium-ion batteries to power small electronics, power tools, and vehicles. The current lithium supply is projected to fall short of meeting the near future demands, such that mineral exploration for new lithium resources is quite active. Substantial lithium resources occur in pegmatites, continental brines, and hydrothermally altered clays. Lithium-rich brines currently represent the most economically recoverable lithium source and contribute three-fourths of the global lithium production.
This project is exploring the application of lithium isotopes to constrain geologic processes responsible for the mobility and concentration of lithium, particularly in ore deposit formation. The initial focus is on technique development to measure lithium isotopes in solution and in-situ by (multi-collector) inductively coupled plasma mass spectrometry (ICP-MS) and establishing a procedure for quantitative separation of lithium from rocks, minerals, and waters. Once technique development is complete, efforts will focus on utilizing lithium isotope systematics to understand ore genesis of lithium and other deposits.
Insights into the Genesis of Magmatic-Hydrothermal Ore Deposits and Related Active Mineralizing Systems From "Heavy Metal" Stable Isotopes: The formation of an economically important metal-rich ore deposit requires a sequence of extraordinary conditions and/or events, the details of which remain poorly understood in most cases. The discovery of new ore deposits, and the full production of existing deposits, depends heavily upon clear knowledge of the mineralizing processes and their geologic context. Heavy metal stable isotopes are a relatively new tool to aid in efforts to investigate the origin of ore deposits.
The advent of multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has made it possible to measure variations in the isotopic composition of a wide array of metals from lithium to uranium. High-precision isotopic measurements of these elements in geological materials have revealed mass-dependent variations due to a range of natural processes, a development that has revolutionized the field of stable isotope geochemistry. These heavy metal stable isotopic systems are particularly well suited to studies of the genesis of ore deposits and related mineralizing systems due partly to the relatively high concentrations of many transition metals and metalloids in these systems. In addition, many of the elements that are amenable to stable isotopic investigations are economically or strategically important (directly, or indirectly because they are key elements that are used to understand the origin of mineral and energy resources). Our primary research objective is to apply “heavy metal" stable isotopic systems to case studies of magmatic-hydrothermal ore deposits, and spatially and genetically related modern (active) natural analogs of ore-forming processes.
Isotopic Compositions of Mercury in Ore Deposits and Evaluation of Mercury Isotope Fractionation During Mining Processes: Mercury is a common contaminant in the environment due to both natural and anthropogenic sources. It is a heavy metal of environmental concern because elevated concentrations can be toxic to living organisms. Tracing natural and anthropogenic sources of mercury is critical to the understanding of mercury contamination in various ecosystems. Measurement of mercury isotope fractionation during various geochemical processes is also critical in order to facilitate tracing of various sources of mercury. Our objectives are to evaluate mercury isotopic variability during mine processes and historical deposition from anthropogenic sources through the measurement of mercury isotopes in 1) ore samples from several mercury mines, 2) ore processed mine wastes, and 3) lake sediment cores.
Return to Mineral Resources Program | Geology, Geophysics, and Geochemistry Science Center
Below are other science projects associated with this project.
Below are publications associated with this project.
The Lepanto Cu–Au deposit, Philippines: A fossil hyperacidic volcanic lake complex
Tracing historical trends of Hg in the Mississippi River using Hg concentrations and Hg isotopic compositions in a lake sediment core, Lake Whittington, Mississippi, USA
Quantifying biomineralization of zinc in the Rio Naracauli (Sardinia, Italy), using a tracer injection and synoptic sampling
Investigation of off-site airborne transport of lead from a superfund removal action site using lead isotope ratios and concentrations
Noble gas geochemistry investigation of high CO2 natural gas at the LaBarge Platform, Wyoming, USA
Formation of a low-crystalline Zn-silicate in a stream in SW Sardinia, Italy
Mercury isotope fractionation during ore retorting in the Almadén mining district, Spain
Pathways of coupled arsenic and iron cycling in high arsenic groundwater of the Hetao basin, Inner Mongolia, China: an iron isotope approach
Identification of contamination in a lake sediment core using Hg and Pb isotopic compositions, Lake Ballinger, Washington, USA
Zinc isotope and transition-element dynamics accompanying hydrozincite biomineralization in the Rio Naracauli, Sardinia, Italy
Zinc isotope investigation of surface and pore waters in a mountain watershed impacted by acid rock drainage
Influence of sulfur-bearing polyatomic species on high precision measurements of Cu isotopic composition
- Overview
Understanding the genesis of ore deposits and their behavior in the environment is a subject of great importance to the Nation. A relatively new tool to aid in these efforts to investigate the origin and environmental effects of ore deposits is the use of "heavy" metal stable isotopes. Our research objectives are to utilize various isotopic systems to advance our understanding of ore genesis and the weathering of mineral deposits. Our studies focus on two themes: 1) use of stable isotopes as source indicators, and 2) use of isotopes as process indicators.
