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.. ~' ~ t~.~;: <br />Ore Deposits: <br />Produdlve veins in Sunnyslde tone can be divided into two groups according to <br />outcrop character. Veins such as the Sungyside, No Name. and iieshingtort shay <br />prominent outcrops at the surface, and occupy structures fordnq either the northern <br />wall of the Eureka Graben or structures closely associated rlth the wall. Other <br />important veins such as the 2150 and Little Hart' have little or no surface <br />expressions. Underground deve]apment also shows that nearly all of these 'blind' <br />veins strike northeasterly. intersecting the Graben wall at high angles (figure 2). <br />A few, hoxever, have northwesterly strikes (e.g. 2250). <br />Additional mineraloglc and econon>ic differences exist between the two vein groups. <br />All are excellent examples of `cortpound' veins showing aultiple stages of <br />mineralization, however the relative abundance of particular stages varies amongst <br />the veins. Six Major stages can be recognized (Casadevall, 1973); their description <br />and mineralogy are summarized in Table II. <br />In general, veins occupying the Graben wall show extensive development of stages <br />2. 3, and 5. while the northeasterly striking veins are distinct 1n the relative <br />abundance of stage ~. <br />Nallrock alteration related to veining consists of intense silicificatlan <br />immediately ad,)acent to vein walls grading outward into an assemblage of quartz- <br />sericite-tunylte and pyrite (Casadeval), 1973). The extern of the alteration halo <br />surrounding a vein is generally proportional to the vein width, the wallrodc having a <br />bleached gray appearance over a few inches to several feet awrly from tlhe vein. <br />Ore Controls• <br />To date, sixteen veins have been prospected by underground workings at Sungystde. <br />Of these. eight have been stoped extensively. At present, wort is being concentrated <br />on the 2150, Portland, Little Hary. No Name, Sunnyside and Kid Yeins (figure 2). <br />Extensive fluid inclusion, light stable isotope and geocheaical wort by Casadevall <br />and Ohrnto (1979) suggest that ore and gangue arinerals within the reins were <br />deposited from dilute fluids composed predominantly of local meteoric water. <br />Temperature ranges of 260 to 320 degrees Celsius are Indicated for Stage 1-5 <br />mineralization and 170 to 245 degrees Celsius for Stage 6 mineralization. A 570 <br />meter +170 peter depth of mineralization is suggested for Stage 1-!i and about 400 <br />meters far Stage 6. Hetals and salts were leached from the surrounding volcanic <br />wallrock and ;n~sibly from underlying Mesozoic and/or Paleozoic sedimentary rocks. <br />Decreasing temperature appears to have been the primary mechanism responsible for <br />precipitation of the ores. <br />The fact that multiple and distinct stages of oineralization occur within the <br />veins suggests an extremely long-lived hydrothermal system undergoing progressive <br />66 <br />