CHEMOSTRATIGRAPHIC, ALTERATION, AND OXYGEN ISOTOPIC TRENDS IN A PROFILE THROUGH THE STRATIGRAPHIC SEQUENCE HOSTING THE HEATH STEELE-B ZONE MASSIVE SULFIDE DEPOSIT, NEW-BRUNSWICK

This detailed study of a drill hole through the middle Ordovician sect ion that hosts the 25 Mt Heath Steele B zone massive sulfide deposit s hows that the local geology has been misinterpreted. Using immobile el ements, their ratios, and oxygen isotopes of quartz phenocrysts, disti nct stratigraphic footwall (FW) and hanging wall (HW) are recognized i n the sedimentary and felsic crystal tuff (CT) sequence; This indicate s that the two FW sedimentary packages and FW and HW CT packages do no t represent fold repetitions of each other, which has obvious explorat ion significance. Although extremely transposed, the stockwork and lar ger alteration system are recognizable with feldspar hydrolysis to phy llosilicates, an increasing proportion of chlorite relative to K mica, and increasing disseminated and vein sulfides toward the ore horizon. These observations are consistent with increasing (>100 m) whole-rock (Fe + Mg)/(K + Na) and Al2O3/(Al2O3 + K2O + Na2O) toward the massive sulfides. In the FW, these trends are coincident with increasing ST, C u, CO, AS, and Sb, reflecting sulfide saturation. The HW alteration is <50 m wide and manifested by slightly higher (Fe + Mg)/(K + Na), Eu/E u, Fe, Mn, Mg, S, Ba, Co, Cu, Zn, Ph, Sb, and Hg than rocks further u p in sequence. The mineralogy and mineral compositions reflect upper g reenschist grade (450 degrees C and 600 MPa) conditions with an overal l equilibrium distribution of major and trace elements. In the least-a ltered FW CT, chessboard albite variably replaces alkali feldspar phen ocrysts and, with increasing alteration, are hydrolyzed to micas. The Fe/(Fe + Mg) and Al contents of K-mica (phengite-muscovite) and chlori te (ferroan clinochlore to chamosite) increase, reflecting alkali-elem ent leaching, variable silica leaching, and Fe and Mg exchange reactio ns at low pH and high Fe/Mg in the fluid. In the HW, hydrolysis of fel dspar to Ba-bearing phengite and Fe clinochlore is indicative of conti nued fluid venting through the CT and mixing with shallowly circulatin g seawater. At an estimated temperature of 400 degrees C, stockwork ch lorite compositions (X-chamosite = 0.8) indicate an Fe/Mg-fluid = 200 for the buoyant mineralizing hydrothermal plume, whereas the distal ch lorite alteration (X-chamosite = 0.3) reflects local seawater-dominate d alteration peripheral to the mineralizing plume. Whole-rock delta(18 )O values, closely reflecting chlorite content, decrease toward the or e horizon to 5.3 parts per thousand (FW chloritite). At inferred tempe ratures between 300 and 400 degrees C, the ore-forming fluid has a hig h O-18 content (approximate to 5 to 7 parts per thousand) indicative o f extensively modified seawater, typical of those forming large VMS de posits. In the HW, whole-rock delta(18)O values decrease toward the or e horizon from >11 parts per thousand to 8.7 parts per thousand in the altered rock.