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P.R. Johnson

Publications and source records attributed to P.R. Johnson.

8 recordsLinked to original sources
USGS2007

Geology and metallogeny of the Ar Rayn terrane, eastern Arabian shield: Evolution of a Neoproterozoic continental-margin arc during assembly of Gondwana within the East African orogen

The Neoproterozoic Ar Rayn terrane is exposed along the eastern margin of the Arabian shield. The terrane is bounded on the west by the Ad Dawadimi terrane across the Al Amar fault zone (AAF), and is nonconformably overlain on the east by Phanerozoic sedimentary rocks. The terrane is composed of a magmatic arc complex and syn- to post-orogenic intrusions. The layered rocks of the arc, the Al Amar group (>689 Ma to ∼625 Ma), consist of tholeiitic to calc-alkaline basaltic to rhyolitic volcanic and volcaniclastic rocks with subordinate tuffaceous sedimentary rocks and carbonates, and are divided into an eastern and western sequence. Plutonic rocks of the terrane form three distinct lithogeochemical groups: (1) low-Al trondhjemite-tonalite-granodiorite (TTG) of arc affinity (632–616 Ma) in the western part of the terrane, (2) high-Al TTG/adakite of arc affinity (689–617 Ma) in the central and eastern part of the terrane, and (3) syn- to post-orogenic alkali granite (607–583 Ma). West-dipping subduction along a trench east of the terrane is inferred from high-Al TTG/adakite emplaced east of low-Al TTG. The Ar Rayn terrane contains significant resources in epithermal Au–Ag–Zn–Cu-barite, enigmatic stratiform volcanic-hosted Khnaiguiyah-type Zn–Cu–Fe–Mn, and orogenic Au vein deposits, and the potential for significant resources in Fe-oxide Cu–Au (IOCG), and porphyry Cu deposits. Khnaiguiyah-type deposits formed before or during early deformation of the Al Amar group eastern sequence. Epithermal and porphyry deposits formed proximal to volcanic centers in Al Amar group western sequence. IOCG deposits are largely structurally controlled and hosted by group-1 intrusions and Al Amar group volcanic rocks in the western part of the terrane. Orogenic gold veins are largely associated with north-striking faults, particularly in and near the AAF, and are presumably related to amalgamation of the Ar Rayn and Ad Dawadimi terranes. Geologic, structural, and metallogenic characteristics of the Ar Rayn terrane are analogous to the Andean continental margin of Chile, with opposite subduction polarity. The Ar Rayn terrane represents a continental margin arc that lay above a west-dipping subduction zone along a continental block represented by the Afif composite terrane. The concentration of epithermal, porphyry Cu and IOCG mineral systems, of central arc affiliation, along the AAF suggests that the AAF is not an ophiolitic suture zone, but originated as a major intra-arc fault that localized magmatism and mineralization. West-directed oblique subduction and ultimate collision with a land mass from the east (East Gondwana?) resulted in major transcurrent displacement along the AAF, bringing the eastern part of the arc terrane to its present exposed position, juxtaposed across the AAF against a back-arc basin assemblage represented by the Abt schist of the Ad Dawadimi terrane. Our findings indicate that arc formation and accretionary processes in the Arabian shield were still ongoing into the latest Neoproterozoic (Ediacaran), to about 620–600 Ma, and lead us to conclude that evolution of the Ar Rayn terrane (arc formation, accretion, syn- to postorogenic plutonism) defines a final stage of assembly of the Gondwana supercontinent along the northeastern margin of the East African orogen.

Precambrian Research↗
USGS2001

Oblique sinistral transpression in the Arabian shield: The timing and kinematics of a Neoproterozoic suture zone

The Hulayfah-Ad Dafinah-Ruwah fault zone is a belt of highly strained rocks that extends in a broad curve across the northeastern Arabian shield. It is a subvertical shear zone, 5-30 km wide and over 600 km long, and is interpreted as a zone of oblique sinistral transpression that forms the suture between the Afif terrane and the Asir-Jiddah-Hijaz-Hulayfah superterrane. Available data suggest that the terranes began to converge sometime after 720 Ma, were in active contact at about 680 Ma, and were in place, with suturing complete, by 630 Ma, The fault zone was affected by sinistral horizontal and local vertical shear, and simultaneous flattening and fault-zone-parallel extension. Structures include sinistral sense-of-shear indicators, L-S tectonite, and coaxial stretching lineations and fold axes. The stretching lineations switch from subhorizontal to subvertical along the fault zone indicating significant variation in finite strain consistent with an origin by oblique transpression. The sense of shear on the fault zone suggests sinistral trajectories for the converging terranes, although extrapolating the shear sense of the suture zone to infer far-field motion must be done with caution. The amalgamation model derived from the chronologic and structural data for the fault zone modifies an existing model of terrane amalgamation and clarifies the definitions of two deformational events (the Nabitah orogeny and the Najd fault system) that are widely represented in the Arabian shield. ?? 2001 Elsevier Science B.V.

