GEOLOGY OF ROUND TOP DEPOSIT
The Round Top porphyry system is composed of two distinctive Late Cretaceous to Early Tertiary felsic intrusives hosted by a Paleozoic quartz mica schist. Age dating returned an age of 74 +/- 2.8 Ma. The “East Lobe” is underlain by a biotite quartz monzonite porphyry that hosts an extensive mineralized hydrothermal alteration system centered around an intense quartz vein stockwork. A coarse grained quartz feldspar porphyry is included in this unit. The “West Lobe” is underlain by a controversial rock type designated “Quartz Latite Fragmental”. It contains quartz fragments and phenocrysts, feldspar phenocrysts, and abundant lithic fragments in a porous “cement-like” groundmass that is intensely altered to quartz-sericite. The unit is interpreted as a late, sub-volcanic explosive/hydrothermal breccia. Numerous dacitic and quartz latite dikes crosscut both the intrusive units.
Surface geologic mapping and geochemistry along with subsurface intercepts in drill holes indicate the existence of a large copper-molybdenum system centered on the oval-shaped “pipe like” quartz stockwork zone, 200 meters in diameter, in the East Lobe. An outer lead-silver zone flanks this central zone. Supergene chalcocite enrichment has been intersected by drilling in both the quartz monzonite porphyry and associated intrusive breccias. Copper-silver bearing semi-massive garnet skarn is an important mineralization target. Although only partially mapped at present, the known area of alteration and mineralization extends 1,000 meters east-west and 1,500 meters north-south. The principal host rock is the quartz monzonite porphyry and the coarser grained quartz feldspar porphyry.
Mineralization and alteration appear to be centered on the quartz stockwork veins and breccia and include Quartz-molybdenum veins and a tight, well developed molybdenum geochemical high. Tungsten, as scheelite in quartz veins, is concentrated about 200 meters out from the quartz stockwork. A copper zone, as defined by soil geochemistry, occurs between 250 and 700 meters outward from the quartz stockwork zone. This area is untested by drilling. A well-developed silver-lead-copper zone occurs between 800 to 900 meters outboard of the quartz stockwork zone along the southern and eastern edge of the East Lobe. This classic zoning pattern could produce a significant lead-silver vein system or a copper-silver replacement target hosted by quartz mica schist at the porphyry contact. Discovery drill hole RT-2 (1981) tested the northern contact of the West Lobe and intercepted three zones of significant copper mineralization that ranged between 0.98 and 1.82% Cu over thicknesses of 4.3 to 7.3 meters. Drill hole RT-7 (1981) was the single deep test of the East Lobe and associated quartz stockwork/quartz-feldspar porphyry. This drill hole cut an 18.6 meter thickness of supergene chalcocite enrichment that averaged .48% Cu. Five additional mineralized zones of 4.3 to 36.6 meters in thickness were hosted in quartz stockwork and quartz feldspar porphyry. Skarn mineralization within schist horizons was intersected near the bottom of the hole at 494.3 meters. Copper values ranged between 0.36% Cu at 227.4 meters and 1.1% Cu at 370.6 meters of depth.
ROUND TOP DRILL INTERCEPTS
Round Top was explored with seven diamond drill holes for a total of 1,668 meters (5,473 feet) in 1981, two shallow “Winkie” drill holes in 1982, 7 drill holes (400 meters) at TG in 1982 and 4 shallow drill holes (270 meters) at TG North in 1984. A survey grid was established for 1:2,000 scale geologic mapping and rock and soil geochemical sampling. An extensive ground geophysics program was conducted in 1981-82. Diamond drilling confirmed the existence of supergene enrichment at a depth of 100 meters. Copper-silver skarn and contact style mineralization occurs along surface exposures of the northern contact of the East Lobe porphyry. Drill hole RT-2 tested the northern contact of the West Lobe and discovered three zones of significant copper mineralization that ranged between 0.98 and 1.82% Cu over thicknesses of 4.3 to 7.3 meters. Silver values were reported up to 23 g/t Ag. Drill hole RT-7 was the single deep test of the East Lobe and associated quartz stockwork and quartz-feldspar porphyry. This drill hole cut an 18.6 meter thickness of supergene chalcocite enrichment that averaged .48% Cu. Five additional mineralized zones of 4.3 to 36.6 meters in thickness were hosted in quartz stockwork and quartz feldspar porphyry. Skarn mineralization within a calcareous schist horizon was discovered near the bottom of the hole at a depth of 494.3 meters. Copper values in each zone ranged between 0.36% Cu at 227.4 meters and 1.1% Cu at 370.6 meters of depth.
