THE WAIHI GOLDFIELD

Evening Post, Volume CIV, Issue 2, 3 July 1922, Page 5

THE WAIHI GOLDFIELD

REJUVENATION POSSIBLE

FURTHER EXPLORATION WORTH

WHILE,

The fascinating pursuit of gold has one of its romances in New Zealand ' in the rioh. production over a long term of years of the great Waihi goldfiejd, which ■until a few years ago was, aa it were, the Bund of the Dominion. The possibility of again striking a richy vein- ofore has always been considered and a preliminary report on -the re-survey of the Waihi. goldaeld by Mr. P. G; 'Morgan, Director of Geological Survey, re-, news hopes of a rejuvenation of the field, making further exploration worth while. The report just published is of a highly technical nature, and only the general conclusions can be properly appreciated by the general reader, but the expert will find the whole extrenfely interesting. , "The Waihi goldfield," declares Mr. Morgan in a summary of his observations, "has apparently reached the stage of old age, and the geologist has been asked to state whether its rejuvenation is possible. On the whole the verdict of the Writer is favourable to a prolobation of the life of the field. Extensive lateral exploration and sinking to a depth of about 2000 feet are recommended." .

The principal reasons for the re-survey, which lasted from November to January last, are given by Mr. Morgan .as follow :—(1) As time- went on some of the conclusions advanced in New Zealand Geological Survey Bulletin No. 15 were found to he untenable, and others at least highly doubtful; (2) new data afforded by the ,deeper workings of the Waihi and Grand Junction mines have become available during the past ten or,; eleven' years;'-'r!(3) the" marked"' tnbiigri'J •perhaps not great evolution of new methods and more especially of new ideas in the science of economic geolosry during the decade 1911-21 has rendered re-. consideration of Waihi geology advisable ; (4) since 1911 work in the deeper levels., of the Waihi. and Grand Junction minos, while continuing to yield considerable amounts of payable ore, has failed to reveal any of those high-grade ore-bodies known as bonanzas,' and therefore it was desirable to ascertain if geological reasoning would point to the presence, or abe«ice ,of such ore-bodies either in.depth or in areas laterally outside the ground already bxplored. Tho area examined dtiring the survey consisted of a belt of. country several miles wide, extending from Owharoa, five miles west of Waihi, to Wiihi Beach, five miles.to the east. The workings of tho Waihi and Grand Junction mines were in part closely examined, and it is intended to make further underground exploration before the main report is finally drafted. Brief visits to Karangahake, Golden Oross, and Maratoto were also mad*. tATERAL EXPLORATION. Tho hypothesis advanced in Bulletin No

15, that an intrusive mass of. dacito contains th« principal ore deposits at Wailii having been completely disproved, the probability of finding ore-bodies by lateral exploration is now much greater than the authors of that bulletin thought. In view, of the great richness of the ground tiiat ,has been worked in. the Waihi and Grand Junction mints, the careful prospecting of all propylitised quartz andpsite outeide tho .knowm lode-bearing aTea is thoroughly justifiabale. In particular, the exploration the western eid© of the Martha Hill, where oxidised rock extends to the greatest depth yet reached in mining operations, is strongly recommendedl. The probability of payable, and even bonanza,, ore at or near the horizon where the oxidised zone merges into the sulphide zone is considerable, andi may even be great. ; DISCOVERING ORE IN DEPTH. As is well known, outcropping oreshoots in tertiary volcanic rocks almost , invariably, ond at depths of from a few hundred feet to 1000 ft, though in a few cases they extend to 1500 ft. In many mines, however, new ore-shoots, not outcropping at the surface, are found even at depths of 1000 ft or more, and some of ..these—rather rarely, it is true—reach depths of over 2000 ft. At Cripple Creek (numerous hidden. ore-shoots have been found during mining op-.ations. The most striking example of bonanza ore in depth appears to be the California and Consolidated Virginia ore-shoot, found towards the northern end of the Coinstock load at. a depth of 1250 ft. It extended to a depth of 1950ffc, and.is said to have yielded one-tenth of the entire production of the lode, or over £6,000,000 worth of gold and silver/. The nearest surface shoot ended more than 1000 ft away. In view of the geological history of the Waihi goldfield, and especially of its former elevation far above sea-level, the existence of rich ore at a depth of 2000 ft is not improbable, though it would be too much to postulate a bonanza so rich and extensive as that of thp California and Consolidated Virginia, , j Against the likelihood of payable gold and silver ore at great depth in tertiary volcanic rocks is urged'not only past experience, but the hypothesis of a critical level for ore-deposition determined by conditions of temperature, and, to a less extent, of pressure. This view is advanced in Bulletin No. 15. While it must /be admitted that if the temperature of the ore-bearing solutions exceeded 365 degrees1 C (the critical temperature of water) ore-deposition. would bs unlikely, there appears to be no reason for supposing that the critical level as. determined '■ by, : temperature was over 5000ft—possibly much more—below the then surface, andi after a liberal allowance for'erosion since the period, of Ibdeformation has: been ■ made, ■it was probably 4000 ft below the present land-sur-face. It is .unlikely that the pressure at that depth would markedly retard oredeposition. A striking' decrease in orevalues has taken place at depths'/over 1000 ft or more, but other causes/ for this can he assigned, and they_ do not exclude,,but rather favour, the view that shoots of payable! ore will be found at still deeper levels. ■. • ■ . , If, however, exploration should be carried to 20(10ft without new ore-shoots being discovered, further, sinking, would not be advisable. From this opinion prob-. ably no one will dissent.