Science Issue and Relevance
Understanding the genesis of ore deposits and their behavior in the environment is a subject of great importance to the Nation. Precious and base metals and critical elements may become enriched in the Earth’s crust by a wide range of processes. However, the formation of an ore deposit requires a sequence of extraordinary conditions and/or events, which remain poorly understood in many cases. A relatively new tool to aid in these efforts to investigate the origin and environmental effects of ore deposits is the use of "heavy" metal stable isotopes. These non-traditional stable isotopes are tracers of specific geologic and biologic processes and can be used to further advance our understanding of metal cycling within magmatic, hydrothermal, and low-temperature systems.
Methodology to Address Issue
Our research objectives are to utilize various isotopic systems to advance our understanding of ore genesis and the weathering of mineral deposits. Our studies focus on two themes: 1) use of stable isotopes as source indicators, and 2) use of isotopes as process indicators.
Isotopic and Geochemical Systematics of Rare Earth Element Ore Deposits: With the increased demand for rare earth deposits outside of China, understanding the processes that control ore-grade enrichment of rare earth elements is critical for not only identifying exploration targets, but also determining which zone of a particular target may be enriched in specific elements of interest. The processes responsible for 1) formation of rare earth element-enriched magmas and 2) ore-grade concentration of rare earth elements are complex and poorly understood. Our objectives are to utilize stable and radiogenic isotopes coupled with trace element geochemistry to constrain the processes responsible for ore-grade enrichment of rare earth elements. We are focusing on carbonatite-related deposits because these deposits:
- are the world's primary source of light rare earth elements (La, Ce, Pr, Nd, Sm, and Eu) and Nb,
- have been mined for PO4, F, Cu, V, Ti, and Ta,
- are potential sources of other critical elements (Th, Y, and other rare earth elements).
Radiogenic, stable, and non-traditional stable isotopic systems can help constrain important questions including:
- what is the source of rare earth elements for these deposits;
- what processes control ore-grade enrichment; and
- why are there over 500 known carbonates but only 9 currently producing ore.
Stable Metal Isotopic and Geochemical Signatures of Element Dispersion from Ore Deposits: Non-traditional isotope ratios may be used to identify sources of metals as they migrate from a mineral deposit or altered area in the weathering environment. These isotopic ratios also may be affected by processes that occur during weathering and transport, including biological uptake. By examining the ratios of selected metal isotopes we hope to resolve the effects of source and process to increase our understanding of the weathering of mineral deposits and surrounding altered areas. Our objectives are to conduct isotopic analyses of several metals, semi-metals, and non-metals, including copper, iron, zinc, and molybdenum to determine the range of values observed in natural systems, and to evaluate the conditions and processes that lead to isotopic fractionation.
We have completed research on the following:
- Method development for copper, iron, and zinc isotopes in complex aqueous samples,
- Fate of metals in mining-affected areas,
- Adsorption of metals onto Fe-oxyhydroxides,
- Using metal isotopes to examine background/baseline conditions and to estimate pre-mining baseline in mined areas.
- Fate of metals in the biosphere - uptake of Zn by aquatic insects.
