La Dicha massive sulfide deposit, Chilpancingo de los Bravo Municipality, Guerrero, Mexicoi

Regional Level Types
La Dicha massive sulfide deposit	Deposit
Chilpancingo de los Bravo Municipality	Municipality
Guerrero	State
Mexico	Country

Volcanogenic massive sulfide (VMS) Cu-Zn-Ag deposit, similar to Campo Morado deposit.
NS-trending, W-dipping massive sulfide horizon, more than 4.5 km long, 4 to 45 m thick.
Estimation for the northern portion of the deposit reserves is of about 10 million tonnes.
All three of the points above are open to serious question - refer to the comments below.

Giles Peatfield comments:
The La Dicha massive sulfide deposit is located immediately north of the small village of Ixcuinatoyac, about 30 kilometres south-west of Chilpancingo, or 27 kilometres north-west of El Ocotito, Guerrero State, Mexico. Access is by road from El Ocotito. Note that Google Earth gives a spelling of Inscuinatoyac for the ‘pueblito’, but the name above was used by Klesse (1968) and is regarded as more valid. The co-ordinates given are for the old smelter site, accurate to a few tens of metres.
The present Mindat posting states that the deposit bears a resemblance to the massive sulfide deposits at Campo Morado. This is not correct, as will be detailed in the following description.
It seemed prudent to examine the old literature for early reports on the deposit. Weed (1908) described La Dicha as follows: “A large orebody of the Huelva type is found at La Dicha, in Guerrero, . . . . The orebody occurs in dark gray to light gray micaceous metamorphic schists, carrying quartzite bands, which are at times conglomeratic, thus proving their original sedimentary condition. The ore belt shows a more or less continuous outcrop for 4 miles, with thin (up to 10 ft.) but persistent stringers of primary ore – chalcopyrite mixed with pyrrhotite in mineralized schist. According to R. T. Hill, the ore belt is proved by natural and artificial exposure for 7700 ft. That author [reference not given by Weed] claims an average width of 12 ft. for the first 2100 ft., 88 ft. for the next 2700 ft. along the vein, and then a width of 61 ft. for the remainder of the distance. The ore consists of pyrrhotite, with chalcopyrite in streaks, bunches, and specks scattered throughout it. It sometimes incloses [sic] angular fragments of schist, proving that the vein, although apparently conformable to the schistosity, occupies a fault fissure. The zoisite, pyroxene, etc., of the Ducktown type have not been noted. The primary ore carries, according to the company’s estimate, about 5 per cent copper.” As will be seen, these numbers are wildly exaggerated. The reference to Ducktown is, however, interesting.
Hill (1904) had prepared a detailed report on the property. This report, prepared for The Mitchell Mining Company, who controlled the deposit through its subsidiary The La Dicha Mining and Smelting Co. of Mexico, was in support of a prospectus document. The report is readily available online (see reference for link), and makes interesting reading. It has details of the numerous workings, the smelter on site, and the basic geological setting. However, it must be taken with several grains of salt, as it is, in my opinion at least, wildly promotional. For example, in the prospectus document is a “Statement from the President of the Mitchell Mining Company”, as follows: “With cheap labor, plenty of fuel, inexpensive timber, and cheap transportation, which will be cheaper when the railroad is finished, the experts who have gone over the property all agree with us that the advantages which nature has given us, not only in the above various ways, but in the vast and indescribably large body of copper ore, we have a proposition which, as a money-maker, has no equal on this continent.” The “calculations” of ore in place would in no way be acceptable today. Also, as described below, it would seem that Hill and other early workers did not really understand the most likely form of the sulfide concentrations, as explained below. What could possibly go wrong?
Emmons (1913), describing the deposit, wrote that: “The Dicha mine, Guerrero, is 150 miles south of the city of Mexico. . . . . According to Weed [1908], the orebody, which occurs in micaceous schist, is parallel to the schistosity. . . . . Mr. Robert T. Hill informs me that the chalcocite zone is encountered at very shallow depths and is itself very thin, ranging in thickness from 6 inches to 6 feet. Its contact with pyrrhotite is sharp.” This is important information. In this period of operation of the mine, the very rich chalcocite ore lying above the pyrrhotite, and incidentally below massive cellular limonite gossan, was the feed for a small smelter. In the Statement from the President alluded to above, the following was reported: “The company has in operation a 200-ton [time interval not stated, probably per day] smelter . . . . The smelter, which has been in operation since August 1, 1905, is giving splendid results. In a run of 21 days the gross value of the matte was $131,600, and the net value $104,000. This output is at the rate of about 1,000,000 pounds a month.” The President’s report stated that the company had plans for a much larger operation, including a converter to produce blister copper. As long as the supply of rich chalcocite ore was available, all was looking well, but when the supply of chalcocite was exhausted, attempts to use the primary sulfide ore in the smelter were apparently not successful and the operation ceased, although I cannot find hard written evidence for this. More information regarding the distribution of the chalcocite concentrations will be given below. Photos #1 to #5 are “before and after” pictures showing the smelter and transportation bridge, taken from Hill (1904), and the same scenes taken by me in 1968. One must at this point ask why, with the dimensions and copper grade quoted, had this deposit not been placed into serious production? The answer to this question became clearer in the late 1960’s.
