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Abstract

Turn off the main road and move toward field stop 1. Participants can observe variations in alteration starting with patchy occurrences of zone I, followed by zone II, and finally by zone iii assemblages, representing a progressive increase in paleohydrothermal fluid temperatures associated with mineralization. Field stop 1 is located above the southern vein system in the vicinity of Takasuka vein and within zone III alteration, which is characterized by the assemblage quartz + interstratified illite/smectite ± kaolinite (Fig. 1). Analyses from these veins have been as high as 27.7 g/t, and fluid inclusions from the same veins indicate temperatures of about 200° to 250°C (figs. 4 and 5, Murakami and Feebrey, 2001). Also at this location silicified Nagano Formation rocks essentially cap and have preserved mineralization. Sedimentary bedding planes of the Nagano Formation are also still evident in some localities (Figs. 2 and 3).

Also of particular interest in terms of exploration for this style of mineralization is the excellent correspondence between the distribution of mapped zone III alteration, characterized by interstratified illite/smectite, and the area of extremely high K counts obtained during a high-resolu- tion airborne radiometric survey. Points of interest include the amount of vegetation that exists and covers zone III alteration, indicating that significant anomalies are still evident in areas with moderate to high vegetation cover.

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