Precambrian Research↗
USGS2001

Implications of SHRIMP and microstructural data on the age and kinematics of shearing in the Asir terrane, southern Arabian Shield, Saudi Arabia

The Asir terrane consists of north-trending belts of variably metamorphosed volcanic, sedimentary, and plutonic rocks that are cut by numerous shear zones (Fig. 1). Previous workers interpreted the shear zones as sutures, structures that modify earlier sutures, or structures that define the margins of tectonic belts across which there are significant lithologic differences and along which there may have been major transposition (Frisch and Al-Shanti, 1977; Greenwood et al., 1982; Brown et al., 1989). SHRIMP data from zircons (Table 1) and sense-of-shear data recently acquired from selected shear zones in the terrane help to constrain the minimum ages and kinematics of these shearing events and lead to an overall model of terrane assembly that is more complex than previously proposed.

Gondwana Research↗
USGS1991

Reconnaissance geochemical survey of the Farah Garan-Kutam mineral belt, Kingdom of Saudi Arabia

Wadi-sediment samples were collected from 450 sites in a 580-km 2 area (sample density 1.5 per km 2 ) as part of an exploration program designed to assess the mineral potential of the Farah Garan - Kutam mineral belt and to evaluate mineralization at the Farah Garan ancient working (DGMR Project 3.03.10). Demagnetized panned-concentrate and sieved minus-80-mesh fractions were prepared from each bulk sample of approximately 5 kg. The fractions were analyzed for silver, arsenic, copper, lead, and zinc by atomic-absorption methods and for gold by an atomic-absorption graphite-furnace method. The analytical results for the panned-concentrate samples delineate 21 anomalies, and the results for the minus-80-mesh samples delineate 24. Of the latter, 14 are coincident with panned-concentrate anomaly areas, whereas 10 are uniquely defined. The analytical results of the panned-concentrate samples uniquely define two areas. Overall, the panned-concentrate samples yielded threshold values higher than the threshold values for the minus-80-mesh samples (for gold, 10 times higher), and delineate anomalous areas larger than those for the minus-80-mesh samples. This observation is consistent with the results of similar surveys elsewhere in the Arabian Shield, which indicate that panned-concentrate samples more effectively outline geochemical anomalies than sieved fine-grained samples. In the present survey, geochemical anomalies locate all the sites of mineralization known from previous work. The survey is therefore technically a success. However, a large number of these anomalies probably result from contamination of the wadi systems by metal dispersed from ancient mine workings, and this particular survey, overall, may be of limited value as a guide to the discovery of hitherto unknown mineralization. Nevertheless, the survey outlines two areas that may mark extensions to known mineralization, and a number of other areas in which no mineralization is known. Based on a consideration of the character of the bedrock geology, the value of each reported analytical result in relation to the respective element thresholds, and the number of anomalous samples that cluster in any given area, four areas are recommended for high-priority follow-up sampling.

Open-File Report↗
USGS1990

Reconnaissance geochemical exploration for gold in the Ad Darb area, Kingdom of Saudi Arabia

Geochemical data were analyzed for 244 wadi-sediment samples in an attempt to locate gold exploration targets in late Proterozoic metasedimentary and metavolcanic rocks in the Ad Darb area of the southern Arabian Shield. The target was gold mineralization in high-alumina alteration zones of the type that occurs in the Carolina Slate Belt, eastern United States. Such a target was sought in the Ad Darb area because of the known presence of kyanite in schist of the Sabya formation, which was interpreted to be a possible indicator of late Proterozoic high-alumina hydrothermal activity in the region. Overall, metal values in the wadi-sediment samples are low. The known kyanite mineral occurrence is marked by anomalous gold and tin in three minus-80-mesh samples. The low values of the metals do not justify further exploration for the sought deposit type in the immediate vicinity, although the signature of anomalous elements in the wider region is comparable to the signature known in the Carolina Slate Belt, and is permissive of the interpretation that a diffuse hydrothermal system operated in the region during the late Proterozoic. A large concentration of polymetallic anomalies (gold, arsenic, copper, antimony, tin, tungsten, and lead) is outlined in the northeastern part of the survey area on the basis of panned-concentrate samples. The source of the concentration is unknown, and further investigations are recommended. Recommended low-priority investigation of the source of lead and zinc anomalies in the western part of the survey area would be justified as part of a larger program designed to evaluate the mineral potential of the entire belt of Sabya formation rock.