In 1968, I was part of an evaluation project for a major mining company, to attempt to answer the question posed above. This involved a team of geologists, assisted by several workers drawn from the local area. Two photos here (#6 & #7) show members of a visitation by the “Brass” from Canada. Concurrently with our investigation, Elmar Klesse (Klesse, 1968) was undertaking regional mapping for his MSc project at the Universidad Nacional Autónoma de Mexico, working with his supervisor Dr. Zoltan de Cserna (photo #8), who was also acting as a consultant to our project.
Our party undertook an integrated program of geological mapping, soil geochemical sampling and geophysics. In the process, we recognized that there were several areas of extensive work, including adits, at roughly equal intervals of several hundred metres along the entire length of the mineralized zone. It soon became obvious to us that the workings were driven below outcrops of massive limonite gossan (photos #9 & #10). The adits were mostly blocked, but a crew of local men was engaged to ‘muck out’ the tunnels to gain access (photo #11). At the same time, I did detailed mapping on an outcrop area exposed in a small stream (The Barranca de Alcaparrosa), immediately across the Rio Alcaparrosa from the old smelter site [note that in Hill (1904), he used the spelling Al Caparosa but on his map the name is Alcaparosa. The modern accepted spelling is Alcaparrosa, which name I have adopted]. This exposure (photo #12) was originally thought to represent a steeply dipping sulfide body some 30 metres thick. Detailed outcrop scale mapping showed, however, that what was really present was a folded layer, with an eastern boundary nearly vertical and a thin sheet lying essentially parallel to the stream surface, with some areas eroded away completely to show the underlying wallrock. This lens, which I interpreted as an overturned isoclinal fold complex, thinned to the west, until it was represented by a thin, rusty layer less than a centimeter thick, in very weakly folded metasedimentary rocks (photos #13 & #14). Using this information, it became apparent that what we had was a relatively thin sulfide layer, strongly folded, with major thickening in the fold noses. These fold noses were marked on surface by the massive limonite gossans, and under them were the workings where the old-timers had mined the very high grade chalcocite material to feed the smelter. The folded nature of the sulfide zone was noted in my diary entry of February 10th, 1968 which reads “Andy and I underground in [adit] #11, and mapped (sort of) the ore horizon, which appears to be about 2-3 ft. thick, and wickedly overfolded.” A final vindication of this interpretation came when we later diamond drilled in a zone not related to a massive limonite gossan outcrop. My diary entry for Friday, April 5, 1968 reads “At about 130’, cut a 6” to 8” band of rotten sulphides, with some fresh pyrite & chalco & pyrrhotite. Below, fresh quartzites (?) with narrow streaks of fine dissem. pyrite.” (Photo #15 shows drill site.) I think this is most likely the true nature of the sulfide bodies at La Dicha. Klesse (1968) put it very nicely when he wrote “The abandoned La Dicha mine was worked at the beginning of this [20th] century, producing supergene, high-grade copper. Lately, its sulphide-ores became attractive for new operations. With the recognition of the true bedded, and hence folded, nature of the ore, it lost further economic interest.” Hill (1904) had made a comment that could be regarded as prescient. He wrote that “Theoretically, the so-called main ledge should be divisible into a number of bodies having the shape of great lenses, which may broaden, narrow or even bifurcate in places, but the development to date, extensive as it has been, fails to differentiate these lenses, and hence we are obliged to discuss the ledge as one continuous body.” A consulting engineer at the present time would not be allowed to do this in a prospectus report.
In 1969, as a graduate student at Queen’s University at Kingston, I prepared a graduate course seminar (Peatfield, 1969) in which I compared the Nairne pyrite deposit in South Australia to the Ducktown, Tennesee copper deposits, described by Magee (1968), and incidentally to the La Dicha deposit. In this report I made the point that “The folding [at La Dicha] is not nearly as strong in the wall rocks, and one is tempted to assume that the sulphide layer acted as an incompetent horizon, with flowage of sulphides into the noses of the folds.” I cited La Dicha as a possible analog of the folded Ducktown lodes, thus harking back to the comment by Weed (1908), as quoted above, and to Hill (1904), who had made this comparison in detail. In the final conclusions of my seminar report, I quoted Haddon F. King (1965) who wrote “Also, we should have in mind that if the reason for conformability of some deposits, as already discussed, is contemporaneity with the enclosing rocks, then many cases of coincidence of structure and ore could be attributable to the ore (as a lubricant) having localized the structure rather than the structure the ore.”
As regards the stated resemblance to the Campo Morado deposits, note that Lorinczi and Miranda V. (1978) describe these deposits as having a distinct felsic volcanic component, which is lacking at La Dicha. The mineralogy is also significantly different from that at La Dicha; Campo Morado has dominant pyrite, minimal pyrrhotite, and strong zinc mineralization. See also Camprubi et al. (2017) for more on the differences.