We are currently working the following:
- Multi-isotope characterization of mineralized areas - what do Fe, Cu, and Zn isotopes tell us?
- Refining methods - lowering blanks for Zn and Cu
Simultaneous Measurement of Trace Elements and Isotopes as Constraints on Ore Forming Processes: In many non-magmatic ore systems the sources of metals and ligands (ion or molecule attached to a metal atom) remains incompletely understood yet is at the heart of determining transport pathways, processes of deposition, and genetic relations between deposits in a mining District. In many deposits the ore minerals are frequently chemically zoned, suggesting complex precipitation mechanisms. Hence, bulk rock compositions, or even bulk chemical properties of individual minerals, cannot reveal the details of the ore forming process or the sources of metals and ligands.
The goal of this study is to evaluate the genetic relations (sources and transport) of metals and ligands that ultimately precipate to form an ore deposits. This task will be looking at the application of the split-flow method to economic geology for simultaneous isotope and trace elements analysis.
Multicollector-Inductively Coupled Plasma-Mass Spectrometry Methods Development: The high resolution multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) laboratory supports the research of this project and other priority research endeavors in the Geology, Geophysics, and Geochemistry Science Center. The laboratory currently houses two MC-ICP-MS units, a quadruple ICP-MS, and a laser ablation system. Isotopic data can assist geologists and geochemists to gain a better understanding in ore genesis processes, metal sources, and metal migration. We focus on continually improving isotope capabilities by improving or developing new methods in support of novel applications for isotope systematics, with current focus on ore genesis and metal mobility and migration.
Completed Activities
Application of Lithium Isotopic Studies to Ore Genesis: Lithium is a rare metal with importance in the nuclear, electronic optical, medical, ceramic, and glass industries. Lithium is identified as an energy-critical element due to the growing demand for lightweight rechargeable lithium-ion batteries to power small electronics, power tools, and vehicles. The current lithium supply is projected to fall short of meeting the near future demands, such that mineral exploration for new lithium resources is quite active. Substantial lithium resources occur in pegmatites, continental brines, and hydrothermally altered clays. Lithium-rich brines currently represent the most economically recoverable lithium source and contribute three-fourths of the global lithium production.
This project is exploring the application of lithium isotopes to constrain geologic processes responsible for the mobility and concentration of lithium, particularly in ore deposit formation. The initial focus is on technique development to measure lithium isotopes in solution and in-situ by (multi-collector) inductively coupled plasma mass spectrometry (ICP-MS) and establishing a procedure for quantitative separation of lithium from rocks, minerals, and waters. Once technique development is complete, efforts will focus on utilizing lithium isotope systematics to understand ore genesis of lithium and other deposits.
Insights into the Genesis of Magmatic-Hydrothermal Ore Deposits and Related Active Mineralizing Systems From "Heavy Metal" Stable Isotopes: The formation of an economically important metal-rich ore deposit requires a sequence of extraordinary conditions and/or events, the details of which remain poorly understood in most cases. The discovery of new ore deposits, and the full production of existing deposits, depends heavily upon clear knowledge of the mineralizing processes and their geologic context. Heavy metal stable isotopes are a relatively new tool to aid in efforts to investigate the origin of ore deposits.
The advent of multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has made it possible to measure variations in the isotopic composition of a wide array of metals from lithium to uranium. High-precision isotopic measurements of these elements in geological materials have revealed mass-dependent variations due to a range of natural processes, a development that has revolutionized the field of stable isotope geochemistry. These heavy metal stable isotopic systems are particularly well suited to studies of the genesis of ore deposits and related mineralizing systems due partly to the relatively high concentrations of many transition metals and metalloids in these systems. In addition, many of the elements that are amenable to stable isotopic investigations are economically or strategically important (directly, or indirectly because they are key elements that are used to understand the origin of mineral and energy resources). Our primary research objective is to apply “heavy metal" stable isotopic systems to case studies of magmatic-hydrothermal ore deposits, and spatially and genetically related modern (active) natural analogs of ore-forming processes.
