DNR Reports 326: Bedrock and Glacial Drift Mapping for Volcanogenic Massive Sulfide and Lode Gold Alteration in the Vermilion-Big Fork Greenstone Belt

Report 326 Part A, Introduction:

INTRODUCTION  The Archean Vermillion-Bigfork granite-greenstone belt of northern Minnesota represents the southern extension of the Wawa Subprovince of the Superior Province of the Canadian shield (Figure 1). In Canada, the Wawa Subprovince is the host for a variety of mineral deposits and showings, most notably lode gold and volcanogenic massive sulfides (VMS). Gold mineralization includes the world class Hemlo mining district (Williams, Golden Giant, and David Bell Mines) which contains more than 616 tons of gold at a grade of 7.7 grams/per ton, and the Renabie district (Renabie Mine, Renabie C-Zone, and Braminco No. 21 vein) which contains 34.7 tons of gold at a grade of 6.1 grams/ton (Fyon et al., 1992). VMS deposits located in the Wawa Subprovince include the Winston Lake, Willroy, Big Nama Creek, Willecho, and Geco deposits. The sizes and grades of these VMS deposits are shown in Table 1.

 

Presently, there are no active gold or VMS mines in the Vermillion-Bigfork area, although favorable lithological and alteration mineral associations are present. As well, mineral occurrences of copper, zinc, and gold have been identified. The lack of economic discoveries in northern Minnesota may, in part, represent the difficulty in performing accurate and efficient mineral exploration due to a paucity of outcrops and the local presence of relatively thick glacial deposits.

The Mineral Resources Division of the Minnesota Department of Natural Resources (MDNR) completed a study (Lawler and Riihilouma, 1997) to evaluate the compositions of glacial clasts from gravel pits in the Vermillion-Bigfork area of northeastern Minnesota. This report indicated the presence of mineralized and altered clasts within the glacial drift. Lawler and Riihilouma (1997) have interpreted the mineralization and alteration present in these clasts to be indicative of lode gold and massive sulfide mineralization. At the present time, however, the sources of these clasts remain problematic.

Project 326, "Bedrock and Glacial Drift Mapping for VMS and Lode Gold Alteration in the Vermillion - Big Fork Greenstone Belt" initially set out to define possible source areas for the mineralized clasts areas (MCA's) defined by Lawler and Riihilouma (1997) by means of evaluating bedrock and glacial drift samples in northeastern Minnesota. A total of twenty two diamond drill holes (Table 2, Figure 2) were relogged to: a) better understand the geology (in particular, the physical volcanology) of these regions; b) identify and evaluate the metamorphosed hydrothermal alteration mineral assemblages that occur at each of these prospects; and c) evaluate the potential for volcanogenic massive sulfide (VMS) and/or lode gold mineralization in these areas...

The following report describes the methodology and findings of detailed investigations of selected diamond drill holes from the Five Mile Lake (Soudan Quadrangle, St. Louis County), Eagles Nest (Eagles Nest Quadrangle, St. Louis County), and Quartz Hill (Shagawa Lake Quadrangle, St. Louis County) prospects in the Vermilion district. These investigations have been performed to: a) better understand the geology (in particular, the physical volcanology) of these regions; b) identify and evaluate the metamorphosed hydrothermal alteration mineral assemblages that occur at each of these prospects; and c) evaluate the potential for volcanogenic massive sulfide (VMS) and/or lode gold mineralization in these areas. Diamond drill core logs, petrographic data, and lithogeochemical data performed on samples from the remaining diamond drill holes investigated during this study are contained in Appendices 2, 3, and 4, but are not described in detail during the remainder of the report.

 

Report 326 Part B, Preface:

PREFACE  Glacial indicator tracing, that is the definition of dispersal trains of distinctive lithologies or geochemical signatures in glacial sediment, has proven to be a valuable tool in exploration for volcanogenic massive sulfide or lode gold deposits. This technique has been widely used in Canada and in Scandinavia. However, the usefulness of similar techniques for exploration in Minnesota has been questioned because of rather different glaciological and glacial geological settings. Several attempts have been made to define geochemical anomalies in till, lake sediment, and soils. Although some unusual concentrations of indicator minerals have been noted, no consistent anomalies or dispersal trains have been defined. Part of the reason for the lack of success is that the relation between bedrock and glacial sediment is complicated and there is little agreement on what constitutes a geochemical or lithologic anomaly.

This investigation consists of two parts; 1) a comprehensive study of lithology, alteration, and geochemistry at the Five Mile Lake, Eagles Nest, and Quartz Hill prospects, and 2) an investigation of the utility of glacial drift prospecting in the Vermilion District. This approach to glacial indicator tracing begins with a fundamentally different approach than previous studies. It begins at a known massive sulfide and lode gold prospect in the vicinity of Five Mile Lake. The unique lithologies are then traced in till from known sources. In this manner the lithologic relation between drift and bedrock can be assessed quantitatively. Length scales of dispersal of rock types of different resistance to erosion can be assessed and dilution and comminution coefficients can be calculated. From theses data sampling strategies can be developed that incorporate appropriate spacing for particular lithologic anomalies.

The results of this investigation indicate that the Vermilion District of northeastern Minnesota contains examples of deep water, flow dominated VMS systems (Noranda type), as well as shallow water, volcaniclastic-dominated (Mattabi type) VMS systems. In addition, glacial indicator tracing of lithologic anomalies in till provide an extremely valuable exploration tool.

 

View or download reports

Report 326 Part A: Discussion of Lithology, Alteration, and Geochemistry at the Five Mile Lake, Eagles Nest, and Quartz Hill Prospects
(139 pages, 8.5 x 11 inches, 6.26 MB)
Authors: 1G.J. Hudak and 2R.L. Morton

Report 326 Part B: The Utility of Glacial Drift Prospecting in the Vermilion District, Minnesota
(78 pages, 8.5 x 11 inches, 2.92 MB)
Authors: 2H.D. Mooers, 2P.C. Larson, and 2B. Shmagin

1University of Wisconsin Oshkosh, Department of Geology
2University of Minnesota Duluth, Department of Geology

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