Comment on Minerals Reported:
Chalcopyrite: This is the copper mineral in the primary ‘ore’. The attached photo (#16) shows a piece of waterworn material from the Barranca de Alcaparrosa exposure, showing the streaky nature.
Chalcocite: This occurs as a thin blanket under the massive limonite gossan, and immediately above the massive pyrrhotite-chalcopyrite ‘ore’. It provided the feed for the local smelter.
Chalcanthite?: Internal company reports mentioned ‘copper sulphates’ (Peatfield, 1969). I have assumed that this was likely chalcanthite.
Galena: Hill (1904) reported that “. . . a few crystals of galena are now and then present.”
Hematite: Reported by Camprubi et al. (2017), quoting Klesse (1968).
Limonite: Common. The massive, cellular material in the ‘gossans’, see photo #17, was derived by weathering of the massive pyrrhotite concentrations in fold noses.
Magnetite: Reported by Camprubi et al. (2017), quoting Klesse (1968).
Malachite: Internal company reports mentioned malachite (Peatfield, 1969).
Mica group: Klesse (1968) described the host rocks as ‘phyllites and quartzites’; Weed (1908) reported ‘dark gray to light gray micaceous metamorphic schists’. There is no information regarding the precise mica species.
Pyrite: This is common, but much subordinate to pyrrhotite.
Pyrrhotite, var. troilite: Klesse (1968) described a ‘pyrrhotite-troilite-chalcopyrite’ mineral bed. Two specimens in my collection, see photos #16 and #18, contain pyrrhotite that is definitely non-magnetic.
Quartz: Peatfield (1969) reported that “Glassy quartz lenses are common in the sulphide bodies.” Quartz veinlets are also common in the quartzites (see Photo #14).
Sphalerite: Reported by Camprubi et al. (2017), quoting Klesse (1968). Hill (1904) mentioned “a slight trace of zinc”.

Other Regions, Features and Areas containing this locality

References