Country Rock Fragments Research Articles

The emplacement of some diatreme-facies kimberlites

Kimberlite breccias from several South African pipes or pipe-like enlargements on dykes contain abundant fragments of country rocks including huge, often brecciated, xenolithic masses which are located at positions well below their original stratigraphic levels. The xenoliths show no pyrometamorphic effects indicative of relatively high temperatures during their incorporation in the kimberlite. A striking petrographic feature of these rocks is an emulsion-like texture resulting from the presence of numerous phlogopite-rich, globular segregations, which are set in a matrix which sometimes consists almost solely of primary calcite but in other instances is comprised of variable proportions of calcite and serpentine. Evidence is presented which indicates that in the latter cases serpentine has replaced calcite. The clear separation of the mica-rich segregations and the essentially calcitic matrix is attributed to the development of low temperature, immiscible K-rich silicate and carbonatitic liquids during emplacement of these rocks. The carbonatitic liquids are considered to have acted as the primary transporting fluids during liquid-solid fluidized intrusion and emplacement of the rocks is considered to have taken place as follows. After the crystallization of early phenocrysts the residual, ascending kimberlite magma differentiates at relatively high crustal levels into immiscible silicate and carbonatitic liquids accompanied by a coexisting gaseous phase. Continued intrusion is accompanied by the separation of the gaseous phase and its accumulation at the head of the magma column. Upon further uprise a stage is reached where the internal gas pressure exceeds the lithostatic load and the diatreme is formed by explosive breaching of the cap rocks. The resulting rapid pressure drop is accompanied by the upsurge of partly degassed magma fractions which incorporate explosively disrupted cap rock fragments and material which slumps from the walls of the diatreme.

Genesis of the melt rocks at Tenoumer Crater, Mauritania

A common problem encountered at probable meteorite impact craters is the occurrence of associated melt rocks, presumably produced by the impact event, that cannot be matched chemically to the target rocks. A similar situation is found at Tenoumer crater, Mauritania. Chemical analyses of 11 melt rocks range from felsic country rock values to much more mafic compositions in two well-defined chemical sequences. Only one of the melt rocks is essentially isochemical with the adjacent granodioritic country rock. This specimen consists of a heterogeneous glassy to cryptocrystalline groundmass commonly containing shocked country rock fragments and lechatelierite. It is clearly an impactite. Conversely, some of the more mafic melt rocks display reasonably normal igneous textures and may show no shock features whatsoever. Without the certain knowledge that all these rocks had a common genesis, some of these mafic rocks could be mistaken for products of endogenous magmatic activity. Neither meteoritic contamination nor vapor fractionation can explain the chemical sequences exhibited by these melt rocks. They most likely result from the combination of felsic country rocks with various amounts of an amphibolite vein that appears to strike through the center of the crater. The hypothesis is advanced that the amphibolite was preferentially incorporated in the impact-generated melt because of favorable kinetic and viscosity characteristics. Since mafic segregations are common in Precambrian metamorphic terrains, this mechanism may also explain other similar occurrences. The clear-cut evidence from Tenoumer makes it difficult to deny an impact origin for certain craters solely on the basis of associated melt rocks that are not isochemical with the country rocks.

Framboidal pyrite associated with lead-zinc mineralisation in mid-Wales

Framboidal pyrite occurs in the breccia fragments of country rock found in lead-zinc veins, while it is generally absent from unmineralised rocks. It is thought to be cogenetic with the main mineralisation, representing an early stage of deposition.

GEOLOGY AND ORE DEPOSITS OF THE ROSARIO MINING DISTRICT AND THE SAN JUANCITO MOUNTAINS, HONDURAS, CENTRAL AMERICA

The Rasario Mining District in south-central Honduras is in the highlands of the rugged San Juancito Mountains. This range consists of a thick section of sediments and volcanics, ranging in age from Upper Triassic to Quaternary, which rests on low-grade metamorphic rocks. It is intruded by plugs and dikes of hypersthene andesites, dacite, and granodiorite. Easterly trending arching, north-south and east-west intrusive trends, and blocks faulting are the dominant structural features. The extensive, steeply dipping, westerly and northerly trending vein systems, consisting of some 85 veins and splits, have developed largely in the major dacite intrusive body of the district and in the adjacent andesitic tuffs and clastic sediments. The mineralization consists of free gold, sulfides and sulfosalts of silver, and the sulfides of lead, zinc, and copper and related oxidation products. The gangue is largely quartz, rhodonite, carbonates, and country-rock fragments. The Rosario mines have produced over $93,000,000.00 in silver and gold from some 7,000,000 tons of ore mined since 1883.

Border Rocks of a Granite Batholith Red Lake, Ontario

Granites intrusive into basic volcanic rocks in the Red Lake area are commonly surrounded by a zone of rocks intermediate in composition between the granite and the volcanic rocks. Inclusions of varying color contrast are abundant in the granite. Study of a specific example, by means of variation diagrams and petrographic observations, leads to the conclusion that the intermediate rocks are the product of contamination of the granite magma by numerous fragments of country rocks. The contamination resulted in final-stage activity of potash-rich solutions.