OF CARBONATE-HOSTED LEAD-ZINC DEPOSITS OF THE KOOTENAY ARC ( 82 FEast Half )
Fieldwork for this study consisted of a preliminary investigation and sampling of carbonate-hosted lead-zinc deposits within the Kootenay arc. Lead isotope data from galena were also determined at The University of British Columbia. Carbonate-hosted lead-zinc deposits in the Kootenay arc have been characterized as either concordant or transgressive types based on field characteristics (Fyles, 1966). In general, the transgressive mineralization occurs as vein or replacement deposits that are enriched in silver. The concordant deposits are stratiform  and  contain  a  simple  base metal sulphide mineral suite with little or no precious metals. The concordant deposits have also been referred to as the Salmo or Remac-type, after several large deposits in the Salmo camp. Lead isotope data from deposits in the Kootenay arc have been used to interpret and explain the source of lead and the ore-forming processes responsible for these deposits. Lead isotope plots for the Remac-type deposits do not conform to lead growth curves constructed for lead-zinc deposits from this region, nor can they be adequately interpreted as the result of lead mixing or lead inheritance. A thorough analysis and comparison of the ore and gangue mineralogy, stratigraphy, and trace element and isotope geochemistry from these conformable Remac-type deposits and the younger Metaline-type deposits will provide an improved understanding of the regional ore-forming process.
Mixing line isochrons; a new interpretation of galena lead isotope data from southeastern British Columbia
Ainsworth Bluebell, Slocan, and Moyie camps. Mixing of lead from two different sources, both of which follow simple growth-curve lead evolution. The more radiogenic source is thought to represent evolution of lead in the upper crustal, continentally derived sedimentary rocks which host these deposits. The two-stage Bluebell growth curve provides an estimate of the uranium-poor lead source which may be the lower crust, the upper mantle, or a mixture of both. Mixing line isochrons drawn between these two curves at ages corresponding to the best estimates for the mineralization ages at these three mining camps closely fit the lead isotope data. This supports the age estimates for all three camps and, in particular, substantiates a Lower Cambrian age for the original mineralization in the Ainsworth and Bluebell deposits. Mixing is assumed to take place at the time of mineralization and may be attributed to magmatic activity in the Moyie and Slocan camps. Applicability of the model to a large part of the Omineca Belt. May be used to date both epigenetic and syngenetic or penecontemporaneous deposits of unknown age.--Modified journal abstract.
Latest Jurassic--Early Cretaceous Regressive Facies, Northeast Africa Craton
Nonmarine to paralic detrital deposits accumulated in six large basins between Algeria and the Arabo-Nubian shield during major regression in latest Jurassic and Early Cretaceous time. The Ghadames (northwestern Libya and adjacent Tunisia and Algeria), Sirte (north-central Libya), and Northern (Egypt) basins lay along the cratonic margin of northeastern Africa. The Murzuk (southwestern Libya and northern Chad), Kufra (southeastern Libya and nearby Sudan), and Southern (Egypt) basins lay in the south within the craton. Data for reconstructing distribution, facies, and thickness of relevant sequences are adequate for the three northern basins only; reconstructions of the southern ones are suggestive at best. High detrital influx near the end of Jurassic time and in mid-Cretaceous time produced regressive nubian facies composed largely of low-sinuosity stream and fan-delta deposits. In the west and southwest the Ghadames, Murzuk, and Kufra basins were filled with a few hundred meters of detritus after long-continued earlier Mesozoic aggradation. The three northeastern basins were filled with 1,000 to 2,000 m of detritus. In northern Egypt the regressive sequence succeeded earlier Mesozoic marine sedimentation; in the Sirte and Southern basins correlative deposits accumulated on Precambrian and Variscan terranes after earlier Mesozoic uplift and erosion. Waning of detrital influx into southern Tunisia and adjacent Libya in the west and into Israel in the east initiated an Albian to early Cenomanian transgression of Tethys. By late Cenomanian time it had flooded the entire cratonic margin, and spread southward into the Murzuk and Southern basins, as well as onto the Arabo-Nubian shield. Latest Jurassic--earliest Cretaceous, mid-Cretaceous, and Late Cretaceous transgressions across northeastern Africa recorded in these sequences may reflect worldwide eustatic sea-level rises. In contrast, renewed large supply of detritus during each regression and a comparable subsidence history of intracratonic and marginal basins imply regional tectonic control.