THE GEOLOGY OF THE BORNITE COPPER-ZINC-COBALT CARBONATE-HOSTED DEPOSIT, SOUTHWESTERN BROOKS RANGE, ALASKA
by Douglas Tynan Conner A thesis submitted to the Faculty and Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Geology).
Golden, Colorado
Date ______
Signed: ______Douglas Tynan Conner
Signed: ______Dr. Murray Hitzman Thesis Advisor
Golden, Colorado
Date ______
Signed: ______Dr. Paul Santi Professor and Head Department of Geology and Geological Engineering
ii iii
iv
v vi vii viii ix
LIST OF ABBREVIATIONS
General: ID – TIMS Isotope dilution-thermal ionization mass spectrometry PPL Plane polarized light RL Reflected light XPL Crossed-polarized light
Units: cm Centimeter km Kilometer m Meter µm Micrometer Ma Million years Mt Million (metric) tons % Percent
Minerals: bn bornite cpy chalcopyrite py pyrite cc chalcocite dol dolomite cal calcite fe-dol ferroan dolomite sid siderite chl chlorite cy Cymrite phg phengite alb albite
x
ACKNOWLEDGMENTS
xi
1 2 3 4 5 6 7
8 9 10 11
12 Up
13 14 15 16 2 cm
17 A
B
C
18 FD
DR
LG
7 mm
19 5 cm
20 21
22 B
23
24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 ü
39 C D
40 41 42 43 44 45 46 A
B
47 48 49 50
51 Inward from the pyrite-rich zone, chalcopyrite is the most volumetrically significant sulfide. It typically replaces fine-grained pyrite and is intergrown with late, normal dolomite and calcite that cuts and replaces light-grey, ferroan replacive dolostone clasts (Fig 5.10A,B). Locally, the less cobaltiferous cores of fine-grained pyrite are preferentially replaced by chalcopyrite, creating “atoll” structures (Bernstein and Cox, 1986; Hitzman, 1986). Increasingly copper-rich mineral assemblages, inward from the chalcopyrite-pyrite zone, contain bornite and anhedral to euhedral carrolite intergrown with chalcopyrite (Fig 5.10C,D,E). Carrolite commonly encases skeletal pyrite grains. Carrollite is almost never observed within chalcopyrite- or bornite-bearing dolomite-(calcite) veins but is restricted to zones where copper sulfides replace pyrite. The distribution and textures of carrollite indicate that it formed due to the breakdown of early precipitated cobaltiferous fine-grained pyrite (Hitzman, 1986). The chalcopyrite-bornite-carrollite assemblage generally contains relatively minor intergrown dolomite and calcite but may contain trace amounts of phengite and albite (Fig 5.10C).
52
53
54
55
56
57
58
59 60
61 62 63
64
65 66 67 68 69 70