Isotopic Compositions of Mercury in Ore Deposits and Evaluation of Mercury Isotope Fractionation During Mining Processes: Mercury is a common contaminant in the environment due to both natural and anthropogenic sources. It is a heavy metal of environmental concern because elevated concentrations can be toxic to living organisms. Tracing natural and anthropogenic sources of mercury is critical to the understanding of mercury contamination in various ecosystems. Measurement of mercury isotope fractionation during various geochemical processes is also critical in order to facilitate tracing of various sources of mercury. Our objectives are to evaluate mercury isotopic variability during mine processes and historical deposition from anthropogenic sources through the measurement of mercury isotopes in 1) ore samples from several mercury mines, 2) ore processed mine wastes, and 3) lake sediment cores.
Return to Mineral Resources Program | Geology, Geophysics, and Geochemistry Science Center
- Science
Below are other science projects associated with this project.
- Publications
Below are publications associated with this project.
Filter Total Items: 28The Lepanto Cu–Au deposit, Philippines: A fossil hyperacidic volcanic lake complex
Hyperacidic lakes and associated solfatara in active volcanoes are the expression of magmatic gas expansion from source to surface. Here we show for the first time, that the vein system that comprises the ~ 2 Ma high-sulfidation, Lepanto copper–gold deposit in the Mankayan district (Philippines) was associated with a contemporary hyperacidic volcanic lake complex—possibly the first such lake recogAuthorsByron R. Berger, Richard W. Henley, Heather A. Lowers, Michael PribilTracing historical trends of Hg in the Mississippi River using Hg concentrations and Hg isotopic compositions in a lake sediment core, Lake Whittington, Mississippi, USA
Concentrations and isotopic compositions of mercury (Hg) in a sediment core collected from Lake Whittington, an oxbow lake on the Lower Mississippi River, were used to evaluate historical sources of Hg in the Mississippi River basin. Sediment Hg concentrations in the Lake Whittington core have a large 10-15 y peak centered on the 1960s, with a maximum enrichment factor relative to Hg in the core oAuthorsJohn E. Gray, Peter C. Van Metre, Michael J. Pribil, Arthur J. HorowitzQuantifying biomineralization of zinc in the Rio Naracauli (Sardinia, Italy), using a tracer injection and synoptic sampling
Streams draining mined areas throughout the world commonly have high concentrations of Zn. Because Zn is not easily removed from stream water and because it can be toxic to aquatic organisms, its presence is a persistent problem. The discovery of biomineralization of Zn-bearing solids in the mine drainage of Rio Naracauli, in Sardinia, Italy, provides insights into strategies for removing Zn and iAuthorsGiovanni De Giudici, Richard B. Wanty, F. Podda, Briant A. Kimball, Philip L. Verplanck, P. Lattanzi, R. Cidu, D. MedasInvestigation of off-site airborne transport of lead from a superfund removal action site using lead isotope ratios and concentrations
Lead (Pb) concentration and Pb isotopic composition of surface and subsurface soil samples were used to investigate the potential for off-site air transport of Pb from a former white Pb processing facility to neighboring residential homes in a six block area on Staten Island, NY. Surface and subsurface soil samples collected on the Jewett White Pb site were found to range from 1.122 to 1.138 for 2AuthorsMichael J. Pribil, Mark A. Maddaloni, Kimberly Staiger, Eric Wilson, Nick Magriples, Mustafa Ali, Dennis SantellaNoble gas geochemistry investigation of high CO2 natural gas at the LaBarge Platform, Wyoming, USA
A regional sampling of gases from thermal springs near the LaBarge Field, Wyoming, USA to determine the extent of the total carbon dioxide system (TCDS) indicates that the system may extend up to 70 km to the northwest of the field. Geochemical evidence from noble gas isotopes, stable element isotopes, and gas composition provide the foundation for these conclusions. Samples from Soda Springs to tAuthorsMatthew D. Merrill, Andrew G. Hunt, Celeste D. LohrFormation of a low-crystalline Zn-silicate in a stream in SW Sardinia, Italy
