Geology Reports⌕ Search

Geology topics

Jeff L. Doebrich

Publications and source records attributed to Jeff L. Doebrich.

12 recordsLinked to original sources
USGS2016

U.S. Geological Survey Mineral Resources Program—Mineral resource science supporting informed decisionmaking

The USGS Mineral Resources Program (MRP) delivers unbiased science and information to increase understanding of mineral resource potential, production, and consumption, and how mineral resources interact with the environment. The MRP is the Federal Government’s sole source for this mineral resource science and information. Program goals are to (1) increase understanding of mineral resource formation, (2) provide mineral resource inventories and assessments, (3) broaden knowledge of the effects of mineral resources on the environment and society, and (4) provide analysis on the availability and reliability of mineral supplies.

Fact Sheet↗
USGS2015

Book review: Natural resources in Afghanistan: Geographic and geologic perspectives on centuries of conflict

This book is the outcome of four decades of work in Afghanistan by the author, John (Jack) Shroder. His travels and research throughout Afghanistan and his understanding of its place in regional and world history provide the foundation for this comprehensive 572-page reference. The book describes the interrelated nature of Afghanistan’s physical and political landscape over time and the role resources have, and have not, played in Afghanistan’s past and could play in its future. Review info: Natural resources in Afghanistan: Geographic and geologic perspectives on centuries of conflict by John F. Shroder, 2014. ISBN 978-01280013356, 572 pp.

Economic Geology↗
USGS2011

Zinc-The key to preventing corrosion

Centuries before it was identified as an element, zinc was used to make brass (an alloy of zinc and copper) and for medicinal purposes. Metallic zinc and zinc oxide were produced in India sometime between the 11th and 14th centuries and in China in the 17th century, although the discovery of pure metallic zinc is credited to the German chemist Andreas Marggraf, who isolated the element in 1746. Refined zinc metal is bluish-white when freshly cast; it is hard and brittle at most temperatures and has relatively low melting and boiling points. Zinc alloys readily with other metals and is chemically active. On exposure to air, it develops a thin gray oxide film (patina), which inhibits deeper oxidation (corrosion) of the metal. The metal's resistance to corrosion is an important characteristic in its use.

Fact Sheet↗
USGS2009

Geology and Nonfuel Mineral Deposits of Africa and the Middle East

A nation's endowment of nonfuel mineral resources, relative to the world's endowment, is a fundamental consideration in decisions related to a nation's economic and environmental well being and security. Knowledge of the worldwide abundance, distribution, and general geologic setting of mineral commodities provides a framework within which a nation can make decisions about economic development of its own resources, and the economic and environmental consequences of those decisions, in a global perspective. The information in this report is part of a U.S. Geological Survey (USGS) endeavor to evaluate the global endowment of both identified and undiscovered nonfuel mineral resources. The results will delineate areas of the world that are geologically permissive for the occurrence of undiscovered selected nonfuel mineral resources together with estimates of the quantity and quality of the resources. The results will be published as a series of regional reports; this one provides basic data on the identified resources and geologic setting, together with a brief appraisal of the potential for undiscovered mineral resources in Africa and the Middle East. Additional information, such as production statistics, economic factors that affect the mineral industries of the region, and historical information, is available in U.S. Geological Survey publications such as the Minerals Yearbook and the annual Mineral Commodity Summaries (available at http://minerals.usgs.gov/minerals).

Open-File Report↗
USGS2004

Ad Duwayhi, Saudi Arabia: Geology and geochronology of a neoproterozoic intrusion-related gold system in the Arabian shield

