Thunderstruck’s 100% owned Rama Porphyry prospect is a large, low-grade copper/gold asset to the south of which the Company has recently delineated its newly discovered Senikura Gold Zone. Since acquiring the asset in 2016, Thunderstruck has undertaken surface geochemical and geophysical exploration campaigns, resulting in numerous anomalous geophysical, rock, trench, and soil samples confirming and extending zones of known gold and copper mineralization.
The Rama Creek porphyry prospect is located on the southern part of Fiji’s largest island, Viti Levu, and is approximately 50 kilometers east south east of Nadi. The prospect sits within Special Prospecting License No. 1425 and, along with landowner agreements, has been renewed for 5 years, the maximum allowed under Fijian exploration policy. The Rama Creek prospect is part of a 148 km2 property located 40 km west of Newcrest’s massive Namosi copper/gold porphyry, in development with Joint Venture partner Mitsubishi, with Proven and Probable Reserves of 1.3 billion tonnes at 0.37 % Cu and 0.12 g/t Au (5.2 M ounces Au and 4.9 M tonnes Cu).
The Rama Creek copper-gold porphyry prospect was discovered in 1971 following reconnaissance stream sediment sampling, geologic mapping and geophysical surveys carried out by Barringer Fiji Ltd. Following discovery, Barringer entered a joint venture with Australian Anglo American Ltd (Anglo).
In 1975 Anglo drilled four vertical diamond drill holes for a total of 802 m (figure 2). Mineralization was intersected in each hole:
- DDH1 intercepted a 244 m interval averaging 0.22 % copper (from 6 to 250 m), ending in mineralization; including an average of 0.23 g/t gold over 66 m (from 6 to 72 m)1.
- DDH3 collared approximately 100 m east of DDH1, returned 60 m averaging 0.098 % copper (from 6 to 66 m) within pyrite and chalcopyrite mineralized volcanics1.
Geology and Mineralization
Geologically, Viti Levu has been exposed to a complexity of crustal plate rotations and has been dominated by island arc development followed by rifting, like that of neighbouring Pacific Rim countries. Magmatism, precious, and base metal emplacement occurred episodically throughout the evolution of the Fijian platform. The oldest know mineral occurrences were emplaced within low-k tholeiitic volcanics of the Wainimala Group, host to the Rama Creek Porphyry prospect.
The prospect occurs in an area where basic and andesitic rocks of the Wainimala Group are intruded by Colo porphyritic intrusions. These intrusions consist of at least two phases. The earlier phase is an ellipsoidal tonalite – quartz diorite intrusion and the later phase is a quartz monzonite which occurs as dykes and small irregular bodies. Stockwork quartz veining occurs mainly in the tonalite, containing the bulk of the copper mineralization.
A recent academic study at Rama Creek confirms that mineralization and alteration styles observed represent a classic porphyry style Cu-Au system. The alteration, the study suggests, is associated with the potassic zone of a porphyry system and has been emplaced through structural controls. The porphyry system is thought to be exposed at moderate depth thus exposing convalesced argillic, phyllic and potassic alteration styles.
Thunderstruck has completed soil, rock, BLEG, trench and geophysical campaigns to better define the porphyry target. Based on the results of geological mapping completed by Thunderstruck, the Rama Creek copper-gold porphyry target exhibits strong a northwest-southeast structural control. The Rama copper-gold stockwork veined porphyry zone is separated from the Senikura Gold Zone to the south by wide northwest trending zone of hydrothermal brecciation.
Senikura Gold Discovery
Thunderstruck’s surface sampling, geophysical and trenching programs have highlighted zones with anomalous gold values and subsequently led to the discovery of the Senikura Gold Zone. The gold zone is an extensive zone of anomalous gold values over a potential 1.5 km strike, offset 1 km to the south of the interpreted centre of the Rama copper-gold porphyry target. Extensive Ridge-and-spur auger soil geochemical sampling has both highlighted and extended the prospect. Furthermore the Senikura Gold Zone target returned trench results of 0.55 g/t gold over 37.6 m; including 0.96 g/t gold over 11 metres (see press release October 22, 2018).
IP/Resistivity Geophysical Surveys
At Rama Creek a series of four survey lines, each 1.5 km in length, were targeted at what is interpreted to be the core of a copper-gold mineralized porphyry system. The results of the survey define an approximately 800 m diameter, bowl shaped, greater than 40 mV/V chargeability high and resistivity low anomaly within resistive, highly altered tonalite intrusive rocks outcropping at surface. The chargeability anomaly extends from surface to a vertical depth of approximately 500 m below surface, the limit of the geophysical inversion.
The chargeability and resistivity low anomalies are coincident at surface with semi-circular, greater than 800 ppm copper and greater-than-200 ppb gold ridge and spur soil geochemical anomalies. In addition, the historic diamond drill hole DDH1, with a 244 m interval averaging 0.22 % Cu and ending in mineralization (including a 72 m interval between 6 m and 78 m averaging 0.23 g/t gold), is coincident with IP chargeability anomalies1.
The exploration of the Rama Creek porphyry prospect has been demonstrated through historic drilling and recent exploration campaigns. The most advanced target is that of the Rama Creek porphyry itself, where historic drilling campaigns were focused, and recent exploration has further highlighted potential. The target warrants additional exploration and deep drilling, and the Company is seeking Joint Venture partners to help facilitate a meaningful program.
Senikura Gold Zone
The predominance of primary mineralization and distal alteration assemblages to the south are interpreted to indicate that the Senikura Gold Zone represents a higher-level manifestation of the same magmatic hydrothermal system as Rama. The implication being that significant potential for the discovery of additional porphyry mineralization at depth beneath the gold zone exists.
Thunderstruck is seeking Joint Venture Partners on Rama.
1 Given the disseminated nature of porphyry copper-gold mineralization, the relationship between the drill intercepts and the true width of mineralization is not known.