n southwestern Sardinia, Italy, the Rio Naracauli drains a catchment that includes several abandoned mines. The drainage from the mines and associated waste rocks has led to extreme concentrations of dissolved Zn, but because of the near-neutral pH, concentrations of other metals remain low. In the reach from approximately 2300 to 3000 m downstream from the headwaters area, an amorphous Zn-silicatAuthorsRichard B. Wanty, G. De Giudici, P. Onnis, D. Rutherford, B. A. Kimball, F. Podda, R. Cidu, P. Lattanzi, D. MedasMercury isotope fractionation during ore retorting in the Almadén mining district, Spain
Almadén, Spain, is the world's largest mercury (Hg) mining district, which has produced over 250,000 metric tons of Hg representing about 30% of the historical Hg produced worldwide. The objective of this study was to measure Hg isotopic compositions of cinnabar ore, mine waste calcine (retorted ore), elemental Hg (Hg0(L)), and elemental Hg gas (Hg0(g)), to evaluate potential Hg isotopic fractionaAuthorsJohn E. Gray, Michael J. Pribil, Pablo L. HiguerasPathways of coupled arsenic and iron cycling in high arsenic groundwater of the Hetao basin, Inner Mongolia, China: an iron isotope approach
High As groundwater is widely distributed all over the world, which has posed a significant health impact on millions of people. Iron isotopes have recently been used to characterize Fe cycling in aqueous environments, but there is no information on Fe isotope characteristics in the groundwater. Since groundwater As behavior is closely associated with Fe cycling in the aquifers, Fe isotope signatuAuthorsHuaming Guo, Chen Liu, Hai Lu, Richard B. Wanty, Jun Wang, Yinzhu ZhouIdentification of contamination in a lake sediment core using Hg and Pb isotopic compositions, Lake Ballinger, Washington, USA
Concentrations and isotopic compositions of Hg and Pb were measured in a sediment core collected from Lake Ballinger, near Seattle, Washington, USA. Lake Ballinger has been affected by input of metal contaminants emitted from the Tacoma smelter, which operated from 1887 to 1986 and was located about 53 km south of the lake. Concentrations and loadings of Hg and Pb in Lake Ballinger increased by asAuthorsJohn E. Gray, Michael J. Pribil, Peter C. Van Metre, David M. Borrok, Anita ThapaliaZinc isotope and transition-element dynamics accompanying hydrozincite biomineralization in the Rio Naracauli, Sardinia, Italy
The Rio Naracauli in SW Sardinia drains part of the Ingurtosu Zn–Pb mining district, and contains extreme concentrations of dissolved Zn at near-neutral pH. In the upper reaches of the stream, pH, alkalinity and Zn concentrations are such that hydrozincite [Zn5(CO3)2(OH)6] precipitates in a biologically mediated process facilitated by a microalga (Chlorella sp.) and a cyanobacterium (Scytonema sp.AuthorsRichard B. Wanty, F. Podda, Giovanni De Giudici, R. Cidu, Pierfranco LattanziZinc isotope investigation of surface and pore waters in a mountain watershed impacted by acid rock drainage
The pollution of natural waters with metals derived from the oxidation of sulfide minerals like pyrite is a global environmental problem. However, the metal loading pathways and transport mechanisms associated with acid rock drainage reactions are often difficult to characterize using bulk chemical data alone. In this study, we evaluated the use of zinc (Zn) isotopes to complement traditional geocAuthorsSuzan Aranda, David M. Borrok, Richard B. Wanty, Laurie S. BalistrieriInfluence of sulfur-bearing polyatomic species on high precision measurements of Cu isotopic composition
An increased interest in high precision Cu isotope ratio measurements using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has developed recently for various natural geologic systems and environmental applications, these typically contain high concentrations of sulfur, particularly in the form of sulfate (SO42-) and sulfide (S). For example, Cu, Fe, and Zn concentrationsAuthorsM.J. Pribil, R. B. Wanty, W.I. Ridley, D.M. Borrok