The Ad Duwayhi gold deposit, located in the east-central part of the Arabian shield, is the newest gold discovery in Saudi Arabia. Exploration to date has identified a gold resource of greater than 1 million ounces (oz) with significant potential for expansion. Gold mineralization was closely associated, in time and space, with emplacement of a late- to postorogenic, intracrustal, northwest-oriented granite body (659 ± 7 Ma) and comagmatic square quartz porphyry (646 ± 11 Ma), a hypabyssal and perhaps younger phase of the granite. Mineralization was largely confined to northeast-striking, southeast-dipping fault zones. Hydrothermal alteration consisted of early biotitic alteration overprinted by sericitic alteration. Sericitic alteration was coincident with gold mineralization and produced a quartz-sericite-carbonate-pyrite-rutile mineral assemblage, found both as vein fill and wall-rock alteration products. Mineralization styles, in the following general paragenetic sequence, include (1) quartz-molybdenite veins in and near the granite stock, (2) low-grade gold-bearing quartz vein breccia in and along the margins of the granite stock, (3) gold-bearing stockwork and sheeted quartz veins, and (4) massive to banded gold-rich tabular quartz veins. The gold-bearing stockwork, sheeted, and tabular veins are generally spatially associated with square quartz porphyry dikes and more distal to the granite stock. Mineralized zones at Ad Duwayhi are characterized by low sulfide and base metal content and gold/silver ratios of approximately 6/1. Gold shows no significant correlation with other metals, except lead, and moderate correlation with silver. Re-Os dating of molybdenite from a quartz-molybdenite vein and a tabular quartz vein with cogenetic gold produced robust ages of 655.6 ± 2.7 and 649.9 ± 2.3 Ma, respectively, documenting that gold mineralization and crystallization of granite and square quartz porphyry were, within uncertainty, coeval events. This age correlation combined with granite textural features, the presence of unidirectional solidification textures in granite and square quartz porphyry, and the nature and time-space distribution of mineralization styles, all indicate that mineralization evolved in and near the interface between a crystallizing magma and the surrounding rocks and, thus, is consistent with an intrusion-related genesis. In light of our findings at Ad Duwayhi, a reassessment of similar intrusion-hosted deposits in the Arabian shield is warranted, and areas of late- to postorogenic plutonism, particularly in the Afif composite terrane, should be considered prospective for intrusion-related gold systems.

Economic Geology↗
USGS1991

Geology of the Ishmas gold district, Kingdom of Saudi Arabia

The Ishmas gold district was mapped at 1:25,000 scale to place auriferous mineralization into geologic perspective, to assist in creating an ore-deposit model, and to aid in devising a strategy for subsequent exploration elsewhere in the Jabal Ishmas-Wadi Tathlith gold belt. The precratonic evolution of the district began with the deposition of a tholeiitic mafic volcanic and volcaniclastic sequence that was intruded by diabase and gabbro. Basaltic to rhyodacitic volcanism following a calc-alkaline evolutionary trend ensued. Subsequent deposition of a thick wacke and sandstone unit represented the final phase in the volcano sedimentary accumulation. The emplacement of a large lopolithic layered-gabbro complex marked the end of the precratonic evolutionary cycle. The district coincides with the boundary of two allochthonous terranes. The collisional Nabitah orogeny represents the suturing of the two terranes. The effects of this event are manifested by numerous north-trending, steeply dipping faults, shear zones, and mylonite belts, as well as diapiric serpentinite. During the waning stages of the orogeny, auriferous quartz pods were precipitated in dilatant structures within the north-trending shear zones by deep-seated circulating fluids. The emplacement of a tonalite stock was closely followed by the formation of N. 20°-35° W. -trending faults. These faults influenced the emplacement of dacite porphyry stocks and associated auriferous quartz veins. The auriferous veins are massive, tabular open-fracture fillings that are spatially, temporally, and genetically related to the dacite porphyry. The emplacement of a quartz monzodiorite stock was responsible for additional auriferous quartz vein mineralization that is almost exclusively hosted by the intrusion. A nearly random orientation of the veins indicates that no regional structure influenced their formation. The formation of a series of N. 60°-80° W -trending faults represents the final episode in the district's geologic history and corresponds to the cratonic reactivation that affected a large part of the Arabian Shield (that is, the Najd faulting event).

Open-File Report↗
USGS1987

Geology of the Ar Rahail ancient gold mine, Kingdom of Saudi Arabia

The Ar Rahail ancient gold mine is a collection of ancient workings in metasedimentary rocks of the Murdama Group adjacent to, and above the flanks of the Rahail stock. Injection of mafic dikes of regional origin was followed by emplacement of the Rahail stock, which consisted of early dikes of porphyritic biotite-bearing hornblende granodiorite, and a later cupola of porphyritic monzogranite. Crystallization of the cupola produced a molybdenum-bearing quartz vein stockwork. Intrusion-heated fluids developed chlorite and sericite alteration zones in the older granodiorite dikes. Pre-existing northwest-trending faults, possibly re-opened by stock emplacement, were invaded by later fluids that precipitated barren quartz veins and, in the adjacent faulted wall rocks, anomalous gold and arsenic. Gold, however, is restricted to the narrow structures, and, although values as much as 4.2 g/t are present, the tonnages are inadequate for profitable mining. No further work is recommended, because the hoped for dissemination of gold between faults does not exist.

Open-File Report↗