TG (TIM’S GOSSAN) AND TG NORTH STRUCTURES
The TG (Tim’s Gossan) and TG North Prospects occur along prominent northwest and north-south trending faults and structural zones, between one to two kilometers west (TG) and northwest (TG North) of the Round Top porphyry system. TG North is on a NW trending alignment from the Round Top Main Discovery Gossan area. Both prospects consist of high grade brecciated silver-gold-lead-zinc veins, breccia hosted and replacement mineralization in graphitic and calcareous quartzite and quartz-muscovite schist and they remain untested except by shallow “winkie” drilling that did not intersect sulfide mineralization. TG and TG North are very similar to the high grade Ag-Au-Pb-Zn Illinois Creek and Waterpump Creek deposits located just west and north of the Illinois Creek airstrip. Grades at Waterpump Creek are reported to be 326 g/t (9.5 oz/ton) silver, 16.1% lead and 5.5% zinc.
The TG Prospect (a.k.a. Tim’s Gossan) is an area of gossan subcrop hosted by quartz-chlorite-muscovite schist. Located west of the Round Top prospect, this prospect was discovered in 1980. A soil sampling program and ground EM geophysical survey was conducted in 1982. This work defined Ag-Pb-Zn replacement type mineralization of chloritic limestone and dolomite units that dip 40-50 degrees to the NE into a fault zone that trends NW-SE (130o). The surface gossan and corresponding geochemical anomaly is 30-110 m wide by 800 m long. Surface gossan samples contain values up to 4.52% Pb, 0.84% Zn and 211 g/ton Ag. Four shallow “Winkie” drill holes explored the surface anomaly and intercepted gossan zones of 5.9 m, 7.8 m and 3.75 m in width. Assay results have not yet been recovered from the data library. Large diameter drilling was recommended, but never completed.
TG North was discovered in 1984, the last year of exploration at Round Top. It is an area of anomalous rock and soil geochemistry that is 2,600 meters by 400 meters. Mineralized float of gossanous, vuggy quartzite breccia occurs along a prominent N-S photo linear north of TG. Surface samples contain between 500 and 5,000 ppm (5.0%) Pb and 2 to 20 ppm Ag. Samples of nearly massive gossan from a prospect pit assayed up to 5.7% Pb and 1,203 g/t Ag. Four shallow “Winkie” drill holes tested a small 100 by 40 meter area in late 1984, but no zones of gossan were intercepted in drilling. One drill hole, DDH TG-10, had anomalous Pb-Ag values over 42.9 meters. The host rock is a brittle quartzite. Bowden (1984) recognized a “preponderance of en echelon and cross trending photo linears contained within this unit suggest a great deal of fracturing has occurred”. Drill core observations noted many cross-cutting features and evidence of both replacement and silicification which indicate a epigenetic hydrothermal style of mineralization.
Exposures at Tim’s Gossan and TG North are sparse to non-existent. Surface exploration and drilling programs ended when Anaconda Copper Company ceased operations in 1985. Potential exists for high grade “Bonanza style” epigenetic Ag-Au-Pb-Zn mineralization in structurally controlled veins, breccias and in replacement deposits similar to the Illinois Creek Mine.
ROUND TOP GEOLOGIC MAP WITH TG NORTH GEOCHEMISTRY
The 1984 soil geochemical grid was digitized and contoured March 2011. Silver (below left), gold (below right), lead, zinc, arsenic and antimony anomalies show a strong positive correlation and a strong north-south orientation.