RECOGNIZING THE TOPS OF DEEP SEDIMENT-HOSTED PORPHYRY COPPER DEPOSITS USING ORBICULAR ACTINOLITE ALTERATION AND HIGH ANTIMONY-ARSENIC-BARIUM VEIN GOSSANS

Deep sediment-hosted porphyry copper deposits occurring within Sevier fold and thrust belt anticlines may be confined and hence exhibit limited upwards manifestation of alteration. Orbicular actinolite alteration with disseminated chalcopyrite occurs as a steeply-dipping distal early-stage alteration at Bingham Utah correlative with Potassic alteration. At the Clementine prospect in Montana surface mapping of the orbicular actinolite alteration shows a large 3 by 5 km ring pattern which dips outward and is co-axially aligned with the axial plane of a doubly-plunging anticlinal dome. Along the apex of the anticline is a system of vein gossans containing up to 3700 ppm As, 2300 ppm Sb, and 3000 ppm Ba with an exposed strike length of 1.5 km and a large elliptical zone containing breccias with two granitic plutons occurring on the east side. U/Pb SHRIMP dating on titanite in the orbicular alteration yield an age of 70.7 +/- 1.3 Ma and 72.7 +/- 1 Ma on the altered pluton which has a zircon age of 76.96 +/- 0.52 Ma. A new chemical thermodynamic phase diagram is presented here in log f CO₂/CH₄) vs log f H₂S coordinates showing redox and sulfidation states of magmatic-hydrothermal and sediment-hosted ore deposits in relation to the sedimentary source rocks of basinal brines and hydrocarbons. Using sulfide, oxide, and alteration silicate mineral assemblages as guideposts, porphyry copper mineral assemblages form a definitive trend line. Butte Montana Central zone high-grade veins define the oxidized, CO₂-dominated end of a linear trend along which the Intermediate zone falls and ends with the early high-temperature pre-Main Stage porphyry copper mineralization at depth. Continuing along this porphyry copper trend line orbicular actinolite alteration at both Bingham and Clementine occurs representing the outer and upper interaction zones with porous wall rock sedimentary formations pore fluids where actinolite forms within orbs by diffusion at the edges of advection. This co-linearity of the orbs with the porphyry copper trend line proves the genetic linkage with early high-temperature porphyry copper mineralization removing any doubt that mapped actinolite orb rings can be indicative of potentially important porphyry copper mineralization. Furthermore, as closed ring patterns co-axial with high Sb, As, and Ba veins, they serve as a powerful targeting tool. Occurrence of barite and Cu-Ag sulfides in pilot diamond drilling at Clementine proves that it is an oxidized porphyry system and not a reduced intrusion-related sheeted quartz vein system.