SEG 45 Final_qx4 - Society of Economic Geologists

More documents

Recommendations

Info

20 SEG NEWSLETTER No 53 • APRIL 2003 SEG STUDENT CHAPTER NEWS SEG NEWS LAURENTIAN UNIVERSITY SECONDARY TEACHER WORKSHOP A SUCCESS The Student Chapter of SEG and the Laurentian University Department of Earth Sciences (LU-DES) contacted the Rainbow district school board about the possibility of running a geoscience education workshop for interested secondary school teachers. The workshop was held on the afternoon of January 9, with teachers from 12 secondary school geography departments and two administrators in attendance. Laurentian earth science department staff, Ontario Geological Survey representatives, and student chapter members gave presentations. Many teachers made positive comments and expressed interest in having further interaction between the high schools and the university. In fact, the administrators spread the word to other school departments, and now teachers in other disciplines are pursuing opportunities for various university departments to hold similar workshops. For complete text, go to the website: http://www.segweb.org/SCReport-LUWorkshop.pdf UNIVERSITY OF TORONTO The chapter held a diamond workshop with Professors Daniel Schulze (University of Toronto) and Herb Helmstaedt (Queen’s University) giving hands-on opportunity for petrographic examination of various host rocks and diamonds. The collection included samples from well known diamond districts. Fifteen industry participants and three students (a free-for-students workshop was presented earlier in January) attended. The workshop complemented a lecture-based course given at the Prospectors and Developers Conference (PDAC) the following day. Participants of the 2003 chapter short course on diamond exploration.
APRIL 2003 • No 53 SEG NEWSLETTER 21 Economic Geology — An Invited Commentary on Journal Papers Donald F. Sangster (SEG 1973 F) The Carbonate-hosted Lisheen Zn-Pb-Ag Deposit, County Tipperary, Ireland (M. W. Hitzman (SEG 1978 F), P. B. Redmond (SEG 1997 S), and D. W. Beaty (SEG 1986 F) The Lower Carboniferous carbonate rocks of Ireland host one of the world’s major base metal districts, and this paper, by Hitzman et al. (Economic Geology, v. 97, 2002), is the first comprehensive description of Lisheen, the second-largest of the Irish Zn-Pb deposits (after Navan). The paper presents new information and data on the structural controls of mineralization and hydrothermal alteration, type, and distribution of hydrothermal dolomite, sulfide petrography and paragenesis, metal zonation, constraints on age of mineralization, and the space-time evolution of the Lisheen mineralizing system. The authors assert that the Irish orefield will be important to readers of Economic Geology because “...of the influence that studies of Irish-type deposits have had on the development of genetic models for sediment-hosted deposits worldwide...” (Hitzman et al., 2002, p. 1627–1628). Although the paper is based mainly on information derived from drill core obtained prior to the start of underground operations in 1997, the work is particularly significant because it presents important information with respect to three features: (1) the nature of the process(es) responsible for an increase in pre-mineralization permeability of host rocks; (2) the spatial and genetic relationship between structure and mineralization; and (3) a detailed description of the “black matrix breccia” (and its postulated origin) which so closely accompanies mineralization at Lisheen. Addressing the first of these features, the authors consider the issue of regional dolomitization. In southcentral Ireland, a Lower Carboniferous mudbank complex (the Waulsortian Limestone) has been completely dolomitized on a regional scale, and Lisheen lies within the westernmost margin of this zone of dolomitization. With the use of an electronic field permeameter, Hitzman et al. (2002) present quantitative data illustrating that the pre-ore regional dolomite possesses permeability (mean = 0.22 millidarcies) 10 times that of undolomitized Waulsortian limestone (mean = 0.2 millidarcies). With the use of bore-hole pumps, larger-scale measurements reflecting fracture permeability show regional dolomite to be up to 10 2 times more permeable than the limestone, although the authors admit much of the fracture permeability might be the result of post-ore jointing and fracture enlargement. Nevertheless, the data are the first (to this reviewer’s knowledge) actual permeability measurements of regional dolomite in the area. The authors regard the higher permeability of the dolomitized Waulsortian to be an important feature that permitted the ingress of mineralizing fluids. The second feature, the relationship between faulting and mineralization, is well illustrated by Hitzman et al. (2002) and, in this reviewer’s opinion, is the main strength of the paper. The Lisheen area is dominated by four major eastwest normal faults, all of which are downfaulted to the north, and which have displacements of up to 220 m. Zones between these major faults are broken by normal east-west and northsouth faults with significantly less displacement. The spatial relationships between these faults and several mineralization parameters are well illustrated by several diagrams. For example, the thickest and highest grades of Zn-Pb mineralization and most of the hydrothermal alteration occur in the hanging wall of the major faults. Sulfide thickness, together with iron, zinc, and lead grades, all increase toward one or another of the major and minor faults. Moreover, mineralization associated with normal faulting is not restricted to the immediate Lisheen area. A large normal fault controls location of the G orebody at Galmoy, 6 km northeast of Lisheen, and similar normal faults occur 20 km farther to the northeast at the subeconomic Derrykearn deposit. In the opposite direction, 10 km southwest of Lisheen, another normal fault system is associated with hydrothermal dolomitization similar to that at Lisheen. These observations, taken together, indicate that the Lisheen fault system is part of a larger northeast-trending system ~40 km in length. The authors suggest the faulting was controlled by reactivation of northeast-trending Caledonian basement structures. Evidence is presented to support the concept that the faulting was synsedimentary in nature, beginning in the late Courceyan and continuing into the Chadian, and that most of the displacement took place prior to mineralization. The third mineralization parameter is wall-rock alteration as exemplified by the “black matrix breccia” which is probably the most distinctive feature of the Lisheen deposit. The significance of the black matrix breccia is that most of the ore sulfides are in direct contact with it, usually within the boundaries of the essentially concordant breccia bodies. In the discussion of genesis, the authors contend that hydrothermal fluids produced both the “dolomitematrix” breccia and sulfides. The black matrix breccia is suggested to have formed by wall-rock replacement and precipitation in open spaces of black dolomite crystals, perhaps triggered by movements along the Lisheen fault system and solution collapse. Alternatively, a debris flow origin for similar dolomite breccias at the Silvermines deposit (~30 km west-northwest of Lisheen) was suggested by Mullane and Kinnaird (1998). The complexity of the various breccias present in the Irish Pb-Zn deposits was highlighted recently by Lee and Wilkinson (2002) to page who, on the basis of 22 ... Economic Geology Commentary
Page 1 and 2: SEG NEWSLETTER SOCIETY OF ECONOMIC
Page 3 and 4: APRIL 2003 • No 53 SEG NEWSLETTER
Page 19: APRIL 2003 • No 53 SEG NEWSLETTER

SEG 45 Final_qx4 - Society of Economic Geologists

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?