Open-File Report↗
USGS1990

Hamdah ancient gold mines, Kingdom of Saudi Arabia

The Hamdah prospect is a 1.5-km 2 area that includes ancient mine workings 15 km southeast of Hamdah in the southern Arabian Shield. The workings cluster at a gently dipping thrust contact between serpentinite (above) and hornblende schist (below) exposed in a window within the serpentinite. Aplite sills intrude the contact, and gold concentrations occur just above or below it. The ancient mine dumps cover >100,000 m 2 of the prospect and are estimated to be 1.5 to 1.8 m thick. They contain nearly 181,000 metric tons of material at an average grade of 4.5 g/t gold, thus having an estimated total gold content of 831 kg, or 26,726 Troy oz. Almost 100 percent of the contained gold is recoverable by cyanide acid leaching. Eleven new diamond-drill holes were completed to supplement information from 12 drill holes completed during an earlier drilling program, in order to enable more detailed evaluation of the prospect. The drill holes are grouped in the northeast, southeast, southwest, and west quadrants of the prospect. In the northeast quadrant, where 9 drill holes intersect gold at shallow (<27 m) depths, the quadrant is calculated to contain nearly 242,300 metric tons of gold-bearing material. The calculations show: (1) 19,144 metric tons high-grade material grading 21.4 g/t Au at 5 g/t cutoff; (2) 39,884 metric tons of material grading 8.5 g/t Au at 5 g/t cutoff; (3) 71,617 metric tons of material grading 3.9 g/t Au at 1 g/t cutoff; and (4) 111,641 metric tons low-grade material grading 2.0 g/t Au at 1 g/t cutoff. Total gold content of this selected block is 1,251 kg gold, or 40,234 Troy oz. The mineralized zone in 1 the northeast quadrant is open to the northeast, east, and southeast. Drill data are insufficient in detail to allow grade or tonnage estimates for the southeast, southwest, and west quadrants of the prospect. Gold is concentrated in surface alluvium in the southeast and southwest of the prospect, in amounts that are of possible economic significance. Ground geophysical surveys (CEM and VJJ7) revealed the presence of electrical conductors at shallow depths, some of which correlate with possible disseminated sulfide minerals, and others with the thrust contact. It is recommended that the thickness of the dumps be more accurately determined, and that the gold leachability of bulk dump material be tested. Shallow reverse-circulation drilling is recommended on 25-m centers in the northeast quadrant, and diamond drilling is recommended elsewhere at the prospect. Geophysical surveys are required to help establish depths to the serpentinite/schist contact on the northeastern, southern, and western margins of the prospect. Detailed mapping should be undertaken at the prospect, and a mineral-belt type of mapping program should be completed over the larger Hamdah region.

Open-File Report↗
USGS1985

Geology of the Precambrian rocks of the Jabal Habashi Quadrangle, sheet 26F, Kingdom of Saudi Arabia

The Jabal Habashi quadrangle contains formations of Lower Paleozoic and Cenozoic age, unconformably overlying part of the Precambrian Arabian Shield. The Precambrian formations include metamorphosed and strongly deformed volcanic and volcaniclastic rocks and plutons of calc-alkalic, mafic to intermediate composition, dated at about 645 Ma. These are unconformably overlain by the Maraghan formation of the Murdama group, composed of sandstone and siltstone deposited in a vast sedimentary basin present in the region between about 640 to 620 Ma. On the northwestern margin of this large basin, a smaller, fault-controlled basin was filled by the Hibshi formation, a sequence of conglomerate, sandstones, tuffs and minor lava flows, which dates from 632 Ma. Other volcanic rocks from this period probably accumulated around caldera-like volcanoes. The deposition of the Maraghan and Hibshi formations was interrupted by moderate deformation and metamorphism. About 617 Ma ago, the region was intruded by many granodiorite, granite and subordinate gabbro, diorite and peralkaline granite plutons. Gold-bearing quartz veins formed as a result of this intrusive event. A gap in the geologic record of nearly 40 Ma followed, ended by the emplacement of more evolved syenogranite and alkali-feldspar granite plutons. Hydrothermal alteration of one of these granites led to the formation of tin greisen; other granites contain anomalous amounts of rare-earth elements. The stable rock mass created by these geologic processes was extensively eroded at the end of the Precambrian, and was subsequently covered by flat-lying Lower Paleozoic clastic rocks, the Saq Sandstone of Cambrian (?) to early Ordovician age, and the Tabuk Formation, largely siltstone and claystone, of early Ordovician to early Silurian age. Another major gap in the record, extending from the early Paleozoic to late Cenozoic, ended by the extrusion of basaltic lava flows and cinder cones in the northern part of the quadrangle, about 1.8 Ma ago. The volcanic activity, preceded by tilting of the region and the stripping back of the Paleozoic rocks to expose the Precambrian again, was linked to rifting in the Arabian Shield and the opening of the Red Sea. The final event in the quadrangle was the onset of the present-day type of climate and erosion pattern, leading to the development of drainage channels partly filled by sand and small sabkhah deposits, and surfaces covered by recently cemented gravels.

Open-File Report↗