16 Figure J. A fresh sample of Monzonite Porphyry is depicted including biotite hornblende and plagioclase phenocrysts, biotite is altering to chlorite and plagioclase is altering to clay. (magnification J5x. crossed nicols) Figure 4. A Phyllically altered Monzonite Porphyry. Sericite aggregates are pseudomorphic after plagioclase. (magnification J5x, crossed nicols)

17 Figure 5. A normally zoned plagioclase phenocryst in the Monzonite Porphyry. The calcic core has been altered to clay plus sericite. (magnification 35x, crossed nicols) Figure 6. Chlorite aggregates pseudomorphic after hornblende in the Monzonite Porphyry. (magnification 35x, crossed nicols)

21 Figure 7. A Latite Porphyry with plagioclase phenocrysts set in a groundmass of acicular plagioclase. (magnification 35x, crossed nicols) Figure 8. HaggIs Hump viewed from the north. Various sills have been identified. For symbol explanations see Plate I.

22 ' F i g u r e 9. Plagioclase and quartz set in an aphanitic groundmass in the Quartz Monzonite Porphyry. (magnification 35x, crossed nicols)

25 Figure 10. Slightly fractured embayed quartz phenocrysts with sericite rims in the Quartz-Eye Porphyry. (magnification 35x, crossed nicols)

27 Figure 11. Euhedral feldspar completely altered to sericite in the Quartz-Eye Porphyry. Note the coarser grained sericite outlining the feldspar. (magnification 35x, crossed nicols) Figure 12. Remnant twins in a plagioclase phenocryst in the Quartz-Eye Porphyry. (magnification loox, uncrossed nicols)

r..: 28 Figure 1 3. Acicular laths of biotite altered to sericite imparts a flow structure to the border zone of the Quartz-Eye Porphyry. (magnification 35x, crossed nicols) Figure 14. Aggregates of sericite pseudomorphic after amphibole in the Quartz-Eye Porphyry. (magnification loox, crossed nicols)

29 The groundmass of the Quartz-Eye Porphyry is aphanitic with an aplitic texture. The general appearance is "dusty" due to the presence of finegrained sericite and clay. The only recognizable primary phase in the groundmass is quartz. Zircon was noted but is uncommon. X-ray diffraction scans indicate that sericite and clays of the kaolin group are indeed present. Orthoclase also observed on the x-ray scan may indicate it is present as a phenocryst phase or in the groundmass, or both. Finally, the x-ray scan indicated the presence of dolomite even though it was not noted petrographically. Breccias Distribution Three types of breccias are present in the Black Gulch area: 1) intrusive breccias, 2) contact breccias and, 3) fault breccias. Two bodies of intrusive breccia were mapped, both of which passively intruded shales and limestone of the Benton-Niobrara undifferentiated (Kbn). One body located on the east end of Haggis Hump (Plate I), is termed the Haggis Hump Breccia (Thhb). The body is a breccia dike approximately 2 meters in width and has extremely sharp contacts with the surrounding rocks (fig. 15). The Kbn to the north was not disturbed by the intrusion of the breccia. However, minor calcite veining-in the Kbn may be related to the emplacement of the breccia body, since the Thhb contains disseminated calcite in the matrix.

30 Figure 15. Hogg's Hump Breccia in contact Benton-Niobrara viewed from the northeast. breccia dike is approximately three met~rs the shale. with the The above

31 Figure 16. The Quartz-Eye Porphyry in close proximity to the Dakota sandstone. Note the flow structure and the presence of a quartzite xenolith. (magnification 3Sx, crossed nicols) Figure 17. A Fault Breccia with fragments of Dakota Quartzite in a gossan matrix. (magnification 3Sx, uncrossed nicols)

34 Figure 18. The Black Gulch Breccia with a quartzpyrite-molybdenite vein cutting the breccia matrix and siltstone fragment. (magnification 35x, crossed nicols) ".

4S F i g u r e 21. Garnet Skarn with calcite and pyrite. Note the garnet is anis otrop ic. (magnification J Sx, cros sed nicols)

76 Figure 37. Fragments within the Black Gulch Breccia that were mineralized prior to brecciation. Figure 38. A quartz-pyrite-molybdenite vein cutting both matrix and fragments in the Black Gulch Breccia.

78 Figur e 39. Ph ot omi c r ogr a ph of a polished section depicting f i ne -gr a i ne d diss eminated Molybdenite in t he quartz st ockwork.